2025 CCHY Prelim P2 Answer

2025 CCHY S4 Chemistry Preliminary Examination P2 Marking Scheme Qn Marking point Marks 1(a) Al or Fe 1 (b) N or O 1 (c) Fe 1 (d) Ca 1 (e) C 1 (f) O or N 1 Total 6 2(a) Alkaline 1 Concentration of hydroxide ions is higher than concentration of hydrogen ions Reject amount 1 (b) Sodium sulfate; Na2SO4 2 Sodium bicarbonate, NaHCO3 Potassium sulfate, K2SO4 Potassium bicarbonate, KHCO3 Magnesium sulfate, MgSO4 Magnesium bicarbonate, Mg(HCO3)2 Calcium sulfate, CaSO4 Calcium bicarbonate, Ca(HCO3)2 Any two of the above (c) Heating to remove all the water/ evaporate to dryness Reject heating/ crystallisation 1 Total 5 3(a) X: concentrated ammonia Y: concentrated hydrochloric acid 1 Mr of ammonia= 17 Mr of HCl= 36.5 Relative molecular mass of ammonia is lower than hydrogen chloride 1 Ammonia (gas from X) will diffuse faster and travel a longer distance [answer can be of gas from Y] 1 (b) Region P: blue 1 Region R: red 1

Qn Marking point Marks (c) A neutral salt formed 1 Total 6 4(a) How: lithium atom loses 1 valence electron to form a positively charged ion 1 How: chlorine atom gains 1 valence electron to form a negatively charged ion 1 Why: to achieve the full valence shell of a noble gas 1 (b) Describe: radius of lithium atom decreases when it forms an ion 1 Explain: Lithium atom loses 1 electron shell when it forms an ion 1 (c) Observation: no visible change/ no observable change/ solution remains reddish brown 1 Explain: bromine is less reactive than chlorine thus no displacement reaction takes place 1 Total 6 5(a)(i) Catalytic converter converted the harmful pollutants, CO, NO and unburnt hydrocarbons into harmless CO2 and H2O 1 (ii) The excess CO2 produced will lead to global warming and thus will lead to extreme weather events 1 (b) The rate of photosynthesis to absorb CO2 cannot catch up 1 with the rate of CO2 being released during the combustion of fossil fuels and animals respiration 1 (c) The amount of CO2 absorbed by the sugar cane during photosynthesis 1 is equal to 1 the amount of CO2 released when ethanol burned as fuel 1 Thus there is net zero change in atmospheric CO2 Total 7 6(a) They formed coloured compounds 1 They form compounds with variable oxidation state 1 (b)(i) 2Cr (s) + 6H+ (aq) → 2Cr3+ (aq) + 3H2 (g) 1: correct balance eqn

Qn Marking point Marks 1: correct state symbol (ii) Cr was oxidized as it lost electrons to form Cr3+/Cr2(SO4)3. 1 H+ / Hydrogen ion / H2SO4 was reduced as H+ gained the elections to form H2 1 (c) Order of metal in increasing reactivity: Iron, Chromium, Manganese 1 Observation in set up A: Green solution turns pink, grey/silvery solid forms 1 Observation in set up B: Green solution turns violet, grey/silvery solid forms. 1 Observation in set up C: No visible / observable change / pale pink solution re