2025 Prelim P4 Mark Scheme (for exchange)_H2 Chem_HCI

1 HCI C2 H2 Chemistry Preliminary Examination/Paper 4 Mark Scheme Hwa Chong Institution 2025 C2 H2 Chemistry Prelim Exam Paper 4 Mark Scheme Qn Mark Scheme 1(a) test observations To 1 cm depth of FA 1, add 1 cm depth of dilute H2SO4. No effervescence To 1 cm depth of FA 1, add aqueous barium nitrate (dropwise). White ppt To 1 cm depth of FA 1, add 1 (or 2) drops of aqueous silver nitrate. followed by aqueous NH3, until there is no further change Pale cream/ cream/ cream-yellow/ pale yellow ppt ppt partially soluble/ insoluble in NH3(aq) [1] for each test and correct observation M1 M2 M3 M4 anion: SO42– [1] and Br– [1] M5 M6 1(b)(i) test observations Transfer all of the solid sample of FA 4 into a boiling tube. Add all of FA 3 to this boiling tube. Gently warm the boiling tube. Turn off the Bunsen flame. White solid dissolves to give a colourless solution. [½] (–½ overall if “effervescence/ gas/ gas bubbles” is written here) Handle the hot boiling tube with care. Add FA 2 to the mixture in three separate portions using a spatula, shaking after each separate addition. Add all the remaining FA 2 to the mixture. Using the glass filter funnel , filter the mixture into a test tube and leave the filtrate to stand. The filtrate contains compound X. Retain this filtrate for use in (b)(ii). Effervescence [½] Gas gives white ppt in limewater [½] Gas is CO2 [½] Colourless/ pale pink/ reddish-pink/ red/ purple-red filtrate [½] Black residue [½] M7 M8 M9 1(b)(ii) test FA 5 filtrate from (b)(i) To 1cm depth of FA 5 in a test tube, add aqueous ammonia dropwise, until in excess. Repeat using filtrate from (b)(i). off-white ppt [½] insoluble in excess [½] turns brown on standing/ on contact with air [1] light brown / brown ppt [½] insoluble in excess [½] turns darker brown on standing/ on contact with air M10 M11 M12 1(b)(iii) Ion present is: Mn2+ [½] Explanation: off-white ppt formed with aqueous NH 3, ppt insoluble in excess aqueous NH3 and turns brown on standing/ on contact with air [½] M13

2 HCI C2 H2 Chemistry Preliminary Examination/Paper 4 Mark Scheme 1(b)(iv) Identity of ion: Mn3+ [½] (Incomplete) reduction of MnO2 (ZO2) (FA 2) to Mn3+ / some of the MnO 2 were reduced (in (b)(i)) to Mn3+ only. Mn3+ formed the brown ppt when aqueous NH3 was added. [½] M14 1(ci) test Observations 1 Add 2 cm depth of aqueous silver nitrate to a clean boiling tube. Add 1 cm depth of aqueous sodium hydroxide slowly to the same tube. brown/black ppt formed Add aqueous ammonia slowly, with shaking, until the precipitate just dissolves. You may use a clean glass rod to help dissolve the precipitate. brown ppt dissolved to form a colourless solution Add 2 cm depth of FA 6 to this mixture and shake the boiling tube. Place the boiling tube in the test-tube rack and leave it f