2024 Fuhua Sec 4E Chemistry P2 answers

1 FUHUA SECONDARY SCHOOL Sec 4E Chemistry 6092 Preliminary Examinations 2024 – Mark Scheme PAPER 2 Section A [70 marks] Q Answer Mark Remarks 1a Q and R 1 b ZT2 1 ci shared electrons non-bonded electrons 1 1 cii Charge of Mg2+ and M3- [;] Three Mg2+ and two M3- [;] Outer electrons for Mg2+ and M3-[;] 2 3; [2] d true false Molecule 3 has lower boiling point then molecule 2. ✓ Molecule 3 is a saturated organic compound. ✓ Element Z reacts with oxygen to form acidic oxide only. ✓ Elements P and T are in Group 16. ✓ 2 4✓ [2] Total 8 marks 2ai Mass of CO2 = 100 – 97.8 = 2.2 g Moles of CO2 = 2.2 / 44 = 0.05 mol 1 ii experiment particle size volume of acid / cm3 concentration of acid / mol/dm3 1 powder 50 e.c.f from (a)(i) mole 2.0 2 lump 200 [1] (must be 4x of expt 1/ 2x of expt 3) 1.0 3 Powder [;] Same as expt 1 100 [;] 2x of expt 1 2.0 [;] Same as expt 1 Expt 1: mole of H+ = 0.05 x 2 = 0.10 mol Volume of acid = 0.10 / 2.0 = 0.05dm3 = 50 cm3 1 1 1 3; [1] 2b As particle size decreases[;], there is a larger exposed surface area[;] of barium carbonate in contact with the acid. There are more collisions and increase in frequency of effective collisions between barium carbonate and H+ ions the acid particles[;], hence increasing speed of reaction[;]. 2 4; [2] 2; [1] c Add aq. silver nitrate solution to each acid, if white ppt forms, the acid is HCl. OR Add aq. NaOH, aluminium foil to each acid and heat the mixture, if ammonia gas is produced (moist red litmus turns blue), the acid is HNO3. 1

2 d Rate will be slower [;] and change in mass will be the same [;] as experiment 1. Ethanoic acid is a weak acid which dissociates partially in aqueous solution to give a lower concentration of H+ ions [;] compared to the strong acid, hence rate is slower. The acid / H+ ions remains as the limiting reagent / same number of moles of H+ ions used [;] hence mass of gas produced is the same and same change in mass of the reaction. 3 4[;] – [3], 2-3[;] – [2], 1[;] – [1] Total 10 marks 3ai Concentration of CuSO4 decreases [;] from 1.10 mol/dm3 until all Cu2+ ions in solution reduced to form Cu at the cathode. [;] Cu2+(aq) + 2e → Cu(s) [;] 2 3; [2] ii Amount of Cu deposited = 200/1000 X (1.10 – 0.22) = 0.176 mol Mass of Cu = 0.176 X 64 =11.3 g 1 1 bi straight horizontal line at 1.10 g 1 ii Colourless gas given off at the graphite electrode. Grey / silvery solid deposited on the iron object. 1 1 c Once tin is scratched, iron will lose electrons / oxidise more readily to form iron(II) ions as tin is less reactive than iron 1 1 Total 9 marks 4a butyl propanoate[;], butanol[;], propanoic acid[;] , addition of hydrogen Accept catalytic hydrogenation. , addition polymerisation Reject ‘additional polymerisation’ , condensation polymerisation 2 1 1 1 3 ; [2], 1; [1] bi Any two of the following: • Members have the same general formula CnH2n+1X • There is gradual increase in boiling point as the number of carbon atoms increases. • Successive members differ from the next by a –CH2 group. 2 ii As the halogen atom changes from Cl to I, the boiling point of the alkyl halide increases. The size of halogen atom increases from Cl to I, molecular mass / molecular size of alky halide increases [1] and hence boiling point increases. Intermoleular forces of attraction between molecules increases and amount of energy taken in to overcome these forces increasesi 1 1 1 iii

3 displayed formulae of organic compounds balanced equation and formulae of other chemicals 1 1 Total 12 marks 5a When chromium is heated with manganese oxide, there is no visible change. [;] as chromium is less reactive than manganese. [;] When chromium is heated with copper (II) oxide, black CuO turned to reddish-brown Cu / grey Cu turned to green Cr2O3../ black CuO and grey Cr turned to green Cr2O3.and reddish-brown Cu. Chromium is more reactive than copper and displaces copper from CuO. 1 1 1 2; [1] b 3Mn (s) + 2Cr3+ (aq) → 3Mn2+ (aq) + 2Cr (s) 1 c Blue solution turned green and grey metal coated with a reddish-brown solid. 1 Total 5 marks 6a colour and state at room temperature and pressure chlorine Greenish-yellow gas [;] bromine Reddish-brown liquid [;] iodine Purplish-black solid [;] 2 2; [1] 3; [2] b In the first experiment, bromine is more reactive than iodine [;] and displaces iodine from aqueous iodide[;] , forming aqueous iodine which is brown. [;] In the second experiment, iodine is less reactive than chlorine [;] and unable to displace chlorine from aqueous chloride ions. Hence aqueous iodine remains and is brown solution. [;] 2 4; [2] 2; [1] c The oxidation state of chlorine increases from 0 in Cl2 to +1 in OCl− and decreases to -1 in Cl−. Therefore, chlorine is oxidised and reduced simultaneously. 1 1 Total 6 marks + HI → + H2O

4 7a Measure each sample of acid with a pH meter AND If the pH reading ranges from 3 to 6, then it is a weak acid OR Add a few drops of Universal indicator to each sample AND If the indicator changes to a yellow or orange colour, it is a weak acid [1] 1 b volume ratio 3: 2 1 c 1. Add aqueous sodium tartarate to a fixed volume of aqueous copper(II) nitrate in a beaker till no more precipitate is formed. [1] 2. Filter the mixture to obtain copper(II) tartarate as a residue 3. Wash the residue with a little distilled water and pat dry between pieces of filter paper. 1 ; ; 2; [1] di 6.0°C [;] Since the magnesium ribbon is the limiting reactant [;], amount of heat energy given out is the same for same no. of moles of Mg [;] 2 3; [2] 1-2; [1] ii 2 3; [2] 1-2;[1] Total 8 marks 8ai HCFC-123 2 ii CFC-114: C2Cl2F4 HFC-125: C2HF5 1 1 b Comparing CFC-11 and HCFC-22 and HFC-23 with one carbon atom in each molecule, [;] HCFC HFC CFC ODP HCFC-22 has ODP of 0.04 less than CFC-11 but more than CFC- 11. [;] HFC-23 has the lowest ODP at less than 4 × l0-4. [;] CFC-11 has the highest ODP of 1.00. [;] 9; [4] 7-8; [3] 4-6; [2] 1-3; [1]

5 GWP HCFC-22 has the least GWP of 1700 [;] HFC-23 has the highest GWP at 12100. [;] CFC-11 has GWP of 4000 [;] Although use of HFC reduces the ozone layer depletion the least, reduces ODP by 2500X but it increases global warming to the greatest extent, increases GWP by 3X. [;] Use of HCFC reduces ODP by 25X reduces GWP by 2.4X.[;] ci CFC molecule releases one Cl atom in presence of sunlight.[;] Chlorine atom consumed by reacting with one O3 molecule [;] is regenerated when ClO reacts with another O3 molecule in the second step. [;] Hence overall Cl atoms is not used up / no net loss of chlorine atoms.[;] 2 4; [2] 1-3; [1] ii Agree C-F and C-H have higher bond energy values (485 and 413 kJ/mol) than C-Cl (328 kJ/mol) , less (light) energy is taken to break the C-Cl bonds to release Cl atoms which react with ozone. 1 1 d Ammonia and carbon dioxide do not react with ozone. 1 Total 12 marks Section B [10 marks] Q Answer M Remarks 9aii C5H12 → C2H6 + C3H6 1 iii Add aqueous bromine to A and B separately. [;] For A, aqueous bromine remained reddish-brown. [;] For B, reddish-brown aqueous bromine turned colourless.[;] 2 3; [2] . iiii Displayed formula for propyl propanoate 1 bi Mr of repeat unit = 114 When Mr = 16 000, number of repeating units = 16 000/114 = 140.35 [;] = 141 [round up] When Mr = 50 000, number of repeating units =