HCI 2024 Prelim P4 Solutions

2024 HCI C2 H2 Chemistry Prelims / Paper 4 HWA CHONG INSTITUTION 2024 C2 H2 CHEMISTRY PRELIMINARY EXAMINATION SUGGESTED SOLUTIONS Paper 4 1 (a) (i) Test 2 Obs 1. white/ off-white ppt formed (with 1cm3 NaOH(aq)) Obs 2. brown ppt formed/ brown ppt observed on the wall of the boiling tube/ white ppt turned brown Obs 3. (white) ppt partially soluble / insoluble in excess NaOH Obs 4. brown residue (on filtering) Obs 5. colourless filtrate (on filtering) Obs 6. white ppt formed (when dilute H2SO4 is added to the filtrate) Obs 7. (white) ppt soluble in excess dilute H2SO4 Test 3 Obs 8. white ppt (with dropwise addition of NH3(aq)) Obs 9. brown ppt/ some white ppt turns brown/ brown ppt observed on the wall of the test-tube Obs 10. ppt insoluble in excess aq. ammonia Test 4 Obs 11. white ppt (with aq. barium nitrate) Obs 12. (white ppt) insoluble in dilute HNO3 There are 12 Obs. They are scaled to 6 marks as follows. 11 – 12 = [6] 9 – 10 = [5] 7 – 8 = [4] 5 – 6 = [3] 3 – 4 = [2] 1 – 2 = [1] 0 = [0] 1 (a) (ii) Al3+: Based on test 2, white ppt forms in the filtrate when dilute H2SO4 is added to it. [1] Mn2+: Based on test 2 (&/or 3), white ppt rapidly turns brown. [1]

1 SO42–: Based on test 4, white ppt insoluble in dilute HNO3 [1] 1 (a) (iii) Effervescence of CO2 is due to the acid-base reaction between CO32– and the H+(aq) produced from the hydrolysis/ionisation of [Al(H2O)6]3+. [1] The white ppt observed is due to the precipitation of insoluble MnCO3 and Al(OH)3. [1] 1 (b) (i) Obs 1. pale brown / colourless filtrate (after dilute HNO3 is added) Obs 2. solution decolourises / remains colourless (when FA 2 is added) Obs 3. brown ppt/solution observed when NaOH(aq) is first added Obs 4. brown ppt dissolves/ brown solution decolorises shortly after appearing Obs 5. this is followed by effervescence of colourless, odourless gas Obs 6. that relights a glowing splint Obs 7. gas evolved is oxygen Obs 8. brown ppt formed dissolves more slowly (with excess NaOH(aq)) / brown ppt remains eventually. (effervescence ceases) There are 8 Obs. They are scaled to 3 marks as follows. 6 – 8 = [3] 4 – 5 = [2] 2 – 3 = [1] 0 – 1 = [0] 1 (b) (ii) The brown intermediate formed on adding NaOH(aq) increases the rate of decomposition of H2O2. [1] OR Since O 2 evolved when NaOH(aq) is added, H 2O2 is being oxidised in an alkaline medium. This implies that H 2O2 is a stronger reducing agent in an alkaline medium. OR H2O2 decomposes more rapidly in alkaline medium as compared to in acidic medium. 2 (a) Headers and units for transfer time and t Transfer time in min & s, to nearest s. t correctly calculated to 1 d.p. [1] Headers and units for burette readings and volume of FA 5 Correct calculation of volume of FA5 added All burette readings and volumes of FA5 to 2 d.p. (0.05 cm3). [1] 5 sets of data, 1st and 2nd aliquot within 1st and 2nd min of exp