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] 5 (a) Oxidation; Hydrogen loss electrons to form H+ ion 1 (b) Water 1 (c) O2(g) + 4H+(aq) + 4e− → 2H2O(g) 1 (d) Advantage: Water is the only product so it is non-polluting unlike pollutant such as carbon dioxide or carbon monoxide or unburnt hydrocarbon which is produced in a petrol engine; Disadvantage: Unlike petrol, hydrogen is a gas making it difficult to be transported/stored OR There is a risk of hydrogen gas leaking and hence reacting explosively; 1 1 [Total: 5] 6 (a) Oxygen produced at the anode reacts with graphite at high temperature to form carbon dioxide; 1 1 (b) Molten cryolite is an impurity in molten aluminium oxide; This mixture melts at a lower temperature than pure aluminium oxide; Hence, less energy is needed to keep the electrolyte molten during electrolysis 1 1 (c) SO42− and OH− ions are attracted to the positive electrode made of aluminium. OH− ions are lower than SO42− ions in the electrochemical series and are selectively discharged, producing oxygen; 4OH−(aq) → O2(g) + 2H2O(l) + 4e− 1m – balanced equation 1m – explanation of OH− being selectively discharged 1 1 (di) Aluminium, metal X and metal Y 1 (dii) Grey solid form; (reject ppt) Green solution turns colourless; 1 1 (diii) Aluminium is more reactive than metal X. Hence, it displaces X from its chloride solution, forming grey solid X and colourless aluminium chloride solution. 1m – more reactive + displace ; 1m – identify products formed 1 1 [Total: 11]

7 (a) Poly(propene) Repeat unit: 1 1 (bi) Depolymerisation is a process in which polymers are broken down into their monomers (using water in the presence of an acid catalyst). 1 (bii) Social issues • more convenient to throw away recyclable plastics • unaware of proper way to recycle plastics • takes time and effort to adopt a recycling lifestyle Economic issues • can be expensive o transport cost o cost of sorting and cleaning o energy cost • recycling plastic waste may cost more than the recycled plastic 1 1 [Total: 5] 8 (ai) Stearic acid has a higher Mr or longer carbon chain than palmitic acid. Hence, more energy is needed to overcome the stronger intermolecular forces of attraction between stearic acid molecules than between palmitic acid molecules. 1 1 (aii) Presence of C=C bonds decreases the melting point of carboxylic acid. 1 (aiii) Add aqueous bromine into the monomer. Reddish brown aqueous bromine will decolourise rapidly. 1m – describe the test 1m – test result 1 1 (b) Reject: 1

(ci) Mass of I2 = 0.237 0.256 x 100 = 92.6 g 1 (cii) No. of moles of I2 = 𝑚𝑎𝑠𝑠 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 = 92.6 2 𝑥 127 = 0.364 mol [1] No. of moles of oil = 100 782 = 0.128 mol [1] 1 mol of I2 exactly saturates 1 mol of C=Cbond Mole ratio of oil : I2 0.128 : 0.364 1 : 2.84 Average number of C=C bonds per oil molecule in olive oil is 2.84 (cao) [1] 3 [Total: 10] 9 (a) Carbon dioxide is a greenhouse gas which traps heat; Having excessive amount of carbon dioxide in the atmosphere may lead to global warming. 1 1 (b) More CO2 per unit volume can be transported. 1 (ci) Mg2SiO4(s) + 2CO2(l) → 2MgCO3(s) + SiO2(s) 1m – balanced chemical equation 1m – correct state symbols (award only if first mark is awarded) 2 (cii) Solid magnesium carbonate and silicon dioxide formed might block the liquid CO2 pipelines. 1 (d) Release of iron compounds pollutes the ocean OR Burning of fossil fuel to run vessels will release excessive CO2 into the atmosphere. 1 (e) Number of moles of 1 kg Fe = 1000 ÷ 56 = 17.857 mol [1] Based on Redfield ratio Fe : C 0.001 : 106 1 : 106 000 17.857 : 106 000 x 17.857 = 1 892 842 No. of mole of C = 1 892 842 [1] (allow ecf) 3

Mass of CO2 = 1 892 842 x 44 = 83 285 048 g = 83 285.048 kg = 83 000 kg [1] (rounded to the nearest thousand) [Total: 10] 10 (a) Sulfur is a solid / precipitate / insoluble substance, preventing light from reaching the sensor. Percentage of light reaching the light sensor decreases because more sulfur is produced with time. 1 1 (bi) Rate of reaction decreases; because concentration of reactants decreases; 1 1 (bii) Hydrochloric acid is used up 1 (c) 1 (d) When temperature increase, reacting particles gain energy and move faster (allow increase KE); More reacting particles will have energy equal to or more than the activation energy; This increases the frequency of effective collision between reacting particles; Speed of reaction increase, hence reaction takes a shorter time to complete. 1 1 (e) When mass of S produced = 0.35 g 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑑𝑖𝑢𝑚 𝑡ℎ𝑖𝑜𝑠𝑢𝑙𝑓𝑎𝑡𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 ℎ𝑦𝑑𝑟𝑜𝑐ℎ𝑙𝑜𝑟