YSS 4E Pure Chem Paper 2 Prelim 2024 MS

Page 1 of 6 Yishun Secondary School Mark Scheme 2024 4E Chemistry Preliminary Examination Date of exam: P2 – 23 Aug 2024 Setter: Paper 2 – Mr Rizal Answers to Paper 2 Qn Key Answers Remarks 1(a) graphite [1] 1(b) calcium chloride / sodium chloride / copper(II) carbonate (reject chemical formula) [1] 1(c) copper(II) carbonate [1] 1(d) copper, zinc [1] 1(e) helium [1] 1(f) zinc [1] 2(a) Experiment 1 Anode: 4OH ̶ (aq) → 2H2O (l) + O2 (g) + 4e ̶ Cathode: Ag+ (aq) + e ̶ → Ag (s) Experiment 2 Anode: Ag (s) → Ag+ (aq) + e ̶ Cathode: Ag+ (aq) + e ̶ → Ag (s) both cathode reactions [1] [1] [1] 2(b) When aqueous silver nitrate reacts with aqueous sodium chloride it will form a white precipitate of silver chloride [1] 2(c) In Experiment 1, no more silver was deposited on the object as all the silver ions in the electrolyte have been discharged. In Experiment 2, this will not be a problem as the silver ions discharged at the cathode are continuously being replaced by the silver anode which ionised / dissociate into silver ions. [1] [1] 2(d) Iron being higher in the reactivity series / more reactive than silver, will be able to displace silver from aqueous silver nitrate. Silver metal is deposited on the iron. But gold is lower than silver in the reactivity series, thus it cannot displace silver from aqueous silver nitrate. [1] [1] 3(ai) salt reagent 1 reagent 2 potassium sulfate sulfuric acid potassium hydroxide lead(II) nitrate nitric acid lead(II) oxide / lead(II) carbonate all correct [2] 2 – 3 correct [1] 0 – 1 correct [0] [2]

Page 2 of 6 Qn Key Answers Remarks 3(aii) K2SO4 + Pb(NO3)2 → PbSO4 + 2KNO3 no. of mole of K2SO4 = 15 1000 x 2 = 0.03 mol K2SO4 : PbSO4 1 : 1 0.03 mol : 0.03 mol mass of PbSO4 = 0.03 x (207+32+4(16)) = 9.09 g [1] [1] [1] 3(bi) willemite: SiO44- castorite: Si4O104- chrysotile: Si2O52- 1 – 2 correct [1] all correct [2] [2] 3(bii) Na Al Si O % by mass 8.7 10.3 32.3 48.7 molar mass 23 27 28 16 no. of moles 8.7 23 = 0.378 10.3 27 = 0.381 32.3 28 = 1.154 48.7 16 = 3.04 smallest ratio 1 1 3 8 Empirical Formula = NaAlSi3O8 (shown) [1] [1] 4(a) calcium [1] 4(b) number of electron shells number of valence electrons 7 4 [1] 4(c) Similarity: They have the same number of protons which is 114. Difference: 289Fl has 175 neutrons but 288Fl has 174 neutrons (must state both) [1] [1] 4(di) correct diagram (ratio of cations : electrons must be 1 : 1 ) correct label [1] [1] 4(dii) good conductor of electricity / good conductor of heat / malleable / ductile / high melting and boiling point [1] 4(diii) It reacts with acids to form salt and water [1] 5(a) Rf value of phaeophytin = 5 / 8 = 0.625 [1] (sea of delocalised) electrons

Page 3 of 6 Qn Key Answers Remarks 5(b)