SNGS 2024 Chem Prelim P2 Answers

2024 Prelim Chem Paper 2 1(a) SO2 or NO2 Poly(ethene) is a macromolecule whereby the boiling point will be higher than those with simple covalent structure. SO2 or NO2 has simple covalent structure with molecular mass greater than 32 so their boiling point will be higher than oxygen due to the stronger intermolecular forces of attraction. 1 1 1 (b) NH3 1 (c) ZnO 1 (d) CO /SO2 1 (e) NH3 1 (f) K2O 1 2(a) H+ any suitable indicator eg moist blue litmus turns red OR Add magnesium , lighted splint extinguished with a pop sound. OR Add carbonate, white precipitate formed in limewater. 1 1 (b) Potassium hydroxide/potassium carbonate and phosphoric acid Titration 1 1 (c) Add acidified aqueous silver nitrate White ppt observed Aqueous lead(II) nitrate max 1m 1 1 (d) Mole of Na+ = (156/1000)/23 OR Mole of Na+ = (48/1000)/23 = 0.00678 mol = 0.00209 mol AND Mole of Cl- = (127/1000)/35.5 OR Mole of Cl- = (39/1000)/35.5 = 0.00358 mol = 0.00110 mol For every mole of NaCl, no of mole of Na+ = No of mol of Cl-. Since there are more mole of Na+ than Cl-, there are other compounds of sodium. 1 1 (e) Energy Shape of graph [1] Label axes with units and include correct reactant and product [1] Enthalpy change and activation energy [1] Level diagram 2m max Endo 1m max 3

(f) (i)yeast, 370C, darkness, no air 0.5 ea (ii) C2H5OH + 3O2 → 2CO2 + 3H2O and C6H12O6 → 2C2H5OH + 2CO2 6CO2 + 6H2O → C6H12O6 + 6O2 not balance -1m 1 1 3(a) Stage IV 1 (b) No, a catalyst provides an alternative pathway with lower activation energy but the enthalpy change remains unchanged as the energy of reactant and product remain the same. 1 1 (c) Stage I: S is oxidised as its oxidation state increases from 0 to +4 in SO2 and O2 is reduced as its oxidation state decreases from 0 to -2 in SO2 OR Stage II: SO2 is oxidised as the oxidation state of S increases from +4 in SO2 to +6 in SO3 and O2 is reduced as its oxidation state decreases from 0 in O2 to -2 in SO3 1 1 (d) Stage II is a reversible reaction. *Some of the sulfur trioxide decompose to sulfur dioxide *at the same time/immediately 1 (e) 2S+ 3O2 + 2H2O → 2H2SO4 Not balanced : 0m 1 (f) (i) NO2 Nitrogen and oxygen in air react in the high temperature at t