NJC 2024 H2 Chem P4 Solutions

1 NJC SH2 Preliminary Examination 9729/04/24 NATIONAL JUNIOR COLLEGE SH 2 Year − End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidate answer on the Question paper. 9729/04 Wednesday 28 August 2024 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 21 and 22. The number of marks is given in brackets [ ] at the end of each question or part question. This paper consists of 22 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 14 2 / 20 3 / 12 4 / 9 Total / 55

2 NJC SH2 Preliminary Examination 9729/04/24 1 Determination of a value for an enthalpy change of solution Via Hess’s Law. FA 1 is solid sodium hydrogencarbonate, NaHCO3. FA 2 is 1.00 mol dm–3 sulfuric acid, H2SO4. Sodium hydrogencarbonate dissolves in water according to equation 1. equation 1 NaHCO3(s) + aq → Na+(aq) + HCO3–(aq) H1 Both solid and aqueous sodium hydrogencarbonate react with sulfuric acid. equation 2 2NaHCO3(s) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) + 2CO2(g) H2 equation 3 2NaHCO3(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) + 2CO2(g) H3 In this question, you will perform an experiment to determine a value for H2. You will use data provided to calculate H3 and hence a value for H1. (a) Determination of the molar enthalpy change of reaction, H2 In this experiment, you will determine the maximum temperature change when a known mass of solid sodium hydrogencarbonate, FA 1, reacts with excess dilute sulfuric acid, FA 2. In an appropriate format in the space provided on page 3, prepare tables in which to record for your experiment: • all weighings to an appropriate level of precision, • all values of temperature, T, to an appropriate level of precision. Procedure 1. Weigh the capped bottle containing FA 1. Record this mass. 2. Place one polystyrene cup inside another polystyrene cup and place both in a glass beaker. 3. Use a 50 cm3 measuring cylinder to transfer 30.0 cm3 of FA 2 into the polystyrene cup. 4. Stir the FA 2 in the polystyrene cup with the thermometer. Read and record its initial temperature, Ti. 5. Slip the thermometer through the lid. Carefully transfer all the solid FA 1 in the bottle to the FA 2 in the polystyrene cup, in small portions, to avoid too much frothing. Secure the lid onto the cup. 6. Use the thermometer to stir the mixture. Observe the temperature until it shows the maximum change from the initial temperature. Record this temperature, Tm. 7. Reweigh the empty capped bottle. Record this mass. Determine the maximum temperature change, T, and the mass of FA 1 used. Results mass of capped bottle / weighing bottle and FA 1 / g 9.391 mass of capped bottle and residual FA 1 OR mass of weighing bottle after transfer / g 5.465 mass of FA 1 used / transferred / g 3.926 Ti / oC 31.2 Tm / oC 24.6 T / oC −6.6 [1]: correct headers and units.

3 NJC SH2 Preliminary Examination 9729/04/24 [Turn over [5] (b) In the following calculations, you should assume that the specific heat capacity of the solution is 4.18 J g–1 K–1, and the density of the solution is 1.00 g cm–3. (i) Use your results from 1(a) to calculate the heat change for your experiment. heat change = ………………………………………. [1] (ii) Hence, determine a value for H2. Include the sign of H2 in your answer. [Ar: H, 1.0; C, 12.0; O, 16.0; Na, 23.0] Mr of NaHCO3 = 84.0 H2 = ………………………………………. [1] The results of an experiment where a solution of 0.690 mol dm −3 aqueous sodium hydrogencarbonate, NaHCO 3(aq) was reacted completely with an excess of dilute sulfuric acid, FA 2, are shown in Table 1.1. Table 1.1 volume of NaHCO3(aq) used / cm3 50.0 initial temperature of NaHCO3(aq) / oC 27.6 volume of FA 2 used / cm3 25.0 initial temperature of FA 2 / oC 31.2 minimum temperature / oC 28.4 (iii) Use the results given in Table 1.1 and the formula below to calculate the weighted average initial temperature, Tav, of the reaction mixture. The formula for Tav is given as Tav = (vol. of FA 2 × initial temp. of FA 2) + (vol. of NaHCO3 × initial temp. of NaHCO3) total volume of reaction mixture heat change (q1) = mcT = (30 x 1.00) x 4.18 x (temp drop) = − ________ J Ignore sign H2 = − 𝑞1 𝑎𝑚𝑡 𝑜𝑓 𝑁𝑎𝐻𝐶𝑂3 x 2 = + _______ J mol-1 or kJ mol-1 with correct sign

4 NJC SH2 Preliminary Examination 9729/04/24 Tav = ………………………………………. [1] (iv) Hence, calculate a value for H3. H3 = ………………………………………. [4] (c) Use your answers from 1(b)(ii) and 1(b)(iv) to calculate a value for H1 for the reaction shown in equation 1. If you are not able to determine a value for 1(b)(ii) and/or 1(b)(iv), you may use x and y to represent the respective enthalpy changes and proceed with this part of the question. Tav = (25 × 31.2) + (50 × 27.6) (25+50) = 28.8 C heat change (q2) = mcT = (25+50) x 1.00 x 4.18 x (28.4 – 28.8) = 125.4 J [1] n(NaHCO3) = 50 1000 × 0.69 = 0.0345 mol H3 = + (125.4 / 0.0345) x 2 = + 7269.6 = + 7270 J mol-1 or + 7.27 kJ mol-1 [1] Final answer to 3 s.f. or 4 s.f. and appropriate units for (b)(i), (b)(ii), (b)(iii), (b)(iv) and (c). [1] H1 = [ H2 − H3] ÷ 2 = ________ J mol-1 or kJ mol-1 [1]: correct application of Hess’ Law H3 = +7.27 H2

5 NJC SH2 Preliminary Examination 9729/04/24 [Turn over H1 = ………………………………………. [2] [Total: 14]

6 NJC SH2 Preliminary Examination 9729/04/24 2 To determine the order of reaction with respect to the concentration of iodine in the iodination of propanone reaction FA 2 is 1.00 mol dm–3 sulfuric acid, H2SO4. FA 3 is 1.00 mol dm–3 propanone, CH3COCH3. FA 4 is an aqueous solution of iodine, I2. FA 5 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3. FA 6 is 0.50 mol dm–3 sodium hydrogencarbonate, NaHCO3. You are also provided with a starch indicator. The equation in reaction 1 represents the reaction between CH3COCH3 and I2. reaction 1: CH3COCH3(aq) + I2(aq) → CH3COCH2I(aq) + HI(aq) This reaction is first order with respect to both CH3COCH3 and H+ ions. You are to investigate the order of reaction with respect to I2. A reaction mixture containing an acidified solution of CH 3COCH3 and I2 is first prepar