CHRISTIAN & MISSIONARY ALLIANCE SUN KEI SECONDARY SCHOOL
MOCK EXAMINATION (2008– 2009 SECOND TERM)
S5 CHEMISTRY
Marking Scheme
Paper 1 (90 marks)
Section A 54 marks
1. / (a) / (i) / Solid krypton is made of monatomic molecules. It has a simple molecular structure with weak van der Waals’ force between the molecules holding them together. / (1)(1)
(ii) / Solid rubidium is made of cations bathed in delocalized electron. It has a giant metallic structure with strong metallic bond between metal ions and delocalized electrons holding them together. / (1)
(1)
(iii) / The metallic bond present in rubidium is stronger than the weak van der Waals’ forces in krypton. This explains the higher boiling point of rubidium. / (1)
(b) / Krypton is very stable because its outermost electron shell is full-filled with an octet of electron. / full-filled with 8 electrons. / (1)
(1)
2. / (a) / (i) / Potassium / sodium / calcium
It reacts with water (in sulphate solution of Q) to give hydrogen. / (1)
(1)
(ii) / Copper
Since copper oxide is reduced by town gas to form reddish brown copper. / (1)
(1)
(b) / R>P>Q>S>T / (1)
3. / (a) / / (1)
(b) / (i) / -1 / (1)
(ii) / +6 / (1)
(c) / (i) / S2O32- + 5H2O → 2SO42- + 10H+ + 8e- / (1)
(ii) / S2O32- + 4H2O2 → 2SO42- + 2H+ + 3H2O / (1)
(1)
(d) / (i) / The thiosulphate ions reduce bromine to bromide ions, which can be washed away by water readily. / (1)+(1)
(ii) / Add an organic solvent (e.g. heptane) to the solution. The organic layer turns from colourless to purple in the presence of iodine. OR
Add starch solution to the solution. It turns from colourless dark-blue in the presence of iodine. / (1)+(1)
4. / (a) / (i) / Ca(OH)2 + CO2 → CaCO3 + H2O / (1)
(ii) / It forms from the incomplete combustion of fossil fuels (petrol). / (1)
(iii) / Sulphur dioxide. It irritates respiratory system / causes lung cancer / causes acid rain. OR
Unburned hydrocarbon. It causes lung cancer / causes photochemical smog. OR
Nitrogen oxides. It irritates respiratory system / causes photochemical smog / causes acid rain. OR
Suspended particulate. It causes breathing problem / causes lung cancer. / (1)+(1)
(b) / (i)
(iii) / Gas G is nitrogen.
Most unreacted nitrogen in air passes out of the combustion engine. / (1)
(1)
(1)
(ii) / Nitrogen can be collected by displacement of water because it is insoluble in water. Nitrogen can also be collected using a gas syringe. / (1)
(1)
5. / (a) / 3Cl2 + 6OH- → ClO3- + 5Cl- + 3H2O / (1)
(b) / (i) / Sodium hypochlorite / hypochlorite ion / (1)
Sodium hypochlorite / hypochlorite ions oxidizes coloured dye to colorless compounds. / (1)
(ii) / Cl2 + 2OH- → ClO- + Cl- + H2O / Cl2 + 2NaOH → NaClO + NaCl + H2O
The reaction in (b) (i) is carried out in hot aqueous solution, whereas the bleach is produced by dissolving chlorine in cold sodium hydroxide solution. / (1)
(1)
6. / (a) / (i) / It is not correct to use calcium nitrate to prepare salt bridge. This is because calcium ions form precipitates with sulphate ions in the 2 half cells. / (1)
(1)
(ii) / Use potassium nitrate / ammonium nitrate / potassium chloride to prepare the salt bridge. / (1)
(iii) / Increase the surface area of electrodes immersed in the electrolyte solutions. OR
Put the 2 electrodes closer to each other. / (1)
(b) / (i) /
/ (1)
(ii) / Using 2 half cells can prevent direct reaction between metal strip and the electrolyte solution. OR
Ensure the chemical energy given out in the reaction is converted to electrical energy, but not to heat energy. / (1)
7. / (a) / (i) / / (1)
(ii) / / (1)
(b) / (i) / It is because both monomers have carbon-carbon double bonds (C=C), they can join together randomly. / (1)
(ii)
(iii) / Yes.
Because they carbon to hydrogen ratio in styrene is higher than that in butadiene. Hence, styrene undegoes incomplete combustion more readily, giving greater amount of particulates. / (1)
(1)
(1)
8. / n Members of homologous series have same general formula. For example, alkanes have the general formula of CnH2n+2.
n Each member in a homologous series differs from the next member by a –CH2– unit. For example, methane (CH4) and ethane (CH3CH3).
n Members in a homologous series also show similar chemical properties. For example, methane and ethane can both turn bromine from brown to colourless in the presence of diffused sunlight.
n Members show gradual change in physical properties as the number of carbon atoms in a molecule increases. For example, as the number of carbon of alkanes increases, their melting boiling point increases.
n Members in a homologous series have same functional group. For example, alkanes have only C-C or C-H single bonds.
(3 marks for effective communication)
/ ANY 3 out of these points
(3 x 2)
(3)
Section B 36 marks
9. / (a) / Catalytic hydration / addition reaction / (1)
(b) / (i) / Excess steam is used because it is cheaper than propene. / (1)
(ii) / / (1)
(iii) / Fractional distillation
The difference in their boiling point makes the method possible. / (1)
(1)
(c) / No. of moles of propene = = 9541.7 mol
According to the equation, the mole ratio of propene to propan-1-ol is 1:1
Theoretically, number of moles of propan-1-ol formed = 9541.7 mol
Actual no. of moles of propan-1-ol obtained = =5833.3 mol
% yield of the reaction = x 100% = 61.1 % / (1)
(1)
(d) / Both propene and propan-1-ol can react with acidified potassium permanganate solution.
/ (1)
(1)
(1)
10. / (a) / (i) / Ester group / (1)
(ii) / The two liquid layers become miscible. / (1)
(iii) / (1)
(iv) / Add concentrated NaCl solution to the reaction mixture to salt out the soap.
Separate the solid soap by filtration. / (1)
(1)
(b) / (i) / Soapy detergent combines with magnesium ions and calcium ions in hard water to form scum.
/ (1)
(1)
(ii) / Soapy detergent combine with H+ ions to form insoluble alkanoic acid. This reduces the cleaning ability of the soapy detergents. / (1)
(iii) / Sodium carbonate / (1)
11. / (a) / Rinse the burette with distilled water first, and then with the 0.445 M sodium hydroxide solution. / (1)
(1)
(b) / (i) / HCl + NaOH → NaCl + H2O
No. of moles of NaOH = 0.445 x = 0.00957 mol
Based on the equation, the mole ratio of NaOH : HCl = 1 : 1
No. of moles of HCl remained = 0.00957 mol / (1)
(1)
(ii) / No. of moles of HCl added = 1.254 x = 0.03135 mol / (1)
(iii) / 2HCl + Na2CO3 → CO2 + 2NaCl + H2O
No. of moles of HCl reacted with Na2CO3 = (0.03135 – 0. 0.00957)
= 0.02178 mol
Based on the equation, the mole ratio of Na2CO3 : HCl = 1 : 2
No. of moles of Na2CO3 in the reaction mixture = = 0.01089 mol
Concentration of Na2CO3 solution x = 4.36 mol dm-3 / (1)
(1)
(c) / The jet of burette should be filled with 0.445 M sodium hydroxide solution also. / (1)
12. /
During electrolysis of concentrated sodium chloride (NaCl) solution, both hydrogen ions (H+) and sodium ions (Na+) are attracted towards the negative electrode, whereas both hydroxide ions (OH-) and chloride ions (Cl-) are attracted towards the positive electrode.
But, because H+ is at a lower position than Na+ in electrochemical series (ECS), H+ will be preferentially discharged to form hydrogen gas (H2) at the negative electrode.
On the other hand, because concentration of Cl- is higher than that of OH-, Cl- will be preferentially discharged to form chlorine gas (Cl2) at the positive electrode.
As both H+ and Cl- ions are discharged, the sodium chloride solution becomes sodium hydroxide solution gradually.
(3 marks for effective communication) / (1)
(1)
(1)
(1)
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(1)
(3)
Paper 2 (50 marks)
1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10B / C / C / C / A / A / A / A / D / C
11 / 12 / 13 / 14 / 15 / 16 / 17 / 18 / 19 / 20
C / C / B / C / D / D / D / B / C / D
21 / 22 / 23 / 24 / 25 / 26 / 27 / 28 / 29 / 30
C / C / B / C / C / C / C / B / C / C
31 / 32 / 33 / 34 / 35 / 36 / 37 / 38 / 39 / 40
A / C / D / A / C / A / B / B / D / A
41 / 42 / 43 / 44 / 45 / 46 / 47 / 48 / 49 / 50
D / A / B / A / B / C / C / C / D / A
MOCK EXAM / 2008-2009 / S5CHEM / Marking Scheme / Page 6 of 6