F.6 / Chemistry/15-16 Ans/ Page 1 of7
S.K.H.LAMKAUMOWSECONDARY SCHOOL
F.6 MOCK EXAMINATION (15-16)
CHEMISTRY (Paper I)
Marking Scheme
Section A
1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 101-10 / A / D / B / D / C / B / D / C / C / D
11-20 / C / D / B / D / A / B / A / C / A / A
21-30 / C / B / B / C / C / A / D / D / D / A
31-36 / A / C / A / B / C / D
Section B
Part I
1.They have the same number of electrons. (1)
They have the same number of protons. (1)
Helium-3 has one neutron while helium-4 has two neutrons. (1)
2a) To prevent it from reacting with the air. (1)
b)2,8,18,8,2 (1)
c) i)(1)
ii) Electrolysis of its molten chloride. (1)
d) Calcium and strontium have similar chemical properties. (1)
Hence, strontium can replace some of the calcium required in teeth. (1)
3 a) Graphene has a giant covalent structure. (1)
It is hard to break the strong covalent bonds between the atoms. (1)
b) Each carbon atom uses three outermost shell electrons in forming covalent bonds. (1)
The remaining outermost shell electrons are delocalized.
These delocalized electrons can move throughout the structure.(1)
4a) X > Z > iron > Y (1)
b) i) Fe(s) + 2Y+(aq) Fe2+(aq) + 2Y(s) (1)
ii) Any one:
A deposit forms on iron. (1)
The colour of the solution changes from colourless to pale green.
5a)i)(1)
ii)12.5 (0.1) cm3(1)
b)When the acid was added, it reacts with the sodium hydroxide solution. Heat was released. Therefore the temperature of the solution mixture increased. (1)
Neutralization was complete when 12.5 cm3 of acid had been added. No more heat was produced. (1)
The excess acid added cooled the solution mixture. (1)
Therefore the temperature of the solution mixture fell after 12.5 cm3.
c)HnX(s) + nNaOH(aq) NanX(aq) + nH2O(l)
2.4 mol dm–3 2.0 mol dm–3
12.5 cm3 30.0 cm3
Number of moles of HnX in 12.5 cm3 solution
= 2.4 mol dm–3×dm3
= 0.030 mol(1)
Number of moles of NaOH in 30.0 cm3 solution
= 2.0 mol dm–3×dm3
= 0.060 mol(1)
= =
n = 2(1)
6a)Mg(s) + X2+(aq) Mg2+(aq) + X(s)(1)
b)i)Direction of electron flow is from strip of metal X to copper strip.(1)
ii)X(s) + Cu2+(aq) X2+(aq) + Cu(s)(1)
c)i)Soak a piece of filter paper with saturated potassiumor sodium nitrate solution. (1)
ii)Any one of the following:(1)
•It completes the circuit by allowing ions to move from one half-cell to the other.
•It provides ions that can move into the half-cells to prevent the build-up ofexcess positively or negatively charged ions in the solutions which would causethe reaction to stop.
7a) Evaporation is a process that takes in heat by system/ molecules leave the surface of liquid. It can cause a temperature decrease. (1)
As the temperature change for pentane (-16.1oC) is greater than that for butan-1-ol (-1.7oC), it can be deduced that pentane evaporates faster than butan-1-ol. (1)
b) Only weak van der Waals’ forces exist between pentane molecules (1)
while there arestrong hydrogen bonds and van der Waals’ forces between butan-1-ol molecules. (1)
Thus pentane molecules can break away from the rest of the liquid more easily.
8a)Hydrocarbons(1)
b)i)Inside car engines, the air gets so hot that nitrogen and oxygen react together, forming NOX. (1)
c)NO reacts with CO as they pass through the catalyst. N2 and CO2 are formed.(1)
2CO(g) + 2NO(g) N2(g) + 2CO2(g)(1)
d)CO is a very poisonous gas. (1)
e)i)Irritate the respiratory system.(1)
ii)The hydrocarbons in diesel contain a much higher percentage of carbon / carbon content. (1)
Incomplete combustion of fuel will give a greater amount of particulates (1)
9a)
(1)
b)Polystyrene is a mixture of polymeric molecules of different chain lengths.(1)
c)Expanded polystyrene is a good insulator of heat.(1)
10a)At electrode X:
The sulphate ions and hydroxide ions in the copper(II) sulphate solution move towards electrode X. (1)
Copper is a stronger reducing agent than sulphate ions and hydroxide ions.(1)
Cu(s) Cu2+(aq) + 2e–
Therefore the copper anode dissolves to form copper(II) ions.
At electrode Y:
The copper(II) ions and hydrogen ions are attracted to the electrode Y. (1)
A copper(II) ion is a stronger oxidizing agent than a hydrogen ion.(1)
Cu2+(aq) + 2e– Cu(s)
Therefore copper(II) ions are preferentially discharged to form a deposit of copper onelectrode Y.
Communication:(1)
(Chemical knowledge = 0, 1 or 2: communication mark = 0
Chemical knowledge = 3 or 4: communication mark = 0 or 1
incomplete answer / difficult to understand, communication mark = 0)
b)The colour of copper(II) sulphate solution would remain unchanged(1)
because the net effect is the transfer of copper from the anode to the cathode/
concentration of Cu2+ is unchanged.(1)
c)Copper(II) ions and hydroxide ions are consumed in the electrolysis process.
Hydrogen ions and sulphate ions remain in the solution.(1)
Thus the solution eventually becomes sulphuric acid.(1)
11a) 3Al(s) + 3NH4ClO4(s) Al2O3(s) + AlCl3(s) + 3NO(g) + 6H2O(g)
ΔHor= Σ ΔHof[products] – Σ ΔHof[reactants]
= ΔHof[Al2O3(s)] + ΔHof[AlCl3(s)] + 3 × ΔHof[NO(g)] + 6 × ΔHof[H2O(g)]
– 3 × ΔHof[NH4ClO4(s)]
= [(–1 676) + (–706) + 3(+90.2) + 6(–242) – 3(–295)] kJ mol–1(1)
= –2 678.4 kJ mol–1(1)
b) Any one of the following reasons:(1)
• The Al(s) / NH4ClO4(s) fuel produces NO(g), which is the precursor to acid rain.
• The Al(s) / NH4ClO4(s) fuel produces solid aluminium compounds, which are suspendedparticulates.
• The Al(s) / NH4ClO4(s) fuel uses Al(s), the production of which causes environmental damage.
12a)i)Higher rate of reaction in A due to higher temperature(1)
ii)Higher rate of reaction in B due to greater surface area of zinc(1)
b)The time taken for the fizzing to stop would be longer.(1)
During the reaction between zinc and the acids, zinc would react with hydrogen ions in the acids.
Hydrochloric acid is a strong acid that completely dissociates in water while ethanoic acid is a weak acid that only partially dissociates in water. (1)
Therefore hydrochloric acid has a higher concentration of hydrogen ions than ethanoic acid. (1)
The reaction rate between zinc and ethanoic acid is thus lower and the reaction takes a longer time to complete.
13a)r has the highest melting point, so r is in Group IV(1)+(1)
/ v has the lowest melting point, so v is in Group O
r is carbonands is nitrogen.(1)
b)w and x are metals. They have metallic bonding.
One outermost shell electron from each atom of w is available for bonding while two electrons of each atom of x are available for bonding. (1)
The atomic radius of x is smaller and it has multi-charged ions.(1)
Thus, the ions of x have a larger charge density.
The attraction between the ions of x and delocalized electrons are stronger/
the metallic bond of x is stronger than that of w.(1)
Thus, the melting point of x is higher than that of w.
14i)MnO4–(aq) ion is a stronger oxidizing agent than Cl2(aq). (1)
It will oxidize Cl–(aq) ions to Cl2(aq).(1)
10 Cl- + 2 MnO4- + 16 H+ 5 Cl2 + 2 Mn2+ + 8 H2O
ii)NO3–(aq) ion is a stronger oxidizing agent than Fe3+(aq) ion.(1)
It will oxidize Fe2+(aq) ions to Fe3+(aq) ions.(1)
2 Fe2+ + NO3- + 3 H+ 2 Fe3+ + HNO2 + H2O
15.i)Let x mol dm–3 be the concentration of SO2(g) and SO3(g).
Kc= 2.70 x 102 dm3 mol–1
= (1)
[O2(g)]=
= 3.70 x 10–3 mol dm–3(1)
ii)
(When O2(g) is added, the system will respond by converting some of the O2(g) to SO3(g).Hence the concentration of SO3(g) will increase while that of O2(g) will decrease, but not to its original value.)
16.a)
or (1)
b)
(1)
c)
or(1)
d)
(1)
17. Add sodium carbonate to the liquids.
Only CH3CH2COOH reacts and give colourless gas bubbles.(1)
Warm the remaining three compounds with acidified potassium dichromate solution separately.
Only CH3(CH2)3OH turns the orange dichromate solution green. (1)
Any one of the following for the remaining two compounds: (1)
‧Mix the remaining two compounds with phosphorus pentachloride separately.
Only (CH3)3COH gives steamy fumes of hydrogen chloride.
‧Warm the remaining two compounds with ethanoic acid in the presence of concentrated sulphuric acid separately.
Only (CH3)3COH gives a sweetish / pleasant smell.
Also accept other positive tests e.g. Lucas test for (CH3)3COH, 2,4-DNP for CH3COCH3
The remaining one is CH3COCH3. (1)
Communication mark(1)
(Chemical knowledge = 0 to 2: communication mark = 0
Chemical knowledge = 3 or 4: communication mark = 0 or 1
incomplete answer / difficult to understand, communication mark = 0)
correct spellings required