2025 ASRJC Prelim H2Chem P4_MS

ASRJC JC2 Prelim 2025 9729/04/H2 ANDERSON SERANGOON JUNIOR COLLEGE 2025 JC2 Preliminary Examination Paper 4 Solutions 1 Determination of an enthalpy change of neutralisation, ∆Hneut, by thermometric titration The enthalpy change of neutralisation between an acid and an alkali can be determined using thermometric titration. This involves using a fixed volume of alkali with progressive addition of small volumes of acid and monitoring the temperature of the reaction mixture during the process. FA 1 is an aqueous solution of 1.80 mol dm-3 of a monobasic acid, HA. FA 2 is aqueous sodium hydroxide, NaOH You are to carry out a thermometric titration to determine the enthalpy change of neutralisation for the reaction given below. HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) Question continues on Page 4.

2 ASRJC JC2 Prelim 2025 9729/04/H2 (a) Prepare a table in the space provided and record, to appropriate level of precision: • all volumes of FA 1 added, V • the maximum temperature, T, reached after each addition of FA 1. It is important that the volume of FA 1 recorded is the total volume you have added up to the point when the temperature reading was made. Note: If you overshoot on an addition, record the actual total volume of FA 1 added up to that point. Procedure 1. Place a polystyrene cup inside a second polystyrene cup and place both cups in a glass beaker. 2. Fill the burette with FA 1. 3. Use a measuring cylinder to transfer 25 cm3 of FA 2 into the polystyrene cup. 4. Stir the FA 2 solution in the cup gently with the thermometer. Read and record its temperature. 5. Use the burette to add 5.00 cm3 of FA 1 into the cup. Stir the mixture gently with the thermometer. Read and record both the maximum temperature and the actual total volume of FA 1 added. 6. Repeat step 5 until a total of 45.00 cm 3 of FA1 has been added. For each addition of FA 1, read and record both the maximum temperature and the actual total volume of FA 1 added up to that point. Results [1] correct headers + units [1] records V to 2 d.p. and T to 1 d.p. (penalised students if recording last decimal recording is consistently .0 or .5 - competency in reading thermometer) [1] complete set of readings that are of 5.00 cm 3 increment, inclusive of V = 0.00 cm3 and 45.00 cm3 V / cm3 T / oC 0.00 30.8 5.00 33.2 … … 40.00 35.0 45.00 34.4 [3]

3 ASRJC JC2 Prelim 2025 9729/04/H2 [Turn Over (b) Plot a graph of temperature, T, on the y-axis, against total volume of FA 1 added, V, on the x-axis on the grid in Fig.1.1. Your scale on the y-axis should allow for extrapolation above the highest temperature recorded. Draw two lines of best fit, taking into account the points when the temperature of the mixture was rising and the points when the temperature was falling. Each line should have a shape best suited to its plotted points. Extrapolate (extend) the two lines until they inte