2023 RI H2 Chem Prelims P4 Answers

2023 H2 Chemistry Preliminary Exams Paper 4 – Suggested Solutions Qn 1 (a) Expt Volume of FA 1 / cm3 Volume of deionised water / cm3 t / s part (c) rate / mol dm−3 s−1 1 5.0 25.0 29.0 2.46 x 10−5 2 8.0 22.0 16.2 4.41 x 10−5 3 11.0 19.0 12.2 5.85 x 10−5 4 15.0 15.0 9.4 7.60 x 10−5 (b) Amount of S2O32− added = 0.00500 × 20 1000 = 1.000 × 10-4 mol Amount of I2 reacted with S2O32− added = 1 2 x 1.000 × 10-4 = 5.00 × 10-5 mol = Amount of I2 produced just before the first appearance of blue-black colour Concentration of I2 in reaction mixture = 5.00 × 10-5 ÷ 70 1000 = 7.14 × 10-4 mol dm−3 (c) See table in part (a).

(d)(i) (d)(ii) Since total volume of reaction mixture is kept constant, volume of FA 1 is directly proportional to [Fe 3+] in the mixture. Since the graph is a straight li ne with a pos itive gradient, rate is directly p roportional to [Fe 3+], the reaction is first order with respect to Fe3+. (e) Agree with student. The reaction time would be longer since more time is required to produce more iodine to react with the larger amount of thiosulfate ions present , and so the percentage error is reduced. (f)(i) rate = k[Fe3+][I−]2 (f)(ii) From the slow step, the rate equation is: rate = k’[[FeI]2+][I−] = k[Fe3+][I−]2. This matches the rate equation for the reaction. Hence the proposed mechanism is consistent with the observed kinetics data. 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 Volume of FA 1/ cm3 rate / 10−5 mol dm−3 s−1

2 Qn 2 (a)(i) Table 2.1 test observations 1 Test solution FA 4 with Universal Indicator paper. UI paper turned orange, pH is 3 2 To 1 cm depth of FA 4 in a clean test- tube, add 1 cm depth of aqueous sodium carbonate. White ppt formed. Effervescence observed. CO2 gas evolved gave a white ppt in limewater. 3 Add 1 cm depth of FA 4 into a clean test-tube. Add aqueous sodium hydroxide, slowly with shaking, until no further change is seen. White ppt. formed, soluble in excess NaOH(aq) to give a colourless solution. (a)(ii) Cation: Al3+ Evidence: In test 2, FA 4 reacted with Na 2CO3 to form a white ppt of Al(OH)3 together with the effervescence of CO2 gas. OR From test 1 / test 2, FA 4 contained an acidic ion. Additionally, in test 3, FA 4 reacted with NaOH to form a white ppt of Al(OH)3 which was soluble in excess NaOH to form the colourless complex ion, [Al(OH)4]−. test observations (a)(iii) To 1 cm depth of FA 4, add a few drops of AgNO3. To 1 cm depth of FA 4 in a boiling tube, add 1 cm depth of aqueous NaOH and a piece of Al foil. Heat cautiously. N