Copy of 2024 CCHM Chemistry 6092 P1 & P2 (MS)

2024 CCHM Chemistry Prelim Mark Scheme 1 (ai) C 1 (aii) E 1 (aiii) B 1 (aiv) D 1 (b) Correct structure of C2+ ion; Correct structure of B- ion; Correct ratio of C2+: B- ; (award only if structure of ions are correct) Allow BeCl2 and [ ]2+ [ ]- [ ]- arrangement 2 [Total: 6] 2 (ai) Filter the mixture; Wash the residue with distilled water and dry the salt between filter paper; 1 1 (aii) Filter the reaction mixture to remove silver dichromate. To the filtrate, add aqueous sodium chloride. (or any chloride or iodide solution) If white precipitate is observed, silver nitrate solution is added in excess. (No visible change is observed in the reaction mixture if silver nitrate solution has been used up.) If student didn’t filter, 1m for suitable reagent, 1m for positive result 1 1 1 (bi) +6 (cao) 1 (bii) 2Ag+(aq) + Cr2O72-(aq) → Ag2Cr2O7(s) 1m – balanced chemical equation 1m – correct state symbols (award only if formula is correct) 2 (ci) The dichromate ion (Mr of 216) has a larger relative molecular mass/molar mass than the silver ion (Ar of 108). Hence silver ion diffuses faster than dichromate ion and the red line is formed nearer to ammonium dichromate. 1 1 (cii) Shorter; When heated, the ions/particles gain energy and moves faster (allow gain KE). Hence, they diffuse faster towards each other. 1 *If answer lack the idea of ‘particles’ in (ci) and (cii), deduct 1m from (c) [Total: 11]

3 (a) Decrease; Water was produced and escaped as vapour / water vapour is produced. 1 (b) To ensure that reaction or decomposition is complete / to completely remove water. 1 (ci) Energy taken in = 88.1 238 x 2 ------------- [1] = 0.740336 = 0.740 kJ (cao) [1] 1 1 (cii) 1m - correct profile with equation; 1m - correct activation energy as shown (reject double-headed arrow); 1m - correct enthalpy change as shown (reject double-headed arrow); Award max 1m for activation energy if profile is wrong 1 1 1 [Total: 7] 4 (a) Cobalt and nickel have identical or similar relative atomic masses. 1 (b) Any two Transition elements Halogens High melting/boiling point Low Melting/boiling point High density Low density Good conductor of heat and electricity Poor conductor of heat, non-conductor of electricity Form positive ions by losing electrons Form negative ions by gaining electrons Good catalysts Not catalyst Form coloured compounds Form white compounds (allow do not) Have variable oxidation states in their compounds Have fixed oxidation state in their compounds 2 (ci) Hydrogen atom loses one electron to form ion of +1 charge OR Hydrogen atom has one valence electron 1

(cii) Any one • Hydrogen atom gains one electron to form ion of -1 charge; • Hydrogen atom can form diatomic molecule through sharing of electrons; • Hydrogen has low melting/boiling point; 1 [Total: 5]