Secondary Chemistry. Student book. Form Four 9781941940136

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SECONDARY

EMISTBY FORMFOUR STUDENT BOOK Chemisffy Writers 1. Hamse 2.

lbrahim Muhumed

Mustafe Mohamed Ahmed (tvl.indho)

3. Abdirashid Abdirahman Hassan

4. Mohamed Ali Omar (Gaas) ::,:::.

Cover and Book Desisners:

1. Hamud Khaireh Yusuf 2. Liban Ali H.Rabi Editors 1.

Ali Mohamed Amiin

2. Hamse lbrahim Muhumed

-HEMA

Chemistry Textbook Ministry of Education and Higher Studies Republic of Somaliland @

Curriculum Development lnstitute is the department of the Somaliland Ministry of Education and Higher Studies that is responsible for the curriculum development and textbook production.

Design and layout @ HEMA Books 2016

First Edition 2076

All rights reserved. No partof this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without prior permission in writing of Ministry of Education and Higher Studies of the Republic of Somaliland, or are expressly permitted by law.

ISBN: 978-L-94L940-L3-6

Printed by HEMA Books Hargeisa, Somaliland

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ACKNOWLEDGEMENT

The development and writing

of national textbooks

based on the Somaliland new outcome-based

curriculum and syllabus is a national responsibility.For a long period, the National lnstitute of Curriculum Development of the Somaliland Ministry of Education and Higher Studies have been putting a lot of

effort to develop and publish textbooks for primary and secondary schools' All praise is due to Allah; thanks to the Almighty Allah for blessing us to finish this rewarding task. We indulge deep gratitude to the government of Somaliland led by His Excellence President Ahmed Mohamed Mohamoud (Siilaanyo) for their encouragement and contribution to realization and development ofprimary and secondary textbooks locally since 1997. My special thanks goes to the Curriculum Development lnstitute led by Mr. Abdillahi Yasin Derie and his team for planning and coordinating the primary and secondary textbooks development process. We appreciate every attempt they have to identify appropriate curriculum advisors, textbooks writers and reviewers with the help of the Ministry of Education who have successfully produced quality textbooks for the primary and secondary schools.

ln addition to that, we are thankful to national curriculum advisors, textbooks writers

(subject

specialists), editors(reviewers) and designers who helped us in recommending effective approach in writing and reviewing the textbookswhile serving as catalysts for the idea to write and publish textbooks

for Somaliland children. Their creative, functional approaches in writing and team work have enabled the curriculum development institute to produce qualiry primary and secondary textbooks.

My sincere appreciation goes to the State Minister Mohamed H. Aden Elmi, Deputy-Minister Yusuf Osman Garas, Director General Abib Ahmed Ali and chairman of the examination board Da'ud Ahmed

Farah for their remarkable contribution in the development of the national curriculum and the production of quality textbooks for Somaliland schools. My acknowledgement is also due to the technical advisors of the ministry and technical review team for their advisory role in reviewing the textbooks.

Also,l would like

to

express my sincere gratitude

to the former Ministers of Education and Higher

Studies Honorable Madam Zamzam Abdi Aden and Mr. Farah Elmi Geedoole for their tremendous effort

of establishing strong foundation for the national curriculum and textbooks production.

Ministry of Education and Higher Studies is grateful to HEMA Books for their contribution towards the development and writing of the Somaliland primary and secondary school textbooks. We're grateful to HEMA's significant contribution to the process of writing textbooks, graphic designing and their great effort in printing and distributingquality and affordableprimary and secondary textbooks. Finally, the MoE&HS is gratefulto any other individual or organizations, who

.r"

no, listed here, for their

contribution to the process of curriculum development. Honorable Abdillahi lbrahim Habane Minister of Education and Higher Studies, Republic of Somaliland

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PREFACE This book is designed for Form 4 Somaliland secondary schools and contains all of the topics they require. Each topic is supported by theories, facts, explanations, analysis and exercises and it's based on newly revised curriculum of the Republic of Somaliland

The objective of this book is to satisfo educational needs of the learners and to help

them to develop self-reliance and complete confidence in their abilities to understand and solve chemistry problems.

In the context of this objective, the book contains:

. . .

Detailed notes, many examples and short clarifications. Summaries and exercises which address all topics covered in the text.

Chapter review questions at the end of each chapter allow students to practice.

This book is an excellent teaching learning tool for both teachers and students.

The language is kept simple, to improve accessibility for all students, Care is taken to

introduce and use all the special terms that students need

to gain a

complete

understanding of the chemical concepts introduced.

In the text, key terms are highlighted in bold. The depth and breadth of each topicis pitched at the appropriate O level students.

The key objective of this book is to improve the quality of secondary chemistry education; all of it has been reviewed and revised, ensuring that the new specification is fully covered.

In addition to the main content in each chapter describing

issues, applications or

events, which put the chemical content introduced into a social context.

in each chapter provide opportunities to check They often address misunderstandings that commonly appear in

Chapter Exercise questions (EQs) understanding.

examination answers, and will help students to avoid such errors.

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The key objective of Modern Education is to give learners the skills, knowledge and attitudes they will need

to

succeed

in a rapidly evolving world. In most developing

countries learning resources are scarce.

It

is therefore necessary that the teacher uses

alternative methods such as: Collection from the environment and Improvisation.

The teacher should have the capacity and attitude to improvise resources from locally available materials which are often considered waste or valueless. Improvisation helps reduce the cost of teaching and learning since improvised resources cost very little or have no cost at all. In addition, improvisation helps demystify science and bring it home

to the learner as part and parcel of everyday life.

Chemistry Textbook Developers

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Table of Contents Chapter One: Atomic Structure...............

1.1

1.2

1.3

1

Atomsand isotopes................

1

What is an atom.

2

Atomic number and mass number......

2

lsotopes....

3

The Electronic structure of the atom.........

5

Sub-shells and orbitals

6

How do two electrons fit into an orbitals?

7

Filling of sub-shells and orbitals......

7

Hund's ru|e............

8

The rule of stability..

10

The periodic table......

12

Group and periods

14

Blocks of elecments in the periodic tab|e..........

15

Electron arrangements and the Periodic Table......

16

Periodicity.

16

lnonization energy.......

16

Periodic patterns of ionization energy....

18

Atomic radius........

23

Periodic petterns of atomic radii...........

23

lonic radii...

25

Properties from structure and Bonding....

26 30

Chapter Two: Moles and Stoichiometry...........

2.1 The masses of atoms and molecules.............

31

Relative and atomic mass (RAM orA,).........

31

Relvative moleculer mass, M................

31

Relative formula mass.........

32

Percentage composition................

33

The mole (The chemical counting unit).......

36

Deflning the mole....

37

Molar mass.........

38

Calculations on the mole..........

38

2.2

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2.3 2.4

Calculation involving gases........

42

Valume from No. of particles

43

Empirical and molecular formula

45

Empirical formula.........

45

Combustion analysis..

49

Molecular formula.........

52

2.5 The mole and chemical equations................

2.6

54

Balancing Chemical Equations..

54

Stoichiometric Calculations..........

55

Mole/mole relationship in stoichiometry............

55

The concentration of solutions

59

ChapterThree: Chemical Bonding & Structure

3.1

64

lntroduction to chemical bonding...

65

The octet ru|e............

65

Types of chemical bonds......

65

Molecular elements...

68

Molecular compounds

69

3.2 Electro-negativity, polarity and polarization.......... 3.3 Shapes of simple molecules. 3.4 lntermolecularforces... 3.5 The structure of sabstances..........

73 78 81

90

Chapter Four: Enthalph Changes..

100

4.1 Heat changes in chemical reactions.............. 4.2 Quantitative determination of enthalpy changes.... 4.3 Bond enthalpies..........

101

106 119

Chapter Five: Organic Chemistry.............

5.1 5.2 5.3 5.4 5.4.1

132

Basic concepts (Revision).

133

Reation machanisms................

147

Polymers...

152

Haloalkanes................

160

Rate of Hydrolysis of Primary Hologenolakanes........

160

Glassary....

162

Reference

166

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Neutron

Nucleus

Proton

\"n*

s".g &&&ffi?s em#. &sax&exg*ew Whm* are mss m*qxm? An atom is the smallest paft of an dement that can exist on its own. Atoms are so tiny that there are more atoms in a full stop than there are people in the world. An atom is the smallest particle of an element that

Sub-atomic particles Atoms are made up of three fundamental particles: protons, neutrons and electrons. Protons and neutrons are found in the nucleus and are collectively called nucleons. Electrons orbit the nucleus in a similar way to that in which planets orbit a sun. ln between the electrons and nucleus there is nothing (empty space)^ The nucleus is very small; if an atom were the size of a football stadium, the nucleus would be the size of a pea!

can take part

in

a

chemical reaction and can exist on its ovm. _'A

./

{ .*

i/\

---,

i'!;ifu

il#!& b ',,,. J,

@.

Y

i@ I

-rr.H i

Protons and neutrons have almost the same mass. Electrons have virtually no mass at all (111840th of the mass of a proton). The other important feature of these particles is their electric charge. Protons and electrons were found to have equal and opposite charges, while neutrons are electrically neutral (have no charge). The properties of these three sub-atomic parlicles are shown in the table below.

n

Property

Protons, p

Neutron,

Mass/kg

1.673r10'zt

1.673'10-zt

Charge/C +1.602 x 10-1e 0

Position

ln the

Electron, e

x

10-30

9 ?11 (almost zero) -1.602 x 10-1e

nucleus ln the nucleus Around the nucleus

These numbers are extremely small. ln practice, we use the relative masses and charges. 1 unit of charge is 1.602 x 10-1e coulombs. 1 unit of mass is 't .661 x 10'27 kg.

Relative

mass

Relative charge

1

1

1t1840

+1

0

-1

A single atom is electrically neutral (it has no overall charge). This means that in any atom there must be equal numbers of protons and electrons. In this way, the total positive charge on the nucleus (due to the protons) is balanced by the total negative charge of the orbiting electrons.

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,{tornic raurxrher axrd xu}&ss xaaeffin}er The number of protons in the nucleus is called the atomic number (or proton number, given the symbol Z) of an atom. The atomic number defines the chemical identity of an element. This means that no two elements can have the same atomic number. Only hydrogen atoms have one proton in their nuclei. Only helium atoms have two protons. lndeed, only gold atoms have 79 protons. This shows that the number of protons in the nucleus of an atom decides which element it is.

-

Atomic number (proton number), Z = number of protons -All atoms of the same element have the same atomic number. Atoms of different elements have different atomic numbers. The atomic number contributes to the mass of an atom. The neutrons in the nucleus also contribute to the total mass. The mass of the electrons can be regarded as so small that it can be ignored. As a proton and a neutron have the same mass, the mass of a particular atom depends on the total number of protons and neutrons present. This number is called the mass number (or nucleon number, given the symbol A) of an atom.

-

The total number of protons plus the neutrons in the nucleus (the total number of nucleons) of an atom is called the mass number, A. Mass number (A) = number of protons + number of neutrons lt is the nucleons that are responsible for almost all of the mass of an atom because electrons weigh virtually nothing. ilasr number

The atomic number, Z and the mass number, A of an atom of an element can be written alongside the symbol for that element, in the general way as ! so the symbol for an atom of lithium is {li. tne following table shows the composition of some common elements.

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Number of protons and neutrons in atom

\\

X

/

Atomlc

iX-m*ffi*

/ numbea

to representatc{n in chemical

formular

Number of proton5 in 6tom

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,frc 6

protons

fl

Gneutrons -4

6elertrons C Gold uranium

U

92

238

92

146

ln a neutral atom, the number of protons and electrons are the same. However, many elements do not exist as neutral atoms, but exist as ions. Ions are species in which the proton and electron numbers are not the same, and hence have an overall positive or negative charge. The number of electrons in a species can be deduced from its charge:

Number of electrons in a positive ion = atomic number - charge on ion e.g. the number of electrons in Na* = 11 - 1 = '10 Number of electrons in a negative ion = atomic number + charge on ion e.g. the number of electrons in Cl- = 17 + 1 = 18

* l-B$

rl:

;

, q :ru rt 4,:i bonded-pai r/l one-pai r > bonded-pai r/bond ed-pai r Lone pairs distort the shape of a molecule and reduce the bond angle. Look at the examples below and notice how each lone pair decreases the bond angle by 2.50

a

a

o

a

o

o

o

O

o

a

O

a

CHr

Molecule

Dot-anddiagram

H{ $c6

H

H

lN( \

X.fi}:X H

Number of lone pairs

IH H

,6

0

7Kir,

2

1

1a1

0s'',

I

Shape and bond angle

H-CT,,,H

H

H

Tetrahedral

H

H

H

of

o

H

across

Name

ILO

NII3

107.8"

Pyramidal

Non-linear

Shape

Ammonia has four pairs of electrons around the central nitrogen atom. Three of these electron pairs are bonded or shared pairs of electrons and the fourth pair that remains is a lone pair. With these four pairs of electrons around the nitrogen atom, the ammonia molecule has a shape based on a tetrahedron. However, there are only three arms so the shape is that of a triangular pyramid. Another way of looking at this is that the electron pairs form a tetrahedron but the bonds form a triangular pyramid. There is an atom at each vertex but, unlike the tetrahedral arrangement, no atom in the center. The angles of a regular tetrahedron are all 109.50 but lone pairs affect these angles. ln ammonia, for example, the shared pairs of electrons are attracted towards the nitrogen nucleus and also the hydrogen nucleus. However, the lone pair is attracted only by the nitrogen nucleus and is therefore pulled closer to it than the shared pairs. So, the repulsion between a lone pair of electrons and a bonding pair of electrons is greater than that between two bonding pairs. This effect squeezes the hydrogen atoms together, reducing all the H -N -H angles. So, the bond angles in ammonia are approximately 1070. Look at the dot-and-cross diagram for water. There are four pairs of electrons around the oxygen atom, so, as with ammonia, the shape is based on a tetrahedron. However, two of the 'arms' of the tetrahedron are lone pairs that are a part of a bond. This results in a V-shaped or a

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CI

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non-linear molecule (also known as bent). As in ammonia, the electron pairs form a tetrahedron but the bonds form a bent shape. With two lone pairs, the H -O -H angle is reduced to 104.50.

Molecules with double bonds ln molecules containing multiple bonds, each double bond is treated in the same way as a bonded pair. ln the diagrarn of carbon dioxide below, each double bond is treated as a 'bonding region'. Examples are given in the table below.

Molecule lion

Number of bonding regions

Name of the shape

Shape and angle

12l-+

Carbonate ion

/o +/ o-c

3

\

TRIGONAL PLANAR

d

?*t{ '' Sulphate ion

4

'"\t*f--fi o-N

Nitrate ion

L

co-ord i nat+ bo nd

TRIGONAL PLANAR

\ Carbon dioxide

TETRAHEDRA

o

,{0" o-c-o

2

LINEAR

9.4 Interrmolecular forces Atoms in molecules and in giant structures are held together by strong covalent, ionic or metallic bonds. These are known as intramolecular forces. Molecules and separate atoms are attracted to one another by other, weaker forces called intermolecular forces. lf the intermolecular forces are strong enough, then molecules are held closely enough together to be liquids or even solids. . rrhe' forces.thatrhold,'atomi,together are,krtqwq.asriiltramolecularforeee, . The forces that hold molecules together are known as intemrolecular forces.

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3

a

a

t

,

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a

a

I

There are three types of intermolecular forces: o Van der waal's forces which act between all atoms and molecules. . Permanent dipole-dipole forces which act only between certain types of molecules. . Hydrogen bonding which acts only between certain types of molecules. These constantly acting attractive forces or intermolecular bonds are very much weaker than covalent or ionic chemical bonds (approximately 1/so to 1/zoth in comparative attractive force).For example, although the oxygen and hydrogen atoms are very strongly bonded in water to make a VERY stable molecule, BUT this does NOT account for the existence of liquid water and ice! It is the weak intermolecular forces that induce condensation below 100oC and freezingsolidification to form ice crystals below 0oC. In the reverse process, when ice is warmed, the intermolecular forces are weakened and at OoC

the intermolecular bonds are weakened enough to allow melting to take place. Above OoC (evaporation), and particularly at '100oC (boiling), the intermolecular forces are weak enough for'intact water molecules' to escape from the surface of the liquid water. It is very important to realise that the chemical hydrogen-orygen covalent bonds (O-H) in water are NOT broken and the state changes are due to the weakening of the intermolecular forces/bonds with increase in temperature OR the strengthening of the intermolecular bonds/forces with decrease in temperature.

Van der waal' forces All atoms and molecules are made up of positive and negative charges even though they are neutral overall. These charges produce very weak electrostatic attractions between all atoms and molecules whether polar or non-polar. Consider a molecule of oxygen, Oz.

The electrons in this molecule are not static; they are in a state of constant motion. lt is therefore likely that at any given time the distribution of electrons will not be exactly symmetrical - there is likely to be a slight surplus of electrons on one of the atoms. + 6

eoe This is known as a temporary dipole. lt lasts for a very short time as the electrons are constantly moving. Temporary dipoles are constantly appearing and disappearing.

o

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a

a

a

a

a

a

t

a

o

.E

Consider now an adjacent molecule. The electrons on this molecule are repelled by the negative part of the dipole and attracted to the positive part, and move accordingly.

+ 6

e e o e ee e eoe e A v6

"f,e s

-e

*( e.

This is known as an induced dipole. There is a resulting attraction between the two molecules, and this known as a Van der Waal's force. These forces are sometimes called instantaneous dipole- induced dipole forces, but this is rather a mouthful. The more usual name is van der waal's forces after the Dutch scientist, Johannes van der Waals. Van der Waal's forces are present between all molecules, although they can be very weak. They are the reason all compounds can be liquefied and solidified. Van der Waal's forces tend to have strengths between 1 kJmol-1 and 50 kJmol-1.

. Van der Waals forces act between all atoms or molecules at all times. . They are in addition to any other intermolecular forces. The strength of the Van der Waal's forces in between molecules depends on the number of electrons in a molecule.

The number of electrons in the molecule The greater the number of electrons in a molecule, the greater the likelihood of distortion, and thus; the greater the frequency and magnitude of the temporary dipoles. Thus the Van der Waal's forces between the molecules are stronger and the melting and boiling points are larger. This explains why: . The boiling points of the noble gases increase as the atomic numbers of the noble gasees increase.

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Substahce

He

Ne

Ar

Kr

Number of electrons Melting point/oC Boiling poinUoC

2

10

18

-272

-252 -250

-189

36 -157 -152

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A rr

rt

-269

s

6

-186

t

o

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I

o

. The boiling points of hydrocarbons increase

with increased chain length. ,CclIis'

Number of electrons Melting poinUoC Boiling poinUoC

10

18

26

-182 -164

-183 -88

-190 -42

34 -1 38 0

-

Permanent dipole dipole forces Temporary dipoles exist in all molecules, but in some molecules there is also a permanent dipole. Most covalent bonds have a degree of ionic character resulting from a difference in electronegativity between the atoms. This results in a polar bond and a dipole.

5+ H _cl

--t 6o

5-

ln many cases, however, the presence of polar bonds (dipoles) does not result in a permanent dipole on the molecule, as there are other polar bonds (dipoles) in the same molecule which have the effect of cancelling each other out. This effect can be seen in a number of linear, trigonal planar and tetrahedral substances: CI

t\

I

_c.-_ 'cr

",t CII COz

B

./'

I

F BFs

CCla

ln all the above cases, there are dipoles resulting from polar bonds but the vector sum of these dipoles is zero; i.e. the dipoles cancel each other out. The molecule thus has no overall dipole and is said to be non-polar. Non-polar molecules are those in which there are no polar bonds or in which the dipoles resulting from the polar bonds all cancel each other out. The only intermolecular forces that exist between non-polar molecules are temporary-induced dipole attractions, or Van der Waal's forces.

ln other molecules, however, there are dipoles on the molecule which do not cancel each other out:

o

o

3

a

a

a

o

a

a

a

t

'E

d^+ H 6

I I

-c.-_ 'cl

",t I

\

CI

-..1"'rr I

+

s%5

H

d

d

H

d^+

il

o NHs

SOz

CHCIe

ln all the above cases, there are dipoles resulting from polar bonds whose vector sum is not zero; i.e. the dipoles do not cancel each other out. The molecule thus has a permanent dipole and is said to be polar. Polar molecules are those in which there are polar bonds and in which the dipoles resulting from the polar bonds do not cancel out.

ln addition to the Van der Waal's forces caused by temporary dipoles, molecules with permanent dipoles are also attracted to each other by permanent dipole-dipole bonding. This is an attraction between a permanent dipole on one molecule and a permanent dipole on another.

6+ 5H-Br

6+ 6H-Br

6+ 6H-Br

Dipole.dipole bonding usually results in the boiling points of the compounds being slightly higher than expected from temporary dipoles alone; it slightly increases the strength of the intermol ecular bondi ng. The effect of dipole-dipole bonding can be seen by comparing the melting and boiling points of different substances which should have Van der Waal's forces of similar strength:

Substance

Clz

ltBr

CrHsG,H(CHgle,Hs

Number of electrons Permanent dipole Melting

34

36

34

32

NO

YES

NO

YES

-101

-88

-159

-95

45

-67

-73

-44

poinUoC

Boiling point/oC

ro

o

o

a

a

o

a

a

a

a

o

a

Hydrogen bonding Hydrogen bonding is a special type of intermolecular force with some characteristics of permanent dipole-dipole forces and some of a covalent bond. lt consists of a hydrogen atom sandwiched between two very electronegative atoms. There are conditions that have to be present for hydrogen bonding to occur. We need a very electronegative atom with a lone pair of electrons covalently bonded to a hydrogen atom. Water molecules fulfill these conditions. Oxygen is much more electronegative than hydrogen so water is polar. We would expect to find weak dipole-dipole attractions, but in this case the intermolecular bonding is much stronger for two reasons: . The oxygen atoms in water have lone pairs of electrons. o In water, the hydrogen atoms are highly electron deficient. This is because the oxygen is very electronegative and attracts the shared electrons in the bond towards it. The hydrogen atoms in water are positively charged and very small. These exposed protons have a very strong electric field because of their small size. The lone pair of electrons on the oxygen atom of another water molecule is strongly attracted to the electron deficient hydrogen atom. This strong intermolecular force is called a hydrogen bond. Hydrogen bonds are considerably stronger than permanent dipole-dipole attractions, though much weaker than a covalent bond. It is only possible if the hydrogen atom is bonded to a very electronegative element; i.e. N, O or F. lt is not fundamentally different from dipole-dipole bonding; it is just a stronger form of it. They are usually represented by dashes - - -,as shown below.

hydrogen bord forme ,itraciion between

dby

r'--\

diPole

:xf,H:i*"

dirrerent

fi-r- .lg^

n.-q ft>-oq

ff-q\o*

\s+

H

A hydrogen bond can be defined as an attraction between an electropositive hydrogen atom (ie covalently'bonded to N; O or F) and an elqctronegatircatom'rbnrahadjacentr,molecule)

When do hydrogen bonds form? Water is not the only example of hydrogen bonding. ln order to form a hydrogen bond, we must have molecules containing the following groups:

fu ..6*

E+ ..6=-

H_N-

H-03

5+ ..E-

H-F: a'

\

a

a

o

a

0

C

o

o

C

a

a

A hydrogen atom that is bonded to a very electronegative atom. This will produce a strong partial charge on the hydrogen atom. . A lone pair-d, -6lelr;trons on a highly electronegative atom of F, O or N on another molecule. These will be attracted to the partially charged hydrogen atom in another molecule and form the o

bond.

The only atoms that are electronegative enough to form hydrogen bonds are oxygen; O, nitrogen; N, and fluorine; F. Examples of substances other than water that contain hydrogen bonds are HF, NHs, alcohols, carboxylic acids, amines, acid amides and urea.

The effect of hydrogen bonding on boiling point The effect of hydrogen bonding on melting and boiling points of substances is huge, unlike other dipole-dipole bonds. Many substances containing hydrogen bonds have much higher boiling points than would be predicted from Van der Waal's forces alone. Substance Structure

CHaCHzOH

CHsCHzCHzCHO

CHgCHzCOOH

HH

cH3-cH2-0H

I I TN H-c-c-C-C-H

26

26

40

40

NO

YES

NO

YES

-95

-117

-81

-21

44

78

56

141

CHgOCHs

CH""\

,/,ZzO C

III

CHu

Number of electrons Hydrogen bonding Melting

lllo ll\o" HH

poinUoC

Boiling poinUoC

The general increase in boiling point down the groups result from the increase in Van der Waal's forces which results from an increasing number of electrons in the molecules. There are permanent dipoles but they are not very strong.

The abnormally high boiling points of HzO, NHg and HF are a result of hydrogen bonding between the molecules. Thus results in very strong intermolecular forces between the molecules despite the fact that the Van der Waal's forces are weaker than in the other hydrides.

The importance of hydrogen bonding Although hydrogen bonds are only about 10o/o of the strength of covalent bonds, their effect can be significant - especially when there are a lot of them. The very fact that they are weaker than covalent bonds, and can break or make under conditions where covalent bonds are unaffected is very significant.

a

a

a

a

o

a

o

a

o

o

o

o

The low density of ice This is due to hydrogen bonding. ln ice, the water molecules arrange themselves in such a way as to maximise the amount of hydrogen bonding between the molecules. This results in a very open hexagonal structure with large spaces within the crystal. This accounts for its low density. When the ice melts, the structure collapses into the open spaces and the resulting liquid, despite being less ordered, occupies less space and is thus more dense. Thus ice floats on water. This means that it forms on top of ponds rather than at the bottom. This insulates the ponds and enables fish to survive through the winter. This must have helped life to continue, in the relative warmth of the water under the ice, during the ice ages. The diagram below shows the open lattice of ice and how it collapses on melting. r-ll-\. \ry

u

hydrogen bonds break*

*

r-\o

4qoq

MELTING OF ICE

iee lattice collaPses:

molecules rnove closer together

tetrahedral oPen lattice in ice

The extra intermolecular bonding from hydrogen bonds also explains the relatively high surface tension and viscosity in water.

Living with hydrogen bonds Proteins are a class of important biological molecules that fulfill a wide variety of functions in living things, including enzyme catalysts. The exact shape of a protein molecule is vital to its function. Proteins are long chain molecules with lots of C = O and N - H groups which can form hydrogen bonds. These hydrogen bonds hold the protein chains into fixed shapes. One common shape is the protein chain that forms a spiral (helix), as shown here.

-fhis is a ribbon mod€l of ttre -Ffre actual molecule c'-frellx. would tre more 'filled in'-

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a

3

3E

Another example is the beta-pleated sheet. Here protein chains line up side by side, held in position by hydrogen bonds to form a two-dimensional sheet. The protein that forms silk has this structure.

The relative weakness of hydrogen bonds means that the shapes of proteins can easily be altered. Heating proteins much above body temperature starts to break hydrogen bonds and causes the protein to lose its shape and thus its function. This is why enzymes lose their effect as catalysts when heated. We say the protein is denatured. We can see this when frying an egg. The clear liquid protein albumen is transformed into an opaque white solid.

Ironing When we iron clothes, the iron provides heat to break hydrogen bonds in the crumpled material and pressure to force the molecules into new positions so that the material is flat. When we remove the iron, the hydrogen bonds reform and hold the molecules in these new positions, keeping the fabric flat. DNA Another vital biological molecule is DNA (deoxyribonucleic acid). lt is the molecule that stores and copies genetic information that makes offspring resemble their parents. This molecule exists as a double-stranded helix. The strands of the spiral are held together by hydrogen bonds. When cells divide or replicate the hydrogen bonds break (but the covalently bonded main chains stay unchanged). The two separate helixes then act as templates for a new helix to form on each, so we end up with a copy of the original helix.

r&

&

r

s

&

&

&

&

&

&

&

3"5 The structure of substance$ Structure describes the arrangement in which atoms, ions or molecules are held together in space. ln the solid state, the particles form a regular arrangement called a lattice. There are four main types of structure: simple molecular, macromolecular (giant covalent), giant ionic and giant metallic. Look at the table below.

The lattice is built up of millions of separate small molecules

The lattice is built up of millions of particles, which can be: o Positive and negative ions, joined by ionic bonds or o Metal ions in a sea of electrons, joined by metallic bonds, or o Non-metal atoms joined by covalent bonds ln sodium chloride (NaCl) the lattice is made up of sodium and chlorine and chlorine ions Particles held together by a network of strong bonds

ln iodine (lz) the lattice is made up of iodine molecules, each containing two atoms What holds the structure Strong covalent bonds within molecules but weak forces together? between molecules The solid usually has a high The solid has a low melting One result of this point, since it does not take melting point, since it take a structure great deal of heat energy to much heat energy to break up break the bonds in the lattice the lattice to form a liquid o A simple molecular structure is composed of small molecules; small groups of atoms strongly held together by covalent bonding. The forces of attraction between molecules are much weaker (often over 50 times weaker than a covalent bond) and are called intermolecular forces. Examples of molecules include Clz, HzO, HzSO+ and NHs. . A giant covalent structure is one in which large numbers of atoms are linked in a regular three-dimensional arrangement by covalent bonds. Examples include diamond, graphite and silicon dioxide (silica), the main constituent of sand. . A giant ionic structure consists of a lattice of positive ions each surrounded in a regular arrangement by negative ions and vice versa. . A giant metallic structure consists of a regular lattice of positively charged metal ions held together by a cloud of delocalized electrons.

a

o

0

o

a

o

a o o a 0 cg

Melting and boiling points The property that best tells us if a structure is giant or simple molecular is the melting (or boiling) point. o Simple molecular compounds have low melting (and boiling) points. . Giant structures generally have high melting (and boiling) points.

We know that if a compound has a low melting (and boiling) point, it has a simple molecular structure. We know that all molecular compounds are covalently bonded. So all compounds with low melting (and boiling) points must have covalent bonding. But take care: a compound with covalent bonding may have either a giant structure or a simple molecular structure and therefore may have either a high or low melting (and boiling) point. Giant ionic structures lonic compounds form giant ionic lattices with ionic bonds holding the particles together. An ionic bond is an attraction between oppositely charged ions. After the ions are formed they all come together to form a Iattice. A lattice is an infinite and repeating arrangement of particles. All the anions are surrounded by cations and all the cations are surrounded by anions. The coordination number of an ion in an ionic solid is the number of oppositely charged ions which surround it. This varies from substance to substance but is usually 4, 6 or 8.

Example - sodium chloride ln sodium chloride, NaCl, each sodium ion is surrounded by six chloride ions and vice versa. The diagram below shows the structure of sodium chloride. The pattern repeats in this way and the structure extends (repeats itself) in all directions over countless ions. You must remember that this diagram represents only a tiny part of the whole sodium chloride crystal.

{J

ua*

0.t

E.

o

o

o

a

t

I

Na+

TJ

CT

Each sodium ion attracts several chloride ions and vice versa so the ionic bonding is not just between one sodium and one chloride ion. There is a 3-D lattice.

o

a

a

o

a

o

It could also be represented as follows:

G)

@YO (r.rJ)

@-I-@ [, Properties of ionic compounds Melting and boiling point The attraction between opposite ions is very strong. A lot of kinetic energy is thus required to overcome them and the melting point and boiling point of ionic compounds is very high. ln the liquid state, the ions still retain their charge and the attraction between the ions is still strong. Much more energy is required to separate the ions completely and the difference between the melting and boiling point is thus large. Compound Melting point/oC

801

Mgo 2852

Boiling poinUoC

1459

3600

NaCl

The higher the charge on the ions, and the smaller they are, the stronger the attraction between them will be and the higher the melting and boiling points. ln MgO, the ions have a 2+ and 2charge and thus the attraction between them is stronger than in NaCl, so the melting and boiling points are higher. E I ectri ca

I

Co n d u cti

vity

Since ionic solids contain ions, they are attracted by electric fields and will, if possible, move towards the electrodes and thus conduct electricity. ln the solid state, however, the ions are not free to move since they are tightly held in place by each other. Thus ionic compounds do not conduct electricity in the solid state. lonic solids are thus good insulators. ln the liquid state, the ions are free to move and so can move towards their respective electrodes. Thus ionic compounds can conduct electricity in the liquid state.

Mechanical properties Since ions are held strongly in place by the other ions, they cannot move or slip over each other easily and are hence hard and brittle.

a

o

o

o

o

o

a

a

o

c

3

t

sf+ss

----#

opposite ions attract

like ions repel

Solubility

-

structure breaks

. The ionic lattice dissolves in polar solvents (e.9. water). o Polar water molecules break down the lattice and surround each ion in solution as shown below for sodium chloride:

5ol*

*>+ *P"* I

*/oo Na+ attracts 6- chaqges on the O atoms of water

CF attracts &r- charges

molecules.

molecules.

Water.molecules attract Na+ and Cl- ions.

lattice break dourn as it dissokes. li\later molecules surround irons.

on the H atoms'of water

Covalent structures Elements and compounds with covalent bonds have either of two structures: . A simple molecular structure . A giant covalent structure Simple molecu lar structures ln many cases, the bonding capacity is reached after only a few atoms have combined with each other to form a molecule. lf no more covalent bonds can be formed after this, the substance will be made up of a larger number of discreet units (molecules) with no strong bonding between them. Such substances are called molecular substances, and there are many examples of them: CH+, Clz, He, Se, Pq, Oz, HzO, NHs etc

The molecules are held together by intermolecular forces, which are much weaker than covalent bonds but are often strong enough to keep the substance in the solid or liquid state.

0

t

O

a

a

t

o

o

t

C

O

Example - lodine

I,

l-

t'

I--

1...1 '

I

|

bonds

-ssYslsnt ..

" intermolecular forces

l'tt...t-t There are attractive forces between these molecules, known as intermolecular forces, but they are weak. ln the gaseous state, the intermolecular forces are broken but the bonds within the molecule remain intact - they are not broken. The gas phase consists of molecules, not atoms.

. When the lz lattice breaks

down, only the weak van der Waals' forces between the lz

molecules break. . ln the lz molecules, the covalent bond, l-1, is strong and does not break when the lattice breaks down. Sim ple molecular su bstances have certain characteristic properties:

Melting and boiling point These are generally low, since intermolecular forces are weak. Low temperatures provide sufficient energy to break the weak intermolecular forces between molecules. Therefore, simple molecular structure have low melting and boiling points. Electri cal conductivity There are no free charged particles and delocalized electrons. Therefore, simple molecular structures are non-conductors of electricity.

Solubility r Van der Waals' forces form between a molecular structure and a non-polar solvent, such as hexane. These weaken the structure. Therefore, simple molecular structures are often soluble in non-polar solvents (e.9. hexane).

Giant covalent structu res ln some cases, it is not possible to satisfy the bonding capacity of a substance in the form of a molecule; the bonds between atoms continue indefinitely, and a large macromolecular structure or lattice is formed. There are no discrete molecules and covalent bonding exists between all adjacent atoms. Such substances are called giant covalent substances, and the most important examples are C, B, Si and SiOz. This type of structure is known by a variety of names: a giant molecular lattice, a giant covalent lattice or a giant atomic lattice.

o

3

c

O

a

a

t

a

a

a

o

a

Carbon: a macromolecular element Carbon is a non-metal. lt occurs in two forms or allotropes: diamond and graphite. These polymorphs or allotropes of carbon are very different materials because their atoms are differently bonded and arranged. They are examples of macromolecular structures. Diamond Diamond consists of pure carbon with covalent bonding between every carbon atom. The bonds spread throughout the structure, which is why it is a giant structure.

A carbon atom has four electrons in its outer shell. ln diamond, each carbon atom forms four single covalent bonds with other carbon atoms. These four electron pairs repel each other, following the rules of the electron pair repulsion theory. ln three dimensions, the bonds actually point to the corner of a tetrahedron (with bond angles of 109.50). Each carbon atom is an identical position in the structure, surrounded by other four carbon atoms. The figure below shows this three-dimensional arrangem ent. Don't forget that this is just a tiny part of a giant structure extending on all 3 dimensions.

The atoms form a giant three-dimensional lattice of strong covalent bonds, which is why diamond has the following properties: lt is very hard material (one of the hardest known). Diamond is in fact the hardest substance known to man. For this reason it is used in drills, glass-cutting and styluses for turntables. o lt has high melting point, over 3700 k . lt does not conduct electricity because there are no free charged particles to carry charge.

.

Graphite Some covalent structures contain an infinite lattice of covalently bonded atoms in two dimensions only to form layers. The different layers are held together by intermolecular forces, and there are often delocalized electrons in between the layers. Examples of these structures are graphite and black phosphorus. Graphite consists of pure carbon but atoms are bonded and arranged differently than in diamond. ln graphite each carbon atom forms three single covalent bonds to other carbon atoms. As predicted by bonding electrons pair repulsion theory, these form a flat trigonal arrangement, sometimes called trigonal planar, with a bond angle of 1200. This leaves each carbon atoms 'spare' electron in a orbital that is not a part of three dimensional bonds.

o

&

a

a

o

c

o

t

o

3

t

o

This anangement produces a two-dimensional layer linked hexagons of carbon atoms, rather like a chicken-wire fence.

atoms

a shck of

i[

ooe

hFc

hyec:

f

.m*z.so

The spare electron is delocalized and occupies the space in between the layers. All atoms in the same layer are held together by strong covalent bonds, and the different layers are held together by intermolecular forces. The p orbital with the'spare' electron merge above and below the plane of the carbon atoms in each layer. These electrons can move anynrhere within the layer. They are delocalised. This adds to the strength of the bonding and it is rather like the delocalised sea electrons in the metals, but in two dimensions only.

A number of characteristic properties of graphite result from this structure: I c o n d u ctivi ty Due to the delocalised electrons in each plane, graphite is a very good conductor of electricity in the x and y directions, i.e. only at the hexagonal planes, even in the solid state (very rare for a non-metal). However, since the delocalisation is only in two dimensions, graphite does not conduct electricity in the z direction (i.e. perpendicular to the planes). E I ectr i ca

Density Graphite has a much lower density than diamond (2.25 gcm-3) due to the relatively large distances in between the planes. Hardness There is no covalent bond between the layers of carbon atoms in graphite. They are held together by the weaker van der waals force.This week intermolecular force of attraction means that the layers can slide across one another and so graphite is much softer than diamond. This results in the widespread use of graphite in pencils and as an industrial lubricant.

o

a

o

o

C

a

a

3

a

o

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The table below compares the properties of diamond and graphite.

AErben aGom shareslll frrrof ibqrEr elEchonarift othercabon abrns, b frorm a ttmedirremrtrral Iattie.

A cafion abrn sharrs orErer

carbm akns,

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of rts q.rter elechons wiih form a layer *urture-

tfe layers and t frre - see pary lU-

The fior.trth EIEEhon exists betrreen

to rnaue {like eHmns in nxtah Giant shuchrm

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luhrka*rtfor engins and lodrs ehctsedes {or ehcbotysis in the lab and in indrrlry

The strong covalent bonds in the lattice are too strong to be broken by polar or non - polar solvents. Therefore, giant molecular structures are insoluble in polar and non - polar solvents.

Silicon dioxide: a macromolecular compound Silicon dioxide, SiOz, is a macromolecular compound. lt occurs naturally as sand and quartz.

*arh sxlgen a!ilm €,fclrm* (a$Elent hs,n& witfu trFJs silirl*n afum:$

s

eech si[icnn atqrn forms f,sffiNent

h*n&

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f*ur orftrgen stnmg

the silicen a*d *xlgen EteffiE fEam a tetmh*drun {lik* the ffirhon atsnix in di*rnund)

.

The bonds are strong, as in diamond. So silicon dioxide has similar properties to diamond: It has a very high melting point, and is very hard w

-

lt is a non-conductor of electricity, and insoluble in water. lt is used in sandpaper, and to line furnaces. (Since it occurs widely in nature, it is cheap.)

.

Metallic bonding

o o

Metallic bonds are the bonds between metal atoms, in a metal or metal alloy. The outer electrons leave the metal atoms, giving metal ions with full outer shells. For example, in silver:

M

ssl$er inn

free el* (Vinyl chloride)

poly (chloroethene) (PVC)

"(-H*)***( a

3

o

o

t

a

rt, ) a

o

c

r

a

a

Tetraflouroethene ------------* poly (tetraflouroethene) (PrFE)

"{.x,}-.**{ f i} PVC and PTFE have monomer units similar to ethene. lf we use chloroethene the polymer we make is slightly stronger and

harder than poly(ethene) and is particularly good for making pipes for plumbing.

PVC is the most versatile plastic and is the second most widely used, after poly(ethene). Worldwide more than 27 million tonnes are produced annually. lf we start from tetrafluoroethene the polymer we make, PTFE, has some slightly unusual

properties: it will withstand very high temperatures, of up to 260 'C it forms a very slippery surface it is hydrophobic (water repellent) it is highly resistant to chemical attack.

. . . .

These properties make PTFE an ideal 'non-stick' coating for frying pans and saucepans. Every year more than 50 000 tonnes of PTFE are made.

ry

s.

w

a

A rp

a

&

erties of some addition

with their uses are

Poly (ethene) (polyethylene, polythene, Poly (propene)

the table below.

plastic bags, bowls, bottles, packaging propene CHsCH

sCHz

Poly (chloroethene) (polyvinyl chloride, PVC) Poly (tetraflouroethene) (polytetrafl ou roethyl en e, teflon, PTFE Poly (phenylethene) (polystyrene, PS)

strong, hard (not as flexible as tetraflouroethene

l,

CF2uef

phenylethene (styrene)

non-stick surface, withstands high

light, poor conductor of heat

insulation, packaging (foam)

CeHECHsCH,

Thermosoftening and thermosetting plastics Plastics can be put into one of two categories. lf they melt or soften when heated (like poly(ethene), PVC and polystyrene) then they are called thermoplastics or thermosoftening plastics. lf they do not soften on heating but only char and decompose on further heating, they are known as thermosefting plastics. Thermoplastics are easily moulded or formed into useful articles. Once they are molten they can be injected or blown into moulds, and a variety of different-shaped items can be produced. Thermosetting plastics can be heated and moulded only once, usually by compression moulding. The figure below shows the different molecular structures for thermosetting and thermosoftening plastics. Thermosetting plastics have polymer chains which are linked or bonded to each other to give a cross-linked structure, and so the chains are held firmly in place and no softening takes place on heating. Thermosoftening plastics do not have polymer chains joined in this way, so when they are subjected to heat their polymer chains flow over one another and the plastic softens.

a

In thermosetting plastic

.*j:"X.J*,inked,

dre

b ln thenosofiening

pl*tfc there

isnocros+linklns-

When identical monomer molecules join together to form an addition polymer, the double bonds break and the monomer molecules just add on to each other to form the polymer chain. No small molecules are lost in the process. ln contrast, many polymers are not composed of just one type of monomer, but often have two monomers joining together ffi

*ry

&

alternately. These polymers are called copolymers and result in chains composed of alternating monomer units, rather than a single repeating monomer. Each time two monomers join together, a small molecule is lost. This reaction is known as condensation polymerization and the polymers formed are condensation polymers..One form of nylon, called nylon 66, is a copolymer of the monomers adipic acid and hexamethylenediamine When nylon forms from these two monomers, a water molecule is expelled for each bond that forms. Polymers that expel atoms or small molecules, usually water, during their formation are called condensation polymers. Nylon is usually drawn into filaments and is best known for its use in nylon stockings, pantyhose, cords, and bristles.

Making nylon Nylon is a copolymer of the monomers hexanedioic acid (a six carbon organic acid with a carboxyl group; -COOH at each end) and a diamine called 1,G-diaminohexane which is an organic compound with one amino group 1-NHz) on each end of the six-carbon chain of the molecule. Under the right conditions, these two monomers join together with the loss of a water molecule each time a bond is formed.

o ",.

o

' "' *l-'1

r ,.: -H*oF[-,"*,,--l{o*a -=N-(trHr).-N-p.

i-

S'iltTT

Y*Tffi FIEIffi

il

tl

"I 5.,f,nfoilng fhe trr*'rfl 6teru tdflEfltdd

I

oc.

UfilET

"

You can simplify this using a 'block diagram' which replaces the -CHz by rectangles and allows you to focus on the part of the reaction in question.

footi

lnli **Ir:sl-i

feffiT*T{-

EGe

ll

H

0 -

llililll

+

e-l-Fc-

$

E

B

# ru- -a l-r-l-t H

c-

C

ru

l-l

'lpun

r*#

ni

Ft

H

-

fl.rf Sk* #f,frrrvt

Other copolymers include polyethylene terephthalate (PET) and polycarbonate. PET is a condensation copolymer formed by the reaction of terephthalic acid with ethylene glycol. Notice the repeating ester groups (rftrQQQrp-r) in the structure of PET; polymers with this structure are called polyesters. PET can be drawn into fibers to make Dacron, often used in clothing; or it can be made into thin sheets called Mylar, used in packaging, as a waterproof coating on sails, as a base for magnetic video and audio tapes, and as a base for

c

c

c

o

a

o

3

I

3

a

a

photographic film. Polycarbonates are condensation copolymers formed by the reaction of carbonic acid with compounds such as bisphenol A. This reaction forms the polycarbonate termed Lexan. Polycarbonates form clear, tough plastic materials that, if thick enough, can even stop bullets. Consequently, polycarbonate is often used in eye protection, as a scratchresistant coating for eye glasses, and in bulletproof windows.

Disposal of plastics ln the last 30 to 40 years plastics have taken over as replacement materials for metals, glass, paper and wood as well as for natural fibres such as cotton and wool. This is not surprising since plastics are light, cheap, relatively unreactive, can be easily moulded and can be dyed bright colours. However, plastics have contributed significantly to the household waste problem, up to 10% in some countries, and it's getting worse (Figure)! fi.gure

i4-IL

This

plxtii

liux'E & ready ta ga to landffii"

ln the recent past, much of our plastic i1:i*i:{Ir ' waste has been used to landfill disused 1l5;:.i1'' ';" quarries. However, all &i over the world these sites are getting harder to flnd and it is becoming more and more expensive. The alternatives to dumping plastic waste are certainly more economical and more satisfactory. o lncineration schemes have been developed to use the heat generated for heating purposes (Figure). However, problems with the combustion process Figure 14J3 A phetk irrircratixr plantin 6ermany. (which can result in the production of toxic gases) mean that especially high temperatures have to be employed during the incineration process. . Recycling - large quantities of black plastic bags and sheeting are produced for resale. . Biodegradable plastics, as well as those polymers that degrade in sunlight (photodegradable, Figure), have been developed. Other common categories of degradable plastics include synthetic biodegradable plastics which are broken down by bacteria, as well as plastics which aThb pb*ic bg isp{rotadegrd&Ha. dissolve in water (Figure 14.24b). The property that allows plastic to dissolve in water has been used in relatively new products, including soluble capsules containing liquid detergent. However, the vast majority of polymers are still non-biodegradable. ,

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CI

o

_C-X I

5.4 Haloalkanes

$

A halqen atom (X) bonded to a carbon gives the haloalkane functional group, and compounds with the general formula R-X. Haloalkanes (common name, alkyl halides) are named by identifying the halogen with a prefix on the hydrocarbon name and numbering the C atom to which the halogen is attached, as in bromomethane,2chloropropane, or 1,3-diiodohexane. Classification of halogenoa lkanes: Primary halogenoalkane such as 1-chlorobutane, the halogen atom is covalently bonded to carbon atom which is bonded to one another carbon atom.

Cl -'\

HHHH \.'r C. -\

c'/

\.'r

-\H

C

c/

l'a, HH

/t'-, HH

I -chlorobutane

(primary)

Secondary halogenoalkane suchas 2-chlorobutane, the halogen atom is covalently bonded to carbon atom which is bonded to two another carbon atoms.

H "\

HCI \.'r .C -\

c'/

l\

H H \.'r

c'/

-\H

C

l\

HH

HH 2

-chlorobutane (secondary)

Tertiary halogenoalkane suchas 2-chloro-2-methyl propane, the halogen atom is covalently bonded to carbon atom which is bonded to three another carbon atoms.

HH

H\1/ CCI L/

H\*c-l

l\

HH

\ri-C.

-H

\c-l

i

l\

HH 2-

cll.l oro -2 -methy

lprcpane (terti ary )

S.4.aRate of Hydrolysis of Primary Halogenoalkanes

The rates of hydrolysis of primary halogenoalkanes increase if you go down the group or it depends on halogen reactivity. The bond enthalpy between carbon and halogen atoms will be stronger if you go up the halogen group. The fastest nucleophilic substitution reactions take place with the iodoalkanes. The slowest nucleophilic substitution reactions take place with the fluoroalkanes.

o

o

0

Role of haloalkanes in ozone layer depletion Chloroalkanes and chlorofluoroalkanes can be used as solvents. One type in particular, known as chlorofluorocarbons (CFCs), are widely used in aerosols and fridges. Chlorofluorocarbons are haloalkanes containing chlorine and fluorine atoms but not hydrogen atoms, eg CClzFz or CClFa. The small chlorofluorocarbons are gases and can escape into the atmosphere. Usually chlorofluorocarbons are very unreactive. However in the upper atmosphere the C-Cl bonds can undergo homolytic fission if exposed to ultra-violet light: e.g. CFzClz CFzCI + Cl' Ozone (Os) is a naturally occurring substance found in the upper atmosphere. lt plays an important role in absorbing ultra-violet radiation from the sun and preventing it from getting to the earth's surface. However if CFCs find their way into the upper atmosphere and the ultra-violet light breaks them down into Cl radicals, these Cl' radicals act as catalysts in the destruction of the ozone layer as follows: Cl'+ Os CIO'+ Oz CIO + Os 2Oz+ Cl This process can repeat itself indefinitely, meaning that even small quantities of chlorine radicals can significantly destroy the ozone layer. This process has resulted in the formation of a hole in the ozone layer. As a result of this, chemists supported legislation to ban CFCs completely and they have been replaced in fridges and aerosols by alternative chlorine-free compounds. The hole in the ozone layer is slowly mending itself.

+

)

)

Uses of halogenoalkanes:

r' ./ r' r'

a

Synthesis of medicine such as ibuprofen Direct application for plastics like poly(chloroethene)

Production of aerosols and foams. W aterproof cl othi ng I ike poly(tetrafl

a

a

o

a

a

ouroethene)

t

a

a

t

t

o

Glossary Addition the joining of two

molecules to form a single product molecule.

Addition polymerization forming

a

polymer by a repeated addition reaction. addition reactions a reaction in which two molecules join to form a single product molecule. alkaline eafth metals the elements found in Group 2 of the Periodic Table. anhydrous without water of crystallisation. Atom the smallest particle of an element atomic number the number of protons in the nucleus of each atom of an element. atomic radius half the distance between the nuclei of two covalently bonded atoms. bond enthalpy the amount of energy needed to break one mole of a pafticular bond in one mole of gaseous molecules. compound a substance made up of two or more elements chemically joined together. cracking the thermal decomposition of an alkane into a smaller alkane and an alkene.

dative covalent bond (coordinate bond) a covalent bond where both electrons come from one atom. d-block elements a block of elements found between Groups 2 and 3 in the Periodic Table. degradable plastics plastics that will rot away once discarded. dehydration a reaction involving the removal of a water molecule. double covalent bond two shared pairs of electrons that bond two atoms together.

electronegativity describes the ability of an atom to attract the bonding electrons in a covalent bond. electrons tiny negatively charged sub-atomic pafticles, found in orbitals around the nucleus of an atom. electrophile an atom (or group of atoms) which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond. elements substances made up of only one type of atom. elimination when a small molecule is removed from a larger molecule. empirical formula the simplest whole-number ratio of the elements present in a compound. endothermic term used to describe a reaction in which heat energy is absorbed from the surroundings (enthalpy change is positive). Enthalpy change is the term used by chemists for heat energy transferred during reactions. enthalpy cycle a diagram displaying alternative routes between reactants and products which allows the determination of one enthalpy change from other known enthalpy changes using Hess's Iaw. exothermic term used to describe a reaction in which heat energy is transferred to the surroundings (enthalpy change is negative). free radical an atom or group of atoms with an unpaired electron. functional group an atom or group of atoms which gives rise to a homologous series. Compounds in the same homologous series show similar chemical propefties.

general formula a formula which may be written for each homologous series (Crllzn+z for alkanes). global warming the increase in average temperature of the Eafth's suface caused by an enhanced greenhouse effect due to increased concentration of greenhouse gases (e.9. carbon dioxide) in the atmosphere. greenhouse effect natural phenomenon by which some gases present in the atmosphere absorb infrared radiation emitted from the Eafth's surface and then re-emit some of this infrared radiation back to the Eafth's suface. Hess's law the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, provided initial and final conditions are the same. heterolytic fission when a bond breaks to form a positive ion and a negative ion. homologous series a series of organic molecules with the same functional group. homolytic fission when a bond breaks to form two free radicals. hydrated with water of crystallisation. hydrocarbon a compound containing only carbon and hydrogen. hydrogen bond a weak intermolecular bond formed between molecules containing a hydrogen atom bonded to one of the most electronegative elements (N, O or F). hydrolysis the breakdown of a compound by water. initiation the first step in a freeradical substitution in which the free radicals are generated by heat or ultraviolet light. ion a positively or negatively charged atom or (covalently bonded) group of atoms.

ionic bonding the electrostatic attraction between oppositely charged ions.

ionisation energy the first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms or ions of an element. isomers compounds with the same molecular formula but with different arrangements of atoms. isotopes atoms of an element with the same number of protons but different numbers of neutrons. mass number the total number of protons and neutrons in the nucleus of an atom. mass spectrometer an analytical instrument in which atoms and/or molecules are ionised, deflected and detected. It can be used to find relative isotopic abundances of elements and to identiff unknown organic compounds. mole the unit of amount of substance (abbreviation: mol). One mole of a substance is the mass that has the same number of pafticles (atoms, molecules, ions or electrons) as there are atoms in exactly t2 g of carbon-

t2. molecular formula shows the total number of atoms present in a molecule of the compound. monomer the small molecule used to build a polymer molecule. neutron a sub-atomic particle found in the nucleus of an atom. It carries no charge and has the same mass as a proton. nucleophile a chemical that can donate a pair of electrons with the subsequent formation of a covalent bond. nucleus the small, dense core at the centre of an atom, containing protons and neutrons (hence a nucleus is always positively charged).

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polar covalent bond a covalent bond in which the two bonding electrons are not shared equally by the two bonded atoms. The atom that gets the bigger share of the two electrons becomes 6. The atom that gets the smaller share of the two electrons becomes 6+. propagation the stage in a freeradical substitution which constitutes the two reaction steps of the chain reaction. protons positively charged sub-atomic pafticles, found in the nucleus of an atom. radicals reactive atoms or molecules with an unpaired electron; also called free radicals. relative atomic mass Ar, the weighted average mass of the atoms of an element taking into account the relative abundance of its naturally occurring isotopes, measured on a scale on which carbon-l2 is given a mass of exactly 12. relative formula mass the mass of one formula unit of a compound relative to an atom of carbon-l2. relative isotopic mass the mass of an isotope of an atom of an element relative to an atom of carbon-l2. relative molecular mass the mass of a molecule of a compound relative to an atom of carbon-l2. single covalent bond a shared pair of electrons that bonds two atoms together. stoichiometry the stoichiometry (or stoichiometric ratio) for a reaction shows the mole ratio of reactants and products in the balanced equation for the reaction.

structural isomerism where two or more structural isomers have the same molecular formula but different structural formulae. substitution reaction when an atom (or group of atoms) is replaced by a different atom (or group of atoms). succssive ionisation energies the sequence of first second, third, fourth, etc. ionisation energies needed to remove the first, second, third, foufth, etc. electrons from each atom in one mole of gaseous atoms of an element. termination the step at the end of a free-radical substitution reaction which occurs when reactants are significantly depleted. titration measurement of the exact amount of one solution needed to react with a fixed amount of another solution. transition elements elements in the d-block that can form at least one ion with a paftially filled d subshell. triple covalent bond three shared pairs of electrons that bond two atoms together. van der Waals'forcc the weak forces of attraction between molecules based on instantaneous and induced dipoles. water of crystallisation water molecules incorporated into the crystal structure of a salt. stereasomerism occurs because a C=C bond cannot freely rotate. In some alkenes (with additional groups either side of the double bond) two isomers (Zand Qare possible.

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