The Free High School Science Texts: Textbooks for High School Students Chemistry

The Free High School Science Texts: Textbooks for High School Students Chemistry
FHSST Authors
The Free High School Science Texts:
Textbooks for High School Students
Studying the Sciences
Chemistry
Grades 10 - 12
Version 0
November 9, 2008
ii
Copyright 2007 “Free High School Science Texts”
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FHSST Core Team
Mark Horner ; Samuel Halliday ; Sarah Blyth ; Rory Adams ; Spencer Wheaton
FHSST Editors
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Whitfield
FHSST Contributors
Rory Adams ; Prashant Arora ; Richard Baxter ; Dr. Sarah Blyth ; Sebastian Bodenstein ;
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Daniels ; Sean Dobbs ; Fernando Durrell ; Dr. Dan Dwyer ; Frans van Eeden ; Giovanni
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Andrew Kubik ; Dr. Marco van Leeuwen ; Dr. Anton Machacek ; Dr. Komal Maheshwari ;
Kosma von Maltitz ; Nicole Masureik ; John Mathew ; JoEllen McBride ; Nikolai Meures ;
Riana Meyer ; Jenny Miller ; Abdul Mirza ; Asogan Moodaly ; Jothi Moodley ; Nolene Naidu ;
Tyrone Negus ; Thomas O’Donnell ; Dr. Markus Oldenburg ; Dr. Jaynie Padayachee ;
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iii
iv
Contents
I
II
Introduction
1
Matter and Materials
3
1 Classification of Matter - Grade 10
1.1
1.2
5
Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
1.1.1
Heterogeneous mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.1.2
Homogeneous mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.1.3
Separating mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Pure Substances: Elements and Compounds . . . . . . . . . . . . . . . . . . . .
9
1.2.1
Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
1.2.2
Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
1.3
Giving names and formulae to substances . . . . . . . . . . . . . . . . . . . . . 10
1.4
Metals, Semi-metals and Non-metals . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.1
Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.2
Non-metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.4.3
Semi-metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.5
Electrical conductors, semi-conductors and insulators . . . . . . . . . . . . . . . 14
1.6
Thermal Conductors and Insulators . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.7
Magnetic and Non-magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . 17
1.8
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 What are the objects around us made of? - Grade 10
21
2.1
Introduction: The atom as the building block of matter . . . . . . . . . . . . . . 21
2.2
Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1
Representing molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3
Intramolecular and intermolecular forces . . . . . . . . . . . . . . . . . . . . . . 25
2.4
The Kinetic Theory of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5
The Properties of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.6
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3 The Atom - Grade 10
3.1
35
Models of the Atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.1
The Plum Pudding Model . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.2
Rutherford’s model of the atom
v
. . . . . . . . . . . . . . . . . . . . . . 36
CONTENTS
3.1.3
3.2
3.3
CONTENTS
The Bohr Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
How big is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.1
How heavy is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.2
How big is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Atomic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.3.1
The Electron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.2
The Nucleus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4
Atomic number and atomic mass number . . . . . . . . . . . . . . . . . . . . . 40
3.5
Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.6
3.7
3.8
3.9
3.5.1
What is an isotope? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.5.2
Relative atomic mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Energy quantisation and electron configuration . . . . . . . . . . . . . . . . . . 46
3.6.1
The energy of electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.6.2
Energy quantisation and line emission spectra . . . . . . . . . . . . . . . 47
3.6.3
Electron configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.6.4
Core and valence electrons . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6.5
The importance of understanding electron configuration . . . . . . . . . 51
Ionisation Energy and the Periodic Table . . . . . . . . . . . . . . . . . . . . . . 53
3.7.1
Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.7.2
Ionisation Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
The Arrangement of Atoms in the Periodic Table . . . . . . . . . . . . . . . . . 56
3.8.1
Groups in the periodic table
. . . . . . . . . . . . . . . . . . . . . . . . 56
3.8.2
Periods in the periodic table . . . . . . . . . . . . . . . . . . . . . . . . 58
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4 Atomic Combinations - Grade 11
63
4.1
Why do atoms bond? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2
Energy and bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3
What happens when atoms bond? . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4
Covalent Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4.1
The nature of the covalent bond . . . . . . . . . . . . . . . . . . . . . . 65
4.5
Lewis notation and molecular structure . . . . . . . . . . . . . . . . . . . . . . . 69
4.6
Electronegativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.7
4.8
4.6.1
Non-polar and polar covalent bonds . . . . . . . . . . . . . . . . . . . . 73
4.6.2
Polar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Ionic Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.7.1
The nature of the ionic bond . . . . . . . . . . . . . . . . . . . . . . . . 74
4.7.2
The crystal lattice structure of ionic compounds . . . . . . . . . . . . . . 76
4.7.3
Properties of Ionic Compounds . . . . . . . . . . . . . . . . . . . . . . . 76
Metallic bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.8.1
The nature of the metallic bond . . . . . . . . . . . . . . . . . . . . . . 76
4.8.2
The properties of metals . . . . . . . . . . . . . . . . . . . . . . . . . . 77
vi
CONTENTS
4.9
CONTENTS
Writing chemical formulae
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.9.1
The formulae of covalent compounds . . . . . . . . . . . . . . . . . . . . 78
4.9.2
The formulae of ionic compounds . . . . . . . . . . . . . . . . . . . . . 80
4.10 The Shape of Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.10.1 Valence Shell Electron Pair Repulsion (VSEPR) theory . . . . . . . . . . 82
4.10.2 Determining the shape of a molecule . . . . . . . . . . . . . . . . . . . . 82
4.11 Oxidation numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5 Intermolecular Forces - Grade 11
91
5.1
Types of Intermolecular Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.2
Understanding intermolecular forces . . . . . . . . . . . . . . . . . . . . . . . . 94
5.3
Intermolecular forces in liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6 Solutions and solubility - Grade 11
101
6.1
Types of solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.2
Forces and solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.3
Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7 Atomic Nuclei - Grade 11
107
7.1
Nuclear structure and stability . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.2
The Discovery of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3
Radioactivity and Types of Radiation . . . . . . . . . . . . . . . . . . . . . . . . 108
7.4
7.3.1
Alpha (α) particles and alpha decay . . . . . . . . . . . . . . . . . . . . 109
7.3.2
Beta (β) particles and beta decay . . . . . . . . . . . . . . . . . . . . . 109
7.3.3
Gamma (γ) rays and gamma decay . . . . . . . . . . . . . . . . . . . . . 110
Sources of radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7.4.1
Natural background radiation . . . . . . . . . . . . . . . . . . . . . . . . 112
7.4.2
Man-made sources of radiation . . . . . . . . . . . . . . . . . . . . . . . 113
7.5
The ’half-life’ of an element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7.6
The Dangers of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.7
The Uses of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.8
Nuclear Fission
7.9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.8.1
The Atomic bomb - an abuse of nuclear fission . . . . . . . . . . . . . . 119
7.8.2
Nuclear power - harnessing energy . . . . . . . . . . . . . . . . . . . . . 120
Nuclear Fusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
7.10 Nucleosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
7.10.1 Age of Nucleosynthesis (225 s - 103 s) . . . . . . . . . . . . . . . . . . . 121
7.10.2 Age of Ions (103 s - 1013 s) . . . . . . . . . . . . . . . . . . . . . . . . . 122
7.10.3 Age of Atoms (1013 s - 1015 s) . . . . . . . . . . . . . . . . . . . . . . . 122
7.10.4 Age of Stars and Galaxies (the universe today) . . . . . . . . . . . . . . 122
7.11 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
vii
CONTENTS
CONTENTS
8 Thermal Properties and Ideal Gases - Grade 11
125
8.1
A review of the kinetic theory of matter . . . . . . . . . . . . . . . . . . . . . . 125
8.2
Boyle’s Law: Pressure and volume of an enclosed gas . . . . . . . . . . . . . . . 126
8.3
Charles’s Law: Volume and Temperature of an enclosed gas . . . . . . . . . . . 132
8.4
The relationship between temperature and pressure . . . . . . . . . . . . . . . . 136
8.5
The general gas equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
8.6
The ideal gas equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
8.7
Molar volume of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
8.8
Ideal gases and non-ideal gas behaviour . . . . . . . . . . . . . . . . . . . . . . 146
8.9
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
9 Organic Molecules - Grade 12
151
9.1
What is organic chemistry? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
9.2
Sources of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
9.3
Unique properties of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4
Representing organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4.1
Molecular formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4.2
Structural formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
9.4.3
Condensed structural formula . . . . . . . . . . . . . . . . . . . . . . . . 153
9.5
Isomerism in organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 154
9.6
Functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
9.7
The Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
9.7.1
The Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
9.7.2
Naming the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
9.7.3
Properties of the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . 163
9.7.4
Reactions of the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . 163
9.7.5
The alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
9.7.6
Naming the alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
9.7.7
The properties of the alkenes . . . . . . . . . . . . . . . . . . . . . . . . 169
9.7.8
Reactions of the alkenes
9.7.9
The Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
. . . . . . . . . . . . . . . . . . . . . . . . . . 169
9.7.10 Naming the alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
9.8
9.9
The Alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
9.8.1
Naming the alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
9.8.2
Physical and chemical properties of the alcohols . . . . . . . . . . . . . . 175
Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
9.9.1
Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
9.9.2
Derivatives of carboxylic acids: The esters . . . . . . . . . . . . . . . . . 178
9.10 The Amino Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
9.11 The Carbonyl Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
9.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
viii
CONTENTS
CONTENTS
10 Organic Macromolecules - Grade 12
185
10.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
10.2 How do polymers form? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
10.2.1 Addition polymerisation . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
10.2.2 Condensation polymerisation . . . . . . . . . . . . . . . . . . . . . . . . 188
10.3 The chemical properties of polymers . . . . . . . . . . . . . . . . . . . . . . . . 190
10.4 Types of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
10.5 Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
10.5.1 The uses of plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
10.5.2 Thermoplastics and thermosetting plastics . . . . . . . . . . . . . . . . . 194
10.5.3 Plastics and the environment . . . . . . . . . . . . . . . . . . . . . . . . 195
10.6 Biological Macromolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
10.6.1 Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
10.6.2 Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
10.6.3 Nucleic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
10.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
III
Chemical Change
209
11 Physical and Chemical Change - Grade 10
211
11.1 Physical changes in matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
11.2 Chemical Changes in Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
11.2.1 Decomposition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 213
11.2.2 Synthesis reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
11.3 Energy changes in chemical reactions . . . . . . . . . . . . . . . . . . . . . . . . 217
11.4 Conservation of atoms and mass in reactions . . . . . . . . . . . . . . . . . . . . 217
11.5 Law of constant composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
11.6 Volume relationships in gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
11.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
12 Representing Chemical Change - Grade 10
223
12.1 Chemical symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
12.2 Writing chemical formulae
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
12.3 Balancing chemical equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
12.3.1 The law of conservation of mass . . . . . . . . . . . . . . . . . . . . . . 224
12.3.2 Steps to balance a chemical equation
. . . . . . . . . . . . . . . . . . . 226
12.4 State symbols and other information . . . . . . . . . . . . . . . . . . . . . . . . 230
12.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
13 Quantitative Aspects of Chemical Change - Grade 11
233
13.1 The Mole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
13.2 Molar Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
13.3 An equation to calculate moles and mass in chemical reactions . . . . . . . . . . 237
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13.4 Molecules and compounds
CONTENTS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
13.5 The Composition of Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
13.6 Molar Volumes of Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
13.7 Molar concentrations in liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
13.8 Stoichiometric calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
13.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
14 Energy Changes In Chemical Reactions - Grade 11
255
14.1 What causes the energy changes in chemical reactions? . . . . . . . . . . . . . . 255
14.2 Exothermic and endothermic reactions . . . . . . . . . . . . . . . . . . . . . . . 255
14.3 The heat of reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
14.4 Examples of endothermic and exothermic reactions . . . . . . . . . . . . . . . . 259
14.5 Spontaneous and non-spontaneous reactions . . . . . . . . . . . . . . . . . . . . 260
14.6 Activation energy and the activated complex . . . . . . . . . . . . . . . . . . . . 261
14.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
15 Types of Reactions - Grade 11
267
15.1 Acid-base reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.1 What are acids and bases? . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.2 Defining acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.3 Conjugate acid-base pairs . . . . . . . . . . . . . . . . . . . . . . . . . . 269
15.1.4 Acid-base reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
15.1.5 Acid-carbonate reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 274
15.2 Redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
15.2.1 Oxidation and reduction
. . . . . . . . . . . . . . . . . . . . . . . . . . 277
15.2.2 Redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
15.3 Addition, substitution and elimination reactions . . . . . . . . . . . . . . . . . . 280
15.3.1 Addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
15.3.2 Elimination reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
15.3.3 Substitution reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
15.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
16 Reaction Rates - Grade 12
287
16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
16.2 Factors affecting reaction rates . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
16.3 Reaction rates and collision theory . . . . . . . . . . . . . . . . . . . . . . . . . 293
16.4 Measuring Rates of Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
16.5 Mechanism of reaction and catalysis . . . . . . . . . . . . . . . . . . . . . . . . 297
16.6 Chemical equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
16.6.1 Open and closed systems . . . . . . . . . . . . . . . . . . . . . . . . . . 302
16.6.2 Reversible reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
16.6.3 Chemical equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
16.7 The equilibrium constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
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16.7.1 Calculating the equilibrium constant . . . . . . . . . . . . . . . . . . . . 305
16.7.2 The meaning of kc values . . . . . . . . . . . . . . . . . . . . . . . . . . 306
16.8 Le Chatelier’s principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
16.8.1 The effect of concentration on equilibrium . . . . . . . . . . . . . . . . . 310
16.8.2 The effect of temperature on equilibrium . . . . . . . . . . . . . . . . . . 310
16.8.3 The effect of pressure on equilibrium . . . . . . . . . . . . . . . . . . . . 312
16.9 Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
16.10Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
17 Electrochemical Reactions - Grade 12
319
17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
17.2 The Galvanic Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
17.2.1 Half-cell reactions in the Zn-Cu cell . . . . . . . . . . . . . . . . . . . . 321
17.2.2 Components of the Zn-Cu cell . . . . . . . . . . . . . . . . . . . . . . . 322
17.2.3 The Galvanic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
17.2.4 Uses and applications of the galvanic cell . . . . . . . . . . . . . . . . . 324
17.3 The Electrolytic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
17.3.1 The electrolysis of copper sulphate . . . . . . . . . . . . . . . . . . . . . 326
17.3.2 The electrolysis of water . . . . . . . . . . . . . . . . . . . . . . . . . . 327
17.3.3 A comparison of galvanic and electrolytic cells . . . . . . . . . . . . . . . 328
17.4 Standard Electrode Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
17.4.1 The different reactivities of metals . . . . . . . . . . . . . . . . . . . . . 329
17.4.2 Equilibrium reactions in half cells . . . . . . . . . . . . . . . . . . . . . . 329
17.4.3 Measuring electrode potential . . . . . . . . . . . . . . . . . . . . . . . . 330
17.4.4 The standard hydrogen electrode . . . . . . . . . . . . . . . . . . . . . . 330
17.4.5 Standard electrode potentials . . . . . . . . . . . . . . . . . . . . . . . . 333
17.4.6 Combining half cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337
17.4.7 Uses of standard electrode potential . . . . . . . . . . . . . . . . . . . . 338
17.5 Balancing redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
17.6 Applications of electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.6.1 Electroplating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.6.2 The production of chlorine . . . . . . . . . . . . . . . . . . . . . . . . . 348
17.6.3 Extraction of aluminium
. . . . . . . . . . . . . . . . . . . . . . . . . . 349
17.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
IV
Chemical Systems
353
18 The Water Cycle - Grade 10
355
18.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
18.2 The importance of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
18.3 The movement of water through the water cycle . . . . . . . . . . . . . . . . . . 356
18.4 The microscopic structure of water . . . . . . . . . . . . . . . . . . . . . . . . . 359
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18.4.1 The polar nature of water . . . . . . . . . . . . . . . . . . . . . . . . . . 359
18.4.2 Hydrogen bonding in water molecules . . . . . . . . . . . . . . . . . . . 359
18.5 The unique properties of water . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
18.6 Water conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
18.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
19 Global Cycles: The Nitrogen Cycle - Grade 10
369
19.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
19.2 Nitrogen fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
19.3 Nitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
19.4 Denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372
19.5 Human Influences on the Nitrogen Cycle . . . . . . . . . . . . . . . . . . . . . . 372
19.6 The industrial fixation of nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . 373
19.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
20 The Hydrosphere - Grade 10
377
20.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
20.2 Interactions of the hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
20.3 Exploring the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
20.4 The Importance of the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . 379
20.5 Ions in aqueous solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
20.5.1 Dissociation in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
20.5.2 Ions and water hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
20.5.3 The pH scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
20.5.4 Acid rain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
20.6 Electrolytes, ionisation and conductivity . . . . . . . . . . . . . . . . . . . . . . 386
20.6.1 Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386
20.6.2 Non-electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
20.6.3 Factors that affect the conductivity of water . . . . . . . . . . . . . . . . 387
20.7 Precipitation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
20.8 Testing for common anions in solution . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.1 Test for a chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.2 Test for a sulphate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.3 Test for a carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
20.8.4 Test for bromides and iodides . . . . . . . . . . . . . . . . . . . . . . . . 392
20.9 Threats to the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
20.10Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394
21 The Lithosphere - Grade 11
397
21.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
21.2 The chemistry of the earth’s crust . . . . . . . . . . . . . . . . . . . . . . . . . 398
21.3 A brief history of mineral use . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399
21.4 Energy resources and their uses . . . . . . . . . . . . . . . . . . . . . . . . . . . 400
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21.5 Mining and Mineral Processing: Gold . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.2 Mining the Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.3 Processing the gold ore . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.4 Characteristics and uses of gold . . . . . . . . . . . . . . . . . . . . . . . 402
21.5.5 Environmental impacts of gold mining . . . . . . . . . . . . . . . . . . . 404
21.6 Mining and mineral processing: Iron . . . . . . . . . . . . . . . . . . . . . . . . 406
21.6.1 Iron mining and iron ore processing . . . . . . . . . . . . . . . . . . . . . 406
21.6.2 Types of iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
21.6.3 Iron in South Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408
21.7 Mining and mineral processing: Phosphates . . . . . . . . . . . . . . . . . . . . 409
21.7.1 Mining phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
21.7.2 Uses of phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
21.8 Energy resources and their uses: Coal . . . . . . . . . . . . . . . . . . . . . . . 411
21.8.1 The formation of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411
21.8.2 How coal is removed from the ground . . . . . . . . . . . . . . . . . . . 411
21.8.3 The uses of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
21.8.4 Coal and the South African economy . . . . . . . . . . . . . . . . . . . . 412
21.8.5 The environmental impacts of coal mining . . . . . . . . . . . . . . . . . 413
21.9 Energy resources and their uses: Oil . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.1 How oil is formed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.2 Extracting oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.3 Other oil products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
21.9.4 The environmental impacts of oil extraction and use . . . . . . . . . . . 415
21.10Alternative energy resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
21.11Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
22 The Atmosphere - Grade 11
421
22.1 The composition of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . 421
22.2 The structure of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.1 The troposphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.2 The stratosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.3 The mesosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
22.2.4 The thermosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
22.3 Greenhouse gases and global warming . . . . . . . . . . . . . . . . . . . . . . . 426
22.3.1 The heating of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . 426
22.3.2 The greenhouse gases and global warming . . . . . . . . . . . . . . . . . 426
22.3.3 The consequences of global warming . . . . . . . . . . . . . . . . . . . . 429
22.3.4 Taking action to combat global warming . . . . . . . . . . . . . . . . . . 430
22.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
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23 The Chemical Industry - Grade 12
435
23.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
23.2 Sasol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
23.2.1 Sasol today: Technology and production . . . . . . . . . . . . . . . . . . 436
23.2.2 Sasol and the environment . . . . . . . . . . . . . . . . . . . . . . . . . 440
23.3 The Chloralkali Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
23.3.1 The Industrial Production of Chlorine and Sodium Hydroxide . . . . . . . 442
23.3.2 Soaps and Detergents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446
23.4 The Fertiliser Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.1 The value of nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.2 The Role of fertilisers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.3 The Industrial Production of Fertilisers . . . . . . . . . . . . . . . . . . . 451
23.4.4 Fertilisers and the Environment: Eutrophication . . . . . . . . . . . . . . 454
23.5 Electrochemistry and batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
23.5.1 How batteries work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
23.5.2 Battery capacity and energy . . . . . . . . . . . . . . . . . . . . . . . . 457
23.5.3 Lead-acid batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457
23.5.4 The zinc-carbon dry cell . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
23.5.5 Environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . 460
23.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461
A GNU Free Documentation License
467
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Chapter 9
Organic Molecules - Grade 12
9.1
What is organic chemistry?
Organic chemistry is the branch of chemistry that deals with organic molecules. An organic molecule is one which contains carbon, and these molecules can range in size from simple
molecules to complex structures containing thousands of atoms! Although the main element in
organic compounds is carbon, other elements such as hydrogen (H), oxygen (O), nitrogen (N),
sulfur (S) and phosphorus (P) are also common in these molecules.
Until the early nineteenth century, chemists had managed to make many simple compounds
in the laboratory, but were still unable to produce the complex molecules that they found in
living organisms. It was around this time that a Swedish chemist called Jons Jakob Berzelius
suggested that compounds found only in living organisms (the organic compounds) should be
grouped separately from those found in the non-living world (the inorganic compounds). He also
suggested that the laws that governed how organic compounds formed, were different from those
for inorganic compounds. From this, the idea developed that there was a ’vital force’ in organic
compounds. In other words, scientists believed that organic compounds would not follow the
normal physical and chemical laws that applied to other inorganic compounds because the very
’force of life’ made them different.
This idea of a mystical ’vital force’ in organic compounds was weakened when scientists began to
manufacture organic compounds in the laboratory from non-living materials. One of the first to
do this was Friedrich Wohler in 1828, who successfully prepared urea, an organic compound in
the urine of animals which, until that point, had only been found in animals. A few years later a
student of Wohler’s, Hermann Kolbe, made the organic compound acetic acid from inorganic
compounds. By this stage it was acknowledged that organic compounds are governed by exactly
the same laws that apply to inorganic compounds. The properties of organic compounds are not
due to a ’vital force’ but to the unique properties of the carbon atom itself.
Organic compounds are very important in daily life. They make up a big part of our own bodies,
they are in the food we eat and in the clothes we wear. Organic compounds are also used to
make products such as medicines, plastics, washing powders, dyes, along with a list of other
items.
9.2
Sources of carbon
The main source of the carbon in organic compounds is carbon dioxide in the air. Plants use
sunlight to convert carbon dioxide into organic compounds through the process of photosynthesis. Plants are therefore able to make their own organic compounds through photosynthesis,
while animals feed on plants or plant products so that they gain the organic compounds that
they need to survive.
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9.3
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
Another important source of carbon is fossil fuels such as coal, petroleum and natural gas. This
is because fossil fuels are themselves formed from the decaying remains of dead organisms (refer
to chapter 21 for more information on fossil fuels).
9.3
Unique properties of carbon
Carbon has a number of unique properties which influence how it behaves and how it bonds with
other atoms:
• Carbon has four valence electrons which means that each carbon atom can form bonds
with four other atoms. Because of this, long chain structures can form. These chains
can either be unbranched (figure 9.1) or branched (figure 9.2). Because of the number of
bonds that carbon can form with other atoms, organic compounds can be very complex.
C
C
C
C
Figure 9.1: An unbranched carbon chain
C
C
C
C
C
C
C
Figure 9.2: A branched carbon chain
• Because of its position on the Periodic Table, most of the bonds that carbon forms with
other atoms are covalent. Think for example of a C-C bond. The difference in electronegativity between the two atoms is zero, so this is a pure covalent bond. In the case of a
C-H bond, the difference in electronegativity between carbon (2.5) and hydrogen (2.1) is
so small that C-H bonds are almost purely covalent. The result of this is that most organic
compounds are non-polar. This affects some of the properties of organic compounds.
9.4
Representing organic compounds
There are a number of ways to represent organic compounds. It is useful to know all of these so
that you can recognise a molecule however it is shown. There are three main ways of representing
a compound. We will use the example of a molecule called 2-methylpropane to help explain the
difference between each.
9.4.1
Molecular formula
The molecular formula of a compound shows how many atoms of each type are in a molecule.
The number of each atom is written as a subscript after the atomic symbol. The molecular
formula of 2-methylpropane is:
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.4
C4 H10
9.4.2
Structural formula
The structural formula of an organic compound shows every bond between every atom in the
molecule. Each bond is represented by a line. The structural formula of 2-methylpropane is
shown in figure 9.3.
H
H
H
C
H
H
H
C
C
C
H
H
H
H
Figure 9.3: The structural formula of 2-methylpropane
9.4.3
Condensed structural formula
When a compound is represented using its condensed structural formula, each carbon atom and
the hydrogen atoms that are bonded directly to it are listed as a molecular formula, followed
by a similar molecular formula for the neighbouring carbon atom. Branched groups are shown
in brackets after the carbon atom to which they are bonded. The condensed structural formula
below shows that in 2-methylpropane, there is a branched chain attached to the second carbon
atom of the main chain. You can check this by looking at the structural formula in figure ??.
CH3 CH(CH3 )CH3
Exercise: Representing organic compounds
1. For each of the following organic compounds, give the condensed structural
formula and the molecular formula.
H
H
H
H
H
C
C
C
C
H
H
(a)
153
H
9.5
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
H
H
H
C
H
C
H
H
C
C
C
H
(b)
H
H
2. For each of the following, give the structural formula and the molecular
formula.
(a) CH3 CH2 CH3
(b) CH3 CH2 CH(CH3 )CH3
(c) C2 H6
3. Give two possible structural formulae for the compound with a molecular formula of C4 H10 .
9.5
Isomerism in organic compounds
It is possible for two organic compounds to have the same molecular formula but a different
structural formula. Look for example at the two organic compounds that are shown in figure
9.4.
H
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
C
H
H
H
H
C
C
C
H
H
H
Figure 9.4: Isomers of a 4-carbon organic compound
If you were to count the number of carbon and hydrogen atoms in each compound, you would
find that they are the same. They both have the same molecular formula (C4 H10 ), but their
structure is different and so are their properties. Such compounds are called isomers.
Definition: Isomer
In chemistry, isomers are molecules with the same molecular formula and often with the same
kinds of chemical bonds between atoms, but in which the atoms are arranged differently.
Exercise: Isomers
Match the organic compound in Column A with its isomer Column B:
154
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.6
Column A
CH3 CH(CH3 )OH
H
H
9.6
Column B
CH3 CH(CH3 )CH3
H
H
H
H
C
C
C
C
H
H
H
H
CH3
H
H
C
C
C
H
H
H
H H
H
H
CH3
C
C
C
H
H
H
H
H
C3 H7 OH
Functional groups
All organic compounds have a particular bond or group of atoms which we call its functional
group. This group is important in determining how a compound will react.
Definition: Functional group
In organic chemistry, a functional group is a specific group of atoms within molecules,
that are responsible for the characteristic chemical reactions of those molecules. The same
functional group will undergo the same or similar chemical reaction(s) regardless of the size
of the molecule it is a part of.
In one group of organic compounds called the hydrocarbons, the single, double and triple bonds
of the alkanes, elkenes and alkynes are examples of functional groups. In another group, the
alcohols, an oxygen and a hydrogen atom that are bonded to each other form the functional
group for those compounds. All alcohols will contain an oxygen and a hydrogen atom bonded
together in some part of the molecule.
Table 9.1 summarises some of the common functional groups. We will look at these in more
detail later in this chapter.
9.7
The Hydrocarbons
Let us first look at a group of organic compounds known as the hydrocarbons. These molecules
only contain carbon and hydrogen. The hydrocarbons that we are going to look at are called
aliphatic compounds. The aliphatic compounds are divided into acyclic compounds (chain
structures) and cyclic compounds (ring structures). The chain structures are further divided into
structures that contain only single bonds (alkanes), those that contain double bonds (alkenes)
and those that contain triple bonds (alkynes). Cyclic compounds include structures such as the
benzene ring. Figure 9.5 summarises the classification of the hydrocarbons.
Hydrocarbons that contain only single bonds are called saturated hydrocarbons because each
carbon atom is bonded to as many hydrogen atoms as possible. Figure 9.6 shows a molecule of
ethane which is a saturated hydrocarbon.
155
9.7
Name of group
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
Functional group
C
Example
Diagram
C
H
Alkane
H
H
C
C
H
H
H
Ethane
H
C
H
C
C
C
H
Alkene
H
Ethene
C
C
H
Alkyne
C
C
H
Ethyne (acetylene)
H
C
CH3
X
X
C
H
(X=F,Cl,Br,I)
Halo-alkane
Chloroethane
C
H
OH
C
C
H
H
OH
H
Alcohol/ alkanol
H
Ethanol
O
O
CH3
C
OH
Carboxylic acid
ethanoic acid
H
O
C
H
R
Amine
OH
N
H
CH3
C
C
H
N
H
Glycine
Table 9.1: Some functional groups of organic compounds
Hydrocarbons that contain double or triple bonds are called unsaturated hydrocarbons because
they don’t contain as many hydrogen atoms as possible. Figure 9.7 shows a molecule of ethene
which is an unsaturated hydrocarbon. If you compare the number of carbon and hydrogen atoms
156
H
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.7
Aliphatic hydrocarbons
Acyclic compounds
(chain structures)
Alkanes (single bonds)
Cyclic compounds
(ring structures e.g. benzene ring)
Alkenes (contain double bonds)
Alkynes (contain triple bonds)
Figure 9.5: The classification of the aliphatic hydrocarbons
H
H
H
C
C
H
H
H
Figure 9.6: A saturated hydrocarbon
in a molecule of ethane and a molecule of ethene, you will see that the number of hydrogen
atoms in ethene is less than the number of hydrogen atoms in ethane despite the fact that they
both contain two carbon atoms. In order for an unsaturated compound to become saturated,
a double bond has to be broken, and another two hydrogen atoms added for each double bond
that is replaced by a single bond.
H
H
C
C
H
H
Figure 9.7: An unsaturated hydrocarbon
teresting Fat that occurs naturally in living matter such as animals and plants is used as
Interesting
Fact
Fact
food for human consumption and contains varying proportions of saturated and
unsaturated fat. Foods that contain a high proportion of saturated fat are butter,
ghee, suet, tallow, lard, coconut oil, cottonseed oil, and palm kernel oil, dairy
products (especially cream and cheese), meat, and some prepared foods. Diets
high in saturated fat are correlated with an increased incidence of atherosclerosis
and coronary heart disease according to a number of studies. Vegetable oils
contain unsaturated fats and can be hardened to form margarine by adding
hydrogen on to some of the carbon=carbon double bonds using a nickel catalyst.
The process is called hydrogenation
157
9.7
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
We will now go on to look at each of the hydrocarbon groups in more detail. These groups are
the alkanes, the alkenes and the alkynes.
9.7.1
The Alkanes
The alkanes are hydrocarbons that only contain single covalent bonds between their carbon
atoms. This means that they are saturated compounds and are quite unreactive. The simplest
alkane has only one carbon atom and is called methane. This molecule is shown in figure 9.8.
H
(a)
H
C
H
(b)
CH4
H
Figure 9.8: The structural (a) and molecular formula (b) for methane
The second alkane in the series has two carbon atoms and is called ethane. This is shown in
figure 9.9.
(a)
H
H
H
C
C
H
H
(b) C2 H6
H
Figure 9.9: The structural (a) and molecular formula (b) for ethane
The third alkane in the series has three carbon atoms and is called propane (Figure 9.10).
(a)
H
H
H
H
C
C
C
H
H
H
H
(b) C3 H8
Figure 9.10: The structural (a) and molecular formula (b) for propane
When you look at the molecular formula for each of the alkanes, you should notice a pattern
developing. For each carbon atom that is added to the molecule, two hydrogen atoms are added.
In other words, each molecule differs from the one before it by CH2 . This is called a homologous
series. The alkanes have the general formula Cn H2n+2 .
The alkanes are the most important source of fuel in the world and are used extensively in the
chemical industry. Some are gases (e.g. methane and ethane), while others are liquid fuels (e.g.
octane, an important component of petrol).
teresting Some fungi use alkanes as a source of carbon and energy. One fungus AmorInteresting
Fact
Fact
photheca resinae prefers the alkanes used in aviation fuel, and this can cause
problems for aircraft in tropical areas!
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.7.2
9.7
Naming the alkanes
In order to give compounds a name, certain rules must be followed. When naming organic
compounds, the IUPAC (International Union of Pure and Applied Chemistry) nomenclature is
used. We will first look at some of the steps that need to be followed when naming a compound,
and then try to apply these rules to some specific examples.
1. STEP 1: Recognise the functional group in the compound. This will determine the suffix
(the ’end’) of the name. For example, if the compound is an alkane, the suffix will be
-ane; if the compound is an alkene the suffix will be -ene; if the compound is an alcohol
the suffix will be -ol, and so on.
2. STEP 2: Find the longest continuous carbon chain (it won’t always be a straight chain)
and count the number of carbon atoms in this chain. This number will determine the prefix
(the ’beginning’) of the compound’s name. These prefixes are shown in table 9.2. So, for
example, an alkane that has 3 carbon atoms will have the suffix prop and the compound’s
name will be propane.
Carbon atoms
1
2
3
4
5
6
7
8
9
10
prefix
meth(ane)
eth(ane)
prop(ane)
but(ane)
pent(ane)
hex(ane)
hept(ane)
oct(ane)
non(ane)
dec(ane)
Table 9.2: The prefix of a compound’s name is determined by the number of carbon atoms in
the longest chain
3. STEP 3: Number the carbons in the longest carbon chain (Important: If there is a double
or triple bond, you need to start numbering so that the bond is at the carbon with the
lowest number.
4. STEP 4: Look for any branched groups and name them. Also give them a number to
show their position on the carbon chain. If there are no branched groups, this step can be
left out.
5. STEP 5: Combine the elements of the name into a single word in the following order:
branched groups; prefix; name ending according to the functional group and its position
along the longest carbon chain.
Worked Example 38: Naming the alkanes
Question: Give the IUPAC name for the following compound:
Note: The numbers attached to the carbon atoms would not normally be shown.
The atoms have been numbered to help you to name the compound.
Answer
Step 1 : Identify the functional group
The compound is a hydrocarbon with single bonds between the carbon atoms. It is
an alkane and will have a suffix of -ane.
Step 2 : Find the longest carbon chain
159
9.7
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
H
H
H
H
C(1)
C(2)
C(3)
C(4)
H
H
H
H
H
There are four carbon atoms in the longest chain. The prefix of the compound will
be ’but’.
Step 3 : Number the carbons in the longest chain
In this case, it is easy. The carbons are numbered from left to right, from one to four.
Step 4 : Look for any branched groups, name them and give their position
on the carbon chain
There are no branched groups in this compound.
Step 5 : Combine the elements of the name into a single word
The name of the compound is butane.
Worked Example 39: Naming the alkanes
Question: Give the IUPAC name for the following compound:
H
H
H
H
C
C
C
H
H
H
H
C
H
H
Answer
Step 1 : Identify the functional group
The compound is an alkane and will have the suffix -ane.
Step 2 : Find the longest carbon chain
There are three carbons in the longest chain. The prefix for this compound is -prop.
Step 3 : Number the carbons in the carbon chain
If we start at the carbon on the left, we can number the atoms as shown below:
Step 4 : Look for any branched groups, name them and give their position
on the carbon chain
There is a branched group attached to the second carbon atom. This group has the
formula CH3 which is methane. However, because it is not part of the main chain,
it is given the suffix -yl (i.e. methyl). The position of the methyl group comes just
160
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
9.7
H
H
H
C(1)
C(2)
C(3)
H
H
H
H
C
H
H
before its name (see next step).
Step 5 : Combine the elements of the compound’s name into a single word in
the order of branched groups; prefix; name ending according to the functional
group.
The compound’s name is 2-methylpropane.
Worked Example 40: Naming the alkanes
Question: Give the IUPAC name for the following compound:
CH3 CH(CH3 )CH(CH3 )CH3
(Remember that the side groups are shown in brackets after the carbon atom to
which they are attached.)
Answer
Step 1 : Draw the compound from its condensed structural formula
The structural formula of the compound is:
H
H
CH3 CH3
H
C(1)
C(2)
C(3)
C(4)
H
H
H
H
H
Step 2 : Identify the functional group
The compound is an alkane and will have the suffix -ane.
Step 3 : Find the longest carbon chain
There are four carbons in the longest chain. The prefix for this compound is -but.
Step 4 : Number the carbons in the carbon chain
If we start at the carbon on the left, carbon atoms are numbered as shown in the
diagram above. A second way that the carbons could be numbered is:
CH3 CH3(4)
H
C(1)
C(2)
C(3)
C
H
H
H
H
H
H
161
H
9.7
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
Step 5 : Look for any branched groups, name them and give their position
on the carbon chain
There are two methyl groups attached to the main chain. The first one is attached
to the second carbon atom and the second methyl group is attached to the third
carbon atom. Notice that in this example it does not matter how you have chosen
to number the carbons in the main chain; the methyl groups are still attached to the
second and third carbons and so the naming of the compound is not affected.
Step 6 : Combine the elements of the compound’s name into a single word in
the order of branched groups; prefix; name ending according to the functional
group.
The compound’s name is 2,3-dimethyl-butane.
Worked Example 41: Naming the alkanes
Question: Give the IUPAC name for the following compound:
H
CH3
H
H
H
C
C
C
C
H
H
CH2
H
H
CH3
Answer
Step 1 : Identify the functional group
The compound is an alkane and will have the suffix -ane.
Step 2 : Find the longest carbon chain and number the carbons in the longest
chain.
There are five carbons in the longest chain if they are numbered as shown below.
The prefix for the compound is -pent.
H
CH3
H
H
H
C(1)
C(2)
C(3)
C
H
H CH2(4)
H
H
CH3(5)
Step 3 : Look for any branched groups, name them and give their position
on the carbon chain
There are two methyl groups attached to the main chain. The first one is attached
to the first carbon atom and the second methyl group is attached to the third carbon
atom.
Step 4 : Combine the elements of the compound’s name into a single word in
the order of branched groups; prefix; name ending according to the functional
group.
The compound’s name is 1,3-dimethyl-pentane.
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.7
Exercise: Naming the alkanes
1. Give the structural formula for each of the following:
(a)
(b)
(c)
(d)
Octane
CH3 CH2 CH3
CH3 CH(CH3 )CH3
3-ethyl-pentane
2. Give the IUPAC name for each of the following organic compounds.
H
H
H
CH3
C
C
C
H
H
H
H
(a)
(b) CH3 CH2 CH(CH3 )CH2 CH3
(c) CH3 CH(CH3 )CH2 CH(CH3 )CH3
9.7.3
Properties of the alkanes
We have already mentioned that the alkanes are relatively unreactive because of their stable
C-C and C-H bonds. The boiling point and melting point of these molecules is determined by
their molecular structure, and their surface area. The more carbon atoms there are in an alkane,
the greater the surface area and therefore the higher the boiling point. The melting point also
increases as the number of carbon atoms in the molecule increases. This can be seen in the data
in table 9.3.
Formula
CH4
C2 H6
C3 H8
C4 H10
C5 H12
C6 H14
C17 H36
Name
methane
ethane
propane
butane
pentane
hexane
heptadecane
Melting point (0 C)
-183
-182
-187
-138
-130
-95
22
Boiling point (0 C)
-162
-88
-45
-0.5
36
69
302
Phase at room temperature
gas
gas
gas
gas
liquid
liquid
solid
Table 9.3: Properties of some of the alkanes
You will also notice that, when the molecular mass of the alkanes is low (i.e. there are few
carbon atoms), the organic compounds are gases because the intermolecular forces are weak. As
the number of carbon atoms and the molecular mass increases, the compounds are more likely
to be liquids or solids because the intermolecular forces are stronger.
9.7.4
Reactions of the alkanes
There are three types of reactions that can occur in saturated compounds such as the alkanes.
1. Substitution reactions
Substitution reactions involve the removal of a hydrogen atom which is replaced by an
atom of another element, such as a halogen (F, Cl, Br or I) (figure 9.11). The product is
called a halo-alkane. Since alkanes are not very reactive, heat or light are needed for this
163
9.7
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
H
H
C
C
+
H
HBr
H
H
H
C
C
H
H
Br
Figure 9.11: A substitution reaction
reaction to take place.
e.g. CH2 =CH2 + HBr → CH3 -CH2 -Br (halo-alkane)
Halo-alkanes (also sometimes called alkyl halides) that contain methane and chlorine
are substances that can be used as anaesthetics during operations. One example is
trichloromethane, also known as ’chloroform’ (figure 9.12).
H
Cl
CHCl3
C
Cl
Cl
Figure 9.12: Trichloromethane
2. Elimination reactions
Saturated compounds can also undergo elimination reactions to become unsaturated (figure 9.13). In the example below, an atom of hydrogen and chlorine are eliminated from
the original compound to form an unsaturated halo-alkene.
e.g. CH2 Cl − CH2 Cl → CH2 = CHCl + HCl
H
H
H
C
C
Cl
Cl
H
H
H
H
C
C
Cl
+
HCl
Figure 9.13: An elimination reaction
3. Oxidation reactions
When alkanes are burnt in air, they react with the oxygen in air and heat is produced. This
is called an oxidation or combustion reaction. Carbon dioxide and water are given off as
products. Heat is also released during the reaction. The burning of alkanes provides most
of the energy that is used by man.
e.g. CH4 + 2O2 → CO2 + 2H2 O + heat
Exercise: The Alkanes
164
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
1. Give the IUPAC name for each of the following alkanes:
(a) C6 H14
H
H
C
H
H
C
H
(b)
(c) CH3 CH3
H
H
H
C
C
C
H
H
H
H
2. Give the structural formula for each of the following compounds:
(a) octane
(b) 3-methyl-hexane
3. Methane is one of the simplest alkanes and yet it is an important fuel source.
Methane occurs naturally in wetlands, natural gas and permafrost. However,
methane can also be produced when organic wastes (e.g. animal manure and
decaying material) are broken down by bacteria under conditions that are anaerobic (there is no oxygen). The simplified reaction is shown below:
Organic matter → Simple organic acids → Biogas
The organic matter could be carbohydrates, proteins or fats which are broken
down by acid-forming bacteria into simple organic acids such as acetic acid or
formic acid. Methane-forming bacteria then convert these acids into biogases
such as methane and ammonia.
The production of methane in this way is very important because methane can
be used as a fuel source. One of the advantages of methane over other fuels like
coal, is that it produces more energy but with lower carbon dioxide emissions.
The problem however, is that methane itself is a greenhouse gas and has a much
higher global warming potential than carbon dioxide. So, producing methane
may in fact have an even more dangerous impact on the environment.
(a) What is the structural formula of methane?
(b) Write an equation to show the reaction that takes place when methane is
burned as a fuel.
(c) Explain what is meant by the statement that methane ’has a greater global
warming potential than carbon dioxide’.
4. Chlorine and ethane react to form chloroethane and hydrogen chloride.
(a) Write a balanced chemical equation for this reaction, using molecular formulae.
(b) Give the structural formula of chloroethane.
(c) What type of reaction has taken place in this example?
5. Petrol (C8 H18 ) is in fact not pure C8 H18 but a mixture of various alkanes. The
’octane rating’ of petrol refers to the percentage of the petrol which is C8 H18 .
For example, 93 octane fuel contains 93% C8 H18 and 7% other alkanes. The
isomer of C8 H18 referred to in the ’octane rating’ is in fact not octane but
2,2,4-trimethylpentane.
(a) Write an unbalanced equation for the chemical reaction which takes place
when petrol (C8 H18 ) burns in excess oxygen.
(b) Write the general formula of the alkanes.
(c) Define the term structural isomer.
(d) Use the information given in this question and your knowledge of naming
organic compounds to deduce and draw the full structural formula for
2,2,4-trimethylpentane. (IEB pg 25)
165
9.7
9.7
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.7.5
The alkenes
In the alkenes, there is at least one double bond between two carbon atoms. This means that
they are unsaturated and are more reactive than the alkanes. The simplest alkene is ethene
(also known as ethylene), which is shown in figure 9.14.
H
(a)
H
C
(b)
C
H
CH2 CH2
(c)
C2 H4
H
Figure 9.14: The (a) structural, (b) condensed structural and (c) molecular structure representations of ethene
As with the alkanes, the elkenes also form a homologous series. They have the general formula
Cn H2n . The second alkene in the series would therefore be C3 H6 . This molecule is known as
propene (figure 9.15). Note that if an alkene has two double bonds, it is called a diene and if
it has three double bonds it is called a triene.
(a)
H
H
H
C
C
H
H
(b) CH3 CHCH2
C
(c)
C3 H6
H
Figure 9.15: The (a) structural, (b) condensed structural and (c) molecular structure representations of propene
The elkenes have a variety of uses. Ethylene for example is a hormone in plants that stimulates
the ripening of fruits and the opening of flowers. Propene is an important compound in the
petrochemicals industry. It is used as a monomer to make polypropylene and is also used as a
fuel gas for other industrial processes.
9.7.6
Naming the alkenes
Similar rules will apply in naming the alkenes, as for the alkanes.
Worked Example 42: Naming the alkenes
Question: Give the IUPAC name for the following compound:
166
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
9.7
H
H
H
H
C(1)
C(2)
C(3)
C(4)
H
H
H
Answer
Step 1 : Identify the functional group
The compound is an alkene and will have the suffix -ene.
Step 2 : Find the longest carbon chain
There are four carbon atoms in the longest chain and so the prefix for this compound
will be ’but’.
Step 3 : Number the carbon atoms
Remember that when there is a double or triple bond, the carbon atoms must be
numbered so that the double or triple bond is at the lowest numbered carbon. In
this case, it doesn’t matter whether we number the carbons from the left to right,
or from the right to left. The double bond will still fall between C2 and C3 . The
position of the bond will come just before the suffix in the compound’s name.
Step 4 : Look for any branched groups, name them and give their position
on the carbon chain
There are no branched groups in this molecule.
Step 5 : Name the compound
The name of this compound is but-2-ene.
Worked Example 43: Naming the alkenes
Question: Draw the structural formula for the organic compound 3-methyl-butene
Answer
Step 1 : Identify the functional group
The suffix -ene means that this compound is an alkene and there must be a double
bond in the molecule. There is no number immediately before the suffix which means
that the double bond must be at the first carbon in the chain.
Step 2 : Determine the number of carbons in the longest chain
The prefix for the compound is ’but’ so there must be four carbons in the longest
chain.
Step 3 : Look for any branched groups
There is a methyl group at the third carbon atom in the chain.
Step 4 : Combine this information to draw the structural formula for this
molecule.
167
9.7
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
H
C
H
H
C
H
H
C
H
C
C
H
H
H
Worked Example 44: Naming the alkenes
Question: Give the IUPAC name for the following compound:
CH3
H
H
CH2
H
H
C(1)
C(2)
C(3)
C(4)
H
Answer
Step 1 : Identify the functional group
The compound is an alkene and will have the suffix -ene. There is a double bond
between the first and second carbons and also between the third and forth carbons.
The organic compound is therefore a ’diene’.
Step 2 : Find the longest carbon chain and number the carbon atoms
There are four carbon atoms in the longest chain and so the prefix for this compound will be ’but’. The carbon atoms are numbered 1 to 4 in the diagram above.
Remember that the main carbon chain must contain both the double bonds.
Step 3 : Look for any branched groups, name them and give their position
on the carbon chain
There is a methyl group on the first carbon and an ethyl group on the second carbon.
Step 4 : Name the compound
The name of this compound is 1-methyl,2-ethyl-1,3 diene.
Exercise: Naming the alkenes
Give the IUPAC name for each of the following alkenes:
1. C5 H10
2. CH3 CHCHCH3
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
H
C
C
H
H
C
C
H
H
3.
9.7.7
9.7
The properties of the alkenes
The properties of the alkenes are very similar to those of the alkanes, except that the alkenes are
more reactive because they are unsaturated. As with the alkanes, compounds that have four or
less carbon atoms are gases at room temperature, while those with five or more carbon atoms
are liquids.
9.7.8
Reactions of the alkenes
Alkenes can undergo addition reactions because they are unsaturated. They readily react with
hydrogen, water and the halogens. The double bond is broken and a single, saturated bond is
formed. A new group is then added to one or both of the carbon atoms that previously made
up the double bond. The following are some examples:
1. Hydrogenation reactions
A catalyst such as platinum is normally needed for these reactions
CH2 = CH2 + H2 → CH3 − CH3 (figure 9.16)
H
H
H
C
C
H
+
H2
H
H
H
C
C
H
H
H
H
C
C
H
H
H
Figure 9.16: A hydrogenation reaction
2. Halogenation reactions
CH2 = CH2 + HBr → CH3 − CH2 − Br (figure 9.17)
H
H
H
C
C
H
+
HBr
H
Figure 9.17: A halogenation reaction
3. The formation of alcohols
CH2 = CH2 + H2 O → CH3 − CH2 − OH (figure 9.18)
169
Br
9.7
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
H
H
C
C
+
H
H2 O
H
H
H
C
C
H
H
OH
Figure 9.18: The formation of an alcohol
Exercise: The Alkenes
1. Give the IUPAC name for each of the following organic compounds:
H
H
H
C
C
C
H
H
H
H
(a)
(b) CH3 CHCH2
C
H
C
H
H
2. Refer to the data table below which shows the melting point and boiling point
for a number of different organic compounds.
Formula
C4 H10
C5 H12
C6 H14
C4 H8
C5 H10
C6 H12
Name
Butane
Pentane
Hexane
Butene
Pentene
Hexene
Melting point (0 C)
-138
-130
-95
-185
-138
-140
Boiling point (0 C)
-0.5
36
69
-6
30
63
(a) At room temperature (approx. 250 C), which of the organic compounds in
the table are:
i. gases
ii. liquids
(b) In the alkanes...
i. Describe what happens to the melting point and boiling point as the
number of carbon atoms in the compound increases.
ii. Explain why this is the case.
(c) If you look at an alkane and an alkene that have the same number of
carbon atoms...
i. How do their melting points and boiling points compare?
ii. Can you explain why their melting points and boiling points are different?
(d) Which of the compounds, hexane or hexene, is more reactive? Explain
your answer.
3. The following reaction takes place:
CH3 CHCH2 + H2 → CH3 CH2 CH3
(a) Give the name of the organic compound in the reactants.
(b) What is the name of the product?
(c) What type of reaction is this?
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.7
(d) Which compound in the reaction is a saturated hydrocarbon?
9.7.9
The Alkynes
In the alkynes, there is at least one triple bond between two of the carbon atoms. They are
unsaturated compounds and are therefore highly reactive. Their general formula is Cn H2n−2 .
The simplest alkyne is ethyne (figure 9.19), also known as acetylene. Many of the alkynes are
used to synthesise other chemical products.
H
C
C
H
Figure 9.19: Ethyne (acetylene)
teresting The raw materials that are needed to make acetylene are calcium carbonate and
Interesting
Fact
Fact
coal. Acetylene can be produced through the following reactions:
CaCO3 → CaO
CaO + 3C → CaC2 + CO
CaC2 + 2H2 O → Ca(OH)2 + C2 H2
An important use of acetylene is in oxyacetylene gas welding. The fuel gas burns
with oxygen in a torch. An incredibly high heat is produced, and this is enough
to melt metal.
9.7.10
Naming the alkynes
The same rules will apply as for the alkanes and alkenes, except that the suffix of the name will
now be -yne.
Worked Example 45: Naming the alkynes
Question: Give the IUPAC name for the following compound:
CH3
CH
CH2
C
CH3
Answer
Step 1 : Identify the functional group
171
C
CH3
9.8
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
There is a triple bond between two of the carbon atoms, so this compound is an
alkyne. The suffix will be -yne. The triple bond is at the second carbon, so the suffix
will in fact be 2-yne.
Step 2 : Find the longest carbon chain and give the compound the correct
prefix
If we count the carbons in a straight line, there are six. The prefix of the compound’s
name will be ’hex’.
Step 3 : Number the carbons in the longest chain
In this example, you will need to number the carbons from right to left so that the
triple bond is between carbon atoms with the lowest numbers.
Step 4 : Look for any branched groups, name them and show the number of
the carbon atom to which the group is attached
There is a methyl (CH3 ) group attached to the fifth carbon (remember we have
numbered the carbon atoms from right to left).
Step 5 : Combine the elements of the name into a single word in the following
order: branched groups; prefix; name ending according to the functional
group and its position along the longest carbon chain.
If we follow this order, the name of the compound is 5-methyl-hex-2-yne.
Exercise: The alkynes
Give the IUPAC name for each of the following organic compounds.
H
H
CH3
C
C
H
H
C
C
H
1.
2. C2 H2
3. CH3 CH2 CCH
9.8
The Alcohols
An alcohol is any organic compound where there is a hydroxyl functional group (-OH) bound to
a carbon atom. The general formula for a simple alcohol is Cn H2n+1 OH.
The simplest and most commonly used alcohols are methanol and ethanol (figure 9.20).
The alcohols have a number of different uses:
• methylated spirits (surgical spirits) is a form of ethanol where methanol has been added
• ethanol is used in alcoholic drinks
• ethanol is used as an industrial solvent
172
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
(a)
OH
H
C
H
H
9.8
(b)
H
OH
H
C
C
H
H
H
Figure 9.20: (a) methanol and (b) ethanol
• methanol and ethanol can both be used as a fuel and they burn more cleanly than gasoline
or diesel (refer to chapter 21 for more information on biofuels as an alternative energy
resource.)
• ethanol is used as a solvent in medical drugs, perfumes and vegetable essences
• ethanol is an antiseptic
teresting
Interesting
Fact
Fact
’Fermentation’ refers to the conversion of sugar to alcohol using yeast (a fungus). The
process of fermentation produces items such as wine, beer and yoghurt. To make wine,
grape juice is fermented to produce alcohol. This reaction is shown below:
C6 H12 O6 → 2CO2 + 2C2 H5 OH + energy
teresting Ethanol is a diuretic. In humans, ethanol reduces the secretion of a hormone
Interesting
Fact
Fact
called antidiuretic hormone (ADH). The role of ADH is to control the amount
of water that the body retains. When this hormone is not secreted in the right
quantities, it can cause dehyration because too much water is lost from the body
in the urine. This is why people who drink too much alcohol can become dehydrated, and experience symptoms such as headaches, dry mouth, and lethargy.
Part of the reason for the headaches is that dehydration causes the brain to
shrink away from the skull slightly. The effects of drinking too much alcohol can
be reduced by drinking lots of water.
9.8.1
Naming the alcohols
The rules used to name the alcohols are similar to those already discussed for the other compounds, except that the suffix of the name will be different because the compound is an alcohol.
173
9.8
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
H
H
OH
H
C1
C2
C3
H
H
H
H
Worked Example 46: Naming alcohols 1
Question: Give the IUPAC name for the following organic compound
Answer
Step 1 : Identify the functional group
The compound has an -OH (hydroxyl) functional group and is therefore an alcohol.
The compound will have the suffix -ol.
Step 2 : Find the longest carbon chain
There are three carbons in the longest chain. The prefix for this compound will
be ’prop’. Since there are only single bonds between the carbon atoms, the suffix
becomes ’propan’ (similar to the alkane ’propane’).
Step 3 : Number the carbons in the carbon chain
In this case, it doesn’t matter whether you start numbering from the left or right.
The hydroxyl group will still be attached to the middle carbon atom, numbered ’2’.
Step 4 : Look for any branched groups, name them and give their position
on the carbon chain.
There are no branched groups in this compound, but you still need to indicate the
position of the hydroxyl group on the second carbon. The suffix will be -2-ol because
the hydroxyl group is attached to the second carbon.
Step 5 : Combine the elements of the compound’s name into a single word in
the order of branched groups; prefix; name ending according to the functional
group.
The compound’s name is propan-2-ol.
Worked Example 47: Naming alcohols 2
Question: Give the IUPAC name for the following compound:
H
OH
OH
H
H
C1
C2
C3
C4
H
H
H
H
174
H
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.8
Answer
Step 1 : Identify the functional group
The compound has an -OH (hydroxyl) functional group and is therefore an alcohol.
There are two hydroxyl groups in the compound, so the suffix will be -diol.
Step 2 : Find the longest carbon chain
There are four carbons in the longest chain. The prefix for this compound will be
’butan’.
Step 3 : Number the carbons in the carbon chain
The carbons will be numbered from left to right so that the two hydroxyl groups are
attached to carbon atoms with the lowest numbers.
Step 4 : Look for any branched groups, name them and give their position
on the carbon chain.
There are no branched groups in this compound, but you still need to indicate the
position of the hydroxyl groups on the first and second carbon atoms. The suffix
will therefore become 1,2-diol.
Step 5 : Combine the elements of the compound’s name into a single word in
the order of branched groups; prefix; name ending according to the functional
group.
The compound’s name is butan-1,2-diol.
Exercise: Naming the alcohols
1. Give the structural formula of each of the following organic compounds:
(a) pentan-3-ol
(b) butan-2,3-diol
(c) 2-methyl-propanol
2. Give the IUPAC name for each of the following:
(a) CH3 CH2 CH(OH)CH3
H
CH3
(b)
9.8.2
C
CH2
CH2
CH3
OH
Physical and chemical properties of the alcohols
The hydroxyl group affects the solubility of the alcohols. The hydroxyl group generally makes
the alcohol molecule polar and therefore more likely to be soluble in water. However, the carbon
chain resists solubility, so there are two opposing trends in the alcohols. Alcohols with shorter
carbon chains are usually more soluble than those with longer carbon chains.
Alcohols tend to have higher boiling points than the hydrocarbons because of the strong hydrogen bond between hydrogen and oxygen atoms.
175
9.9
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
Alcohols can show either acidic or basic properties because of the hydroxyl group. They also
undergo oxidation reactions to form aldehydes, ketones and carboxylic acids.
Activity :: Case Study : The uses of the alcohols
Read the extract below and then answer the questions that follow:
The alcohols are a very important group of organic compounds, and they have
a variety of uses. Our most common use of the word ’alcohol’ is with reference
to alcoholic drinks. The alcohol in drinks is in fact ethanol. But ethanol has
many more uses apart from alcoholic drinks! When ethanol burns in air, it produces
carbon dioxide, water and energy and can therefore be used as a fuel on its own,
or in mixtures with petrol. Because ethanol can be produced through fermentation,
this is a useful way for countries without an oil industry to reduce imports of petrol.
Ethanol is also used as a solvent in many perfumes and cosmetics.
Methanol can also be used as a fuel, or as a petrol additive to improve combustion. Most methanol is used as an industrial feedstock, in other words it is used
to make other things such as methanal (formaldehyde), ethanoic acid and methyl
esters. In most cases, these are then turned into other products.
Propan-2-ol is another important alcohol, which is used in a variety of applications as a solvent.
Questions
1. Give the structural formula for propan-2-ol.
2. Write a balanced chemical equation for the combustion reaction of ethanol.
3. Explain why the alcohols are good solvents.
9.9
Carboxylic Acids
Carboxylic acids are organic acids that are characterised by having a carboxyl group, which has
the formula -(C=O)-OH, or more commonly written as -COOH. In a carboxyl group, an oxygen
atom is double-bonded to a carbon atom, which is also bonded to a hydroxyl group. The simplest
carboxylic acid, methanoic acid, is shown in figure 9.21.
O
C
H
OH
Figure 9.21: Methanoic acid
Carboxylic acids are widespread in nature. Methanoic acid (also known as formic acid) has the
formula HCOOH and is found in insect stings. Ethanoic acid (CH3 COOH), or acetic acid, is
the main component of vinegar. More complex organic acids also have a variety of different
functions. Benzoic acid (C6 H5 COOH) for example, is used as a food preservative.
teresting
Interesting
Fact
Fact
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.9
A certain type of ant, called formicine ants, manufacture and secrete formic acid, which is
used to defend themselves against other organisms that might try to eat them.
9.9.1
Physical Properties
Carboxylic acids are weak acids, in other words they only dissociate partially. Why does the
carboxyl group have acidic properties? In the carboxyl group, the hydrogen tends to separate
itself from the oxygen atom. In other words, the carboxyl group becomes a source of positivelycharged hydrogen ions (H+ ). This is shown in figure 9.22.
H
H
C
H
O
C
H
Acetic acid
H
OH
C
O
+
C
H+
O−
H
Acetate ion
Hydrogen ion
Figure 9.22: The dissociation of a carboxylic acid
Exercise: Carboxylic acids
1. Refer to the table below which gives information about a number of carboxylic
acids, and then answer the questions that follow.
Formula
Common Source
name
formic
acid
boiling
point
(0 C)
101
butter
melting
point
(0 C)
methanoic 8.4
acid
ethanoic 16.6
acid
propanoic -20.8
acid
-5.5
valerian
root
goats
pentanoic -34.5
acid
-4
186
ants
CH3 CO2 H
vinegar
propionic
acid
CH3 (CH2 )2 CO2 H butyric
acid
valeric
acid
CH3 (CH2 )4 CO2 H caproic
acid
enanthic
acid
CH3 (CH2 )6 CO2 H caprylic
acid
milk
IUPAC
name
118
141
164
205
vines
-7.5
223
goats
16.3
239
(a) Fill in the missing spaces in the table by writing the formula, common
name or IUPAC name.
(b) Draw the structural formula for butyric acid.
(c) Give the molecular formula for caprylic acid.
(d) Draw a graph to show the relationship between molecular mass (on the
x-axis) and boiling point (on the y-axis)
177
9.10
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
i. Describe the trend you see.
ii. Suggest a reason for this trend.
9.9.2
Derivatives of carboxylic acids: The esters
When an alcohol reacts with a carboxylic acid, an ester is formed. Most esters have a characteristic and pleasant smell. In the reaction, the hydrogen atom from the hydroxyl group, and
an OH from the carboxlic acid, form a molecule of water. A new bond is formed between what
remains of the alcohol and acid. The name of the ester is a combination of the names of the
alcohol and carboxylic acid. The suffix for an ester is -oate. An example is shown in figure 9.23.
H
H
C
H
O
O
H + H
O
C
H
H
H
C
O
O
C
H + H2 O
H
methanol
methyl methanoate
methanoic acid
Figure 9.23: The formation of an ester from an alcohol and carboxylic acid
9.10
The Amino Group
The amino group has the formula -NH2 and consists of a nitrogen atom that is bonded to
two hydrogen atoms, and to the carbon skeleton. Organic compounds that contain this functional group are called amines. One example is glycine. Glycine belongs to a group of organic
compounds called amino acids, which are the building blocks of proteins.
H
O
C
OH
C
H
H
N
H
Figure 9.24: A molecule of glycine
9.11
The Carbonyl Group
The carbonyl group (-CO) consists of a carbon atom that is joined to an oxygen by a double
bond. If the functional group is on the end of the carbon chain, the organic compound is called
a ketone. The simplest ketone is acetone, which contains three carbon atoms. A ketone has
the ending ’one’ in its IUPAC name.
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.12
Exercise: Carboxylic acids, esters, amines and ketones
1. Look at the list of organic compounds in the table below:
Organic compound
CH3 CH2 CH2 COOH
NH2 CH2 COOH
propyl ethanoate
CH3 CHO
Type of compound
(a) Complete the table by identifying each compound as either a carboxylic
acid, ester, amine or ketone.
(b) Give the name of the compounds that have been written as condensed
structural formulae.
2. A chemical reaction takes place and ethyl methanoate is formed.
(a) What type of organic compound is ethyl methanoate?
(b) Name the two reactants in this chemical reaction.
(c) Give the structural formula of ethyl methanoate.
9.12
Summary
• Organic chemistry is the branch of chemistry that deals with organic molecules. An
organic molecule is one that contains carbon.
• All living organisms contain carbon. Plants use sunlight to convert carbon dioxide in the
air into organic compounds through the process of photosynthesis. Animals and other
organisms then feed on plants to obtain their own organic compounds. Fossil fuels are
another important source of carbon.
• It is the unique properties of the carbon atom that give organic compounds certain
properties.
• The carbon atom has four valence electrons, so it can bond with many other atoms,
often resulting in long chain structures. It also forms mostly covalent bonds with the
atoms that it bonds to, meaning that most organic molecules are non-polar.
• An organic compound can be represented in different ways, using its molecular formula,
structural formula or condensed structural formula.
• If two compounds are isomers, it means that they have the same molecular formulae but
different structural formulae.
• A functional group is a particular group of atoms within a molecule, which give it certain
reaction characteristics. Organic compounds can be grouped according to their functional
group.
• The hydrocarbons are organic compounds that contain only carbon and hydrogen. They
can be further divided into the alkanes, alkenes and alkynes, based on the type of bonds
between the carbon atoms.
• The alkanes have only single bonds between their carbon atoms and are unreactive.
• The alkenes have at least one double bond between two of their carbon atoms. They
are more reactive than the alkanes.
• The alkynes have at least one triple bond between two of their carbon atoms. They are
the most reactive of the three groups.
• A hydrocarbon is said to be saturated if it contains the maximum possible number of
hydrogen atoms for that molecule. The alkanes are all saturated compounds.
179
9.12
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
• A hydrocarbon is unsaturated if it does not contain the maximum number of hydrogen
atoms for that molecule. The alkenes and alkynes are examples of unsaturated molecules.
If a double or triple bond is broken, more hydrogen atoms can be added to the molecule.
• There are three types of reactions that occur in the alkanes: substitution, elimination
and oxidation reactions.
• The alkenes undergo addition reactions because they are unsaturated.
• Organic compounds are named according to their functional group and its position in the
molecule, the number of carbon atoms in the molecule and the position of any double
and triple bonds. The IUPAC rules for nomenclature are used in the naming of organic
molecules.
• Many of the properties of the hydrocarbons are determined by their molecular structure,
the bonds between atoms and molecules, and their surface area.
• The melting point and boiling point of the hydrocarbons increases as their number of
carbon atoms increases.
• The molecular mass of the hydrocarbons determines whether they will be in the gaseous,
liquid or solid phase at certain temperatures.
• An alcohol is an organic compound that contains a hydroxyl group (OH).
• The alcohols have a number of different uses including their use as a solvent, for medicinal
purposes and in alcoholic drinks.
• The alcohols share a number of properties because of the hydroxyl group. The hydroxyl
group affects the solubility of the alcohols. Those with shorter carbon chains are generally
more soluble, and those with longer chains are less soluble. The strong hydrogen bond
between the hydrogen and oxygen atoms in the hydroxyl group gives alcohols a higher
melting point and boiling point than other organic compounds. The hydroxyl group also
gives the alcohols both acidic and basic properties.
• The carboxylic acids are organic acids that contain a carboxyl group with the formula
COOH. In a carboxyl group, an oxygen atom is double-bonded to a carbon atom, which is
also bonded to a hydroxyl group.
• The carboxylic acids have weak acidic properties because the hydrogen atom is able to
dissociate from the carboxyl group.
• An ester is formed when an alcohol reacts with a carboxylic acid.
• The amines are organic compounds that contain an amino functional group, which has
the formula NH2 . Some amines belong to the amino acid group, which are the building
blocks of proteins.
• The ketones are a group of compounds that contain a carbonyl group, which consists of
an oxygen atom that is double-bonded to a carbon atom. In a ketone, the carbonyl group
is on the end of the carbon chain.
Exercise: Summary exercise
1. Give one word for each of the following descriptions:
(a)
(b)
(c)
(d)
(e)
The
The
The
The
The
group of hydrocarbons to which 2-methyl-propene belongs.
name of the functional group that gives alcohols their properties.
group of organic compounds that have acidic properties.
name of the organic compound that is found in vinegar.
name of the organic compound that is found in alcoholic beverages.
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CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.12
2. In each of the following questions, choose the one correct answer from the
list provided.
(a) When 1-propanol is oxidised by acidified potassium permanganate, the
possible product formed is...
i. propane
ii. propanoic acid
iii. methyl propanol
iv. propyl methanoate
(IEB 2004)
(b) What is the IUPAC name for the compound represented by the following
structural formula?
H
Cl
Cl
H
C
C
C
H
H
Cl
C
H
H
H
i.
ii.
iii.
iv.
1,2,2-trichlorobutane
1-chloro-2,2-dichlorobutane
1,2,2-trichloro-3-methylpropane
1-chloro-2,2-dichloro-3-methylpropane
(IEB 2003)
3. Write balanced equations for the following reactions:
(a) Ethene reacts with bromine
(b) Ethyne gas burns in an excess of oxygen
(c) Ethanoic acid ionises in water
4. The table below gives the boiling point of ten organic compounds.
1
2
3
4
5
6
7
8
9
10
Compound
methane
ethane
propane
butane
pentane
methanol
ethanol
propan-1-ol
propan-1,2-diol
propan-1,2,3-triol
Formula
CH4
C2 H6
C3 H8
C4 H10
C5 H12
CH3 OH
C2 H5 OH
C3 H7 OH
CH3 CHOHCH2 OH
CH2 OHCHOHCH2 OH
Boiling Point (0 C)
-164
-88
-42
0
36
65
78
98
189
290
The following questions refer to the compounds shown in the above table.
(a) To which homologous series do the following compounds belong?
i. Compounds 1,2 and 3
ii. Compounds 6,7 and 8
(b) Which of the above compounds are gases at room temperature?
(c) What causes the trend of increasing boiling points of compounds 1 to 5?
(d) Despite the fact that the length of the carbon chain in compounds 8,9
and 10 is the same, the boiling point of propan-1,2,3-triol is much higher
than the boiling point of propan-1-ol. What is responsible for this large
difference in boiling point?
(e) Give the IUPAC name and the structural formula of an isomer of butane.
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9.12
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
(f) Which one of the above substances is used as a reactant in the preparation
of the ester ethylmethanoate?
(g) Using structural formulae, write an equation for the reaction which produces ethylmethanoate.
(IEB 2004 )
5. Refer to the numbered diagrams below and then answer the questions that
follow.
1
O
H
H
H
HO
C
C
C
C
H
H
H
H
Br
C
C
H
Br
3
H
C
C
H
H
2
H
H
HO
C
C
H
H
4
H
O
H
C
C
H
H
H
O
C
H
H
(a) Which one of the above compounds is produced from the fermentation of
starches and sugars in plant matter?
i. compound 1
ii. compound 2
iii. compound 3
iv. compound 4
(b) To which one of the following homologous series does compound 1 belong?
i. esters
ii. alcohols
iii. aldehydes
iv. carboxylic acids
(c) The correct IUPAC name for compound 3 is...
i. 1,1-dibromo-3-butyne
ii. 4,4-dibromo-1-butyne
iii. 2,4-dibromo-1-butyne
iv. 4,4-dibromo-1-propyne
(d) What is the correct IUPAC name for compound 4?
i. propanoic acid
ii. ethylmethanoate
iii. methylethanoate
iv. methylpropanoate
IEB 2005
6. Answer the following questions:
(a) What is a homologous series?
(b) A mixture of ethanoic acid and methanol is warmed in the presence of
concentrated sulphuric acid.
i. Using structural formulae, give an equation for the reaction which takes
place.
182
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
9.12
ii. What is the IUPAC name of the organic compound formed in this
reaction?
(c) Consider the following unsaturated hydrocarbon:
H
H
H
H
C
C
C
C
H
H
H
i. Give the IUPAC name for this compound.
ii. Give the balanced equation for the combustion of this compound in
excess oxygen.
(IEB Paper 2, 2003)
7. Consider the organic compounds labelled A to E.
A. CH3 CH2 CH2 CH2 CH2 CH3
B. C6 H6
C. CH3 -Cl
D. Methylamine
E
H
H
C
H
H
C
C
H
(a)
(b)
(c)
(d)
H
H
O
H
C
C
C
H
H
C
H
H
C
H
H
H
H
Write a balanced chemical equation for the preparation of compound C
using an alkane as one of the reactants.
Write down the IUPAC name for compound E.
Write down the structural formula of an isomer of compound A that has
only FOUR carbon atoms in the longest chain.
Write down the structural formula for compound B.
183
9.12
CHAPTER 9. ORGANIC MOLECULES - GRADE 12
184
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