Richard Harwood and Ian Lodge
Cambridge IGCSE®
Physical Science Chemistry Workbook
University Printing House, Cambridge CB2 8BS, United Kingdom One Liberty Plaza, 20th Floor, New York, NY 10006, USA 477 Williamstown Road, Port Melbourne, VIC 3207, Australia 4843/24, 2nd Floor, Ansari Road, Daryaganj, Delhi – 110002, India 79 Anson Road, #06–04/06, Singapore 079906 Cambridge University Press is part of the University of Cambridge. It furthers the University’s mission by disseminating knowledge in the pursuit of education, learning and research at the highest international levels of excellence. www.cambridge.org Information on this title: www.cambridge.org/9781316633519 © Cambridge University Press 2017 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2017 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Printed in Malaysia by Vivar Printing A catalogue record for this publication is available from the British Library ISBN 978-1-316-63351-9 Paperback ii
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Contents Introduction Periodic Table
iv v
C7 Acids, bases and salts
46
C7.1 Acid and base reactions – neutralisation
46
C1 The particulate nature of matter
1
C7.2 Types of salt
47
C1.1 Changing physical state
1
C7.3 Descaling a coffee machine
48
C1.2 Plotting a cooling curve
3
C7.4 Thermochemistry – investigating the neutralisation of an acid by an alkali
50
C2 Experimental techniques
7
C8 The Periodic Table
54
C2.1 Diffusion, solubility and separation
7
C8.1 Trends in the halogens
54
C8.2 Displacement reactions of the halogens
56
C2.2 Chromatography at the races
10
C3 Atoms, elements and compounds
12
C8.3 Group I: The alkali metals
58
C3.1 Atomic structure
12
C9 Metals
59
C3.2 The first four periods
13
C9.1 The reactivity series of metals
59
C3.3 The chemical bonding in simple molecules
15
C9.2 Energy from displacement reactions
61
C3.4 The nature of ionic lattices
16
C9.3 Metals and alloys
64
C4 Stoichiometry
17
C10 Air and water
66
C4.1 Formulae of ionic compounds
17
C10.1 Atmospheric pollution, industry and transport
66
C10.2 Clean water is crucial
69
C11 Carbonates
72
C11.1 The action of heat on metal carbonates
72
C11.2 Concrete chemistry
73
C12 Organic chemistry
76
C4.2 Making magnesium oxide – a quantitative investigation
19
C4.3 The analysis of titration results
22
C4.4 Calculating formula masses
24
C4.5 A sense of proportion in chemistry
26
C4.6 Finding the mass of 5 cm of magnesium ribbon
27
C12.1 Families of hydrocarbons
77
C4.7 Reacting volumes of gases
29
C12.2 Unsaturated hydrocarbons (the alkenes)
78
C5 Electricity and chemistry
31
C12.3 The alcohols as fuels
80
C5.1 The nature of electrolysis
32
C12.4 Essential processes of the petrochemical industry
83
C5.2 Making and ‘breaking’ copper chloride
33
C12.5 Addition polymerisation
85
C6 Energy changes in chemical reactions
36
C13 Analysis
86
C6.1 Energy diagrams
36
C13.1 Titration analysis
86
C6.2 The collision theory of reaction rates
38
C13.2 Chemical analysis
88
C6.3 The influence of surface area on the rate of reaction
C13.3 Planning a controlled experiment
92
39
C13.4 Chemical testing and evaluation
95
C6.4 Finding the rate of a reaction producing a gas
43
C13.5 Experimental design
98
Answers
103
iii
Introduction This workbook contains exercises designed to help you develop the skills needed for success in Cambridge IGCSE® Physical Science. The examination tests three different Assessment Objectives, or AOs for short. These are: AO1 Knowledge with understanding AO2 Handling information and problem solving AO3 Experimental skills and investigations. In the examination, about 50% of the marks are for AO1, 30% for AO2 and 20% for AO3. Just learning your work and remembering it is therefore not enough to make sure that you get the best possible grade in the exam. Half of all the marks are for AO2 and AO3. You need to be able to use what you’ve learnt in unfamiliar contexts (AO2) and to demonstrate your experimental skills (AO3). This workbook contains exercises to help you to develop AO2 and AO3 further. There are some questions that just involve remembering things you have been taught (AO1), but most of the questions require you to use what you’ve learnt to work out, for example, what a set of data means, or to suggest how an experiment might be improved. These exercises are not intended to be exactly like the questions you will get on your exam papers. This is because they are meant to help you to develop your skills, rather than testing you on them. iv
There’s an introduction at the start of each exercise that tells you the purpose of it – which skills you will be working with as you answer the questions. For some parts of the exercises, there are self-assessment checklists. You can try marking your own work using these. This will help you to remember the important points to think about. Your teacher should also mark the work and will discuss with you whether your own assessments are right. The exercises cover both Core and Supplement material of the syllabus. The Supplement material can be identified by the Supplement bar in the margin (as shown). This indicates that the exercise is intended for students who are studying the Supplement content of the syllabus as well as the Core.
Ba
barium 137 88
Ra
radium –
Cs
caesium 133 87
Fr
francium –
actinoids
lanthanoids
actinoids –
89–103
lanthanoids
24
141 91
Pa protactinium 231
cerium 140 90
Th thorium 232
lanthanium 139 89
AC
actinium –
59
25
–
61
uranium 238
U
144 92 neptunium –
Np
93
promethium
Pm
60
Nd
hassium –
Hs
osmium 190 108
Os
ruthenium 101 76
Ru
iron 56 44
Fe
26
hydrogen 1
H
1
bohrium –
Bh
rhenium 186 107
Re
75
technetium
Tc
manganese 55 43
Mn
praseodymium neodymium
Pr
58
Ce
57
LA
seaborgium –
Db dubnium –
Sg
tungsten 184 106
W
molybdenum 96 74
Mo
chromium 52 42
Cr
Rf
tantalum 181 105
Ta
niobium 93 73
Nb
vanadium 51 41
V
23
rutherfordium –
hafnium 179 104
Hf
zirconium 91 72
Zr
Y
yttrium 89 57–71
Sr
titanium 48 40
scandium 45 39
strontium 88 56
calcium 40 38
potassium 39 37
22
Ti
21
Sc
Rb
Ca
K
rubidium 85 55
Mg
magnesium 24 20
Na
sodium 23 19
Be
beryllium 9 12
Li
lithium 7 11
name relative atomic mass
atomic symbol
4
3
Key
atomic number
II
I
27
28
Pt
Mt
Ds
63
Am americium –
Pu plutonium –
europium 152 95
Eu
Pb
dysprosium 163 98
Cf
terbium 159 97
Bk berkelium –
gadolinium 157 96
Cm curium –
californium –
Dy
65
Tb
64
Gd
einsteinium –
Es
holmium 165 99
Ho
67
–
Fl
lead 207 114 flerovium
66
thallium 204
tin 119 82
copernicium
–
Cn
Tl
Hg mercury 201 112
Au gold 197 111
Rg
Sn
In indium 115 81
Cd cadmium 112 80
Ag silver 108 79
Ge germanium 73 50
zinc 65 48
copper 64 47
Ga gallium 70 49
30
Zn
29
S
P
Si silicon 28 32
Al aluminium 27 31
As
fermium –
Fm
erbium 167 100
Er
68
bismuth 209
Bi
antimony 122 83
Sb
arsenic 75 51
mendelevium –
nobelium –
No
ytterbium 173 102
thulium 169 101
Md
Yb
70
astatine –
At
iodine 127 85
I
bromine 80 53
Br
chlorine 35.5 35
Cl
fluorine 19 17
F
9
VII
Tm
69
–
livermorium
Lv
polonium – 116
Po
tellurium 128 84
Te
aelenium 79 52
Se
sulfur 32 34
O oxygen 16 16
N nitrogen 14 15
C carbon 12 14
B boron 11 13 phosphorus 31 33
8
7
6
5
VI
V
IV
III
Cu
darmstadtium roentgenium – –
samarium 150 94
Sm
62
meitnerium –
platinum 195 110
Ir indium 192 109
palladium 106 78
Pd
nickel 59 46
Ni
rhodium 103 77
Rh
cobalt 59 45
Co
Group
lawrencium –
Lr
lutetium 175 103
Lu
71
radon –
Rn
xenon 131 86
Xe
krypton 84 54
Kr
argon 40 36
Ar
neon 20 18
Ne
helium 4 10
He
2
VIII
Glossary
The Periodic Table
v
C1: The particulate nature of matter DEFINITIONS TO LEARN
physical state: the three states of matter are solid, liquid and gas condensation: the change of state from gas to liquid melting: the change of state from solid to liquid freezing: the change of state from liquid to solid at the melting point boiling: the change of state from liquid to gas at the boiling point of the liquid evaporation: the change of state from liquid to gas below the boiling point sublimation: the change of state directly from solid to gas (or the reverse) crystallisation: the formation of crystals when a saturated solution is left to cool
Exercise C1.1 Changing physical state This exercise will develop your understanding of the kinetic model and the energy changes involved in changes of physical state.
1 Temperature / ยบC
The graph shows the heating curve for a pure substance. The temperature rises with time as the substance is heated.
D
115
C B
17
A 0
0
a What physical state(s) is the substance in at points A, B, C and D? A
C
B
D
b What is the melting point of the substance? c What is its boiling point? d What happens to the temperature while the substance is changing state?
Time
Cambridge IGCSE Physical Science
e The substance is not water. How do we know this from the graph?
f
Complete the passage using the words given below. different
diffusion
gas
spread
particles
diffuse
random
lattice
vibrate
temperature
in a liquid and a
The kinetic model states that the
are in constant motion. In a gas, the particles are far apart from each other and their motion is said to be . The particles in a solid are held in fixed positions in a regular . In a solid, the particles can only
about their fixed
positions.
Liquids and gases are fluid states. When particles move in a fluid, they can collide with each other. When they collide, they bounce off each other in mixed, the different types of particle
directions. If two gases or liquids are out and get mixed up. This process is called
. 2
At the same
particles that have a lower mass move faster than those with higher
mass. This means that the lighter particles will spread and mix more quickly; the lighter particles are said to faster than the heavier particles.
g Use the data given for the substances listed below to answer the questions that follow on their physical state at a room temperature of 25 °C and atmospheric pressure.
Substance sodium
Melting point / °C
Boiling point / °C
98
883
−71
−62
ethanol
−117
78
cobalt
1492
2900
nitrogen
−210
−196
propane
−188
−42
16
118
radon
ethanoic acid
C1: The particulate nature of matter
i
Which substance is a liquid over the smallest range of temperature?
ii
Which two substances are gaseous at −50 °C? and
iii
Which substance has the lowest freezing point?
iv
Which substance is liquid at 2500 °C?
v
A sample of ethanoic acid was found to boil at 121 °C at atmospheric pressure. Use the information in the table to comment on this result.
Exercise C1.2 Plotting a cooling curve This exercise presents data obtained practically for plotting a cooling curve. It will help develop your skills in handling the data and interpreting what changes the different regions of the curve represent. Examples of sublimation are also discussed. A student, carried out the following data-logging experiment using the apparatus shown below, as part of a project on changes of state. An organic crystalline solid was melted by placing it in a tube in a boiling water bath. A temperature sensor was placed in the sample tube.
computer
temperature sensor
bath of boiling water
magnetic stirrer
The temperature change was followed as the liquid was allowed to cool down. The data shown in the table below are taken from the computer record of the temperature change as the liquid cooled down to room temperature. Time / min
0
0.5
1.0
1.5
2.0
2.2
2.4
2.6
2.8
3.0
3.5
4.0
4.5
5.0
Temperature / °C 96.1 89.2
85.2
82.0
80.9
80.7
80.6
80.6
80.5
80.3
78.4
74.2
64.6
47.0
3
Cambridge IGCSE Physical Science
a On the grid below, plot a graph of the temperature change taking place in this experiment.
4
b What change is taking place over the second minute of the experiment?
c Why does the temperature remain almost constant over this period of time? Give your explanation in terms of what is happening to the organisation of the molecules of the substance.
C1: The particulate nature of matter
d What change would need to be made to carry out the experiment using a compound with a melting point greater than 100 °C?
e A similar experiment was carried out to demonstrate the cooling curve for paraffin wax.
f
i
In the space below, sketch the shape of the graph you would expect to produce.
ii
Explain why the curve is the shape you have drawn.
Sublimation occurs when a substance passes between the solid and gaseous states without going through the liquid phase. Both carbon dioxide and water can sublime under certain conditions of temperature and pressure. ‘Dry ice’ is the solid form of carbon dioxide used in commercial refrigeration. At atmospheric pressure it has a ‘sublimation point’ of –78.5 °C. i
What difference can you see between solid carbon dioxide and water ice at atmospheric pressure?
CO2
ii
If you gently shake a carbon dioxide fire extinguisher, you will feel the presence of liquid within the extinguisher. What conditions within the extinguisher mean that the CO2 is liquid in this case?
5
Cambridge IGCSE Physical Science
iii
Complete the following paragraph about a particular type of frost using the words listed below. surrounding
liquid
colder
white
crystals
ice
Hoar frost is a powdery
going through the
6
humid
frost caused when solid
forms from
air. The solid surface on which it is formed must be
than the
air. Water vapour is deposited on a surface as fine ice
without
phase.
C2: Experimental techniques DEFINITIONS TO LEARN
filtration: the separation of a solid from a liquid using filter paper distillation: the separation of a liquid from a mixture using differences in boiling point fractional distillation: the separation of a mixture of liquids using differences in boiling point diffusion: the random movement of particles in a fluid (liquid or gas) leading to the complete mixing of the particles chromatography: the separation of a mixture of soluble (coloured) substances using paper and a solvent
Exercise C2.1 Diffusion, solubility and separation The processes of diffusion and dissolving in a solvent are linked. This exercise explores the basis of these processes in terms of the kinetic (particle) theory. The separation of a solvent mixture by fractional distillation is also discussed. A student placed some crystals of potassium manganate(VII) at the bottom of a beaker of distilled water. She then left the contents of the beaker to stand for one hour. a The diagram below shows what she saw during the experiment. After one hour, all the solid crystals had disappeared and the solution was purple throughout.
distilled water
purple crystals at start
after 15 minutes
after one hour
i
Use the ideas of the kinetic theory to explain her observations.
ii
If warm water at 50 °C had been used, would the observations have taken place in a longer or shorter time? Explain your answer.
7
Cambridge IGCSE Physical Science
b The process of dissolving can be used to separate and purify chemical compounds. Organic solvents such as propanone can be used to extract pigments from plants. Some grass is crushed and mixed with the propanone. The colour pigments are extracted to give a dark green solution.
8
i
Given a pure sample of chlorophyll, describe how could you show that the green solution from the grass contained chlorophyll and other coloured pigments?
ii
Draw a labelled diagram that describes the method of separating coloured pigments that you have discussed in part i.
C2: Experimental techniques
Use the checklist below to give yourself a mark for your drawing. For each point, award yourself:
• 2 marks if you did it really well • 1 mark if you made a good attempt at it, and partly succeeded • 0 marks if you did not try to do it, or did not succeed. Self-assessment checklist for drawings
Marks awarded Check point You
Your teacher
You have made a large drawing, using the space provided. There are no obvious errors – liquids missing, flasks open when they should be closed, etc. You have drawn single lines with a sharp pencil, not many tries at the same line (and erased mistakes). You have used a ruler for the lines that are straight. Your diagram is in the right proportions. You have drawn label lines with a ruler, touching the item being labelled. You have written the labels horizontally and neatly, well away from the diagram itself. Total (out of 14) 12–14 Excellent. 10–11 Good. 7–9
A good start, but you need to improve quite a bit.
5–6
Poor. Try this same drawing again, using a new sheet of paper.
1–4
Very poor. Read through all the criteria again, and then try the same drawing.
9
Cambridge IGCSE Physical Science
c Propanone is a very useful solvent that mixes well with water even though it is an organic compound. A propanone:water (65%:35%) mixture used for cleaning laboratory apparatus can be separated using fractional distillation. A total volume of 80 cm3 of the mixture was distilled. Sketch below a graph of the temperature readings against the volume of distillate collected for the distillation carried out. The thermometer is placed at the connection between the fractionating column and the condenser. The boiling point of propanone is 56 °C.
Exercise C2.2 Chromatography at the races This exercise will help you understand aspects of chromatography by considering an unfamiliar application of the technique. Chromatography is used by the ‘Horse Racing Forensic Laboratory’ to test for the presence of illegal drugs in racehorses. A concentrated sample of urine is spotted on to chromatography paper on the start line. Alongside this, known drugs are spotted. The chromatogram is run using methanol as the solvent. When finished, the paper is read by placing it under ultraviolet light. A chromatogram of urine from four racehorses is shown below. solvent front
10 9
Spot
Description
8
1
caffeine
2
paracetamol
3
urine sample horse A
4
urine sample horse B
5
urine sample horse C
6
urine sample horse D
7 Scale / cm
10
6 5 4 3 2 1 start line
0 1
2
3
4
5
6
C2: Experimental techniques
a State two factors which determine the distance a substance travels up the paper.
b The results show that the sample from one horse contains an illegal substance. State which horse and the drug that is present.
c Give a reason for the use of this drug.
d The results for known drugs are given as ‘Rf values’. Rf =
distance travelled by the substance distance travelled by the solvent
Calculate the Rf value for caffeine.
11
C3: Atoms, elements and compounds DEFINITIONS TO LEARN
atom: the smallest part of an element that can take part in a chemical change proton number (atomic number): the number of protons in the nucleus of an atom of an element nucleon number (mass number): the number of protons and neutrons in the nucleus of an atom electron arrangement: the organisation of electrons in their different energy levels (shells) isotopes: atoms of the same element which have the same proton number but a different nucleon number element: a substance containing only one type of atom compound: a substance made of two, or more, elements chemically combined together Periodic Table: the table in which the elements are organised in order of increasing proton number and electron arrangement group: a vertical column of elements in the Periodic Table; elements in the same group have similar properties period: a horizontal row of elements in the Periodic Table valency: the number of chemical bonds an atom can make
Exercise C3.1 Atomic structure 12
This exercise helps familiarise you with aspects of atomic structure including the organisation of electrons into energy levels (or shells), and the uses of radioactivity.
a Choose from the words below to fill in the gaps in the passage. Words may be used once, more than once or not at all. proton
electrons
nucleon
neutrons
nucleus
energy levels
isotopes
Atoms are made up of three different particles: which have no charge; and
which are positively charged; which are negatively charged.
(shells) around the
The negatively charged particles are arranged in different of the atom. The particles with a negligible mass are the of the same element contain the same number of the same element with different numbers off
protons
and are known as
. All atoms . Atoms of .
C3: Atoms, elements and compounds
b This part of the exercise is concerned with electron arrangements and the structure of the Periodic Table. Complete these sentences by filling in the blanks with words or numbers.
The electrons in an atom are arranged in a series off also called
around the nucleus. These shells are
levels. In an atom, the shell
to the nucleus fills first,
then the next shell, and so on. There is room for:
• • •
up to
electrons in the first shell
up to
electrons in the second shell
up to
electrons in the third shell.
(There are 18 electrons in total when the three shells are completely full.)
The elements in the Periodic Table are organised in the same way as the electrons fill the shells. Shells fill from to
• • • •
across the
The first shell fills up first from
of the Periodic Table.
to helium.
The second shell fills next from lithium to Eight
.
go into the third shell from sodium to argon.
Then the fourth shell starts to fill from potassium. 13
Exercise C3.2 The first four periods This exercise is aimed at developing your knowledge of the basic features of the Periodic Table and the properties of an element that relate to its position in the table.
The diagram below shows the upper part of the Periodic Table with certain elements selected. I
II
III
IV
V
VI
VII
H
He C
Li Na
Mg Ca
0
Al Cr
Cu
N P
F
Ne
Br
Kr
S
Using the elements shown above, write down the symbols for the elements which answer the following questions.
Cambridge IGCSE Physical Science
a Which two elements are stored under oil because they are very reactive?
b Which two elements are transition metals?
c Which element has just two electrons in the full outer shell of its atom?
d Which element is a red-brown liquid at room temperature and pressure?
e Which element has four electrons in the outer energy level of its atom?
f
14
Which element is a yellow solid at room temperature?
g Which elements are noble gases?
h Which element has compounds that produce blue solutions when they dissolve?
i
Which element has the electron arrangement 2.8.8.2?
j
Which element burns with a brilliant white flame when ignited?
C3: Atoms, elements and compounds
Exercise C3.3 The chemical bonding in simple molecules This exercise will familiarise you with the structures of some simple covalent compounds and the methods we have for representing the structure and shape of their molecules. a Many covalent compounds exist as simple molecules where the atoms are joined together with single or double bonds. A covalent bond, made up of a shared pair of electrons, is often represented by a short straight line. Complete the table by filling in the blank spaces.
Name of compound
Formula
hydrogen chloride water
Drawing of structure H
H2O
Molecular model
Cl O
H
H
ammonia
CH4
ethene
H
H C
C
H
O
H
C
O
b Graphite is one of the crystalline forms of carbon. Two of the distinctive properties of graphite are:
• •
it conducts electricity even though it is a non-metal, and it can act as a lubricant even though it has a giant covalent structure.
Give a brief explanation of these properties in the light of the structure of graphite. i
Graphite as an electrical conductor
ii
Graphite as a lubricant
15
Cambridge IGCSE Physical Science
Exercise C3.4 The nature of ionic lattices This exercise will help you relate the structures of ionic compounds to some of their key properties. The diagram shows a model of the structure of sodium chloride and similar ionic crystals. The ions are arranged in a regular lattice structure − a giant ionic lattice. The boxes below contain properties of ionic compounds and their explanations. Draw lines to link each pair.
+
– + + –
Property The solution of an ionic compound in water is a good conductor of electricity − such ionic substances are electrolytes.
16
Ionic crystals have a regular shape. All the crystals of each solid ionic compound are the same shape. Whatever the size of the crystal, the angles between the faces of the crystal are always the same.
– +
– +
–
–
+ –
+
+
–
–
+
+ –
+
– +
– +
– +
–
Explanation The ions in the giant ionic structure are always arranged in the same regular way − see the diagram. The giant ionic structure is held together by the strong attraction between the positive and negative ions. It takes a lot of energy to break down the regular arrangement of ions.
Ionic compounds have relatively high melting points.
In a molten ionic compound, the positive and negative ions can move around − they can move to the electrodes when a voltage is applied.
When an ionic compound is heated above its melting point, the molten compound is a good conductor of electricity.
In a solution of an ionic compound, the positive metal ions and the negative nonmetal ions can move around − they can move to the electrodes when a voltage is applied.
C4: Stoichiometry DEFINITIONS TO LEARN
indicator: a substance that changes colour depending on whether it is in an acid or alkali salt: an ionic substance produced from an acid by neutralisation with a base neutralisation reaction: a reaction between an acid and a base to produce a salt and water only relative atomic mass: the average mass of naturally occurring atoms of an element on a scale where the carbon-12 atom has a mass of exactly 12 units relative formula mass: the sum of all the relative atomic masses of all the atoms or ions in a compound chemical formula: the formula of an ionic compound shows the ratio of the atoms in a compound in whole numbers; for a simple covalent compound the formula shows the numbers of each atom present in the molecule. mole: the relative formula mass of a substance in grams molar gas volume: the volume occupied by one mole of any gas (24 dm3 at room temperature and pressure)
Exercise C4.1 Formulae of ionic compounds The writing of chemical formulae is central to chemistry. This exercise will help you understand how to work out the formulae of ionic compounds and what such formulae mean. The table below shows the valencies and formulae of some common ions. Valency
Positive ions (cations)
Negative ions (anions)
1
2
3
metals
sodium (Na+) potassium (K+) silver (Ag+)
magnesium (Mg2+) copper (Cu2+) zinc (Zn2+) iron (Fe2+)
aluminium (Al3+) iron (Fe3+) chromium (Cr3+)
compound ions
ammonium (NH4+)
non-metals
chloride (Cl−) bromide (Br−) iodide (I−)
oxide (O2−) sulfide (S2−)
nitride (N3−)
compound ions
nitrate (NO3−) hydroxide (OH−)
carbonate (CO32−) sulfate (SO42−)
phosphate (PO43−)
17
Cambridge IGCSE Physical Science
a Use the information in the table to work out the formulae of the following ionic compounds. i
Copper oxide
ii
Sodium carbonate
iii
Zinc sulfate
iv
Silver nitrate
v
Magnesium bromide
vi
Ammonium sulfate
vii
Magnesium nitride
viii Potassium phosphate ix
Iron(III) hydroxide
x
Chromium(III) chloride
b Use the information in the table and your answers in a above to give the ratio of the different atoms in the following compounds.
18
i
Copper oxide
Cu : O
ii
Magnesium bromide
Mg : Br
iii
Magnesium nitride
Mg : N
iv
Iron(III) hydroxide
Fe : O : H
v
Ammonium sulfate
N:H:S:O
c The diagram below shows a representation of the structure of an ionic oxide. K+
K+
K+ O2–
O2– K+
K+
K+
O2–
K+
K+
O2– K+
i
What is the ratio of K+ ions to O2− ions?
ii
What is the formula of this compound?
K+
O2– K+
K+
K+
C4: Stoichiometry
d The following diagram shows the structure of common salt. i
Extend the structure to the right, by adding four more ions. Cl–
Na+ Cl–
Na+
Cl–
Na+ Cl–
Na+
ii
Complete the diagrams below for the ions in the structure to show their electron arrangement. Draw in any missing electron shells, showing clearly the origin of the electrons involved. +
Na+
—
Cl–
19
iii
Draw an ionic diagram similar to the one above for the structure of magnesium chloride.
Exercise C4.2 Making magnesium oxide − a quantitative investigation This exercise will develop your skills in processing and interpreting results from practical work. Magnesium oxide is made when magnesium is burnt in air. How does the mass of magnesium oxide made depend on the mass of magnesium burnt? The practical method is described below.
Cambridge IGCSE Physical Science
Method
• Weigh an empty crucible and lid. • Roll some magnesium ribbon around a pencil, then remove • • • • • •
crucible
the coiled ribbon and place it in the crucible and re-weigh (not forgetting the lid). Place the crucible in a pipeclay triangle sitting safely on a tripod. (The lid should be on the crucible.) pipeclay Heat the crucible and contents strongly, occasionally triangle lifting the lid to allow more air in. When the reaction has eased, take off the lid. tripod Heat strongly for another three minutes. Let the crucible cool down and then weigh it. Repeat the heating until the mass is constant.
magnesium ribbon
heat
Results The table shows a set of class results calculated from the weights each student group obtained using this method. Mass of magnesium / g
0.06
0.05
0.04
0.18
0.16
0.10
0.11
0.14
0.15
0.14
0.08
0.10
0.13
Mass of magnesium oxide / g
0.10
0.08
0.06
0.28
0.25
0.15
0.15
0.21
0.24
0.23
0.13
0.17
0.21
20
Use these results to plot a graph on the grid below relating mass of magnesium oxide made to mass of magnesium used. Remember there is one point on this graph that you can be certain of − what point is that? Include it on your graph.
C4: Stoichiometry
Use the checklist below to give yourself a mark for your graph. For each point, award yourself:
• 2 marks if you did it really well • 1 mark if you made a good attempt at it, and partly succeeded • 0 marks if you did not try to do it, or did not succeed. Self-assessment checklist for graphs:
Check point
Marks awarded You
Your teacher
You have drawn the axes with a ruler, using most of the width and height of the grid. You have used a good scale for the x-axis and the y-axis, going up in 0.01 s, 0.05 s or 0.10 s. You have labelled the axes correctly, giving the correct units for the scales on both axes. You have plotted each point precisely and correctly. You have used a small, neat cross for each point. You have drawn a single, clear best-fit line through the points − using a ruler for a straight line. You have ignored any anomalous results when drawing the line. Total (out of 14) 21
12−14 Excellent. 10−11 Good. 7−9
A good start, but you need to improve quite a bit.
5−6
Poor. Try this same graph again, using a new sheet of graph paper.
1−4
Very poor. Read through all the criteria again, and then try the same graph again.
Cambridge IGCSE Physical Science
a How does the mass of magnesium oxide relate to the starting mass of magnesium?
b Work out from the graph the mass of magnesium oxide that you would get from 0.12 g of magnesium (show the lines you use for this on your graph).
g
c What mass of oxygen combines with 0.12 g of magnesium? d What mass of oxygen combines with 24 g of magnesium?
g g
e What is the formula of magnesium oxide, worked out on the basis of these results? (Relative atomic masses: Mg = 24, O = 16.)
Exercise C4.3 The analysis of titration results 22
This exercise will develop your understanding of some of the practical skills involved in acid–base titrations and the processing and evaluation of experimental results. A student investigated an aqueous solution of sodium hydroxide and its reaction with hydrochloric acid. He carried out two experiments.
Experiment 1 Using a measuring cylinder, 10 cm3 of the sodium hydroxide solution was placed in a conical flask. Methyl orange indicator was added to the flask. A burette was filled to the 0.0 cm3 mark with hydrochloric acid (solution P). The student added solution P slowly to the alkali in the flask until the colour just changed. Use the burette diagram to record the volume in the results table and then complete the column for experiment 1.
9
10
11 Experiment 1 final reading
C4: Stoichiometry
Experiment 2
14
35
Experiment 1 was repeated using a different solution of hydrochloric acid (solution Q).
15
36
16
37
initial
final
Use the burette diagrams to record the volumes in the results table and complete the column.
Experiment 2 readings
Table of results Burette readings / cm3
Experiment 1
Experiment 2
final reading initial reading
0.0
difference
a What type of chemical reaction occurs when hydrochloric acid reacts with sodium hydroxide?
23
b Write a word equation for the reaction.
c What was the colour change of the indicator observed?
d Which of the experiments used the greater volume of hydrochloric acid?
e Compare the volumes of acid used in experiments 1 and 2 and suggest an explanation for the difference between the volumes.