1 April 2007 sundayherald 17
Photograph: Rex
Chemistry
ABOUT THE
AUTHOR
JOSEPH Farrell has
taught Chemistry for
33 years, with 22
years of that as a
principal teacher. He
currently teaches at St
Columba’s High School,
Gourock.
He has been a
Higher marker since
1990 and in 2000
joined the team setting
the Higher paper for
2002 and 2003. Since
2000 he has been a
member of the Higher
Chemistry examining
team.
C
HEMISTRY appears to be difficult because
of the language used both in the
framework of the questions and in the
coursework involving formulae and
equations. Because of this, time spent thinking
about the questions in the paper and working out
exactly what is being asked will be worthwhile.
Many questions are best answered by considering
the facts you have learned and applying
chemical principles. In order to be able to do
this, however, you have to have these principles
and facts firmly established in your mind and the
aim of this guide is to help you study and learn
them.
In general, as with the study of other subjects,
chemistry should be studied in short periods of
time – 30 to 40 minutes is probably the length of
your concentration span – with breaks of about 10
minutes in between. Study by concentrating upon
the information you are trying to learn and avoid
distractions, such as TV or talking with friends,
while you are studying.
Study with an enquiring mind – ask yourself
questions such as: if atoms are made of positive
and negative particles then how can they be
neutral? Look for connections between facts and
try to build up mind maps.
No matter which chemistry course you are
studying, a useful technique is to use lists of
learning outcomes for each part of the course and,
using highlighters, colour each learning outcome
as green (fully understood), orange (not too sure)
or red (not understood at all).
Another useful tip for studying is always to have
at hand a pen or pencil for jotting down important
facts as you read your notes or textbook. This way
you can build up a set of essential facts which can
be revised in a relatively short time.
In all chemistry exams, different parts of the
course can be asked about within one question.
This makes trying to predict questions and concentrating most of your efforts on a few topics
unlikely to be of any value. Familiarity with the
whole course is needed.
Again mind maps, in which many facts and
concepts can be linked pictorially, are a good way
of remembering how different aspects relate to
each other. An example is given elsewhere in these
pages.
When you start revising it is best to keep a
record of your studies, such as the plan shown for
Standard Grade elsewhere on these pages.
Finally, since 10% of the Higher paper is
devoted to Standard Grade revision questions,
Higher candidates should read and think about
all parts of this study guide.
STANDARD GRADE
You should learn the facts about the structure of
atoms (protons, electrons and neutrons etc) and
bonding (ionic and covalent) right at the start,
since all chemistry is based on these.
When you start studying the topics it is best to
keep a record of your studies such as the study
plan for Standard Grade shown overleaf.
Turn to page 18
18 sundayherald 1 April 2007
mass (g) so 49g of H SO = 49(g) = 0.5 moles
2
4
gfm
98(g)
From page 17
STANDARD GRADE STUDY PLAN
Group
Reaction rates
Periodic Table
Bonding (covalent, ionic)
Hydrocarbons (including synthetic polymers)
Acids & alkalis
Metals & corrosion
Cells
Fertilisers
Cardohydrates
Times studied
1 2 3 4 5 6
e) Calculating the mass of a number of moles:
7 8 9
Calculations
Many calculations are included under Knowledge
and Understanding because the method for
carrying out the calculation should be learned
and practised.
Chemists use the concept of the “mole” in
almost all calculations. You should therefore
become familiar with the following procedures:
a) Writing formulae (using valencies)
b) Using Relative Formula Masses from the data
book to calculate formula masses. Formula mass
is the sum of the RAMs of all of the atoms in the
formula eg the formula mass for H2SO4 is:
(1 x 2) + 32 + (16 x 4) = 98 amu
c) Finding one mole (the gram formula mass).
This is the formula mass in grams, eg:
number of moles x g f m
so 0.1 moles of H2SO4 = 0.1 x 98g = 9.8g
f) Calculating percentage composition
% composition =
mass of element in compound(g)
x 100
gram formula mass (g)
so the percentage by mass of S in H2SO4 =
32(g) x 100 = 32.7%
98(g)
Sometimes the question will not be straightforward. For example, in the following question the
candidate has to realise that the pie chart shows
that gold forms 37.5% of the total mass of the alloy.
g) Calculating mass of a product or reactant,
knowing the balanced equation, for example:
Calculate the mass of magnesium chloride
produced when 3g of magnesium react with excess
hydrochloric acid according to the equation.
Mg + 2HCl
1 mole
24.3g
Standard Grade, 1997, Credit paper, Question 11
1g
Pure gold is known as 24-carat gold. In
jewellery, it is often mixed with other metals
to make it harder.
One common mixture is 9-carat gold. Its
composition by mass is shown in the pie-chart.
3g
31%
31.5%
gold
silver
copper
37.5%
One mole of H2SO4 is 98g
d) Finding the number of moles in a mass of a
substance:
Answer: Alloy
(b) A 9-carat gold ring weighs 7.88g
Calculate the number of moles of gold in
the ring.
Show your working clearly.
Answer: Mass of gold in ring = 37.5% of 7.88g
= 37.5 x 7.88g = 2.96g
100
Mass of 1 mole of gold = 197g
Therefore, number of moles
of gold in ring
= 2.96g
197g =0.015moles
(a)What name is given to a mixture of metals
such as 9-carat gold?
MgCl2 + H2
1 mole
24.3 + (2 x 35.5)g
= 95.3g
95.3g
24.3g
95.3g x 3
24.3g
= 11.8g
Note: The balanced equation tells us the
relationship between the number of moles of
reactant and the number of moles of product. In
the above example, one mole of magnesium gives
one mole of magnesium chloride. Only these two
substances need be considered. Since the masses
of both compounds are involved in the question,
both numbers of moles are changed into
grammes and then a proportion calculation is
carried out.
h) calculating the number of moles in a solution
= C x V (litres)
1 April 2007 sundayherald 19
MIND MAP FOR HYDROCARBONS
Unleaded
petrol
Kerosene
for jets
feedstock
petrol
naphtha
diesel
Crude oil
Fuel for
ships
Heavy
fuel oil
Making wax
bitumen
Cracking
AI203 catalyst
Redox: O
x
i
d
a
t
i
o
n
Tar for
roads
Roof felt
alkenes
Unsaturated,
double bonds
j) calculating the concentration (C) of a solution:
C = number of moles
Volume (in litres)
= mol/l
k) calculating the empirical formula of a
compound:
Elements
Mass or % mass
Divide by relative atomic mass
Divide all by smallest
Ratio
Empirical Formula
Some questions about the electrochemical series
or reactions in cells will require you to write an
ion-electron equation for reduction or oxidation.
A reliable rule to follow when dealing with ionelectron equations is that for reduction, the
electrons appear on the left side of the equ ation,
but for oxidation they appear on the right side.
New data books
This year’s candidates will be given a new edition of
the data book. This edition has only two changes:
the names and information of new elements 104 to
109 have been added to the Periodic Table on the
last page of the book and on page 6 the melting
and boiling points of calcium oxide and sodium
chloride have been added or amended.
Drawing apparatus
In all chemistry courses pupils should know how
to draw apparatus for dealing with gases.
G
a
i
n
H H
C=C
0 H
Gas in
Gas out
H H
C=C
0 H
addition
Liquid
Note that the tube bringing the gas must dip into
the liquid but the tube removing the gas must not
dip into the liquid.
H
C
Gas collection:
If the gas is insoluble it can be collected over
water. If the volume is to be measured it must be
collected in a vessel with volume markings on the
side. The end of the delivery tube must be under
the collecting vessel.
0
gas
gas in
H
C
H
H H
C=C
0 H
polymerisation
H
C
0
H
C
H
H
C
0
H
C
H
INTERMEDIATE 1
You should split the course up into several small
sections of work and study each until you feel
confident enough to answer questions on it. The
exam lasts for one hour 30 minutes and consists of
20 multiple choice questions worth 20 marks and
extended answer questions worth 40 marks.
INTERMEDIATE 2
water
If the gas is soluble in water it should be collected
in a syringe with volume markings.
gas
syringe with
volume markings
reactants
heat
This course is designed as an intermediate
between Standard Grade and Higher and
therefore consists of mostly Credit level work with
some parts of the Higher course.
The exam lasts for two hours and consists of
two sections: Section A has 30 multiple choice
questions worth 30 marks and Section B has
extended answer questions worth 50 marks.
Although the course is a bridge between
Standard Grade and Higher and therefore
contains some Higher material, the exam does not
have the same emphasis on problem-solving and
so knowledge and understanding of the
coursework is tested more thoroughly.
The questions in the Intermediate 2 exam
relating to Higher learning outcomes are
presented in a straightforward fashion, as can be
seen in the following example:
Intermediate 2, 2005, Question 11
Polymers
Remember that, to draw the structure of part of a
polymer formed by addition polymerisation
between three monomers, you have to draw the
monomers in the form of an H, with the double
bond represented by the horizontal line and
nothing else on this line.
In the following example, three molecules of
phenylethene (old name: styrene) are drawn so
that it can be easily seen how they can form bonds
between the carbons by addition across the
double bonds to form a part of the polymer
polyphenylethene (old name: polystyrene)
Fats are broken down in the body by hydrolysis
fat
H
H C OH
H C OH + fatty acids
H C OH
H
Turn to page 20
Succeed
Graphs
You should become familiar with and practise
drawing line graphs, bar graphs, pie charts and
tables of information as well as obtaining
information from these types of presentation.
I
s
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Problem solving
Often the information needed to solve the
problem can be found in the question. Remember
to read back through the whole question when
you get to the end since very often the information needed to solve a problem in the last part of
the question has been given in the first part.
R
e
d
u
c
t
i
o
n
Test
Sometimes mnemonics can be very helpful in
remembering lists and the order in which certain
chemicals are arranged and mind maps can help
to show how certain items are connected. See the
panel above for examples.
Passing a gas through a liquid:
L
o
s
s
Practise
i) using titration results to carry out a calculation:
use the equation above to find the number of
moles, then use the chemical equation to relate
the numbers of moles of each substance.
Note: In titration calculations the “rough titre” is
not used to calculate the average volume used in
the titration. Only the “concordant” results (within
0.2ml of each other) are used.
I
s
Revise
Hydrocarbons: Monkeys Eat Peanut Butter
E
T R
U
T
H O
T
H
A P
A
A
N A
N
N
E N
E
E
E
20 sundayherald 1 April 2007
From page 19
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PASS
(a) When one mole of fat is hydrolysed, how many
of the moles of fatty acids are produced?
________ moles
(b) Name the molecule with the structure shown.
_____________________
H
H C OH
H C OH
H C OH
H
(c) Lipase is an enzyme which can catalyse the
hydrolysis of fats in milk. Complete the diagram
to show how the indicator colour would change
after lipase was added to the test-tube.
milk,
lipase and
Universal
indicator
milk and
Universal
indicator
Colour: green
Colour:
As well as knowing the information given above on
polymerisation, Higher candidates have to know
some factual material about the properties of some
important polymers and their uses. A table, such as
the one below could prove a helpful means of listing several connected facts in a compact fashion:
POLYMER
NAME
Kevlar
SPECIAL
PROPERTY
strength
Poly(ethenol) soluble
in water
Poly(ethyne) conducts
electricity
Polyvinyl
light
carbazole
sensitivity
to move
Biopol
biodegradable
podcast
bulletproof
vests
laundry bags
for hospitals
and stitching of
of internal
wounds
speakers
photocopiers
internal stitching
but expensive
New data book
Although a new data book will be used there are
no changes affecting the information required by
the Higher pupils. The changes relate only to that
needed by Advanced Higher.
Calculations and moles
Units One and Three of the Higher course contain all of the calculations except for percentage
yield calculations.
For Higher all calculations for Standard Grade
must be known except “empirical formula”-type
calculations. Here, the unit called the mole is considered in more detail. One mole of any substance
contains 6.02 x 1023 “elementary entities” of that
substance. We encounter a lot of different types of
entities, ie particles, in the Higher course, so there
are several ways to apply the term “one mole”.
6.02 X 1023 entities
(molecules, atoms, formula units, ions)
Molar volume (volume
of 1 mole of any
gas if temperature
and pressure are same)
gram formula mass(gfm)
ie: the mass of one mole
in grams
o
ONE MOLE o
o
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iTunes
Part two is now
available from
OH group
o
EXAM GUIDE
delocalised
electrons
light causes
electrons
USES
Examination technique
Some questions extend over one page. Each
question should be read from start to finish so that
an overall picture can be formed in your mind. Then
answer the question carefully, taking into account
the information given in all parts of the question.
o
Get the
sundayherald
CHEMICAL
FEATURE
hydrogen
bonds
has many
OH groups
1 litre of solution of
concentration 1mol/l
(a) The balanced equation for the reaction is:
Higher 2005, Section B, Question 11 (a)
Mg(s) + 2HCl(aq) –> MgCl2(aq) + H2(g)
Respiration provides energy for the body through
“combustion” of glucose. The equation for the
enthalpy of combustion of glucose is:
Show your working clearly.
C6H12O6(s) + 6O2(g) –> 6CO2(g) + 6H2O(l)
sH = –2807kJ
mol–1
(a) Calculate the volume of oxygen, in litres,
required to provide 418kJ of energy. (Take the
molar volume of oxygen to be 24 litres mol -1.)
96,500 coulombs = charge
in 1 mole of electrons
Higher 2003, Section B, Question 7 (a) and (b)
7. A student added 50cm3 of 4.0 mol 1–1 hydrochloric acid to 4.0g of magnesium ribbon.
Show by calculation which reactant was in excess.
(b) The hydrogen produced in the reaction can be
contaminated with small quantities of hydrogen
chloride vapour.
This vapour is very soluble in water.
Complete the diagram to show how the hydrogen
chloride can be removed before the hydrogen is
collected.
Show your working clearly.
In this calculation, we have to relate the volume
of oxygen to the number of kilojoules of
heat produced. However, the equation tells us
that six moles of oxygen produce 2807kJ and so
we have to use the molar volume to change
the number of moles of oxygen to a certain
volume.
sH = CMs9 is only to be used if temperature is
given.
HIGHER
TABLE OF POLYMERS
In general, only if you are asked for an actual
number of particles do you use Avogadro’s Number, 6.02 x 1023.
In many calculations molar quantities are used.
This is seen in questions involving sH value or
molar volumes of gases.
Redox calculations involving moles
Chemical reactions can often be broken down to
an oxidation step and a reduction step. These are
often shown as ion-electron equations.
In an ion-electron equation, if the electrons
appear on the right hand side of the equation then
it represents oxidation. If electrons are on the left
then equation shows reduction.
If you are asked to write an ion-electron
equation, remember that the total number of
charges on each side must be the same.
Moles are once again used in redox calculations, as shown in this example:
Higher, 2005, Section B, Question 15 (b) (iii)
An average of 21.4cm3 of the iodine solution was
required for the complete reaction with the vitamin
C in 50.0cm3 of orange juice.
Use this result to calculate the mass of vitamin C, in
grams, in the 500cm3 carton of orange juice.
hydrochloric
acid
gas syringe to
collect hydrogen
hydrochloric
acid +
magnesium
This answer illustrates important practical points
made earlier about passing gases through liquids
– the tube delivering the gas to the water must dip
into the water so that the gas and water come into
contact over as large an area as possible. The tube
removing the gas must be out of the water so that
the gas can enter it.
The completed diagram should look like this:
hydrochloric
acid
gas syringe to
collect hydrogen
hydrochloric
acid +
magnesium
Water
Show your working clearly.
In this question, an average of the concordant
readings from the burette is used for the volume
of iodine.
Earlier in the question we are told the concentration of iodine solution.
Therefore, we can use the equation n = C x
V(litres) to find the number of moles of iodine
used in the titration. The chemical equation tells
us that this is equal to the number of moles of
vitamin C in 50cm3 of orange juice and by multiplying by 10 we can find the number of moles in
500cm3 . Using the equation:
mass of vitamin C =
(number of moles) x (mass of one mole)
we can calculate the mass of Vitamin C.
Reactants in excess
Moles can also be used to find out which reactant
is in excess.
Questions which have been set since 2000 ask
the candidate to calculate which reactant is in
excess. Usually one reactant is in the form of a
solid while the other is in solution. The number of
moles of each reactant should be found using
these equations:
number of moles = mass (g)
gfm
n = concentration x volume (litres)
and then using the equation for the reaction
(usually given) the numbers of moles can be
compared to see which one is in excess. If the
amount of product is then required this can be
calculated using the number of moles of the reactant not in excess.
PPAS
National Qualifications test your practical work
by including a number of marks related to the
PPAs carried out during the course. In Higher
chemistry there are nine PPAs (three per unit) and
six marks of part B of the exam paper are more
likely to be obtained by those who have done the
PPAs and learned them in detail.
Often the types of questions asked refer to the
experimental procedures, such as indicating
which readings have to be taken or the meaning of
an instruction or even which solution has to be
used.
PPA reports have to be kept by the school in
case of moderation but each candidate should
keep a note about the details of the experiments
including an evaluation of results.
EXTENDED WRITING ANSWER
Since 2002 the paper for Higher has contained a
question requiring extended writing in the answer
for four marks. These require the candidate to link
certain ideas and to explain the connection. A
convenient way of studying for questions of this
type is making mind maps, which show how ideas
can be connected. For an example of a mind map,
see the one for hydrocarbons at Standard Grade
on page 19.
Carbon chemistry – homologous series
Candidates must be familiar with what is meant
by a homologous series – compounds with similar
chemical properties which can be represented by
a general formula are usually the accepted criteria
– and must know the information shown in the
following table:
1 April 2007 sundayherald 21
Revise
NAME
ENDING
FUNCTIONAL
GROUP
Alkenes
–ene
C=C
Alkynes
–yne
C C
Alcohols
–ol
–C–OH
Aldehydes
–al
–C=O
H
Ketones
–one
0
–oic acid
–C=O
OH
Esters
–yl ; –oate
0
–C–O–C–
Amines
Prefix: amino–
–NH2
Much practice is needed in drawing structural
formulae of the organic molecules. It is
worthwhile to practise drawing the structures of
molecules from all homologous series. Remember
that every carbon must have four bonds,
hydrogen one bond and oxygen two bonds the
same as their valencies.
Using information from questions
In many problem-solving questions, the
information needed to solve the problem is
(b) An acid can be thought of as a chemical which
can release H+ ions.
In an acid-base reaction the H+ ions released by
the acid are accepted by the base.
Some acid-base reactions are reversible. In these
reactions both forward and reverse reactions
involve the transfer of H+ ions from the acid to the
base.
(i) Using the information given above, complete
the table showing the acid and base produced
when HS- ions react with H3O+ ions.
Acid
Base
Acid
Base
H20
NH3
NH4+
OH
H30+
HS
_
_
(ii) Another reversible acid-base reaction is shown.
HCO3- + OH-
CO3 2- + H2O
In the reverse reaction, state whether the water is
acting as an acid or a base.
CHEMISTRY EXAM
TIMETABLE
Level/Paper
General
Credit
Time
Thursday May 10
9am-10.30am
10.50am-12.20pm
Tuesday May 29
Intermediate 1
9am-10.30am
Intermediate 2
9am-11am
Higher
9am-11.30am
Advanced Higher
9am-11.30am
Succeed
Carboxylic acids
(a) Write the balanced equation for the reaction
between copper(II) oxide and nitric acid.
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C–C–C
Higher 2005, Section B, Question 8
Acid-base reactions are common in chemistry.
In the table, the H3O+ acts as an acid – it releases
H+ and turns into a base because it can now
accept an H+. The reverse is true for HS-.
In part (b)(ii), the water becomes OH- and so it
must release H+ ie it must act as an acid.
These answers are applications of the information in the question.
Test
HOMOLOGOUS
SERIES
contained within the question. In the following
example question, part (b) can be answered by
using information given in the question.
Practise
Photograph: Simon Townsley/Rex
TABLE OF HOMOLOGOUS SERIES