2022 RI H2Chem Prelims P4 Answers

-1- 2022 RI H2 Chemistry Prelim Paper 4 – Suggested Solutions 1(a) Expt Tinitial / oC Tfinal / oC t / s 1/t / s−1 lg (1/t) Tave / oC TK / K 1/TK / K−1 1 30.4 30.6 136 0.00735 -2.13 30.5 304 0.00329 2 62.4 50.0 22 0.0455 -1.34 56.2 329 0.00304 3 39.2 36.0 72 0.0139 -1.86 37.6 311 0.00322 4 48.2 40.8 45 0.0222 -1.65 44.5 318 0.00315 1(b)(i) (b)(ii) t = 3  60 = 180 s lg (1/t) = lg (1/180) = –2.26 From the graph, 1/TK = 0.00334 K−1 Tave = 26.4 oC (c)(i) Mole ratio of MnO4− : D-glucose : e− = 1 : 2.5 : 5 Hence, mole ratio of D-glucose : e− = 1 : 2 1 mol of D-glucose loses 2 mol of e−. lg (1/t) 1/TK / K-1 -2.80 -2.60 -2.40 -2.20 -2.00 -1.80 -1.60 -1.40 -1.20 -1.00 0.00290 0.00300 0.00310 0.00320 0.00330 0.00340 0.00350 0.00334 –2.26 0.00324 –1.94

-2- (c)(ii) Structure of P: Since 1 mol of D -glucose loses 2 mol of e− and only one carbon atom in each glucose molecule is oxidised, the oxidation state of this carbon must have increased by 2. Hence, the aldehyde group of D-glucose is oxidised to carboxylic acid as the oxidation state of carbon increases from +1 in aldehyde to +3 in carboxylic acid. (d)(i) Maximum total percentage uncertainty = ( 2(0.05)+(0.5)+(0.5) 60 )  100% = 1.83% (d)(ii) Although Tave was calculated, the difference between Tinitial and Tfinal was quite significant, especially for experiments conducted at temperatures above room temperature. This in turn resulted in inaccuracy in the measurement of the temperature of the reaction/measurement of the time required for the reaction to complete at Tave. A modification to the experimental procedure would be to use a water bath maintained at a fixed temperature for each experiment, to equilibrate the reactants at the same fixed temperature before starting the reaction, and to ensure that the temperature of the reaction mixture stayed constant throughout the reaction. OR The human perception of when the reaction mixture turned colourless may be inconsistent/vary across different experiments , hence resulting in inaccuracy in the measurement of the time required for the reaction to complete at Tave. A modification to the experimental procedure would be to use a colorimeter to monitor the absorbance of the reaction mixture, and stopping the stopwatch only when there was no more absorbance (i.e. reaction mixture had turned colourless). 2(a)(i) Mass of capped weighing bottle and FA 4 / g 9.135 Mass of capped weighing bottle after emptying FA 4 / g 5.211 Mass of FA 4 used / g 3.924 Titration number 1 2 Final burette reading / cm3 19.90 39.80 Initial burette reading / cm3 0.00 19.90 Volume of FA 1 used / cm3 19.90 19.90 Values used ✓ ✓ 2(a)(ii) Average volume of FA 1 used = (19.90 + 19.90) / 2 = 19.90 cm3

-3- 2(b)(i) n(MnO4−) = 19.90 1000  0.0100 =0.