VS 2016 O Level Pure Chemistry P1+P2 Answer Scheme

2016 Answer 1 Victoria School2016 Chemistry O Levels / 5073Suggested Answer Key PAPER 1Q1Q2Q3Q4Q5Q6Q7Q8Q9Q10Q11Q12Q13Q14Q15Q16Q17Q18Q19Q20CDBCBDACBDCADCCABBACQ21Q22Q23Q24Q25Q26Q27Q28Q29Q30Q31Q32Q33Q34Q35Q36Q37Q38Q39Q40CBCCAABDCBCACBDDDADCPAPER 2 (SECTION A)QuestionAnswerMarkA1atrendonly true down a grouponly true across a periodtrue for bothThe number of electron shells increases.✔The number of valency electrons increases.✔Proton number increases.✔There is a change in character from metallic to non-metallic. ✔[2 correct – 1 mark]2

2016 Answer 2 biMelting point decreases down Group I and increases down Group VII.1biiReactivity increases down Group I and decreases down Group VII. 1cCatalysts increase the rate of reaction hence the industrial processes can be completed in a shorter time. This will help to reduce energy usage and save costs. Only a small amount of catalysts is needed and they remain chemically unchanged and unchanged in mass at the end of the reaction, so they can be reused indefinitely and there is no recurring costs.Hence, they reduce costs in the long run.11A2aRusting involves oxidation. The oxidation state of iron increases from 0 in Fe to +3 in Fe2O3. Extraction involves reduction. The oxidation state of iron decreases from +3 in Fe2O3 to 0 in Fe. 11biAn alloy is a homogeneous mixture of metal with (small amounts of) other elements. 1biiThe oil, the paint and the plastic coating acts as a protective layer to prevent the iron parts of the bicycle from rusting as they prevent the iron from being exposed to water and oxygen. 11biiiZinc is more reactive than iron and hence its atoms will lose electrons more readily and corrodes in place of iron. 11A3aThe actual surface temperature of Mercury is close to its predicted surface temperature as there is no atmosphere of gases. [must state but no marks]The actual surface temperatures of Earth and Venus are higher than their predicted surface temperatures. This is because their atmospheres contain carbon dioxide which is a greenhouse gas. Carbon dioxide traps excess heat which increases the surface temperature. The difference between the actual surface temperature and predicted surface temperature is much higher for Venus than Earth (by 500°C) as the atmosphere of Venus contains 97% carbon dioxide but the atmosphere of Earth contains only 0.04% carbon dioxide. 11

2016 Answer 3 biVolcanic eruptions1biiThe pH of the water will be low/ less than 7.1biiiThe surface temperature of Venus is too high at 462°C hence the water would have boiled as its boiling point is 100°C and formed steam/water vapour. 1ciPlants undergo photosynthesis where they take in carbon dioxide and release oxygen. Hence the percentage of carbon dioxide decreased and the percentage of oxygen increased. 11ciiIncrease in industrial activities, use of machinery and vehicles which results in increased combustion of fossil fuels or carbon containing fuels which releases (excess) carbon dioxide. 1ciiiOCOCarbon dioxide:OOOxygen:21A4aname of saltformula of saltname of acid used to make saltname of other compound used to make saltpotassium sulfateK2SO4sulfuric acidpotassium hydroxidepotassium phosphateK3PO4phosphoric acidpotassium hydroxideammonium nitrateNH4NO3nitric acidammonium carbonate/aqueous ammonia4

2016 Answer 4 calcium phosphateCa3(PO4)2phosphoric acidcalcium hydroxide[1 mark per row]bNH4NO3 and K3PO4 1ciSalt: Ca3(PO4)2Reason: It contains phosphorus but does not contain nitrogen and potassium. 11ciiMr of K3PO4 = 3(39) + 31 + 4(16) = 212% by mass of K = [3(39) / 212] x 100% = 55.2%% by mass of P = [31 / 212] x 100% = 14.6%% by mass of N = 0.00%Ratio of N : P : K = 0 : 15 : 55111A5aSalts: silver chloride, barium sulfate, lead sulfate[3 correct – 2 marks, 2 correct – 1 mark, 1 correct – 0 marks]Reason: They are insoluble salts and hence can be prepared by adding two soluble reactants together. 21bcompound useto make detergentsto make polymers for packagingfor flue gas desulfurisationas a solvent in inksfor road surfacesbitumen✔sulfuric acid✔ethene✔ethyl ethanoate✔calcium carbonate✔ 3

2016 Answer 5 [5 correct – 3 marks, 4 or 3 correct – 2 marks, 2 correct – 1 mark, 1 correct – 0 marks]A6aThey have the same general formula CnH2n+1CHO.They increase by a –CH2 group down the homologous series.*They have the same functional group –CHO.[first two points are more essential. any two points – 1 mark each] 2bFormula: C3H7CHOPredicted boiling point: 75°C [between 70°C to 80°C]11ciIsomers are compounds with the same molecular formula but different structural formula. Ethanol has a molecular formula of C2H6O and structural formula of CH3CH2OH. Methoxymethane has a molecular formula of C2H6O but different structural formula of CH3OCH3 from ethanol. Hence they both are isomers of each other. Ethanal has a molecular formula of C2H4O which is different from that of ethanol and methoxymethane hence it is not an isomer of the other two compounds. 111cii2C2H4O + 5O2  4CO2 + 4H2OCorrect reactants and products [1 mark]Balanced equation [1 mark]2ciii 2 +  2 1

2016 Answer 6 PAPER 2 (SECTION B)QuestionAnswerMarkB7anylon X: Nylon 4, 6nylon Y: Nylon 1121b (CH2)8 C (CH2)5NN[1 mark per product]2cMr of repeating unit = 2(14) + 12(12) + 2(16) + 22 = 226Minimum number of repeating units = 10000 / 226 = 45 11dDiamine: As the chain length of the diamine increases, the melting point decreases. The melting point nylon 4,6 is 275°C but the melting point of nylon 6,6 is 269°C.Dicarboxylic acid:As the chain length of the dicarboxylic acid increases, the melting point decreases. The melting point of nylon 6,6 is 269°C but melting point of nylon 6,10 and nylon 6,12 is 220°C and 218°C respectively. [Use of data to support reasoning]111eMr of 1 repeating unit of nylon 6 = 6(12) + 11 + 14 + 16 = 113No. of moles of repeating units of nylon 6 in 1 kg = 1000/113 = 8.8496 molNo. of moles of CO2 = 8.8496 x 6 = 53.0976 molMr of CO2 = 12 + 2(16) = 44Carbon footprint = 53.0976 x 44 = 2336.29 g = 2.34 kg (to 3 s.f.)111B8aN2 + 3H2 ⇌ 2NH3 1

2016 Answer 7 By Avogadro’s law, 1 mole of nitrogen reacts with 3 moles of hydrogen to form 2 moles of ammonia. Hence, the mole ratio is 1 : 3. 1biAt a lower pressure, there are fewer nitrogen and hydrogen molecules per unit volume. This decreases the frequency of effective collisions between the molecules and hence the rate of reaction deceases. 11biiFrom the graph, the percentage yield of ammonia decreases as pressure decreases. For example, at 500°C, the yield of ammonia is about 24% at 400 atm but is about 8% at 100 atm. Hence, there will be a smaller volume of ammonia and larger volumes of unreacted nitrogen and hydrogen leaving the main reactor. 11biiiThere will be no effect on the final amount of ammonia formed. This is because the unreacted nitrogen and hydrogen gases will be recycled again to react to form ammonia. 11EITHERB9asolutionname of products of electrolysisionic equation for reaction at each electrodeconcentrated aqueous copper(II) chlorideat negative electrodecopper metalCu2+(aq) + 2e-  Cu(s)at positive electrodechlorine gas2Cl-(aq)  Cl2(g) + 2e-dilute copper(II) sulfateat negative electrodecopper metalCu2+(aq) + 2e-  Cu(s)at positive electrodeoxygen gas4OH-(aq)  O2(g) + 2H2O(l) + 4e-[1 mark for each correct product + ionic equation][State symbols not required for ionic equations]1111biA and DbiiReagent A:When A is added to both copper(II) chloride and copper(II) sulfate, a reddish brown solid will be formed and the solutions turn from blue to colourless. 2

2016 Answer 8 Reagent B:When B is added to copper(II) chloride, a white precipitate will be formed. When B is added to copper(II) sulfate, there is no visible change. Reagent C:When C is added to copper(II) chloride, there is no visible change.When C is added to copper(II) sulfate, a white precipitate will be formed. Reagent D:Whe