NYJC H2 Chem 2013 Prelim P2 Soln

1 H2 Chemistry 9647/02 NYJC J2/2013 Prelim [Turn Over For Examiner's Use Answer all questions in the spaces provided. 1 Planning (P) Acids can be classified in terms of t heir basicity depending on the number of protons that can be donated to a base. A st udent is investigating the basicity of a solution of 0.80 mol dm-3 acid, A, using just the acid and sodium hydroxide of the same concentration. He conducted two ex periments by mixing different volumes of A and sodium hydroxide, and taking the temperature rise for each experiment. (a) Based on the definition of the enthalpy change of neutralisation (∆Hn) and his results, deduce the basicity of acid A. Experiment Volume of acid Volume of NaOH ∆T /oC 1 40 20 3.6 2 20 40 7.2 If it is a dibasic acid, n(H 2O) formed in experiment 2 is twice that of experiment 1. Since total volume is constant, ∆T for experiment 2 is doubled. Hence it is dibasic. If it is a m onobasic acid, n(H 2O) formed in expt 1 = n(H 2O) formed in expt 2 Since total volume is constant, ∆T must be the same for both experiments. (b) (i) The temperature rise of the reaction can be det ermined graphically by measuring how the temperature changes with time.   You are to plan an experiment to determine the ∆Hn between a solid sample of A and sodium hydroxide. You are provided with a solid sample of A (Mr = 174) and sodium hydroxide solution of 0.80 mol dm-3. In your plan, you should give:  details including calculations to determine the quantities of the reactants to use;  choice of apparatus (You may use apparatus normally found in the school laboratory.);  the essential details for obtaining t he graph of temperature against time in order to determine the temperature rise;  table(s) of readings to taken. 51

Calculations: Volume of NaOH to use = 40 cm3 Amount of acid used in experiment 2 = 0.016 mol Mass of acid used in experiment 2 = 0.016 x 174 = 2.784 g To ensure acid is limiting here, we use 2.5 g. Procedure: 1. Using a weighing balance, we igh accurately about 2.5 g of A. 2. Using a 50 cm 3 measuring cylinder, measure 50 cm 3 of NaOH and add into a Styrofoam cup and cover with a lid. 3. Using a 0.2 oC division thermometer, note the tem perature of the solution at 1 min interval for the first 3 minutes. 4. At 3.5 min, rapidly add sample A into the cup, close the lid. 5. Stir the mixture. 6. Measure the temperature of the mixture at the 4 th minute, and at every 1 minute interval until the 10th minute. Apparatus: weighing balance, 50 cm 3 measuring cylinder, Styrofoam cup, 0.2 oC division thermometer. (2 items missing, 0) Timing: take initial temp for at least 3 min, time intervals (at most 1 min), read minimum of 5 readings after mixing. (2 items missing, 0) General procedure: rapidly pour, stir, close lid. (2 missing, 0) Quantities: vol. of NaOH, mass of X (no penalty since cal