Cambridge International AS & A Level
Roger Norris and Mike Wooster
Chemistry
Practical Teacher’s Guide
SAMPLE
Original material © Cambridge University Press
Roger Norris and Mike Wooster
Cambridge International
AS & A Level
Chemistry
Practical Teacher’s Guide
SAMPLE
Original material © Cambridge University Press
ii
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Original material © Cambridge University Press
iii
Contents
Introduction v
Safety vi
AS Practical Skills vii
A Level Practical Skills x
1 Masses, moles and atoms
1.1 Empirical formula of hydrated copper(II) sulfate crystals 2
1.2 Relative atomic mass of magnesium using molar volumes 4
1.3 Percentage composition of a mixture of sodium hydrogen
carbonate and sodium chloride 7
1.4 Relativeatomicmassofcalciumbytwodierentmethods:
molar volume and titration 9
2 Structure and bonding 2.1 Physicalpropertiesofthreedierenttypesofchemicalstructure 13
2.2 Eectoftemperatureonthevolumeofaxedmassofgas 15
2.3 Eectofpressureonthevolumeofaxedmassofgas 17
3 Enthalpy changes 3.1 Enthalpy change for the reaction between zinc and
aqueous copper(II) sulfate solution 19
3.2 Enthalpy change of combustion of alcohols 22
3.3 Enthalpychangeofthermaldecomposition 25
3.4 Change in enthalpy of hydration of copper (II) sulfate 27
4 Redox reactions 4.1 Understandingredox(I):investigatingareactivity
series and displacement reactions 30
4.2 Understandingredox(II):investigatingfurtherreactions 32
5 Chemical equilibrium 5.1 ApplyingLeChatelier’sprincipletoagaseousequilibrium 35
5.2 ApplyingLeChatelier’sprincipletoanaqueousequilibrium 37
5.3 Theequilibriumconstantforthehydrolysisofethylethanoate 39
6 Rates of reaction 6.1 Eectsofconcentrationonrateofchemicalreaction 43
6.2 Eectsoftemperatureandahomogeneouscatalyst
on the rate of chemical reaction 46
6.3 Observing a catalysed reaction 47
7 The properties of metals 7.1 Properties of metal oxides and metal chlorides across Period 3 49
7.2 Relativeatomicmassofmagnesiumusingaback-titrationmethod 51
7.3 Separationoftwometalionsinsolution 52
7.4 Identicationofthreemetalcompoundsusingqualitativeanalysis 54
Contents
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Cambridge International AS & A Level Chemistry
8 The properties of
8.1 Formulaofhydratedsodiumthiosulfatecrystals 56
non-metals 8.2 Preparationandpropertiesofthehydrogenhalides 58
8.3 Reactionofbrominewithsulteions(sulfate(IV)) 60
8.4 Identicationofunknownscontaininghalideions 62
9 Hydrocarbons and 9.1 Cracking of hydrocarbons 64
halogenoalkanes
9.2 Howhalogenoalkanestructureaectstherateofhydrolysis 66
10 Organic compounds 10.1 Identifying four unknown organic compounds
containing oxygen
68
11 More about enthalpy 11.1 Enthalpy change of vaporisation of water 72
changes 11.2 Enthalpy change of solution of chlorides 74
11.3 Thermaldecompositionofiron(II)ethanedioate 77
11.4 Thermaldecompositionofmetalcarbonates 80
11.5 Enthalpychangeofmixing 82
12 Electrochemistry 12.1 Determining the Faraday constant 86
12.2 Comparing the voltage of electrochemical cells 88
12.3 Half-cells containing only ions as reactants 90
12.4 Changing the concentration of ions in an electrochemical cell 91
12.5 Electricalconductivityofethanoicaid 93
13 Further aspects 13.1 ChangeinpHduringanacid–basetitration 95
of equilibria 13.2 Partition of ammonia between water and trichloromethane 97
13.3 Anestericationreactionatequilibrium 99
13.4 TheeectoftemperatureontheN
2
O
4
⇌ 2NO
2
equilibrium 101
13.5 Equilibrium,entropyandenthalpychange 104
14 Reaction kinetics 14.1 Kinetics of the reaction between propanone and iodine 107
14.2 Rate of decomposition of an organic compound 109
14.3 Determination of the order of a reaction 111
14.4 Eectoftemperatureonrateofreaction 114
15 Transition elements 15.1 Coppercontentofcopperore 118
15.2 Analysisofirontablets 120
15.3 Formulaofacomplexion 122
15.4 Reactionofcopperwithpotassiumdichromate(VI) 124
16 More about organic 16.1 Making an azo dye 127
chemistry 16.2 Acylationofanucleicacid 129
16.3 Nitrationofbenzene 131
17 Identifying organic 17.1 Extracting an amino acid from hair 133
compounds
17.2 Identicationofawhitecrystallinesolid 134
17.3 Preparationandidenticationofacolourlessliquid 136
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Original material © Cambridge University Press
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Contents
Introduction
Practical work is an essential part of any advanced
Chemistry course. For Cambridge International AS &
A Level Chemistry, Paper 3 and Paper 5 focus on the
assessment of practical skills.
The practical investigations in the workbook have been
carefully chosen to:
• provide progressive guidance and practice of
Assessment Objective 3 (AO3) skills.
The skills grids at the front of this guide summarise the
practical skills that are assessed in Paper 3 (AS) and
Paper 5 (A level). You can use these grids to search for
practical investigations that involve a particular skill or
skills. At the beginning of each practical investigation,
the learning objectives and skills that are supported are
also listed.
The order of the investigations presented follows the
order of the topics in the Cambridge International AS
& A Level Chemistry coursebook, but please note that
this does not mean that they must be completed in
that order. Some coursebook chapters involve the
the practical guidance are covered in the workbook.
Each chapter of the workbook has more than one
investigation so do choose those that you feel suits the
been chosen to be as commonly available as possible
in the practical guidance. We have, however, provided
a set of sample results for each Practical Investigation,
which you can give to learners who have not managed
to obtain a complete set of results themselves, so that
they can continue to answer all of the Data analysis and
the theory they have learned works in practice. Because
of this, the details of the theory are more easily retained.
The important learning experiences, when carrying out
practical work, are the range of skills that are being used
and developed – the processes of planning, carrying
out, observing, recording, and analysing. The workbook
gives the learners experience in developing these skills.
It is not a series of mock practical exam papers! But, in
carrying out the investigations, the learners will practise
These points have been provided to give extra
support to students who may be struggling with the
investigation.
These points provide additional tasks to extend
more able learners.
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Cambridge International AS & A Level Chemistry
Working safely in a science laboratory is an essential
aspect of learning which characterises practical work. It
is the duty of the school to make it clear to learners just
what is expected of them when they are working in a
laboratory.
In every investigation, every learner is expected to
wear eye protection and long hair should be tied back.
Safety goggles generally give more protection than
safety spectacles. It is also advisable for them to wear a
laboratory coat to protect their clothing from chemical
splashes.
All chemicals should be treated as hazardous and
whenever they are spilt on the skin they should be
dangers of a chemical may not have been realised and
therefore using them without safety precautions can
lead to unforeseen problems.
Learners should also take responsibility for working
safely. It is advisable that learners are made aware of the
hazard signs on reagent bottles and in the laboratory.
Table S1. shows some of the most common hazard
symbols. This is not an exhaustive list, but it does list the
most common hazard symbols encountered in school
hazards is available for students to download.
Safety
Hazard symbol What does it mean? Special points
The substance is corrosive. It will damage your
skin and tissues if it comes into contact with
them.
Always wear safety goggles and if possible
gloves when using corrosive substances.
The substance is an irritant. If it comes into
contact with your skin it can cause blisters and
redness.
Always wear safety spectacles when using
irritants.
The substance is toxic and can cause death if
swallowed, breathed in or absorbed by skin.
Wear gloves and eye protection.
The substance is fl ammable
easily.
and when heating reaction mixtures use the hot
water from a kettle rather than using Bunsen
burners.
The material is a biohazard. Examples are
bacteria and fungi.
The substance is an oxidising agent. It will
liberate oxygen when heated or in the presence
of a suitable catalyst.
Keep oxidising agents well away from
Table S1
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Contents
AS Practical Skills
Manipulation, measurement and observation (MMO)
SKILL CHAPTER
1 2 3 4 5 6 7 8 9 10
Collection of data and observations
(a) set up apparatus correctly 1.1; 1.2;
1.3; 1.4
2.1; 2.2;
2.3
3.1; 3.2;
3.3; 3.4
5.1; 5.2;
5.3
6.1; 7.2; 7.3;
7.4
8.1; 8.2 9.1; 9.2 10.1
(b) follow instructions in written
form or from diagrams
1.1; 1.2;
1.3; 1.4
2.1; 2.2;
2.3
3.1; 3.2;
3.3; 3.4
4.1; 4.2 5.1; 5.2;
5.3
6.1; 6.2 7.1; 7.2;
7.3; 7.4
8.1; 8.3;
8.4
9.1; 9.2 10.1
(c) use apparatus to collect an
of data and observations
1.1; 1.2;
1.3; 1.4
2.1; 2.2;
2.3
3.1; 3.2;
3.3; 3.4
4.1; 4.2 5.1; 5.2;
5.3
6.1; 6.2 7.1; 7.2;
7.3; 7.4
8.1; 8.2;
8.3: 8.4
9.1; 9.2 10.1
(d) make measurements using
pipettes, burettes, and other
common laboratory apparatus
1.1; 1.2;
1.3; 1.4
2.1; 2.2;
2.3
3.1; 3.2;
3.3; 3.4
5.3 6.1 7.2 8.1
(e) make accurate and consistent
measurements and observations
1.1; 1.2;
1.3; 1.4
2.1; 2.2;
2.3
3.1; 3.2;
3.3; 3.4
4.1; 4.2 5.2; 5.3 6.1; 6.2;
6.3
7.1; 7.2;
7.3; 7.4
8.1; 8.2;
8.3; 8.4
9.1; 9.2 10.1
Decisions relating to measurements or observations
(a) decide how many tests or
observations to perform
1.1; 1.3;
1.4
2.2 4.1; 4.2 5.1; 5.3 7.2; 8.1; 8.2;
8.3
9.1 10.1
(b) make a suitable range and
number of measurements
suitable for the experiment
1.1; 1.3;
1.4
2.2 3.2; 3.4 4.2 5.3 7.2; 8.1; 8.3 9.1
(c) decide how long to leave
experiments running before
making readings
1.1; 1.2;
1.4
2.1; 2.2 3.4 4.2 5.1; 5.2;
5.3
6.1 7.2; 7.3;
7.4
8.2; 8.3;
8.4
9.1; 9.2 10.1
(d) make as many repeated
readings or observations as
appropriate
1.1;
1.2;1.3;
1.4
2.1; 2.2 3.3 4.1 5.1; 5.2 7.1; 7.2 8.1; 8.2 9.1 10.1
tests are needed and the nature
of these tests
1.3 4.1; 4.2 10.1
(f) choose reagents to distinguish
between given ions
4.1; 4.2 7.3; 7.4 8.3 9.2
Presentation of data and observations (PDO)
SKILL CHAPTER
1 2 3 4 5 6 7 8 9 10
Recording data and observations
(a) present data, values or
observations in a single table of
results
1.2; 1.3 2.1; 2.2 3.1; 3.2;
3.3; 3.4
4.2 5.1; 5.2
5.3
6.1; 7.2; 8.1 9.1; 9.2 10.1
The following grids map the practical investigations from the workbook to the mark categories for Papers 3 and 5, as
listed in the Cambridge International AS & A Level Chemistry syllabus.
The grids are designed to aid you when planning practical and theory lessons, to ensure learners develop the practical
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Cambridge International AS & A Level Chemistry
SKILL CHAPTER
1 2 3 4 5 6 7 8 9 10
Recording data and observations (Continued)
(b) draw up a table in advance of
taking readings
4.2 8.1 9.1 10.1
(c) include in the results table:
columns for raw data, calculated
values and analyses
1.3; 1.4 2.1; 2.2 3.3; 3.4 4.2 6.1; 6.2 7.2 8.1 10.1
(d) use column headings
1.4 3.3; 3.4 6.2 8.1 9.1
(e) record readings to the
same degree of precision and
observations to the same level
of detail
1.1; 1.2;
1.3; 1.4
2.1; 2.2 3.1; 3.3;
3.4
5.1; 5.2;
5.3
6.1; 7.1; 7.2 8.1; 8.3;
8.4
9.1
Display of calculations and reasoning
(a) show all working in
calculations and the key steps in
your reasoning
1.1; 1.2;
1.3; 1.4
2.2; 2.3 3.1; 3.2;
3.3; 3.4
5.3 6.1 7.2 8.1
(b) use the correct number of
1.1; 1.2;
1.3; 1.4
2.2; 2.3 3.1; 3.2;
3.3; 3.4
5.3 6.1 7.2 8.1
Data layout
(a) choose a suitable, clear way
of presenting data, for example,
tables, graphs or a mixture
3.1; 3.4 4.2 6.1 7.1 8.1 9.1; 9.2 10.1
(b) decide how to plot the
variables and whether a graph
should be a straight line or a curve
1.1; 1.2 2.2 3.1; 3.4 6.1
(c) plot appropriate variables on
clearly labelled xy
1.1; 1.2 2.2 3.1; 3.4 6.1
(d) choose suitable scales for
graph axes
1.1; 1.2
2.2 3.1; 3.4 6.1
(e) plot all points or bars to an
appropriate accuracy
1.1; 1.2 2.2 3.1; 3.4 6.1
account the anomalous points
1.1; 1.2 2.2 3.1; 3.4 6.1;
Analysis, conclusions and evaluation (ACE)
SKILL CHAPTER
1 2 3 4 5 6 7 8 9 10
Data interpretation and sources of error
(a) describe the patterns and
trends shown by tables and
graphs
2.2; 2.3 3.1; 3.4 5.1; 5.2 6.1; 6.2 7.1 9.2
(b) describe and summarise
the key points of a set of
observations
2.1; 2.2;
2.3
5.1; 5.2 6.1 7.1 9.2
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AS Practical Skills
SKILL CHAPTER
1 2 3 4 5 6 7 8 9 10
Data interpretation and sources of error (Continued)
a graph including the drawing of
intersecting points
1.1; 1.2 2.2 3.1; 3.4
from data or the mean from
reproducible values
1.1; 1.2;
1.3;
2.2; 2.3 3.4 5.3 6.1 7.2 8.1 9.2
(e) determine the gradient of a
6.1
sources of error in an experiment
1.3; 1.4 2.2 3.1; 3.2
3.3
5.3 7.2
(h) estimate in terms of number
values the uncertainty in
1.2; 1.3;
1.4
3.1; 3.2;
3.3
5.3
(i) express these uncertainties as an actual or percentage error
(j) show and understand the
and random errors
1.2;
1.3; 1.4
3.1; 3.2
Drawing conclusions
(a) consider to what extent the
experimental data support a
given hypothesis
1.2;
1.3
2.3 4.2 5.1; 5.2;
5.3
6.1; 6.2;
6.3
7.1 8.3 9.2 10.1
(b) make further predictions
arising from the experiment
4.2 5.1; 5.2 6.1; 6.3
(c) draw conclusions from
observations, data and
calculated values
1.1; 1.2;
1.3; 1.4
2.1; 2.2;
2.3
3.1; 3.2;
3.3
4.1; 4.2 5.1; 5.3 6.1; 6.2;
6.3
7.1; 7.2 8.1; 8.2;
8.3; 8.4
9.1; 9.2 10.1
arising from the data,
observations and conclusions
1.3; 1.4 2.1; 2.2 4.1; 4.2 5.1; 5.3 6.1; 6.2;
6.3
7.1 8.2; 8.3;
8.4
9.1; 9.2 10.1
Suggesting improvements
improve the accuracy of the
experiment/observations
1.1;
1.3; 1.4
6.1; 6.2
(b) suggest how to extend the
investigation to answer a new
6.2 7.3
experiment in words or diagrams
1.4 6.2
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Planning an investigation (PI)
SKILL CHAPTER
11 12 13 14 15 16 17
Selecting information
(a) locate, select, organise and present
information from a variety of sources
14.3 17.1; 17.2;
173
(b) construct arguments to support hypotheses
and justify a course of action
11.1; 11.3;
11.5
14.3; 14.4 15.2 16.1; 16.3 17.1; 17.2;
17.3
(c) apply knowledge, including principles,
to new situations
11.1; 11.3;
11.5
12.3;
12.5
13.4; 13.5 14.1;
14.2; 14.4
15.1;
15.3; 15.4
16.1;
16.2; 16.3
17.1; 17.2;
17.3
Dening the problem under investigation
to a reliable result
11.3; 11.4, 12.5 13.3; 13.4 14.3; 14.4 15.1; 15,4 16.1; 16.3
(b) express the aim in terms of a prediction (in
words or as a predicted graph)
14.3; 14.4
(c) identify the steps needed to carry out the
procedure
11.3; 11.4 12.4; 12.5 13.3; 13.4 14.3; 14.4 15.1; 15.4 16.1; 16.3
(d) identify apparatus that is suitable for
carrying out each step of the procedure
11.3; 11.4 12.3; 12.4;
12.5
13.3; 13.4 14.3; 14.4 15.1; 15.4 16.1; 16.3 17.1
(e) indicate how and why the procedure
13.2; 13.4 14.4 15.3 16.1
Control experiments and identication of variables
(a) identify the independent variable and
the dependent variable in an experiment or
investigation
11.5 12.4 14.4; 14.4
(b) explain how control experiments verify that
12.5 15.3; 15.4 16.1; 16.2
(c) identify any variables that are to be
controlled
11.1; 11.3;
11.4
12.1; 12.5 13.2; 13.3 14.1;
14.3; 14.4
15.1;
15.3; 15.4
16.2
Considering hazards
(a) assess the risks of the proposed experiment 11.1; 11.3;
11.4; 11.5
12.1; 12.5 13.3; 13.4 14.3; 14.4 15.1; 15.4 16.1; 16.3
(b) describe precautions that should be taken
to keep risks to a minimum
11.4 12.5 13.2;
13.3; 13.4
14.3; 14.4 15.1; 15.4 16.1; 16.3
Carrying out an investigation (COI)
SKILL CHAPTER
11 12 13 14 15 16 17
Methods used
(a) describe the method to be used when
carrying out an investigation
11.1; 11.3;
11.4
12.3;
12.4; 12.5
13.3; 13.4 14.3; 14.4 15.1; 15.4 16.1 17.1
(b) describe the arrangement of the apparatus
and the steps in the procedure to be followed
11.3; 11.4 12.4; 12.5 13.3; 13.4 14.3; 14.4 15.1; 15.4 16.1 17.1
A Level Practical Skills
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A Level Practical Skills
SKILL CHAPTER
11 12 13 14 15 16 17
Methods used (Continued)
(c) arrange and use the apparatus provided
correctly
11.1; 11.2;
11.4
12.1;
12,2;
12.3; 12.4
13.1 14.3; 14.4
(d) suggest and use appropriate volumes and
concentrations of reagents
11.3; 11.4 12.3;
12.4; 12.5
13.3; 13.4 14.3; 14.4 15.4 16.1; 16.3
Carrying out the experiment
(a) carry out the experiment by varying the
independent variable and measuring the
dependent variable
11.1; 11.2;
11.4
12.1;
12,2;
12.3;
12.4
13.1 14.3; 14.4
(b) carry out the experiment so that key
variables are controlled
11.1; 11.2;
11.4
12.1;
12,2;
12.3;
12.4
13.1 14.3; 14.4
degree of accuracy
11.1; 11.2;
11.4
12.1;
12,2;
12.3;
12.4
13.1 14.3; 14.4
(d) carry out the experiment safely 11.1; 11.2;
11.4
12.1;
12,2;
12.3;
12.4
13.1 14.3; 14.4
Handling information (HI)
SKILL CHAPTER
11 12 13 14 15 16 17
Collecting and displaying data
(a) describe the outcome of steps in the
procedure relevant to the experiment
11.3; 11.4 14.3; 14.4
(b) handle information, distinguishing the
relevant from the irrelevant
11.1;
11.2;
11.3; 11.5
12.1;
12.3;
12.4; 12.5
13.1;
13.2;
13.3; 13.4
14.1; 14.2;
14.3; 14.4
15.2; 15.3 17.1; 17.2;
17.3
(c) draw up tables for data that need to be
recorded
11.1; 11.2 12.4 13.1 14.3; 14.4
Manipulating data
(a) describe how the data might be used in
order to reach a conclusion
13.1 14.1; 14.3;
14.4
15.2 17.2; 17.3
(b) manipulate numerical and other data 11.1;
11.2;
11.4; 11,5
12.1;
12.2,
12.4; 12.5
13.1;
13.2;
13.3; 13.5
14.1; 14.2;
14.3; 14.4
15.1;
15.2;
15.3; 15.4
16.2 17.2; 17.3
(c) translate information from one form to
another including graphical information
11.1;
11.2; 11.5
12.4; 12.5 13.1;
13.2; 13.5
14.2;
14.3; 14.4
15.1; 15.3 16.2
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Cambridge International AS & A Level Chemistry
Data analysis (DA)
SKILL CHAPTER
11 12 13 14 15 16 17
Identifying trends and patterns
(a) analyse information so as to identify patterns
and report trends
11.2;11.5 12.1; 12.2;
12.4; 12.5
13.1;
13.2; 13,3
14.1;
14.2;
14.3; 14.4
15.3; 15.4 16.2 17.1;
17.2; 17.3
draw attention to key points
11.1 14.3; 14.4 15.4 16.2 17.2; 17.3
(c) comment, where necessary on the variability
of the data
11.5 12.4 13.3 14.2;
14.3; 14.4
15.2; 15.3
(d) analyse data from spectra or other published
data to reach appropriate conclusions
15.3; 15.4 17.2; 17.3
Identifying and using calculations
of presentation of data provided
11.3 12.1 13.1 15.1; 15.2
(b) AL3.2.3 use calculations to enable
11.1;
11.3
12.4 13.3; 13.5 14.1; 14.2 15.2; 15.3
Conclusions and Predictions (CP)
SKILL CHAPTER
11 12 13 14 15 16 17
Drawing conclusions
conclusions
11.2; 11.4 13.1 15.1; 15.4
conclusions
11.3;
11.4; 11.5
12.3;
12.4; 12.5
13.1;
13.2; 13.5
14.1; 14.3;
14.4
15.2; 15.3 16.2 17.1; 17.2;
17.3
(c) draw conclusions to describe the key
features of the data and analyses
13.1 14.1; 14.3;
14.4
17.2; 17.3
(d) make detailed explanations of the data,
analyses and conclusions
14.1; 14.3 17.2; 17.3
(e) consider whether the experimental data
supports the conclusion reached
11.2; 11.3 12.2 13.3 14.3; 14.4 15.3 17.1; 17.2;
17.3
Making predictions
(a) make further predictions, ask relevant
11.2 12.1; 12.4 13.4 14.4
(b) suggest improvements by asking relevant
11.2 12.1; 12.4 13.4 14.4
SAMPLE
Original material © Cambridge University Press
xiii
A Level Practical Skills
Evaluating investigations (EI)
SKILL CHAPTER
11 12 13 14 15 16 17
Identifying problems with the procedure
(a) identify and explain the weaknesses of the
experimental procedure used
11.1; 11.2;
11.4; 11.5
12.1;
12.2; 12.4
13.1;
13.3; 13.4
14.1,14.2;
14.3,14.4
15.1; 15.2
apparatus on the results
12.4 14.1 15.3
(c) use information provided to assess the
14.2; 14.3;
14.4
16.2
(d) explain how changes in the conditions
12.1 14.3; 14.4
(e) explain how changes in concentration of
12.2; 12.4 14.3
Identifying problems with the data
(a) identify anomalous values in data provided,
give possible explanations and suggest how to
deal with these.
11.5 12.5 13.1; 13.5 15.3
(b) identify the extent to which readings
provided have been reproduced
11.5 13.3 15.2
(c) describe whether the range of data
11.5 13.2; 13.5 14.2; 14.3;
14.4
15.2; 15.3
Making a judgement on the conclusions
(a) evaluate information and hypotheses 11.1; 11.2;
11.3; 11.4
12.1;
12.3;
12.4; 12.5
13.1;
13.2;
13.3; 13.4
14.1; 14.2;
14.3; 14.4
15.2;
15.3; 15.4
17.1
(b) evaluate information to make judgements
11.1; 11.2;
11.4; 11.5
14.1;14.2;
14.4
15.1; 15.3 16.2 17.1
SAMPLE
Original material © Cambridge University Press
Chapter 12:
Electrochemistry
I
Chapter outline
In this chapter you will complete investigations on:
■ 12.1 Determining the Faraday constant
■ 12.2 Measuring the voltage of some electrochemical cells
■ 12.3 Changing ionic concentrations in a electrochemical cell
■ 12.4 Half cells containing only ions as reactants
Chapter outline
In this chapter you will complete investigations on:
■
12.1 Determining the Faraday constant
■
12.2 Measuring the voltage of some electrochemical cells
■
12.3 Changing ionic concentrations in a electrochemical cell
■
12.4 Half cells containing only ions as reactants
Chapter 1:
Masses, moles and atoms
I
Chapter outline
This chapter relates to Chapter 1: Moles and equations, Chapter 2: Atomic structure and Chapter 3:
Electrons in atoms in the coursebook.
In this chapter learners will complete practical investigations on:
■ 1.1 Empirical formula of hydrated copper(II) sulfate crystals
■ 1.2 Relative atomic mass of magnesium using molar volumes
■ 1.3 Percentage composition of a mixture of sodium hydrogen carbonate and sodium chloride
■ 1.4Relativeatomicmassofcalciumbytwodierentmethods:molarvolumeandtitration
Practical investigation 1.1:
Empirical formula of hydrated
copper(II) sulfate crystals
Introduction
In this investigation learners determine the empirical formula
(see Chapter 1 of the coursebook) of hydrated copper(II)
x in CuSO
4
.xH
2
O. They weigh
out some hydrated copper(II) sulfate in an evaporating basin,
heat it to constant mass, determine the mass of water present
4
: H
2
O.
Skills focus
The following skill areas are developed and practised (see
the skills grids at the front of this guide for codes):
MMO Collection of data and observations: (a), (b), (c),
(d) and (e)
Decisions relating to measurements of
observations: (a), (b) (c) and (d)
PDO Recording data and observations: (e)
Display of calculations and reasoning: (a) and (b)
Data layout: (b), (c), (d), (e) and (f)
ACE Data interpretation and sources of error: (c) and (d)
Drawing conclusions: (c)
Suggesting improvements: (a)
Duration
This investigation should take no more than 1 h to complete.
error calculations you may need another hour to go through
the errors involved.
Preparing for the investigation
• Learners should be made aware of the ‘Skills Chapter’
be using.
• They will also need to have an awareness of the sources
of errors.
• Learners will need to understand the concept of an
empirical formula and be able to calculate the number of
moles present.
• They should revise the concepts of moles and molar
ratios.
Equipment
Each learner or group will need:
•
• an evaporating basin
• Bunsen burner and tripod
• tongs
• glass stirring rod
•
• spatula
Access to:
• a supply of gas
•
places
2
SAMPLE
Original material © Cambridge University Press
Safety considerations
• Learners must wear eye protection at all times in this
experiment and tie back long hair.
• When weighing the evaporating basin and copper sulfate
•
The c
opper(II) sulfate is an environmental hazard and
should be recycled. It can be used as a test for water or
dissolved in water and recrystallised. It could also be
used in a Hess’ Law determination.
Carrying out the investigation
• They may need help to understand what is meant by
‘water of crystallisation’ and how it is loosely bound
to the copper(II) sulfate and that the number of water
molecules per formula is a whole number.
• Assuming that the length of the practical time available is
one determination.
•
All
ocate a given mass to each group. It is a good idea
to give the larger masses of copper sulfate to the more
able learners or more patient ones because they will
obviously need more time in heating the copper(II)
sulfate to give the anhydrous form.
• If they heat the copper sulfate properly there will be
some at the beginning that will stick to the stirring rod
and the basin and when this ceases to happen it shows
that they are removing the water from the salt.
• The anhydrous salt should be as near white as possible
and constant mass is obtained.
•
Ens
ure that if more than one balance is used, the learners
should use the same balance throughout. By doing this
any errors in the balances are reproducible.
Some learners will need help on why some points
on their graph lie above and below the line.
• Some will also need help on heating the copper(II) sulfate
as gently as possible (see above) so will need to be trained
•
Lea
rners who struggle with the practical, especially the
theoretical part, should be given the lowest value masses
processing their results.
More able learners should, if possible, be allowed to
work on their own.
Common learner misconceptions
• When instructed to ‘heat gently’ some learners will still
Sample results
Mass of crystals/g
Mass of anhydrous
copper(II) sulfate/g
0.20 0.12
0.50 0.32
0.80 0.51
2.50 1.60
Table 1.1
0.0 0.5 1.0
0.0
0.4
0.2
0.6
0.8
Mass of anhydrous copper(II) sulfate / g
1.5 2.0
2.5
1.0
1.2
Mass of
water / g
Figure 1.1
3
Chapter 1: Masses, moles and atoms
SAMPLE
Original material © Cambridge University Press
Practical Investigation 1.2:
Relative atomic mass of
magnesium using molar volumes
Skills focus
The following skill areas are developed and practised (see
the skills grids at the front of this guide for codes):
MMO
Col
lection of data and observations (a), (b), (c),
(d) and (e)
Dec
isions relating to measurements of
observations (c) and (d)
PDO Recording data and observations (a) and (e)
Disp
lay of calculations and reasoning (a) and (b)
Data layout (b), (c), (d), (e) and (f)
ACE Data interpretation and sources of error (c), (d), (h),
(i) and (j)
Drawing conclusions (a) and (c)
Duration
This investigation should take approximately 1.5 h to
complete.
Preparing for the investigation
• Learners should, ideally, have a good understanding of
moles and molar volumes. The crucial relationships are:
A
r
=
mass (m)
and n =
Volume of gas in cm
3
number of moles (n) 24 000
Equipment
Each learner or group will need:
• either a trough;
a selection of measuring cylinders (10
cm
3
; 25 cm
3
and
50
cm
3
); OR a 100 cm
3
gas syringe
•
150 cm
3
•
sma
ll piece of steel wool
• 25 cm
3
measuring cylinder for acid
•
one 1
0.0
cm le
ngth of magnesium ribbon
• 30 cm ruler
•
pla
stic gloves (see safety considerations)
Answers to the workbook questions
(using the sample results)
a & b
the ideal answer for this practical (see Table 1.1 and
Figure 1.1).
c
As ca
n be seen from the graph, the mass of water that
combines with 1.60 g of copper(II) sulfate is 0.90 g
d
Res
ults shown in Table 1.2
Copper(II)
sulfate (CuSO
4
) Water (H
2
O)
Mass/g 1.60 0.90
Number of
moles
1.60
= 0.0100
159.6
0.90
= 0.0500
18
Simplest ratio
(divide by
lowest number)
0.0100
= 1
0.0100
0.0500
= 5
0.0100
Table 1.2
e This means that the formula for the hydrated
copper(II) sulfate is CuSO
4
.5H
2
O
f
0, 0 be
cause if there is no copper(II) sulfate then there
will be no water attached to it.
g
i If a po
int lies above the line then it could have
been heated too much and the copper (II) sulphate
has decomposed to some extent.
ii If a point lies below the line there has been
crystallisation is still attached to them. However, it
and absorbed water from the atmosphere.
h
The b
est alternative is to use an oven. The
temperature of the oven can be adjusted to one
where the water of crystallisation will be removed but
it will not be hot enough to decompose the copper(II)
accurate. A possible way of determining the Bunsen
burner temperature is to use a thermocouple to give
a reading of the temperature. Even using this method
is inaccurate because any slight change in the extent
to which the air hole is opened will lead to a change in
temperature.
4
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Access to:
•
at least two decimal places
• 2 mol dm
–3
hydrochloric acid
Alternative equipment
• Of the two sets of apparatus suggested, the easiest to
set up is the one using the gas syringe. However, if gas
syringes are not available, then the displacement of
water in a measuring cylinder works very well.
Safety considerations
• Learners must wear eye protection at all times and tie
hair back if it is long.
•
•
• 2 mol dm
–3
hydrochloric acid is an irritant.
•
Ste
el wool sometimes splinters and some learners could
should be worn when using the steel wool to clean the
magnesium.
Carrying out the investigation
• The point of weighing out 10 cm lengths of magnesium
ribbon is that 10
cm wi
ll give a valid reading on the
two decimal places. The masses of the shorter lengths
are then calculated using the relationship:
mass =
length
10
x mass of 10 cm length.
•
Ple
ase note that if learners are measuring the gas volume
is making sure that the measuring cylinder is full of water
when it is put in the trough and that none or very little
escapes. This can be done by either learners holding
their hands over the end of the measuring cylinder or
placing a piece of plastic wrap over the open end and
then turn the measuring cylinder upside down when it
starting the actual measurement. A boiling tube will do
• The main problem with the practical is the purity of
the magnesium ribbon. If you have fresh ribbon then
omit the cleaning. If it is visibly oxidised then it will need
cleaning and that is done using the steel wool. This
should be done by holding the ribbon using the wool and
then drawing it through. Once should be enough. Any
more than that will lead to irregularities in the thickness
of the ribbon and inaccuracy when estimating the
masses of the individual lengths.
Evaluation of a practical method always presents
problems to learners and they will need help when
apparatus.
• Before the practical, a short demonstration will give
learners some idea of the volumes of gas that they will
be dealing with. This can be their trial run but more able
learners can be asked to do this for themselves. If the
volume of gas for a 1 cm length of ribbon is found then
they should be able to estimate the volumes for the other
lengths and adjust their choice of measuring cylinder (if
these are used) accordingly.
If learners are measuring the gas volume by
displacement of water then they can be marked on
which measuring cylinder they use for the most accurate
measurements of gas volumes.
• Learners can be asked to analyse their results in
Sample results
Mass of 10 cm length of magnesium ribbon = 0.160 g
The results from one set of measurements are shown in
Table 1.3
Length of Mg
ribbon/cm
Mass of
Mg/g Expt. 1 Expt. 2 Average
0.00 0 0 0 0
0.50 0.008 8 8 8.0
1.00 0.016 16 17 16.5
1.50 0.024 23 25 24.0
2.00 0/032 30 31 30.5
Table 1.3
5
Chapter 1: Masses, moles and atoms
SAMPLE
Original material © Cambridge University Press
Answers to the workbook questions
(using the sample results)
a Please see Figure 1.2
0
5
10
15
20
25
30
35
0 0.005 0.01 0.015 0.02 0.025 0.03 0.03
5
Volume of gas/cm
3
mass of mg ribbon/g
Figure 1.2
b Using Figure 1.2: 24.0 cm
3
of H
2
is produced from
0.0245 g of magnesium
c 24.0 cm
3
=
24
24,000
mol = 0.001 mol of H
2
= number of
mol of magnesium
Therefore, mass of 1 mol
=
m
n
=
0.0245
0.001
= 24.5 g mol
–1
d Percentage error =
| Actual value – experimental value |
Actual value
x 100
=
24.5 – 24.3
24.3
x 100% = 0.823%
The mass of 10 cm of magnesium ribbon is around
0.15–0.17 g.
In this experiment, the systematic errors come from
cylinder (or gas syringe).
e
maximum error can be estimated to be ±0.005 g. If
we think our 10 cm length of magnesium will weigh
in the region of 0.15 g then the percentage
error = 2 x
0.005
0.15
x 100% = 6.67%
f Percentage error from measurements of lengths
For example, if the length is 1 cm then the maximum
0.05
1.0
x 100% = 5%
g Total error from length measurements
i The measurement of the lengths of magnesium
ribbon. If we go by the rules that the maximum
error or uncertainty is half the smallest possible
measurement then the ruler reads to ±0.5 mm.
The length measurements will give the greatest
error.
ii If they use measuring cylinders, learners should
be marked on their choice. For example, if they
estimate from their trial run that they will obtain
around 20 cm
3
from the reaction, then choosing
a 50 cm
3
measuring cylinder that is graduated in
2.0 cm
3
divisions will give a maximum error of
±1.0 cm
3
(half the graduation’s reading).
iii Total possible percentage error from apparatus
readings. In this case, the maximum percentage
error is
1
20
x 100% = 5%. This error is halved if a
25 cm
3
measuring cylinder is used.
h Other factors that limit accuracy and contribute to
the error
• Because of the cleaning by the steel wool, the
thickness of the magnesium ribbon is not the
same along its whole length.
•
cleaning.
6
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Practical Investigation 1.3:
Percentage composition of a
mixture of sodium hydrogen
carbonate and sodium chloride
Introduction
composition of a mixture of sodium hydrogen carbonate and
sodium chloride. They will do this by titrating the sodium
hydrogen carbonate against standard hydrochloric acid.
Skills focus
The following skill areas are developed and practised (see
the skills grids at the front of this guide for codes):
MMO
Col
lection of data and observations (a), (b), (c),
(d) and (e)
Dec
isions relating to measurements of
observations: (a), (b) (d) and (e)
PDO Recording data and observations (a), (c) and (e)
Disp
lay of calculations and reasoning (a) and (b)
ACE
Dat
a interpretation and sources of error (d)
Drawing conclusions (a), (c) and (d)
Sug
gesting improvements (a)
Duration
making up the solution of the mixture, then 1 h for the
titrations and calculations.
Preparing for the investigation
•
calculated from the projected titre values. For example,
if you calculate that the sodium hydrogen carbonate
3
of acid for complete reaction,
3
3
per learner/group would be an
Equipment
Each learner or group will need:
• 150 cm
3
•
250 cm
3
•
was
h bottle of distilled water
• burette stand
•
25.0 cm
3
pipette
•
whi
te tile
• 250 cm
3
beaker and 100 cm
3
beaker
•
sti
rring rod and small dropper
•
• weighing boat
•
50 cm
3
burette
Access to:
•
a mix
ture of sodium hydrogen carbonate and sodium
classes are doing the same practical they can be given
•
0.1
00
mo
l
dm
–3
hydrochloric acid
•
calculated from the projected titre values. For example,
calculations might show that the sodium hydrogen
3
of acid for a complete
titrations, 85 cm
3
3
per student/
• methyl orange indicator and dropper
•
• distilled water
Safety considerations
• They must wear eye protection and tie hair back if it is long.
•
The a
cid is an irritant at the concentration used in the
experiment.
•
Carrying out the investigation
• As far as the mixture is concerned, a typical calculation is
as follows:
i Let us suppose we want the titre to be 17.20 cm
3
.
burette twice at most.
ii The number of moles of sodium hydrogen carbonate
present in 25.00
cm
3
is: 17.20 x 10
–3
x 0.1 = 1.72 x 10
–3
mol
7
Chapter 1: Masses, moles and atoms
SAMPLE
Original material © Cambridge University Press
iii Therefore, in 250 cm
3
the learner has 1.72 x 10
–2
mol
or 1.72 x 10
–2
x 84.1 g = 1.445 g
iv
sodium chloride should contribute 2.00–1.45
g = 0.5
5
g.
v The p
ercentage composition of the mixture = 72.5%
NaHCO
3
and 27.5% NaCl. If you have 20 learners then
you will need 20 x 2.00
g of mi
xture but allow for
more because learners are still in the early stages of
vi
results obtained because the solid mixture may not
be homogeneous. The only way to ensure complete
homogeneity is to make up a solution of the mixture.
This makes a very good discussion point at the end.
vii It is important that learners express the burette
readings to ±0.05
cm
3
. For example, if they get two
readings such as 17.00 and 17.10, then the average
is 17.05 because burettes usually read to 0.05 cm
3
,
which is one drop.
•
Ple
ase be aware that learners tend to ‘blow out’ or expel
the last drop of solution from their pipette. The pipette
is calibrated so that this last remaining drop is not in the
25.00 cm
3
used.
•
colour. If a red colour is obtained, then they have overshot.
•
The b
iggest problem is how well you have mixed the
sodium hydrogen carbonate and sodium chloride. It
is not that big a problem because the apparatus used
is overall very accurate and therefore the systematic
errors are small. It is a random source of error and a
systematic error could be the learner who does the same
thing wrong for every titration.
As already mentioned, making the whole mixture
into a solution would remove the possibility of random
distribution of the solids. Ask them to put forward one
way to overcome the problem and see if they come up
with a plausible method.
Common learner misconceptions
• The most common error is that learners forget that
25
cm
3
is only
1
10
th of the total amount of solution they
have prepared.
Sample results
Rough titration/cm
3
First accurate
titration/cm
3
Second accurate
titration/cm
3
Third accurate
titration/cm
3
Final burette
reading/cm
3
18.00 35.20 19.80 37.00
Starting burette
reading/cm
3
0.00 18.00 2.20 19.80
Titre/cm
3
18.00 17.20 17.60 17.20
Table 1.4
Answers to workbook questions (using the
sample results)
a Change in enthalpy of hydration of copper (II) sulfate
b
i Vol
ume of 0.100
mo
l
dm
–3
hydrochloric acid needed
to react completely with the sodium hydrogen
carbonate present in 25 cm³ of the mixture = 17.20 cm
3
ii Number of moles of hydrochloric acid reacting =
number of moles of sodium hydrogen carbonate
present in 25.00 cm
3
= 17.20 × 10
–3
× 0.100 = 1.72 × 10
–3
= number of moles of sodium hydrogen carbonate
present in 25.00 cm³ of solution.
The
refore, in 250
cm³ o
f solution the number of
moles of sodium hydrogen carbonate present =
1.72 × 10
–3
× 10 = 1.72 × 10
mol
iii Mas
s of sodium hydrogen carbonate present
(m = n x M
r
) = 1.72 x 10
–2
× 84.1 = 1.45 g
iv Tot
al mass of mixture = 2.00
g
v Mass of sodium chloride present in mixture =
vi Percentage of sodium hydrogen carbonate
present mixture =
1.45
2.00
× 100% = 72.5%
8
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
vii What is the actual percentage composition of the
mixture? = 72.5% NaHCO
3
and 27.5% NaCl
If yo
u have 20 students then you will need 20 x 2.00
g
of mi
xture but allow for more because students are
still in the early stages of learning in detail about
not yet be up to scratch.
c Percentage error =
| Actual value – experimental value |
Actual value
x 100
d
The s
ystematic errors:
i The top pan balance: if 2 readings are taken and
the balance reads to 0.01 g then the percentage
error for a mass of 2.00 g the percentage error =
2 ×
0.005
2.00
× 100% = 0.5%
ii The p
ipette: if this reads to ± 0.05
cm³ t
hen the
percentage error =
0.05
25.00
× 100% = 0.200%
iii The burette readings
It is important that the students express the burette
readings to ±
0.
05
cm
3
. For example, if they get two
readings such as 17.00 and 17.10 then the average is
17.05 because burettes usually read to 0.05 cm
3
, which
is approximately one drop of solution.
The uncertainty for a burette is ±0.05 cm
3
for each
reading. Therefore, the uncertainty associated with
= 2 × 0.0
5 = ± 0.10
cm³
Therefore, the error =
0.10
17.20
× 100% = 0.58%
e The main random error depends on the homogeneity
of the mixture. Another possible error is in the purity
of the sodium hydrogen carbonate. Over time it can
decompose to give sodium carbonate.
f The main contribution to any percentage error is due
to the solid mixture not being homogeneous.
g The only way to ensure complete homogeneity is to
make up a solution of the mixture. This makes a very
good discussion point at the end.
Practical investigation 1.4:
Relative atomic mass of calcium
by two dierent methods: molar
volume and titration
Introduction
In this investigation, learners will react calcium with water
to give hydrogen. The volume of hydrogen given by a known
mass of calcium is measured and, using molar ratios, the
number of moles of calcium is found and from this the
relative atomic mass. The reaction of calcium with water also
gives the alkali calcium hydroxide, which is titrated against
standard hydrochloric acid. Again, the number of moles of
calcium hydroxide (and therefore calcium) is determined
and this will give another value for the relative atomic mass.
Skills focus
The following skill areas are developed and practised (see
the skills grids at the front of this guide for codes):
MMO
Col
lection of data and observations: (a), (b), (c),
(d) and (e)
Dec
isions relating to measurements of
observations: (a), (b) (c) and (d)
PDO Recording data and observations: (c) and (e)
Disp
lay of calculations and reasoning: (a) and (b)
ACE
Dat
a interpretation and sources of error: (g) (h),
(i) and (j)
Dra
wing conclusions: (c) and (d)
Suggesting improvements: (a) and (c)
Duration
This investigation is a summative exercise as it uses
learners to use several formulae and relationships.
Preparing for the investigation
• Of the two sets of apparatus suggested for collecting the
gas, the easiest to set up is the one using the gas syringe.
However, if gas syringes are not available, then the
displacement of water in a measuring cylinder works
very well.
Equipment
Each learner or group will need:
• apparatus for measuring gas volumes as used in
Investigation 1.2
9
Chapter 1: Masses, moles and atoms
SAMPLE
Original material © Cambridge University Press
•
•
50 cm
3
burette
•
weig
hing boat
• 150 cm
3
•
was
h bottle of distilled water
• burette stand
•
25 cm
3
pipette
•
whi
te tile
• 250 cm
3
beaker
•
25 cm
3
measuring cylinder (for water)
•
met
hyl orange indicator in dropper bottle
Access to:
• at least two decimal places.
preferable.
•
0.2
00
mo
l
dm
–3
hydrochloric acid
•
acc
ess to fresh calcium granules
• distilled water
Safety considerations
• Learners must wear eye protection and tie their hair back
if it is long.
• Calcium reacts vigorously with water. Emphasise that
learners should not handle it with wet hands.
•
•
0.2 mo
l
dm
–3
hydrochloric acid is an irritant.
•
It is im
portant that if learners are using gas syringes they
do not clamp the syringe too tightly. Firstly, they could
crack the glass and it may also hinder the movement of
the piston.
•
The c
alcium hydroxide is an alkali and should be
immediately if spilt on the skin.
• Methyl orange indicator is poisonous. If any is splashed
Carrying out the investigation
•
sure that the measuring cylinder is full of water when it
is put in the trough and that none or very little escapes.
This can be done by either learners holding their hands
over the end of the measuring cylinder or placing a piece
of plastic wrap over the open end and then turn the
measuring cylinder upside down when it is in the trough.
measurement.
•
The m
ain problem with the practical is the freshness
of the calcium. If it is visibly oxidised, then the results
will be inaccurate and this is one of the random errors
encountered. If the top portion of your calcium looks to
be oxidised then use the lower portions. An alternative is
that if you know you are going to use calcium for Group
II experiments, then as soon as it is bought, divide it up
into smaller portions and store in small containers until
ready to use. It is the constant exposure to air that leads
in the end to its oxidation.
Make sure that learners have at least two sets of
will get better the more practice they have.
•
Onc
e learners have started, then one of the group can do
the determination of gas volumes while the other can
swap over.
•
Bef
ore the practical, a short demonstration with
an approximate mass of calcium will give learners
some idea of the volumes of gas that they will be
dealing with.
•
Als
o, unless there is time for a trial run, learners could
titration.
• Evaluation of a practical method always presents
problems to learners and they will need help when
apparatus.
If you want to extend the more able learners, you
can state that they know what the answer should be and
they can work back to see what readings they should get.
However, in this case it should be emphasised that the
methods are not perfect and therefore cheating will give
them fewer marks.
Common learner misconceptions
• Learners may need to be reminded that the calcium
hydroxide is formed from the same mass of calcium as in
learners are doing their calculations.
10
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Sample results
Part 1: Determination by molar volume
Example measurements shown in Table 1.5.
Learner Mass of Ca/g Volume of H
2
/cm
3
At mass of
Ca/g mol
–1
Burette readings Vol of HCl/cm
3
At mass of Ca/g mol
–1
1
0.050 30.00 40.00
2nd
1st
12.50
0.00
12.50 40.00
0.048 28.0 41.10
2nd
1st
23.90
12.50
11.40 42.30
2
0.040 24.0 40.0
2nd
1st
10.00
0.00
10.00 40.00
0.055 31.0 42.5
2nd
1st
24.50
10.00
14.50 37.70
3
0.060 37.0 38.9
2nd
1st
17.20
2.00
15.20 39.50
0.071 42.0 40.57
2nd
1st
36.20
18.00
18.20 39.01
Table 1.5
Answers to the workbook questions
(using the sample results)
Weight of calcium = 0.048 g
a i
experiment:
n
hydrogen
=
28
24 000
= 1.17 x 10
–3
mol
ii Number of moles of calcium:
n
calcium
= n
hydrogen
= 1.17 x 10
–3
mol
iii Rel
ative atomic mass of calcium:
A
r
(Ca) =
0.048
1.17 x 10
–3
= 41.0 g mol
–1
b The percentage error in your result
Perc
entage error =
| Actual value – experimental value |
Actual value
x 100
The re
lative atomic mass result for this experiment is
41.0 which should be 40.1.
This gives a percentage error of
41.0 – 40.1
40.1
x 100%
= 2.24%
c Systematic errors in the apparatus:
i
pan balance reading to ± 0.001 g then the possible
error is ½ x 0.001 = 0.0005.
A mass of 0.048 g has a possible error of
0.0005
0.048
x
100% = 1.04%. This will rise to 10.4% if you use a
two decimal
places.
ii The measurement of gas volume
A 100 cm
3
measuring cylinder reads to ±2.00 cm
3
and therefore has a maximum error of ±1.00
cm
3
. A
volume of 28.0 cm
3
has a possible error of
1
28.0
x
100% = 3.60%.
iii Random errors in the method:
The calcium is possibly oxidised. In this case,
the volume of hydrogen will be less than ideal
and the value of n will be lower than expected.
Therefore
m
n
will give a value of the relative
atomic mass higher than the published value.
This method also assumes that the hydrogen is
collected at R.T.P.
iv Imp
rovements to Method 1:
If th
e calcium is oxidised than some of the mass
weighed out is not calcium. The best way round this
is to not use the calcium at the top of the container
but use the calcium below it because it is less exposed
to air.
11
Chapter 1: Masses, moles and atoms
SAMPLE
Original material © Cambridge University Press
Part 2: Determination by titration
Answers to the workbook questions (using
2nd set of sample results for learner 1 in
Table 1.5)
Weight of calcium = 0.048 g
d
The n
umber of moles of hydrochloric acid reacting
with the calcium hydroxide:
n
calcium
= ½ n
HCl
= ½ x 11.40 x 10
–3
x 0.200 = 1.14 x 10
–3
i The number of moles of calcium hydroxide and
therefore the number of moles of calcium:
n
calcium
= ½ n
HCl
= ½ x 11.40 x 10
–3
x 0.200 = 1.14 x 10
–3
ii The relative atomic mass of calcium:
A
r
(Ca) =
0.048
1.14 x 10
– 3
= 42.1 g mol
–1
e The percentage errors:
i Wei
ghing out of the calcium: 2 x (
0.0005
0.048
) x 100
%
= 2.08%.
ii The t
itrations: The burettes read to ± 0.05
cm³
a
nd therefore in a titration where two readings are
made, the error = 2 × 0.05 = ± 0.10
cm³
. This means
that in the set of readings used, the error
=
0.10
11.40
× 100% = 0.88%
iii Total systematic error due to apparatus
= 2.08 + 0.88 = 2.96%
iv If the value for the relative atomic mass is greater
than it should be then
m
n
is greater than it should
be and we have overestimated the number of
moles of calcium. This may be due to oxidation of
the calcium so that n is smaller leading to a greater
than expected value for A
r
.
f Imp
rovements to Method 2: If the concentration
of the hydrochloric acid is reduced then more of
the percentage error in the results due to the
titrations.
12
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Chapter 2:
Structure and bonding
Chapter outline
This chapter refers to Chapters 4: Chemical bonding and Chapter 5: States of matter in the coursebook.
In this chapter learners will complete practical investigations on:
■ 2.1Physicalpropertiesofthreedierenttypesofchemicalstructure
■ 2.2Eectoftemperatureonthevolumeofaxedmassofgas
■ 2.3Eectofpressureonthevolumeofaxedmassofgas
Preparing for the investigations
• Learners should be familiar with types of structure and
• Learners should be familiar with the behaviour of
conductivity and melting point.
• The theory underlying the Gas Laws is fairly
straightforward but the practical work will underpin what
is learned in the theory lessons.
• If there are enough data loggers for a class practical
then learners should be made familiar with how to use
is demonstrated then linking the data logger to a
whiteboard would help learners follow the practical.
Practical investigation 2.1:
Physical properties of three
di erent types of chemical
structure
Introduction
In this investigation, learners will carry out tests on three
substances and make sense of their observations by using
their knowledge of structures.
Skills focus
The following skill areas are developed and practised (see
the skill grids at the front of this guide for codes):
MMO Collection of data and observations (a, b, c, d and e)
Decisions relating to measurements of
observations (c) and (d)
PDO Recording data and observations (a), (c) and (e)
ACE Data interpretation and sources of error (b)
Drawing conclusions (c) and (d)
Duration
•
and the rest of the hour for discussion of results and a
plenary.
Preparing for the investigation
•
needs to be washed thoroughly with distilled water and
dried in an oven before use.
Equipment
Each learner or group will need:
• Bunsen burner, tripod, gauze and heatproof mat
•
•
• two graphite rods in a holder
• three spatulas
• three leads and two crocodile clips
• 12 V bulb
• power pack
•
• small evaporating basin
• tongs
13
SAMPLE
Original material © Cambridge University Press
Access to:
• cyclohexane
•
wax
•
white sand
•
pota
ssium iodide
Safety considerations
•
which can then be disposed of safely, or it can be distilled
•
•
Lea
rners should be told to obey safety instructions
especially when heating some of the solids very strongly.
Carrying out the investigation
• The methods used are very simple but they need to be
aware that apart from the investigation of electrical
conductivity, they need to use small amounts of the
three solids.
Ask learners to research the electrolysis of molten
potassium iodide on the internet.
Common learner misconceptions
• Learners need to be aware of what constitutes a soluble
solid. As stated above they need to use small amounts
of solid.
Sample results
Please refer to Table 2.1
Substance Type of structure Summary of observations
Wax Simple molecular Melts easily therefore low melting point. Does not conduct electricity as a solid or in
Silicon dioxide Giant covalent Does not melt therefore very high melting point. Does not conduct electricity as a
solid or in water. Does not dissolve in water or cyclohexane.
Potassium iodide Giant ionic Melts if heated very strongly; high melting point. Does not conduct in solid state but
does so in water. Soluble in water, insoluble in cyclohexane.
Table 2.1
Answers to the workbook questions (using
the sample results)
a Explain your observations for each of the three
substances
i Wax
Wax h
as a simple molecular structure. It does not
conduct electricity in a solid or in solution because
cyclohexane but not in polar water.
ii Potas
sium iodide
pot
assium iodide has a giant ionic structure. It does
not conduct electricity as a solid because the ions
cannot move and carry the current, but they can
do when they are dissolved in water and therefore
potassium iodide solution is a conductor. High
melting point because of the strong electrostatic
attraction between the oppositely charged ions.
cyclohexane.
iii Silicon dioxide
Silicon dioxide has a giant covalent structure. It
has a high melting point because all the strong
covalent bonds have to be broken when it melts.
Because all the bonds in the giant structure are
covalent, it will not dissolve in polar water or in
nonpolar cyclohexane.
14
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Practical investigation 2.2:
Eect of temperature on the
volume of a xed mass of gas
Introduction
In this investigation, learners will investigate how changing
is air which is a mixture of gases but it is assumed that it
will obey the ideal gas laws under these conditions. This is
extrapolation back to zero volume will give a temperature
Skills focus
The following skill areas are developed and practised (see
the skill grids at the front of this guide for codes):
MMO
Col
lection of data and observations: (a, b, c, d and e)
Decisions relating to measurements of
observations: (a, b, c and d)
PDO
Rec
ording data and observations: (a), (c), (d) and (e)
Display of calculations and reasoning: (a) and (b)
Dat
a layout: (b), (c), (d), (e) and (f)
ACE
Dat
a interpretation and sources of error: (a), (c) and (e)
Drawing conclusions: (c) and (d)
Duration
Practical investigation 2.3 because this will take a very short
familiar with using the data logger.
Preparing for the investigation
•
tubing attaching it to the gas syringe. If the round bottom
method used is described in the practical method. The
permanent marker pen can be used to show how far the
•
way of doing it is to measure the internal diameter of the
tubing and its length and using the following formula:
2
x length of tube) where r is the internal
radius of the tube.
• If the same length of tubing is used for each group then
this value can be given to the whole class.
Equipment
Each learner or group will need:
• Bunsen burner, tripod, gauze and heatproof mat
•
100 cm
3
•
rubber tubing
•
100 cm
3
measuring cylinder
•
perm
anent marker pen
• dropper
•
100 cm
3
gas syringe or apparatus to measure gas volume
by displacement of water (see Skills chapter)
• metal container for heating water
•
•
either a stirring rod or a small ‘paddle’ for stirring water in
metal container
• water supply
Safety considerations
• Towards the end of the experiment there will be hot
water which needs to be stirred on a tripod and gauze.
Therefore, learners need to be very careful when they stir.
Carrying out the investigation
• It will take a little while for learners to understand how
rise in the water and inevitably some of them will go
past their desired temperature. Firstly, make sure they
realise that it is not absolutely essential for them to get
go way past a desired temperature, then as described in
the method they can add a small amount of cold water to
get back near to the desired temperature.
•
is to draw small circles round their points and sometimes
this helps with ascertaining the best line to draw. The other
way is to export their results into an Excel document and
then draw a scatter chart to give them the desired line.
• The instructions in the method do help learners draw
their line by stating the scales that need to be adhered to
when drawing their graph.
Ask learners to write their own law using their
results.
Chapter 2: Structure and bonding
15
SAMPLE
Original material © Cambridge University Press
Common learner misconceptions
•
why their line does not go through the origin.
Sample results
Please refer to Table 2.2
Temp/°C 18 32 39 45 54 60 65 72 80 85
Reading on syringe/ml 0 4 7 9 12 14 16 18 21 22
Total volume of gas/cm
3
100 104 107 109 112 114 116 118 121 122
Table 2.2
Answers to the workbook questions (using
the sample results)
a Figure 2.1 shows the sample results plotted.
i
ii
Volume of gas /
cm
3
−300 −200 −100
−20
20
40
60
Temperature / °C
0
100
200
80
100
120
Figure 2.1
b The extrapolated value for the temperature when the
gives an experimental error of 6.22%.
c
The m
ain sources of error in the experiment are
the stirring and the synchronisation between the
temperature and the volume measurement.
d The name given to the temperature when the volume
is zero is absolute zero.
e A reasonable law is that the volume of gas is directly
proportional to the temperature of the gas if a scale
16
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Practical investigation 2.3:
Eect of pressure on the volume
of a xed mass of gas
Introduction
In this investigation, learners or the teacher are asked to use
loggers for a class practical and therefore it can be viewed
of data in order to establish a relationship between pressure
Skills focus
The following skill areas are developed and practised (see
the skill grids at the front of this guide for codes):
MMO
Col
lection of data and observations: (b) and (c)
PDO
Rec
ording data and observations: (a), (c) and (e)
Display of calculations and reasoning: (a) and (b)
Dat
a layout: (b), (c), (d) (e) and (f)
ACE
Dat
a interpretation and sources of error: (a)
Drawing conclusions: (a) and (c)
Duration
•
partly allocated to this investigation because this will
learners are familiar with using the data logger.
Equipment
Each learner or group will need:
• a laptop or other device that will interface with a data
•
•
a 60 cm
3
plastic syringe attached to a small length of
data logger.
Safety considerations
• The only thing that can possibly happen is that the tube
likely to cause any great problems in terms of safety.
Carrying out the investigation
•
realise that the unit of volume is m
3
and not cm
3
. The
units on the data logger are probably given in kPa and
they should also realise that this needs converting to Pa.
The more able learners will realise that P α
1
v
and that PV = constant. Therefore, once the results are
obtained they can be asked to plot the results in any way
they wish. The correct way is to plot
1
v
(horizontal axis)
against P (vertical axis). The more able learners could
also be asked to use an Excel spreadsheet to process
the results.
Sample results
A sample set of results is shown in Table 2.3:
Volume of
gas/cm
3
1
volume
Pressure of
gas /kPa
60 0.0167 102.1
55 0.0182 108.6
50 0.0200 117.4
45 0.0222 127.2
40 0.0250 141.9
35 0.0286 155.2
30 0.0333 176.2
27 0.0370 190
Table 2.3
17
Chapter 2: Structure and bonding
SAMPLE
Original material © Cambridge University Press
Answers to the workbook questions
(using the sample results)
a P α
1
v
b
Ref
er to data in Table 2.3.
c Please refer to Figure 2.2, obtained from one set of
results.
Pressure / kPa
0.00 0.01 0.02
0
100
50
150
200
1/V / cm
3
0.03
0.04
Figure 2.2
Note the R
2
value of 0.9983 is very close to 1 and therefore
there is a great deal of certainty that it is a straight line.
d
The re
sults give a straight line and therefore the slope
of the line is constant.
e
The s
lope of the line =
P
1/v
= PV = constant
f
P = 102.1 kPa = 1.021 × 10
5
Pa
V = 60 cm³ = 6
0 × 10
–6
m
3
; n =
60
24
000
= 0.
0025
mol
;
g
R =
PV
nT
=
1.021 × 10
5
× 60 × 10
–6
0.0025 × 293
= 8.36
Jmo
l
–1
K
–1
h The value of R given in the literature is 8.31 Jmol
–1
K
–1
The percentage error =
8.36 – 8.31
8.31
× 100% = 0.60%
i
units of R we have:
R =
PV
nT
; 1 Pa= 1 N m
–2
(1 Newton per m
2
);
Uni
ts of R = N
m
–2
x m
3
/mol x K= N m mol
–1
K
–1
1
N m =
1
J
Therefore, units of R = J mol
–1
K
–1
18
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Chapter 3: Enthalpy changes
Chapter 3:
Enthalpy changes
Chapter outline
This chapter relates to Chapter 6: Enthalpy changes in the coursebook
In this chapter, learners will complete practical investigations on:
■ 3.1 Enthalpy change for the reaction between zinc and aqueous copper(II) sulfate solution
■ 3.2 Enthalpy change of combustion of alcohols
■ 3.3 Enthalpy change of a thermal decomposition
■ 3.4 Change in enthalpy of hydration of copper (II) sulfate
Preparing for the investigations
• Learners should be familiar with terms such as enthalpy
enthalpy changes are always expressed in terms of kJ mol
–1.
• Learners can refer to the Skills chapter for the theory
Practical Investigation 3.1.
•
and realise that their initial heat calculations are expressed
in J.
• Hess’ Law investigations are straightforward to carry
theory behind what they are doing.
Practical investigation 3.1:
Enthalpy change for the reaction
between zinc and aqueous
copper(II) sulfate
Introduction
time graphs and use these to determine the temperature
changes in the reaction. The two investigations are
reactants for each determination.
The reaction taking place is:
Zn(s) + CuSO
4
ZnSO
4
Or more accurately Zn(s) + Cu
2+
Zn
2+
Skills focus
The following skill areas are developed and practised (refer
the skills grids at the front of this guide for codes):
MMO Collection of data and observations (a), (b), (c) (d)
and (e)
PDO Recording data and observations (a) and (e)
Display of calculations and reasoning (a) and (b)
Data layout (a) (b), (c), (d), (e) and (f)
ACE Data interpretation and sources of error (a) and (c)
Drawing conclusions (c)
Duration
• The practical work will take approximately half an hour to
complete.
• The necessary introductions and plenaries will also
take half an hour in total so one hour will probably be
Preparing for the investigation
•
data for a temperature–time graph and they may not get
it right straightaway.
Equipment
Each learner or group will need:
• two small polystyrene beakers
• glass beaker large enough to hold the polystyrene beakers
•
• 25 cm
3
measuring cylinder
19
SAMPLE
Original material © Cambridge University Press
• plastic covers for polystyrene beakers
•
a sma
ll spatula
• two weighing boats
•
1 mo
l
dm
–3
copper(II) sulfate solution
•
zinc
powder
•
places
Safety considerations
• The copper(II) sulfate is both an irritant and harmful and
Bunsen burners.
•
Be ca
reful when disposing of any copper(II) sulfate
because it is regarded as an environmental hazard.
Carrying out the investigation
•
polystyrene beaker does need to be stable and intact
when the mixture is swirled.
•
The s
econd part of the investigation has the copper(II)
sulfate as the limiting reactant and this necessitates the
copper(II) sulfate being pure and the concentration of the
solution being accurate.
Initially, some learners do not understand the
reasons for doing the experiment using a temperature–
however, and the results are explained they can usually
understand the rationale.
Some learners may need help with the calculation
but it is probably more important that they complete
both practical assignments before they start the
calculations.
working out the actual enthalpy change for the
reaction.
Cu
2+
Cu(s) + Zn
2+
H
reaction
= –219 kJ mol
–1
The more able learners can calculate their values
for the enthalpy change of reaction and work out their
percentage accuracy using the theoretical results above
and their experimental results.
Common learner misconceptions
• Learners must be able to convert heat/enthalpy changes
from J to kJ. Occasionally they fail to do so.
Sample results
Part 1
Table 3.1 gives an idea of the results learners should end the
investigation with.
Copper(II) sulfate was in excess. The mass of the zinc was:
0.66
g =
0.66
65.4
= 0.010
mol
T
he number of moles of copper(II) sulfate = C x V = 1.00 x
0.025 = 0.025 mol.
Therefore, the copper(II) sulfate is in excess and the limiting
reactant is the zinc. This means that the number of moles
reacting = 0.010 mol
Time /min Temp. /°C
0 19
1 19
2 19
3 X
4 34.5
5 36
6 36
7 35
8 34.5
9 34
10 33.5
Table 3.1
20
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Sample results
Part 2
Table 3.2 shows the results when zinc is in excess (6.50 g = 0.10 mol). This means that the number of moles reacting = 0.025 mol.
Time /min 0 1 2 3 4 5 6 7 8 9
Temp. /°C 19 19 19 X 64 62.5 60.5 59 57 55
Table 3.2
Answers to the workbook questions (using
the sample results)
Part 1
a See Figure 3.1
Key
Zinc in excess
Copper(II) sulfate
in excess
Temperature / °C
0 2 4
0
40
20
60
Time / min
6 8
10
30
10
50
70
Figure 3.1
When the graph is plotted the initial temperature is
extrapolation is 37.6. The thermometer only reads to
b
Enthalpy change: q = m × cT = 25 × 4.18 × 19 =
1985.5
J
c
The number of moles of CuSO
4
present: n =C × V
= 0.100 × 0.025 = 0.0250
mol
d
The number of moles of zinc present: n =
m
A
r
=
0.66
65.4
=
0.0101
e
of moles of copper(II) sulfate. In this experiment there
are fewer moles of zinc than copper(II) sulfate so the
zinc is the limiting reactant.
f
The s
tandard enthalpy change in kJ
mol
–1
The
refore, 1
mol o
f reactants produce
1985.5
0.0101
H =
–1
. This is the standard enthalpy
change of reaction. The thermometer only reads to
–1
.
Answers to the workbook questions
(using the sample results)
Part 2
a The number of moles of CuSO
4
present: n = C x V =
0.100 × 0.025 = 0.0250 mol
b The number of moles of zinc present: n =
m
A
r
=
6.6
65.4
=
0.101
c See graph drawn in answer to Part 1.
d
Whe
n the graph is drawn for these results, the initial
e
47 = 4911.5
J
f
The n
umber of moles of CuSO
4
present: n = C × V = 1.00
× 0.0250
mol = 0
.0250
mol
g
The number of moles of zinc present
n =
m
A
r
=
6.50
65
= 0.0994
h
In th
is experiment there is a greater number of moles
of zinc and therefore the limiting reactant is the
copper(II) sulfate solution.
i The standard enthalpy change in kJ mol
–1
∴ The standard enthalpy change of reaction
∆H
Θ
r
=
4911.5
0.025
–1
(3 s.f.)
Chapter 3: Enthalpy changes
21
SAMPLE
Original material © Cambridge University Press
Practical investigation 3.2:
Enthalpy change of combustion
of alcohols
Introduction
This practical is as much about what is wrong with it as
what is good about it. Ideally, a bomb calorimeter would be
practical, however, spirit burners or micro burners are used.
For all four alcohols the temperature of water is raised by the
same amount each time. This means that the heat /enthalpy
change will be the same each time because the apparatus being
free by placing them around the spirit burner and calorimeter.
should be changed but the calculation remains the same.
Skills focus
The following skill areas are developed and practised (refer
the skills grids at the front of this guide for codes):
MMO
Col
lection of data and observations (a), (b), (c). (d)
and (e)
Dec
isions relating to measurements of
observations (c)
PDO Recording data and observations (a)
Disp
lay of calculations and reasoning (a) and (b)
Data layout (b), (c), (d), (e) and (f)
ACE Data interpretation and sources of error (g) (h),
(i) and (j)
Drawing conclusions(c)
Duration
• The practical can be completed easily within one hour.
•
can be shared amongst the groups of learners. Groups of
two are ideal.
Preparing for the investigation
•
combustion
Equipment
Each learner or group will need:
• spirit burners containing the four alcohols
•
cop
per wire stirrer
• clamp stand, boss and clamp
•
• thermometer
•
100 cm
3
measuring cylinder
•
lid
/cover for spirit burner
• wooden splint
Access to:
•
– two balances strategically placed would be ideal
•
a suppl
y of water
• a Bunsen burner (for lighting splints)
j The reaction is the same both in both experiments
and the values are expressed in terms of 1
mol .
k
The accepted value for H
Θ
r
–1
.
–1
the percentage error
for our results =
219 – 200
219
× 100% = 8.7%
l The highest degree of error will be for Part 1 because
the lower temperature rise is recorded and the
smallest mass of zinc is weighed.
The b
alance weighs to 0.010
g and t
herefore its
maximum error is ± 0.005 g.
The
refore, the maximum percentage error from
weighing = 2 ×
0.005
0.66
× 100% = 1.52%
Therefore, the maximum percentage error
=
1
19
× 100% = 5.3%
The v
olume can be read to 0.5
cm
3
Therefore percentage error from volume
measurement =
0.5
25
× 100% = 2%
The t
otal percentage error from apparatus
measurement = 1.52 + 5.30 + 2.00% = 8.82%
m
loss through the apparatus.
22
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Safety considerations
• They must wear eye protection at all times.
•
• All the alcohols should be treated as harmful.
•
Whe
n weighing the alcohols learners must transport
Carrying out the investigation
•
the calorimeter must be taken into consideration.
• Learners need to realise that as the spirit burners will
almost certainly be shared amongst the group; they
could well be holding up other groups if they do not
complete their determination in a reasonable time.
•
height for each burner and adjusting the calorimeter
position so that its bottom is the same distance from the
• The temperature rise advocated in the workbook is
determination.
by now in which they have been shown how to calculate
should at least be able to work out the percentage error
of their results compared with the values available in
due to their apparatus, there should still be a certain
percentage error which is not accounted for.
•
The m
ost obvious source of error is heat loss through
the apparatus. Also, the combustions of the alcohols,
especially those with a higher relative molecular mass,
are not complete and the learner should be reminded of
• This incomplete combustion can be demonstrated by
inspection of the underside of the calorimeter, where a
carbon deposit is a good sign of incomplete combustion.
One way to extend the learner is to encourage
them to use spreadsheets in order to automate their
calculations and save themselves time. One could say
that this is taking them away from carrying out the
calculations. However, in constructing their spreadsheet
and getting it working they are in fact doing the
calculations and putting in formulae that they need to
understand.
• To extend the more able learners, they can either take
their values for the standard enthalpies of combustion
or the values available in the literature and plot them
against the relative molecular mass. From their graph
2
–
is burned
and from this calculate the bond enthalpy of the C–H
bond.
Common learner misconceptions
• Learners sometimes fail to remember that they are
heating up the calorimeter as well as the water. Because
of this they must remember that the calorimeter has a
heat capacity of copper is 0.385 J g
–1
K
–1
and that of glass
is 0.84
J g
–1
K
–1
.
Answers to the workbook questions (using the sample results)
a The standard enthalpy changes of combustion for all four alcohols
Mas
s of copper calorimeter = 198.00
g
= 0.3
85 × 20 × 198.00 + 4.18 × 20 x 100
= 1524.6 + 8360 = 9884.6 J
This e
nthalpy change is the same for all four alcohols. In the results given below spirit burners were not available and
microburners were used. The time taken for each determination was approximately 1–2 mins
23
Chapter 3: Enthalpy changes
SAMPLE
Original material © Cambridge University Press
Table 3.3 shows the results for all four alcohols taken from a spreadsheet.
Alcohol
Mass of
burner +
alcohol
before
burning
Mass of
burner +
alcohol
aer
burning
Mass
burned
RMM of
alcohol
no. of
moles
burned
Enthalpy
change/J
Standard
enthalpy
change in
kJ
mo
l
-1
methanol 5.41 4.79 0.62 32 0.019375 9884.6
ethanol 6.05 5.57 0.48 46 0.010435 9884.6
6.20 5.79 0.41 60 0.006833 9884.6
6.27 5.87 0.40 74 0.005405 9884.6
Table 3.3
b The percentage errors for each alcohol are shown in Table 3.4.
Alcohol
Standard enthalpy
change in kJ mol
-1
Literature values for
standard enthalpy of
combustion in kJ mol
-1
percentage error
methanol 29.7
ethanol 30.7
28.4
31.7
Table 3.4
c The maximum percentage error from apparatus shown in Table 3.5
Apparatus/reading
Reading
error
Reading
taken
Percentage
error Comments
0.01 g error = 0.005
0.01 0.62 1.61
There are two mass readings with
an error of ± 0.005
g eac
h time
Measuring cylinder reading
to ± 2
cm
3
1.00 100 1.00
The measuring cylinder measures to
2 cm
3
therefore uncertainty is ±1 cm
3
Thermometer reading to ± 0.5 1.00 20.00 5.00
Two thermometer readings taken
– both giving maximum error of
To
tal % 7.61
Table 3.5
d Measurement of uncertainty for the mass of alcohol burned for each alcohol.
Methanol Percentage error =
0.01
0.62
× 100% = 1.61%
Ethanol Percentage error =
0.01
0.48
× 100% = 2.08%
0.01
0.41
× 100% = 2.44%
0.01
0.40
× 100% = 2.50%
24
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Practical investigation 3.3:
Enthalpy change of thermal
decomposition
Introduction
The enthalpy change for some reactions is impossible to
measure. Because thermal decomposition is an endothermic
this investigation we look at the thermal decomposition of
potassium hydrogen carbonate.
2KHCO
3
(s) K
2
CO
3
(s) + CO
2
(g) + H
2
O(l)
Skills focus
The following skill areas are developed and practised (see
the skill grids at the front of this guide for codes):
MMO
Col
lection of data and observations (a), (b), (c), (d)
and (e)
Dec
isions relating to measurements of
observations (d)
PDO Recording data and observations (a) and (c)
Disp
lay of calculations and reasoning (a) and (b)
ACE
Dat
a interpretation and sources of error (a), (c)
(d) and (e)
Dra
wing conclusions (c) and (d)
Duration
• The practical can be completed easily within half an
hour.
• Learners can work individually.
•
If a le
sson lasts for one hour then the majority of the
time can be spent explaining the theory behind the
method used and, in the plenary, how they are going to
calculate the changes.
Preparing for the investigation
• Learners need to know the theory behind Hess’ Law and
how it can be used to determine enthalpy changes that
otherwise would be impossible to determine.
•
Lea
rners need to revise reactions between acids and
carbonates or hydrogen carbonates.
Equipment
Each learner or group will need:
• polystyrene beaker and lid with hole for thermometer
•
gla
ss beaker to hold the polystyrene beaker
•
divisions is preferable
• spatula
•
weig
hing boats
• 50 cm³ measuring cylinder
•
cot
ton wool to act as extra insulation
Access to:
•
– two balances strategically placed would be ideal.
• a supply of water
•
2.0
0
mo
l
dm
–3
hydrochloric acid
•
pot
assium hydrogen carbonate and potassium
carbonate
e Maximum percentage error for one alcohol
Example = methanol
Total error due to measuring apparatus = 7.61%
f
(through sides of calorimeter) and convection (hot waste gases not heating up calorimeter).
25
Chapter 3: Enthalpy changes
SAMPLE
Original material © Cambridge University Press
Safety considerations
• Learners must wear eye protection at all times
•
The a
cid is an irritant
•
therefore care must be taken in replacing the lid as soon
as the solids are added to the acid to minimise exposure
to acid spray.
Carrying out the investigation
• H
1
in their calculations. They should also realise that the
sign given to the enthalpy change (plus or minus) is vitally
A number of learners struggle with Hess’ law and
therefore practice in these calculations will obviously
(2HCl +) 2KHCO
3
K
2
CO
3
+ H
2
O + CO
2
(+ 2HCl)
2KCl + 2H
2
O + 2CO
2
∆H
2
2 × ∆H
1
∆H
reaction
Figure 3.2
Encourage learners to use spreadsheets in order to
automate their calculations and save themselves time.
If you are worried that this will take them away from
carrying out the calculations themselves, remember that
in constructing their spreadsheet and getting it to work
they are in fact doing the calculations and putting in
formulae that they need to understand.
Common learner misconceptions
• The Hess cycle used for this practical investigation is
shown in Figure 3.2.
Answers to the workbook questions (using the sample results)
a–f The results shown in Table 3.6 are from a typical laboratory investigation of this topic.
Mas
s of KHCO
3
= 2.55 g Mass of K
2
CO
3
= 3.46 g
Init
ial
temp.
°C
Final
temp.
°C
Change in
temp. °C
Enthalpy
change (q)/J
Mass of
KHCO
3
/g
Relative
formula
mass/
g
mo
l
–1
Number
of moles
Standard
enthalpy
change
Reaction 1 18 14.4 –3.6 +752.4 2.55 100.1 0.025475 +29.5 kJ mol
–1
Reaction 2 17.9 22.2 +4.3 –898.7 3.46 138.2 0.025036 –35.9 kJ mol
–1
Table 3.6
g The standard enthalpy change for the reaction:
H
r
H
2
H
1
;
H
r
H
1
H
2
–1
The accepted values using the standard enthalpies of formation = +93.4 kJ mol
–1
h The percentage error = [
94.9 –93.4
93.4
i
Max
imum percentage error
Reaction 1
The percentage error = (
0.2
3.6
) × 100% = 5.56%
26
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
For the weighing, there are two readings being made and for each one the uncertainty is ±.005 g.
Therefore the percentage error = 2 ×
0.005
2.55
× 100% = 0.39%
For measurement of acid using measuring cylinder the measuring cylinder measures to 1 cm
3
and therefore
uncertainty = ± 0.5
cm
3
Percentage error = (
0.5
50
) x 100% = 1%
Therefore, the total percentage error for reaction 1 = 5.56 + 0.39 + 1 = 6.95%
Reaction 2
Percentage error from temperature measurement = (
0.2
4.3
) × 100% = 4.65%
Percentage error from weighing = 2 ×
0.005
3.46
× 100% = 0.29%
Percentage error due to measurement of acid = 1%
Therefore, total percentage error = 4.65 + 0.29 + 1.0 = 5.94%
The total possible error due to measuring apparatus = 5.94 + 6.95 = 12.89%
This means that the actual error for the experiment (1.6%) is well within the error due to the measuring apparatus.
Practical investigation 3.4:
Change in enthalpy of hydration
of copper (II) sulfate
Introduction
This practical will complete this series of experiments
already encountered and enables them to make calculations
based on Hess’s law.
The reaction studied is the hydration of copper(II) sulfate:
CuSO
4
(s) + 5H
2
O(l) CuSO
4
.5H
2
O
The Hess cycle used is shown in Figure 3.3.
CuSO
4
(s) + 5H
2
O + aq CuSO
4
.5H
2
O(s) + aq
CuSO
4
(aq)
∆H
1
∆H
2
∆H
reaction
route 1
route 2
Figure 3.3
Skills focus
The following skill areas are developed and practised (refer
the skills grids at the front of this guide for codes):
MMO
Col
lection of data and observations (a), (b), (c),
and(d)
Dec
isions relating to measurements of
observations (b)
PDO Recording data and observations (a), (c), (d)and (e)
Disp
lay of calculations and reasoning (a) and (b)
Data layout (a), (b), (c), (d), (e) and (f)
ACE Data interpretation and sources of error (a), (c), (d)
and (e)
Drawing conclusions (c) and (d)
Duration
This practical takes one hour to compete.
Preparing for the investigation
• Preparation of anhydrous copper(II) sulfate. To save
time and reduce inaccuracies, it is best to take some
copper sulfate crystals and heat them in an oven which
check the accuracy of the oven’s temperature.
• Each learner or group of learners will need something
in excess of 4
g to wei
gh out for their 0.025
mol o
f the
anhydrous salt. Therefore, this must be taken into
account when deciding how much needs to be roasted in
the oven. The solid will need to be stirred at various times
to make sure that the loss of water is uniform throughout
the solid.
27
Chapter 3: Enthalpy changes
SAMPLE
Original material © Cambridge University Press
Equipment
Each learner or group will need access to:
• two polystyrene beakers plus lids
•
di
visions
•
spa
tula
• wash bottle containing distilled water
•
gla
ss beaker large enough to hold the polystyrene
beakers
•
cot
ton wool to improve the insulation of the polystyrene
beakers
•
a 50 cm³ m
easuring cylinder
• weighing boat x 2
•
places
•
anh
ydrous copper(II) sulfate
• hydrated copper(II) sulfate crystals
•
dis
tilled water
• paper towels
Safety considerations
• Eye protection must be worn at all times during this
experiment.
• The copper(II) sulfate solution is an irritant and
copper(II)
su
lfate is an environmental poison; any
solution formed should be poured into a bottle.
This
co
pper sulfate solution can be used to crystallise
out pure copper sulfate which can be used for other
experiments.
Carrying out the investigation
• The temperature change for the dissolving of the
hydrated copper(II) sulfate crystals is small and therefore
if at all possible a thermometer (or temperature data
• Some learners will still have problems with
understanding the Hess cycle. The cycle, however, is
amount of water present in the hydrated crystals.
Learners have had a reasonable amount of
in this practical. Therefore, it may be a good idea to use
this practical investigation to gauge their progress.
Ask learners to explain how the apparatus could be
and immersion heater can be used to measure the
by the temperature measured in the reaction. This then
well as the water. Ask them to write an explanation of
Common learner misconceptions
•
impossible to determine the enthalpy change for the
hydration of anhydrous copper(II) sulfate.
Sample results
The following results shown in Table 3.7 were obtained for
H
2
Mass of anhydrous copper(II) sulfate = 3.99 g =
3.99
159.6
=
0.025 mol
Time /min Temperature/°C
0 17.5
1 17.6
2 17.8
3 18.1
4 X
5 25.4
6 25.8
7 25.7
8 25.5
9 25.3
10 25.1
Table 3.7
28
Cambridge International AS & A Level Chemistry
SAMPLE
Original material © Cambridge University Press
Answers to the workbook questions (using the sample results)
Part 1: For the determination of ∆H
2
.
a See Figure 3.4
b
c
d ∴
e
H
2
= = (
–1672
0.025
) ÷10
00 =
–1
For ∆H
1
; Mass of copper(II) sulfate crystals = 6.24 g
=
6.24
249.6
= 0.025
mol
Part 2: For the determination of ∆H
1
.
a
b
∴
c
H
2
= (+209/0.025) ÷ 1000 = +8.36 kJ mol
–1
d ∴H
reaction
H
2
H
1
e ∴ percentage error =
78.2 –75.3
78.2
× 100% = 3.71%
f
The e
rrors due to the apparatus are shown in Table 3.8.
Apparatus/reading Reading to Reading taken Percentage error Comments
to 0.01 g error = 0.005
0.01 3.99 0.25 There are two mass readings
with an error of ± 0.005 g each
time
0.01 6.24 0.16
Measuring cylinder
reading to ± 1 cm
3
1.0 50.0 1.00 The uncertainty is ± 0.5 cm
3
1.0 50.0 1.00
Thermometer reading
to ± 0.2
0.2 8.2 2.44 Two thermometer readings
taken for each experiment –
both giving maximum error of
0.
2 1.0 20.00
Total % 24.85
Table 3.8
The potential errors due to the measuring apparatus can explain the percentage error in the experiment.
Temperature /
°C
0 2 4
16
20
18
22
24
Time / min
6 8
10
26
Figure 3.4
29
Chapter 3: Enthalpy changes
SAMPLE
Original material © Cambridge University Press
