2021 HCI Prelim Paper 4 Solutions

2021 HCI C2 H2 Chemistry Prelim / Paper 4 HWA CHONG INSTITUTION 2021 C2 H2 CHEMISTRY PRELIM PAPER 4 SUGGESTED SOLUTIONS 1(a) Sample answer: experiment 1 experiment 2 TFA1 / oC 29.4 29.2 TFA2 / oC 29.6 - Tmax / oC 38.2 35.0 Tave / oC 29.5 - 'Tmax / oC 8.7 5.8 mass of capped bottle and FA3 = 7.56 g mass of capped bottle and residual FA3 = 5.50 g mass of FA3 added = 2.06 g Records temperature / oC and mass / g Correct headers and units (included in the header or with each entry in the table) [1] All temperatures recorded to 0.1 oC, and mass to 0.01 g [1] Correctly calculates Tave and 'Tmax for experiment 1 and 'Tmax for experiment 2 and mass of FA3 added is about 2.1-2.2 g. [1] In general, the recording of data is well done – the requisite data was captured, and the precision used was appropriate. There was a wide variety of formats but they were generally acceptable. It is highly recommended that recordings should be grouped in a systematic manner, so that data can be retrieved quickly for calculations. The most common omission was the absence of 'Tmax for experiment 1 and experiment 2, though they may be determined in the calculation of q1 and q2. Candidates were not penalized for this, but it is a good experimental practice to record and process all the necessary data coherently, especially when the question asks for it. The instructions requested for candidates to weigh the bottle, the contents and the cap, but some candidates either failed to include the mass of the cap before, or after emptying the bottle. This caused the measurement of the mass of solid used to be inaccurate. This is a reminder to be systematic in your experimental procedures. 1(b)(i) q1 = mc'T = (50.0 + 20.0) × 4.18 × '𝑇௠௔௫ J (accept kJ) [1] nH2SO4 = 1.00 × 20/1000 = 0.02 mol 'H1 = −௤భ ଴.଴ଶ J mol1 (accept kJ mol1) [1]

The quantity, q1, refers to the amount of energy transferred to the mixture by the (exothermic) chemical reaction. This quantity is measured in joules or kilojoules, and does not have a sign. The mass, m, refers to the total mass of mixture present, i.e. 70.0 g, which is calculated using the total volume of mixture multiplied by density of mixture. '𝑇௠௔௫ refers to the temperature change of the mixture, i.e. the difference between Tmax and Tavg. The enthalpy change of reaction, 'H1, is the amount of heat given out per mole of the reaction in equation 1, i.e. when 2 mol of NaOH reacts with 1 mol of H2SO4. To find 'H1, in this case, we divide q1 by the amount of H2SO4 since it is the limiting reagent. 'H1, is negative because this is an exothermic reaction, i.e. the temperature of the reaction mixture increases. The appropriate sign has to be incorporated into the calculation at this point. 1(b)(ii) q2 = mc'T = 50.0 × 4.18 × '𝑇௠௔௫ J (accept kJ) [1] n(citric acid) = mass added ÷ 210 = c mol 'H2 = −௤మ ௖ J mol1 (accept kJ mol1) [1] All comments about q1 are applicable t