Copy of VS 2024 Sec 4 Chem Prelim Paper 1 and 2 Answers

1 Victoria School 2024 Secondary 4 Chemistry Prelim Answer Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 C D B B B A C A C D 11 12 13 14 15 16 17 18 19 20 D B A B C C C D D D 21 22 23 24 25 26 27 28 29 30 B A D C D B C C D B 31 32 33 34 35 36 37 38 39 40 A A D D A C B C D C Paper 2 Section A Qn Suggested answers Mark A1 2 Across: fluoride 1 3 Down: addition 1 4 Down: alloy 1 5 Down: sublimation 1 6 Down: fixed 1 A2a Sodium: The indicator would turn from green to purple REJECT: Blue Carbon: The indicator would turn from green to orange REJECT: Red 1 A2b [1m] – cation [1m] – anion 2 A2c 4Na + O2  2Na2O No of moles of sodium= 2.30 / 23 = 0.100 mol 4mol Na ≡ 2mol Na2O 0.100mol Na ≡ 0.05 mol Na2O Theoretical yield of Na2O = 0.0500 mol x 62 = 3.10 g 4Na + O2  2Na2O 1

2 Qn Suggested answers Mark Mass gained = 94.82 – 94.50 = 0.32g No of moles of oxygen = 0.32 / 16+16 = 0.01 mol 1mol O2 ≡ 2mol Na2O 0.0100mol O2 ≡ 0.0200 mol Na2O Actual Yield = 0.0200 x 62 = 1.24 g Percentage Yield = 1.24 / 3.10 x 100% = 40.0 % 1 1 A3ai Bond breaking absorbs energy, hence is endothermic. Bond forming releases energy, hence is exothermic. Since more energy is released than absorbed, the reaction is exothermic. OR The energy absorbed when breaking the bonds in 1 mole of carbon monoxide and 1 mole of chlorine is less than the energy released when forming the bonds in 1 mole of phosgene, hence making it an exothermic reaction. 1 1 1 1 A3aii The oxidation state of chlorine decreased from 0 in Cl2 to -1 in COCl2. This is reduction. The oxidation state of carbon increased from +2 in CO to +4 in COCl2. This is oxidation. Since oxidation and reduction occurs at the same time, this is a redox reaction. (Must state but no marks) 1 1 A3b At –128 oC, molecules are packed tightly and very close together in a regular/orderly manner and they can only vibrate about their fixed positions. (As temperature decreases, the molecules move faster and faster) and begin to settle in fixed positions. At –108 oC, the phosgene molecules are packed closely together in a disorderly manner and they are able to slide over each other. 1 1 A4a • Catalysts can be regenerated/are not consumed at the end of a reaction. • Thus they can be (reused), so they only need to be purchased once • small amount need to be used. • 1 1 A4b In pellet form, there is a higher surface area (per unit volume). Higher frequency of effective collisions between reacting particles (and the catalyst), leading to a faster reaction rate. 1 1

3 Qn Suggested answers Mark A4ci 1 A4di Methyl ethanoate 1 A4dii [1m] – balanced equation with irreversible arrow [1m] – full structural formula 2 A5a 1 A5bi 1 A5bii React