RI 2021 Prelim P3 Answers

26 2021 H2 Chemistry Prelim Paper 3 Suggested Solutions C1 (a)(i) Cl2(aq) is added to solution of KI, followed by hexane. The mixture is shaken and the hexane layer turns purple, indicating the presence of I2. Cl2 + 2I– ⎯→ I2 + 2Cl– Examiner Comments • This question was poorly attempted as many students were unable to properly describe a redox reaction involving the two halogens. In order to show the relative oxidizing abilities of Cl2 and I2, you must choose a redox reaction. • It is also necessary to describe the use of hexane for a visual observation of the reaction. (a)(ii) Cl2 + 2e – ⇌ 2Cl– EӨ = 1.36 V I2 + 2e– ⇌ 2I– EӨ = 0.54 V EӨcell = 1.36 – 0.54 = +0.82 V Since the EӨcell value is positive, the reaction is spontaneous. OR Since the EӨ value for C l2 is higher than that of I2, chlorine has a higher tendency for reduction and is hence, a stronger oxidising agent. Examiner Comments • Many students were able to quote the correct EӨ to use and/or calculate the correct EӨcell value. However, do remember to compare them and state a conclusion. • Unfortunately, many students did not apply this knowledge into answer part (1). (a)(iii) Half-equation for iodine: I2(aq) + 2e– ⇌ 2I–(aq) Hence, ascorbic acid and iodine react in a 1:1 mole ratio. Amount of iodine titrated = 0.005  0.02205 = 1.1025  10-4 mol Amount of ascorbic acid present in 10.0 cm3 = 1.1025  10-4 mol Amount of ascorbic acid present in 100 cm3 = 10  1.1025  10-4 = 1.1025  10-3 mol Mass of ascorbic acid present = 1.1025  10-3  176.0 = 0.194 g Mass of ascorbic acid present in 100.0 g of candy = 0.194 g % by mass of ascorbic acid = 0.194 100.0 × 100% = 0.194% Examiner Comments • Generally well done. • Students are reminded to include units in the intermediate steps of your working. (b) Add equal amounts of bromopropane, chloropropane and iodopropane in separate test tubes and warm the test tubes in a water bath maintained at 50 °C. Then add 5.0 cm3 of silver nitrate solution in ethanol to each compound and note the time taken for the precipitates to first appear. The mixture containing iodopropane will form a yellow ppt of AgI first, followed by the mixture containing bromopropane with a pale cream ppt of AgBr and in the mixture containing chloropropane with a white ppt of AgCl.

27 Examiner Comments • Many students who attempted this question gave reasonable answers but left out important details, and hence, did not earn full credit. While this is not a full planning question as you would see in Paper 4, your experiment outline must still include steps to ensure the aim of the experiment can be achieved. • For instance, many students left out the heating step so the reaction did not even take place. Note that heating in a water bath is recommended for this experiment as temperature should be kept constant for all three reactions (sin