2022 SASS A-Z Chem Prelims Ans Pt3

1 A – Z School’s Preliminary Papers 4 Express Chemistry ANSWERS: Part 3 K CHIJ Katong Convent School L Loyang View Secondary School M Montfort Secondary School N Nanyang Girls’ High School O Orchid Park Secondary School CHIJ KATONG CONVENT SCHOOL Paper 1 Paper 2 1 Sec 4 Express Pure Chemistry (6092) Preliminary Examination 2022 Marking Scheme Paper 1 (40 marks) 1 2 3 4 5 6 7 8 9 10 D D B B A C A B A A 11 12 13 14 15 16 17 18 19 20 B C D A C C A D B D 21 22 23 24 25 26 27 28 29 30 C A D C A B B B B D 31 32 33 34 35 36 37 38 39 40 D A D D A C A B A D Paper 2, Section A (50 marks) Qn Correct Response Marks A1a nitrogen dioxide 1 A1b iron(III) oxide 1 A1c lithium oxide 1 A1d calcium oxide 1 A1e silicon dioxide 1 Total 5 A2a F 1 A2b A 1 A2c B 1 A2d D 1 Total 4 A3a H2 and CrCl2 1 A3b Aluminium has an oxide layer / aluminium oxide on the surface. This forms a protective barrier/ layer around the wire, preventing the acid from reacting with it. 1 1 1 Sec 4 Express Pure Chemistry (6092) Preliminary Examination 2022 Marking Scheme Paper 1 (40 marks) 1 2 3 4 5 6 7 8 9 10 D D B B A C A B A A 11 12 13 14 15 16 17 18 19 20 B C D A C C A D B D 21 22 23 24 25 26 27 28 29 30 C A D C A B B B B D 31 32 33 34 35 36 37 38 39 40 D A D D A C A B A D Paper 2, Section A (50 marks) Qn Correct Response Marks A1a nitrogen dioxide 1 A1b iron(III) oxide 1 A1c lithium oxide 1 A1d calcium oxide 1 A1e silicon dioxide 1 Total 5 A2a F 1 A2b A 1 A2c B 1 A2d D 1 Total 4 A3a H2 and CrCl2 1 A3b Aluminium has an oxide layer / aluminium oxide on the surface. This forms a protective barrier/ layer around the wire, preventing the acid from reacting with it. 1 1 SASChemistry 2022edition

2 2 Qn Correct Response Marks A3c labelling, lesser copper atoms than zinc atoms [1] disrupted arrangement of atom layers [1] 2 Total 5 A4a no. of mol of H2SO4 =45 1000 × 0.20 = 0.009 mol [1] no. of mol of KOH =0.78 39+16+1 = 0.0139 mol (3 sf) [1] From equation, mole ratio of KOH:H2SO4 = 2:1 sulfuric acid is in excess because (2 × 0.009 = 0.018 mol) is greater than 0.0139 mol [1] / sulfuric acid is in excess because ( 0.0139 2 = 0.00695 mol) is less than 0.009 mol 3 A4b ammonia (gas) 1 Total 4 A5a Higher pressure of (gaseous) reactant(s)/ H2 and Cl2 / H2/ Cl2 There are more reactant particles/ H2 and Cl2 molecules per unit volume of the gas and they collide more frequently and there is an increase in frequency of effective collision. 1 1 A5b(i) Energy released = 184 x 2 = 368 kJ 1 A5b(ii) Energy taken in to break bonds in reactants/ hydrogen and chlorine is less than energy given out to form bonds in products/ hydrogen chloride. 1 1 Total 5 A6ai The percentage yield of ammonia decreases with increasing temperature. 1 A6aii Percentage yield of ammonia increases with increasing pressure. 1 A6aiii Catalyst speeds up the reaction / lowers activation energy. Less energy is used and lowers energy cost. 1 1 zinc atoms copper atoms

3 3 Qn Correct Response Marks A6b NH3 + 2O2 → HNO3 + H2O 1 Total 5 A7a Viscosity. 1 A7b(i) Substitution reaction. 1 A7b(ii) Chlorine (or any halogen) 1 A7c any one from: C13H28 → C4H8 + C9H20 C13H28 → 2C4H8 + C5H12 C13H28 → 3C4H8 + CH4 1 A7d C H % by mass 88.9 100 - 88.9 = 11.1 no. of moles 88.9 12 = 7.41 11.1 1 = 11.1 [1] mole ratio 7.41 7.41 = 1 11.1 7.41 = 1.5 simplest mole ratio 2 3 empirical formula C2H3 [1] 2 Total 6 A8a Acidified potassium manganate(VII) / acidified potassium dichromate(VI) 1 A8b 1 A8c H+ 1 A8d 2HCOOH + Zn → (HCOO)2Zn + H2 / 2HCOOH + Zn → Zn (HCOO)2 + H2 correct chemical formulae [1] balance correctly [1] 2 Total 5 A9a Number of repeat units = 14 000 4(12)+1 (8) = 250 Number of C atoms = 250 x 4 =1000 1 1

4 4 Qn Correct Response Marks A9b true false Polymer D is a polyester.  Water is formed as a by-product when monomers react to form polymers A and B.  The empirical formula of polymer C is the same as its monomer.  One of the monomers that react to form polymer B is an alcohol.  4 corr – 2 2 – 3 corr – 1 1 corr – 0 A9c 1 1 A9di 1 A9dii 1 1 A9e Advantage: The polymers are durable/ resistant to corrosion. Disadvantage: These polymers when disposed cause land pollution as they take up space in landfills / These polymers when burnt in incinerator produce greenhouse gases which causes climate changes Reject: The polymers will cause environmental problems when they are improperly disposed. 1 1 Total 11 (CH2)2 OH HO / H C C H CH3 COOCH3

5 5 Paper 2, Section B (30 marks) Qn Correct Response Marks B10a(i) Anhydrous copper(II) sulfate changes from white to blue. 1 B10a(ii) By heating to dryness / evaporating to dryness the hydrated copper(II) sulfate. 1 B10a(iii) By determining the boiling point which is 100 C for pure water. / By determining the freezing point which is 0 C for pure water. 2 B10b(i) number of moles of Cu(NO3)2 used = 4.7 64 + 2(14)+ 6(16) = 0.025 mole ratio = Cu(NO3)2 : NO2 1 : 2 number of moles of NO2 formed = 0.025 x 2 = 0.050 [1] volume of NO2 formed = 0.050 × 24 = 1.2 dm3 [1] 2 B10b(ii) mole ratio = Cu(NO3)2 : O2 2 : 1 number of moles of O2 formed = 0.025 ÷ 2 = 0.0125 [1] mass of O2 formed = 0.0125 × 2(16) = 0.4 g [1] 2 B10c CuO(s) + H2(g) → Cu(s) + H2O(l) correct chemical formulae [1] correct state symbols and balance correctly [1] 2 Total 10

6 6 Qn Correct Response Marks B11a Cell 1. The electrodes are made up of two (different) metals / The electrodes are not inert. 1 1 B11b Copper(II) ions, nitrate ions, hydrogen ions, hydroxide ions [Must get all the name of ions correct] 1 B11c At zinc electrode: Zn(s)→ Zn2+ (aq) + 2e- At copper electrode: Cu2+ (aq) + 2e- → Cu(s) 1 1 B11di Chlorine/ Cl2 gas is produced at electrode Y. Due to the higher concentration of chloride ions compared to hydroxide ions, chloride ions are preferentially discharged/discharged in preference/ preferentially oxidised. 1 1 B11dii Disagree. The volume/number of moles of hydrogen gas produced at electrode X should be the same as the volume of chlorine gas produced at electrode Y. / The volume/number of moles of gas produced should be the same at both electrodes. 1 B11diii Oxygen gas is produced at electrode Y. Since concentration of chloride ions is lower than hydroxide ions, hydroxide ions are preferentially discharged/ discharged in preference / preferentially oxidized. Reject: concentration of Cl- ions decreased. 1 1 Total 10

7 7 Qn Correct Response Marks EITHER B12a Nitrogen makes up 78% of air. 1 B12b Carbon dioxide. Reject: other greenhouse gases Average global temperature increases/ More occurrences of unusual weather conditions/ Melting of icebergs that causes ocean levels to rise and cause flood. / AVP 1 any 1 - 1 B12ci Temperature of the gases from engine is high. The reactant particles have more kinetic energy and move faster. The powdered catalyst. Increasing total surface area / surface area to volume ratio. Increasing collision and effective collision frequency. 1 1 B12cii Oxygen/ unburnt hydrocarbons 2C8H18 + 25O2 → 16CO2 + 18H2O Carbon monoxide 2CO + O2 → 2CO2 2CO + 2NO → 2CO2 + N2 4CO + 2NO2 → 4CO2 + N2 Nitrogen oxides / nitrogen monoxide / nitrogen dioxide 2CO + 2NO → 2CO2 + N2 4CO + 2NO2 → 4CO2 + N2