Mark Scheme with Examiners’ Report
IGCSE Chemistry (4335)
June 2006
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IGCSE CHEMISTRY 4335, MARK SCHEME
Paper 1F
1.
(a)
atomic number (second box)
1
(b)
hydrogen / H / H2
1
(c)
silicon / Si
1
(d)
lithium / Li
1
(e)
three / all three correctly listed
1
Total 5 marks
2.
3
1
1
from top to bottom: proton – electron - neutron
8
Be/ Beryllium
(a)
(i)
(ii)
(iii)
(b)
same number of protons /atomic number
different number of neutrons / atomic mass / mass number / nucleon
number
1
1
Total 7 marks
3.
(a)
(i)
(ii)
(iii)
bubbles / fizzing / effervescence / magnesium gets smaller /
disappears NOT dissolves / gas made
increases / goes up NOT heat produced
magnesium + hydrochloric acid → magnesium chloride + hydrogen
1
1
1
(b)
lighted splint / flame / burn
(squeaky) pop (ONLY if 1st mark awarded)
1
1
(c)
ticks in 3rd, 4th and last boxes
3
Total 8 marks
4.
(a)
(iodine)
element
(magnesium oxide) compound
(hydrogen chloride) compound
covalent
ionic
covalent
bonding: all 3 correct = 2; 2 correct = 1
element/compound: all three correct = 2, 2 correct = 1
4
(b)
(i)
(ii)
allotropes
sulphur ALLOW phosphorus / oxygen / other correct
1
1
(c)
(i)
two electrodes/wires in solution (but not touching)
power supply in circuit
bulb/buzzer/ammeter in circuit
fizzing / bubbles at electrodes / bulb lights etc.
electrolyte(s) ONLY
1
1
1
1
1
(ii)
Total 11 marks
5.
1
1
1
A, C and D (any order)
C (accept B)
A and D (either order)
(a)
(i)
(ii)
(iii)
(b)
alkene(s)
1
(c)
CnH2n+2
1
(d)
add bromine (water) / Br2
decolourised / (goes from orange to) colourless with A NOT ‘clear’
remains orange/yellow/brown (or combination) / no change with C
1
1
1
(e)
fruity / pleasant
1
(f)
46
1
(g)
(i)
(ii)
C2H4 + H2O → C2H5OH
(concentrated) phosphoric acid
1
1
Total 12 marks
6.
(a)
aqueous / dissolved in water
gas
solid
1
1
1
(b)
(i)
(ii)
heat / heated
green
(to) black
carbon dioxide
1
1
1
1
(iii)
(c)
nitric acid
(d)
magnesium oxide
copper
1
(either order)
1
1
(e)
nitrogen dioxide
1
(f)
sulphuric acid / H2SO4
1
Total 12 marks
7.
(a)
1
2
(b)
(i)
(ii)
1
1
1
sodium + water → sodium hydroxide + hydrogen
sodium moves around / floats
melts / becomes a ball / gets smaller / disappears
NOT dissolves
effervescence / fizzing / bubbles NOT gas made
any two – max one from each line
2
(c)
indicator NOT ‘universal indicator’
blue
1
1
(d)
(i)
(ii)
1
1
(e)
potassium / K
magnesium / Mg
Mg + H2O → MgO + H2
white
1
1
Total 11 marks
8.
(a)
X: hydrochloric acid / HCl
Y: / limestone / calcium carbonate / marble / CaCO3
1
1
(b)
in a syringe / by downward delivery or recognisable diagram / by
upward displacement of air
1
(c)
(i)
(ii)
yellow / orange NOT red
carbonic (acid)
H2CO3
proton/H+ donor/source OR provides/loses/gives protons
1
1
1
1
(iii)
(d)
ionic
covalent
1
1
(e)
carbonating drinks / fizzy drinks / fire extinguishers / dry ice
1
(f)
amount/percentage too small (any stated % under 1%)
1
Total 11 marks
9.
(a)
carbon and hydrogen
1
(b)
(i)
(ii)
(iii)
fractional distillation
(group of) compounds with same / similar boiling points
crude oil heated / boiled
(vapour) passed into column / tower
fractions collect at different heights
1
1
1
1
1
(c)
(i)
(ii)
(iii)
gasoline
fuel oil
(refinery) gases NOT natural gas
bitumen
naphtha
1
1
1
1
(d)
(i)
(ii)
carbon monoxide
poisonous / toxic / lethal / causes death
reduces capacity of blood to carry oxygen / combines with
haemoglobin
1
1
1
Total 13 marks
10.
(a)
acts as solvent
mixture melts at lower temperature / reduces operating temperature
/ allows lower temperature to be used
increases conductivity of mixture
(Any two)
2
1
1
1
carbon / graphite / C
oxygen
they burn/combine with oxygen/form carbon dioxide
(b)
(i)
(ii)
(iii)
(c)
(aluminium) more reactive than carbon / too reactive
1
(d)
electricity / replacing anodes
1
(e)
(aeroplanes)
(overhead power cables)
1
1
(pans for cooking food)
low density NOT light
(good) conductor of electricity
low density (if not scored above)
(good) conductor of heat
1
(Accept resists corrosion once as alternative for any of the above)
Total 10 marks
PAPER TOTAL 100 MARKS
Paper 2H
1.
(a)
1
2
(b)
(i)
(ii)
1
1
1
sodium + water → sodium hydroxide + hydrogen
sodium moves around / floats
melts / becomes a ball / gets smaller / disappears
NOT dissolves
effervescence / fizzing / bubbles NOT ‘gas made’
any two – max one from each line
2
(c)
indicator NOT ‘universal indicator’
blue
1
1
(d)
(i)
(ii)
1
1
(e)
potassium / K
magnesium / Mg
Mg + H2O → MgO + H2
white
1
1
Total 11 marks
2.
(a)
X: hydrochloric acid / HCl
Y: / limestone / calcium carbonate / marble / chalk / CaCO3
1
1
(b)
in a syringe / by downward delivery or recognisable diagram / by
upward displacement of air
1
(c)
(i)
(ii)
yellow / orange NOT red
carbonic (acid)
H2CO3
proton/H+ donor/source OR provides/loses/gives protons
1
1
1
1
(iii)
(d)
ionic
covalent
1
1
(e)
carbonating drinks / fizzy drinks / fire extinguishers / dry ice
1
(f)
amount/percentage too small (any stated % under 1%)
1
Total 11 marks
3.
(a)
carbon and hydrogen
1
(b)
(i)
(ii)
(iii)
fractional distillation
(group of) compounds with same / similar boiling points
crude oil heated / boiled
(vapour) passed into column / tower
fractions collect at different heights
1
1
1
1
1
(c)
(i)
(ii)
(iii)
gasoline
fuel oil
(refinery) gases NOT ‘natural gas’
bitumen
naphtha
1
1
1
1
(d)
(i)
(ii)
carbon monoxide
poisonous / toxic / lethal / causes death
reduces capacity of blood to carry oxygen / combines with
haemoglobin
1
1
1
Total 13 marks
4.
(a)
acts as solvent
mixture melts at lower temperature / reduces operating temperature
/ allows lower temperature to be used
increases conductivity of mixture
(Any two)
2
1
1
1
carbon / graphite / C
oxygen
they burn/combine with oxygen/form carbon dioxide
(b)
(i)
(ii)
(iii)
(c)
(aluminium) more reactive than carbon / too reactive
1
(d)
electricity / replacing anodes
1
(e)
(aeroplanes)
(overhead power cables)
1
1
(pans for cooking food)
low density NOT light
(good) conductor of electricity
low density (if not scored above)
(good) conductor of heat
1
(Accept resists corrosion once as alternative for any of the above)
Total 10 marks
5.
(a)
(b)
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
all formulae correct
state symbols correct
balanced
(i)
(ii)
(c)
(d)
1
1
1
1
1
1
1
line steeper
same final volume
line not as steep
produces half the final volume of gas
particles/ions move faster / have more energy
more collisions per second / more frequent collisions / greater chance
of collisions
more successful/effective/fruitful collisions / idea of more collisions
with EA
1
1
add nitric acid
and silver nitrate (solution)
white ppt (ONLY if silver nitrate mark awarded)
1
1
1
1
Total 13 marks
6.
(a)
(i)
(ii)
(iii)
titanium
electrons
Na+ / sodium ions
Cl- / chloride ions
1
1
1
1
(b)
(i)
(ii)
(iii)
uv light / sunlight / sun
(goes red then) bleached / goes white / decolorised / colourless
goes red / pink
1
1
1
(c)
(i)
division of percentages by Ar values
division of numbers of moles by the smallest
CH2Cl
C2H4Cl2 only
1
1
1
1
(ii)
Total 11 marks
7.
(a)
(b)
(c)
Company A
• fermentation
• (agricultural area so) grows sugar (cane)
Company B
• reaction of ethene with steam
• (crude) oil available / needs pure ethanol / ethene comes from oil
1: conc sulphuric acid/conc phosphoric acid/aluminium oxide(+heat) /
pumice / porous pot
2: acidified potassium dichromate(VI) / potassium manganate(VII)
3: sodium
(i)
correct (ester) linkage between monomer units
repeat unit correct (with continuation bonds)
O
(ii)
H
H
C
C
H
H
O
O
O
C
C
1
1
1
1
1
1
1
1
1
n
condensation / polyester
1
Total 10 marks
8.
(a)
(b)
C + O2 → CO2
C / carbon reacted with oxygen
equation correct
1
1
ZnO + CO → Zn + CO2
Fe2O3 + 3CO → 2Fe + 3CO2
all formulae correct
balancing correct
1
(c)
limestone decomposes
or CaCO3 → CaO + CO2 (2)
to make CaO
this reacts with silicon dioxide
or CaO + SiO2 → CaSiO3 (2)
to form slag / calcium silicate
1
1
1
1
(d)
zinc has lower boiling point than silicon dioxide
evaporates / vaporises
leaving impurities behind
(last two points could be awarded by saying ‘zinc distils off’)
1
1
1
(e)
prevents rusting
zinc more reactive than iron
oxidises /corrodes instead of iron
1
1
1
1
1
Total 15 marks
9.
Cu2O / Cu+
it gains an electron / loss of oxygen / causes (Mg) to lose electrons /
oxidation number decreases
1
(b)
brown gas / fizzing / bubbling / effervescence
blue / blue-green solution
1
1
(c)
32 x 300 seconds = 9600 coulombs
9600/96000 = 0.1 faradays
0.1/2 = 0.05 moles of copper
0.05 x 63.5 = 3.175g / 3.2g copper
1
1
1
1
(d)
(i)
(a)
(ii)
atoms/particles/ions in layers
slip / move / slide over each other (can get this from diagram)
tin atoms/particles/ions large(r)
prevents (layers) sliding / slipping / moving
1
1
1
1
1
Total 12 marks
10.
(a)
stoichiometric coefficients are: 2:3:2:2
(b)
(i)
(ii)
(c)
(i)
(ii)
(iii)
(iv)
1
energy in = 2468 / correct working
energy out = 2958 / or correct working
energy change = − 490(kJ/mol)
exo/endothermic diagram
enthalpy change and vertical energy axis labelled
reagents / products labelled (names or formulae)
1
1
1
1
1
1
pipette to measure sulphuric acid
sodium hydroxide in burette
indicator used and colour change (NOT universal indicator)
add sodium hydroxide gradually near end point (and swirl)
0.00167 (3 or 4 s.f.)
(ii) ÷ 2
(iii) x 100 = 0.0835
1
1
1
1
1
1
1
Total 14 marks
PAPER TOTAL 120 MARKS
Paper 3
1.
(a)
A
B
C
D
(b)
(i)
(ii)
(iii)
1
1
1
1
pipette
fractionating column
syringe
conical flask
1
1
1
A / name
C / name
B / name
Total 7 marks
2.
(a)
wear eye protection/gloves / wipe up spills
NOT glasses / don’t get on skin
1
(b)
20.2
1.6
18.6
1
1
1
(c)
(i)
(ii)
1
1
ticks under 27.45 and 27.25
27.35 (to 2 or 3 decimal places)
Total 6 marks
3.
1
1
(a)
2.7 (g)
45 (%)
(b)
(i)
(ii)
(c)
dry the filter paper / residue
THEN
weigh filter paper with insoluble impurities (1)
weigh the original/new filter paper/subtract mass of filter paper (1)
OR
remove insoluble impurities from filter paper (1)
weigh insoluble impurities (1)
it would dissolve more quickly / would take less time
less
1
1
1
2
Total 7 marks
4.
(a)
polystyrene is a (better) insulator / to reduce heat loss / glass
conducts heat
1
(b)
18.6
22.8
4.2
1
1
1
IGNORE sign
(c)
points for 1-3
line (NOT curve) of best fit for 1-3 MUST use ruler
points for 4-6
line (NOT curve) of best fit for 4-6 MUST use ruler
1
1
1
1
(d)
27.2 - 27.4 °C
44 - 45 (cm3)
56 - 55 (cm3)
1
1
1
(e)
use 44 cm3 of KOH and 56 cm3 of nitric acid
MUST give two volumes, which total 100 cm3
1
(f)
KOH, because smaller volume than acid
1
Total 13 marks
5.
(a)
(i)
(ii)
5 (cm)
40 (seconds)
1
1
(b)
(i)
1 cm represents 1 cm for y axis
all points correct (deduct 1 for each error)
smooth line of best fit
any time between 85 and 90 (s) / cq on graph
1
2
1
1
same surface area / powdered
same amount / same number of moles
NOT same mass or same quantity
same proportions OR volumes of acid and detergent
same volume of mixture
same concentration of acid
(any of these could be scored in (iii) instead)
temperature
(this could be scored in (ii) instead of here)
could score metal points here if not in (i)
1
1
(ii)
(c)
(i)
(ii)
(iii)
(d)
(i)
(ii)
(iii)
(iv)
(v)
any two
2
1
2
Metal 3 for student S / 105 sec
clock read incorrectly / thought 100 sec = 1 min / used too
little metal or mixture / did not use powder
2 and 4
results overlap / some times are same for both metals / results
similar
1
1
1
1
1
Total 17 marks
PAPER TOTAL 50 MARKS
CHEMISTRY 4335, CHIEF EXAMINER’S REPORT
Paper 1F, Section A
Questions in this section are targeted at grades E, F and G.
Question 1
This question was designed to test candidates’ knowledge of the Periodic Table.
Most scored well on this question with no parts proving to be more demanding than
others.
Question 2
This question was designed to test candidates’ knowledge of atomic structure.
Most candidates scored all three marks available for labelling the diagram of the
atom. A few candidates miscalculated the mass number of the atom, either by
counting all of the particles or by counting only one type of particle. One or two
candidates were confused by (a)(iii), since no atom with this atomic mass is shown on
the Periodic Table provided. However, candidates are expected to know that it is the
atomic number that determines which element an atom is. In (b), some candidates
got the protons and neutrons the wrong way round. One common error was to say
that isotopes of the same element had different relative atomic masses - this must
be wrong since the relative atomic mass is the average atomic mass of the atoms of
an element allowing for the abundance of each isotope.
Question 3
This question focused on the reaction of magnesium with sulphuric acid.
In giving observations, it must be remembered that if a colourless gas is produced,
then the gas cannot be seen (and so saying “a gas is made” is not a correct
observation and so does not score). What is seen are bubbles or effervescence and
we conclude that a gas is being produced. Likewise, we can not see the metal
dissolve – what we do see is the lump of metal get smaller; in any case ‘dissolve’ is
not a correct term here, since if the metal were dissolving we would have a solution
of magnesium produced – dissolving is a physical process and this is a chemical
reaction. With gas tests, the general rule is that if the test is wrong then no marks
can be awarded for the result. Hence if the test is given as ‘apply a glowing spill’, no
mark will be awarded for the result of the test, since an incorrect test has been
used. Part (c) was testing candidates’ knowledge of factors that change the rate of a
reaction. Many candidates selected incorrect choices and full marks were not as
common as expected.
Question 4
In (a), candidates were required to complete the table to show whether a substance
was an element or a compound, and to state the type of bonding in that substance.
The first column of the table, as expected, proved to be straightforward, since if a
substance is an element it will be found on the Periodic Table provided. More errors
were evident in the bonding column, with the main error being the assigning of
hydrogen chloride as ionic. Many candidates failed to score in (b), with some giving
two isotopes in (ii). In (c), candidates were expected to draw a diagram of an
electric circuit. Many of the diagrams seen suggested that candidates were not
familiar with testing the conductivity of a solution. All that was required was a power
supply (such as a cell or battery), something to detect a flow of charge (such as a
light bulb) and a complete electric circuit with electrodes dipping into the solution.
Many simply left this blank or drew circuits which were little more than wires or
electrodes dipping into the solution.
Candidates were not expected to know the correct symbols for items in the
electronic circuit, but they are advised to label apparatus so that the examiner can
give full credit for their knowledge. The observation had to be linked to the circuit
drawn – if a light bulb was not in the circuit then credit could not be given for saying
the bulb lights. Few correct answers were seen for (c)(ii) – near misses such
‘electrolysis’ were seen, but could not gain a mark since this is the process and not
the name of the solution.
Question 5
This question dealt with organic chemistry.
Part (a) was generally well answered, but candidates should be reminded to look
carefully at the wording of the question; if the question asks for one substance then
giving two is not a good idea, since if the extra substance given is incorrect no mark
can be awarded as the wrong answer negates the correct answer given. Part (b) was
often correct, but some candidates then did not read (c), and gave the general
formula for an alkene rather than for compound C; some candidates gave the
molecular formula of compound C. The remainder of the question, with exception of
the formula mass determination in (f), was very low scoring. The organic chemistry
of this course, even at Foundation Tier, is not restricted to hydrocarbons.
Question 6
This question was based on a reaction scheme for copper compounds.
In (a), most candidates scored well, although (aq) was sometimes given as aquatic
(which has a rather different meaning) and some thought that (s) stood for solution.
Part (b) was very poorly answered with most candidates picking up zero marks, or
one mark (that being for (iii) – although even here some candidates thought that W
must be the symbol of the gas and so claimed it was tungsten). Candidates often
failed to name nitric acid correctly, with answers based on nitrate or oxides of
nitrogen not being uncommon. Part (d) proved more rewarding for many with the
reaction products often correct, although nitrogen dioxide was rarely seen in (e).
Candidates had more success with working out the acid was sulphuric acid.
Paper 1F, Section B / Paper 2H, Section A
Questions in this section are targeted at grades C and D.
Paper 1F Question 7 / Paper 2H Question 1
This question was based on the elements in Groups 1 and 2.
Few candidates had difficulty with (a), although in (b)(i) some candidates ignored the
instruction and tried to give a symbol equation; in a few instances the gaseous
product, hydrogen, was missing. In (b)(ii), the required observations were things that
could be seen, a colourless gas cannot be seen being given off, the observation is
bubbles or effervescence; other neutral answers (ones that did not gain credit but
were not deemed to be wrong and so did not cancel out a correct observation)
included ‘dissolving’ (since this can not be seen, and sodium solution is not the
product) and ‘flame’ (since a small piece of sodium on water will not ignite unless it
is restrained in some way). Part (c)(i) caused some students problems: some
described the use to which litmus is put, but did not use the all-important word
‘indicator’; those who added ‘pH’ or ‘universal’ to indicator did not gain the mark
since litmus does not indicate pH. In (c)(ii), some students hedged their bets and
gave the colour in both acid and alkali. Two answers, one of which is wrong, will
never gain credit in this type of question. Parts (d) and (e) were well answered with
a minority of candidates having the hydroxide as the product in (d)(i).
Paper 1F Question 8 / Paper 2H Question 2
The majority of this question tested candidates’ knowledge and understanding of the
preparation and properties of carbon dioxide.
Part (a) was well answered by most candidates, although some tried to add the solid
through the tap funnel. The method of gas collection expected was the use of a gas
syringe. ‘Downward delivery’ or ‘upward displacement of air’ were also accepted,
but some candidates got these two names mixed up and gave answers such as
‘downward displacement of air’; some candidates gave two contradictory answers
and so failed to score. Part (c) was very poorly answered: a common error was to see
the word ‘acid’ in the introduction and so say the indicator turned red, whereas
carbon dioxide is only a weak acid. Candidates are advised to give only one colour in
answer to this sort of question – if they give two (or more) colours than ALL must be
correct to score the point. In (c)(ii), a common error was to focus on the hydrochloric
acid and not on the required product of the dissolution. Part (d) was nearly as often
wrong as it was right – suggesting a degree of guesswork rather than trying to use the
information provided on melting points. Most scored in (e), but in (f) many
candidates incorrectly focussed on the fact it was impure rather than on the fact
there was very little of it. A few candidates seemed concerned that removal of
carbon dioxide from the air would kill plants! In these days of the enhanced
greenhouse effect, students would be expected to know that increasing carbon
dioxide levels are thought to be a problem.
Paper 1F Question 9 / Paper 2H Question 3
This question was about the industrial separation of crude oil.
It proved to be low scoring for the majority of candidates. Most scored the first two
marks but the remainder of (b) often yielded only one further mark. Many gave a
mathematical or dictionary definition of a fraction rather than linking it to the stem
of the question which was about fractions of crude oil. Very few correctly stated that
a fraction was a collection of compounds with similar boiling points. There was a
confusion between the industrial and laboratory process in (b)(iii). This question
required the industrial process in which the oil is heated (not burnt as some
candidates claimed) prior to the vapour being passed into a fractionating tower.
Some candidates made life a little easier for themselves by drawing a diagram which
made it clear that the fractions were obtained at different levels. The main problem
in (c) was in (iii), where candidates gave the names of fractions already given
(paraffin, for example, is kerosene). Part (d) provided a happier ending to the
question for most candidates. Carbon monoxide as the product was well known and
some detailed explanations were seen in (d)(ii). However, it should be noted that
carbon monoxide does not destroy haemoglobin or cells: it can, however, lead to
death and this simple statement was missing from many answers.
Paper 1F Question 10 / Paper 2H Question 4
This question was about aluminium.
Very few good answers were seen in (a), although almost no candidates were able to
state that the cryolite acted as solvent. The mark for reducing the melting point of
the mixture or lowering the temperature required to conduct the process were given
more frequently. Some students referred to boiling points, and others seemed to
think that cryolite was a coolant since it cooled the process down – neither of these
approaches gained credit. While many students scored well on (b), many scored
poorly; incorrect electrode materials were often seen (aluminium being common). It
should be noted that the gas produced by electrolysis is not carbon dioxide – this is
made by oxygen reacting with the carbon of the anode. Some misconceptions were
evident in (b)(iii)- the electrodes need to be replaced due to a chemical reaction in
which they are used up to make carbon dioxide; the electrodes are not worn away
(this is a frictional process) nor are they eroded or eaten.
Imprecise answers also cost some candidates marks in (c). The important point is that
aluminium is more reactive than carbon – saying that aluminium is very reactive is
insufficient. Some answers to (d) incorrectly focused on various costs (such as the
need to obtain cryolite). Those who chose to use replacement of the electrodes as a
reason needed to specify that it is the anodes that are replaced (since the cathodes
are not replaced frequently). All that was required in (e) was to relate the uses of
aluminium to its properties. This proved unexpectedly difficult. Two important points
to note are firstly, that since only iron can rust any answer stating that aluminium
did not rust (rather than did not corrode) was incorrect; and secondly that aluminium
is not a light metal - the mass also depends on how much you have – it has a low
density. A variety of odd answers were seen - a number of candidates suggested that
aluminium was used for power cables because it did not conduct electricity!
Paper 2H, Section B
Questions in this section are targeted at grades A*, A and B.
Question 5
Part (a) required candidates to write a chemical equation with state symbols. Many
candidates ignored the instruction to include state symbols and a large number of
others used monovalent magnesium in the equation and so were unable to score any
marks. Where state symbols were included, common errors included stating HCl was
a liquid or that MgCl2 was a solid. Most candidates managed to score some marks
when drawing the sketch lines on the graph. While the relative gradients of the lines
were usually correct there were more problems with predicting the final volume of
gas collected. Line B very rarely indicated that the final volume of gas would be half
that shown by the printed line. Most candidates managed to gain at least one mark in
(c), although it was common for candidates to claim that atoms or molecules were
moving faster, rather than ions: in this situation it is safer to use the general term
particles. Part (d) required a chemical test for chloride ions. Tests for chlorine gas
were depressingly frequent and performance was rather centre dependent.
Question 6
This question was based on chorine.
Very few candidates knew the metal from which the anode is made, so they guessed.
Sadly, when guessing they did not use their chemical knowledge and so common
answers were: sodium (which would react with the water, or the chlorine produced);
carbon (not a metal) and chlorine (neither a metal nor a conductor). Many candidates
were able to state that electrons were responsible for the conduction in a metal, but
very few gave the names of both ions responsible for the conductivity of the brine –
candidates should look at the number of marks available to help them in this sort of
situation. Part (b) was concerned with the chlorination of an alkane. Few candidates
could recall the need for uv light to initiate the reaction. To answer (b)(ii) and (iii),
candidates were expected to use the information in the equation provided and
realise that Cl2 and HCl would both change the colour of blue litmus - it was not
uncommon for the two observations to be the wrong way round. Some excellent
calculations were seen in (c). Where a candidate failed to obtain the correct answer,
credit was given for evidence of some correct working. However, credit for correct
working can only be given if the examiner can follow what has been done – spaces
filled with seemingly random calculations with no indication as to what the candidate
thought they were doing failed to score. Some candidates had the empirical and
molecular formula the wrong way round.
Question 7
This question was designed to test candidates’ knowledge and understanding of the
chemistry of ethanol.
Most candidates scored well in (a), though some chose to use methods of production
other than the two given in the introduction. Part (b) elicited very few correct
answers: what should have been straightforward recall of reagents proved beyond
the knowledge of most candidates. The best candidates scored full marks when
drawing the polymer structure, although a common error was to add a hydrogen
atom to one of the oxygen atoms making it either trivalent, or converting it into an
alcohol group. Part (c) (ii) required the name of the type of polymer: trade names
(such as Terylene) were not credited.
Question 8
This question was about the extraction and uses of metals.
While many candidates scored both marks in (a), a number wrote equations which did
not involve carbon at all. The two balanced symbol equations in (b) proved to be very
difficult for some candidates. Of the two equations, the first one was most often
correct. Many candidates then chose to make something other than iron in the
second equation – oxides of iron or oxygen gas were not unusual products. Some
excellent answers were seen to (c) with some very logical structures: those who did
not know the chemistry involved often picked up a mark for forming slag. In contrast,
(d) was probably the worst answered question on the paper. The extraction of zinc is
on the specification and so the provided data should have been an additional aid to
candidates. However, it was evident that very few candidates either knew the
process of extracting zinc in a blast furnace, or how to use the data. All that was
required was an understanding that the temperature in the blast furnace was above
the boiling point of zinc, but below that of silicon dioxide (the main impurity); as a
result the zinc evaporates leaving the impurities behind. Part (e) proved to be more
difficult than expected; some candidates claimed that ‘zinc rusted instead of iron’ –
since only iron can rust, this statement is incorrect. The correct answer is ‘zinc
corrodes instead of iron’, which should then have been linked with the idea that zinc
is more reactive than iron and that the iron will not rust. Some very bizarre answers
were seen: some candidates thought zinc had a low density and so it helped ships
float, while others thought zinc had a high density and so it acted as a ballast to
keep the ships upright.
Question 9
This question was based on the chemistry of copper.
In (a), a common error was to give the name of the species that had been oxidized,
rather than the oxidising agent. Candidates were expected to use the equation to
help, the expected answer being Cu+, although copper(I) oxide was also accepted.
The observations in (b) were not well known. Many stated that a blue colour would
be seen but did not gain the mark because they did not specify it would be the
solution as opposed to the solid or a gas. The reaction of copper with concentrated
nitric seems to be unfamiliar to many students – a common error was (correctly) to
state that there would be effervescence, but then incorrectly to call the gas
colourless (hydrogen); it is worth noting that while ‘gas produced’ is not an accepted
observation for the production of a colourless gas (since the gas cannot be seen, its
production is concluded from the fact that effervescence is seen) here stating ‘brown
gas made’ is accepted as an observation since the gas can be seen.
Many candidates gained full marks for the calculation in (c), while others picked up
some credit for correct working. As in Q 6(c), credit for correct working can only be
given if the examiner can follow what has been done. Common errors included not
converting the time to seconds and not using the equation to see that the ratio of
moles Cu: moles electrons was 1:2. In (d), students were expected to explain why
metals are ductile or malleable (based on the layers of atoms being able to slide over
each other) and then to use the diagram provided to work out why alloys are harder
to deform. Answers that implied the random motion of particles did not gain credit.
Question 10
Some candidates failed to attempt (a): this can only be due to not reading the
examination paper with sufficient care. The calculation in (b)(i) was often very well
done, with the most common error being the final subtraction of energies, where
candidates got the numbers the wrong way round. As a general rule, there will not be
any marks for units in calculations unless the units are specifically asked for.
However, the inclusion of incorrect units will mean that the final answer is wrong
and so a mark is lost – in this question a number of candidates claimed their answer
was in joules rather than kilojoules. The energy level diagram in (b)(ii) was a good
discriminator: some candidates left it blank or rewrote how they worked out the
answer to (i). Amongst partially correct answers, a common omission was not to label
the vertical direction as ‘energy’ or to label the enthalpy change incorrectly on the
diagram. Descriptions of how to conduct a titration in (c) were very poor. Most
candidates scored 1 or zero marks. The following should be borne in mind:
• The volume of acid used was 10.0 cm3 and so this solution should be placed in
a conical flask; its volume should be measured with a pipette since a
measuring cylinder is not sufficiently accurate for titration work.
• The volume of sodium hydroxide used was 16.70 cm3 and so it must have been
in the burette. There is no problem with using dilute alkalis in modern
burettes since they have Teflon taps. Using the alkali in the burette gives a
better colour change for phenolphthalein indicator.
• An indicator is required. If one was named it had to be an acid/alkali
indicator, and a correct colour change was required at the endpoint.
Universal Indicator is not suitable for titrations since it does not have a sharp
colour change at the endpoint.
• The solution in the burette should be added slowly near the endpoint (not all
the way through). Only candidates who wrote about doing repeat runs tended
to score this point.
Some candidates clearly did not read the question and filled the space with a moles
based calculation based on the volumes and concentrations given the question. Some
of these candidates then failed to attempt the calculation in (d). The titration
calculation was often well done: common errors were to fail to convert the volume
from cm3 to dm3 or to make an error with the magnitude of the answer.
Paper 3
General Comments
Questions in this paper are targeted at full range of grades from A* to G.
Question 1
This question was designed to test candidates' familiarity with common examples of
laboratory apparatus and their uses. High scores were expected, and few candidates
lost more than 1 or 2 of the 7 marks available. Common errors included naming the
pipette as a burette, and using the gas syringe to measure the volume of a solution.
The main cause of lost marks was the fractionating column, partly due to a poor
diagram, which is regretted.
Question 2
In (a), the safety precaution hoped for was one specific to the corrosive nature of
sodium hydroxide, so general precautions such as "wear a lab coat" or "tie your hair
back" were not accepted, although "wear eye protection" was. The burette readings
in (b) were generally correct, with only a minority reading the scales upwards instead
of downwards, or writing the readings in the wrong boxes. In this particular question,
although the liquid levels were actually on the 0.1 cm3 divisions, candidates should
remember that they are expected to read to the nearest half-division, so it is good
practice to quote values such as 20.20 rather than 20.2 (as shown in the table in (c)).
In (c), very few candidates seemed to be familiar with the concept of concordance in
burette readings, which in this paper means that readings should differ by no more
than 0.20 cm3. Every possible combination of ticks was seen, with the ticking of the
last three being the most common (perhaps on the assumption that the first result
will be inaccurate, or perhaps because this was the only one beginning with 28,
rather than 27). Some candidates failed to place ticks as requested, but were able to
score this mark when their choice was clear from their subsequent working in
calculating the average. A small minority of candidates seemed unable to calculate
an average from their ticked results correctly.
Question 3
The vast majority of candidates correctly calculated the mass and percentage in (a).
Part (b) was also well answered, with the commonest error being to state that
crushing the rock salt would make more dissolve. Quite a few candidates wrote about
the reaction, instead of the dissolving being faster, although this was not penalised.
Very few candidates scored all 3 marks in (c). Quite a number got no further than
just weighing, with many of these failing to score because it was not clear what was
being weighed. Others repeated the method shown in the diagrams.
Question 4
The use of a polystyrene cup in (a) elicited a surprising variety of responses. Although
many correctly mentioned its insulating properties, some went on to suggest that the
glass beaker would break because of the great heat, while others thought that the
polystyrene cup prevented the reactants from attacking the glass. A few claimed that
as the polystyrene cup could be thrown away it would avoid having to wash the glass
beaker! The thermometer readings in (b) were generally correct, with errors similar
to those found in the burette readings in Q 2. The drawing of the two-line graph in
(c) will have been unfamiliar to most candidates, and it is pleasing to report that so
many produced excellent lines that scored all 4 marks.
The weakest candidates plotted 12 points instead of 6, or drew curves that started at
15°C, or had lines curving towards each other rather than crossing. Some of the
general errors common to all graph-plotting questions were seen, such as misreading
the scales, using large blobs for points, or drawing multiple lines. Most who
succeeded in (c) had little trouble in (d), although some misreading of scales was
again in evidence. Most candidates mentioned repeating in (e), but this response did
not score unless it was related to the volumes in (d). A pleasing number of
candidates gave a correct reason for their choice of potassium hydroxide in (f).
Question 5
Part (a) was generally correct, with just a few candidates giving the depth of liquid,
or of liquid-plus-foam. Relatively few candidates scored full marks in (b). Quite a
number of points were misplotted, and the drawing of the smooth line of best fit was
poorly done. Identifying the features in (c) proved a challenge for candidates, and
full marks were rare, with the weakest scoring only with temperature. A minority
thought that the other metals should be in the same group of the Periodic Table as
magnesium, or that they should have the same reactivity. Perhaps the most common
unacceptable answer was that the masses of metal should be the same, rather than
the number of moles. Although hardly any candidates mentioned moles, those who
wrote "amount" scored because this term has the meaning of amount in moles.
Common acceptable answers were that the metals should be powdered like
magnesium, that the same volumes of acid-plus-detergent should be used, and that
the proportions of acid and detergent should be the same. The full range of marks
was seen in (d), but there were many common errors. These included identifying
Metal 5 as having the most reliable results because it had the most results, rather
than Metal 2 (where there were 4 results much closer together). The anomalous
result (105) stood out from the others, but even so was often inadequately identified
just as Student S or Metal 3, rather than as a combination of the two. The most
reactive metals were frequently taken as the two with the longest times, rather than
those with the shortest times. Many of those who made the correct identification
gave as their reason that there was a result missing, rather than that the results
were similar.
COURSEWORK (PAPER 4), PRINCIPAL MODERATOR’S REPORT
General Comments on Science Coursework
The coursework component is only available to centres which are recognised by
Edexcel as International Teaching Institutions.
The number of students entered for this component of the iGCSE examination was as
follows:
Code
Subject
4335
Chemistry
Number entered
in 2006
193
Number entered
in 2005
79
All of the centres that entered students for this component of the examination had
their science coursework moderated by Edexcel’s co-ordinating Principal Moderator
for GCSE. The moderating instrument used was the Sc1 criteria as used by Home
centres, using exemplars provided by the JCQ (Joint Council for Qualifications) as a
guide. Centres entering students for the coursework component of the iGCSE
examinations in 2006 therefore had their coursework moderated to the same
standards as for all Home centres.
Chemistry 4335
The most common task seen this year was a rates task – almost invariably sodium
thiosulphate / hydrochloric acid.
This is a very common task in UK centres, but it does have some disadvantages.
Firstly, if the students (or teacher) decide to investigate the effect of varying the
concentration of sodium thiosulphate solution, it is difficult for the students to
incorporate sufficient scientific knowledge to access P8a fully. It is more appropriate
to study temperature as the variable, so that students can discuss exo- and
endothermic steps, as well as the concept of activation energy.
Centres who awarded full marks for the visual disappearance of a cross in the
thiosulphate / acid task were too generous. The observation of a cross disappearing
as the precipitate of sulphur forms is a subjective matter, and therefore lacks
precision. (Precision is a key factor in the award of O8a). Please note also that the
requirements of O6a and O6b must be fully met before an award of O8a is
considered.
Students who choose to investigate the effect of varying temperature on the reaction
rate should be encouraged to record the actual temperatures used. Quoting the
temperatures to the nearest ten degrees (perhaps following the range of
temperatures stated in the planning phase) lacks precision, and such students had
their marks reduced in consequence.
Most centres did use the approved annotation method when marking scripts, but one
centre used an inappropriate method of awarding marks after every paragraph
written by the students.
CHEMISTRY 4335, GRADE BOUNDARIES
Option 1 : with Written Alternative to Coursework (Paper 3)
A*
A
B
C
D
E
F
G
54
44
35
26
17
49
36
25
19
B
C
D
E
F
G
Foundation
Tier
Higher
Tier
75
62
Option 2 : with Coursework
A*
A
Foundation
Tier
Higher
Tier
No candidates this session
75
63
51
39
28
22
Note: Grade boundaries may vary from year to year and from subject to subject,
depending on the demands of the question paper.
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