2023 EJC H2 Chem Prelims P4 Solution

© EJC 9729/04/J2PE/23 [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2023 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 2 – INDEX NUMBER CHEMISTRY Paper 4 Practical 9729/04 29 August 2023 2 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidental Instructions

2 © EJC 9729/04/J2PE/23 READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on the work you hand in. Give details of the practical shift and laboratory, where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Shift Laboratory Do not use paper clips, highlighters, glue or correction fluid. For Examiner’s Use 1 / 21 2 / 13 3 / 11 4 / 10 Total / 55 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. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 19 printed pages and 1 blank page. Answer all the questions in the spaces provided. 1 Determination of the kinetics of the reaction between thiosulfate ions an d acid FA 1 is aqueous sodium thiosulfate, Na2S2O3.

3 © EJC 9729/04/J2PE/23 [Turn Over FA 2 is 2.00 mol dm−3 dilute hydrochloric acid, HCl. Solid sulfur is one of the products formed in the reaction between sodium thiosulfate and hydrochloric acid, as shown in equation 1. The presence of sulfur causes the solution to become opaque. equation 1 S 2O2–3(aq) + 2H+(aq) ฀ SO2(g) + S(s) + H2O(l) In this experiment, the rate of this reaction is studied by measuring the time taken for the reaction mixture to become opaque. (a) Determination of order of reaction with respect to thiosulfate ions You will perform a series of five experiments. Then, you will graphically analyse your results in order to determine the order with respect to the concentrati on of thiosulfate ions, [S2O2–3]. For each experiment, you will note the volume of FA 1 added, VFA 1, and the time in seconds, t, taken for the reaction mixture to become opaque. In each experiment, you will need to ensure that the same total volume of reaction mixture is used by adding an appropriate volume of deionised water, Vwater, as required. You will then calculate values for ● 1000 t , to 1 decimal place ● 1000lg t    and  lg VFA 1 , to 3 significant figures Prepare a table in the space provided on page 4 in which to record, to an appropriate level of precision: ● all volumes, ● all values of t, ● all calculated values of 1000 t , 1000lg t    and  lg VFA 1 . Notes: In each of these experiments, you will need to place the conical flask containing the reaction mixture on the printed page on page 2 of the insert. You will view the page by looking vertically down through the mixt ure. You will stop the stopwatch when the mixture first becomes opaque. This will be the first instant when you can no longer see the printed numbers on the page.

4 © EJC 9729/04/J2PE/23 Experiment 1 1. Fill the burette labelled FA 1, with FA 1. 2. Transfer 50.00 cm 3 of FA 1 into a clean, dry 100 cm3 conical flask. 3. Measure 5.0 cm 3 of FA 2 using a 25.0 cm3 measuring cylinder. Note: Small amounts of SO 2 will be produced during the reaction. Minimise inhalation of SO2. 4. Pour the FA 2 rapidly into the conical flask. Start the stopwatch during this addition. 5. Mix the contents thoroughly by swirling the flask. Then place the fl ask on the printed page of page 2 of the insert. 6. Stop the stopwatch when the solution first becomes opaque. 7. Note the time taken, t, to the nearest second, in your table. 8. Discard the reaction mixture immediately down the sink. Wash out the conical flask and stand it upside down on a paper towel to drain. Experiment 2 Repeat experiment 1, adding 10.00 cm3 of FA 1 and 40.0 cm3 of deionised water using a 50.0 cm 3 measuring cylinder at point 2, while keeping the volume of FA 2, VFA 2, the same at point 3. Record all required volumes, time taken and calculated values in your table. Experiments 3 to 5 In experiment 1, you would have obtained the time taken for the ‘fastest’ reaction and in experiment 2, the time taken for the ‘slowest’ reaction. Repeat experiment 1 three more times, with different volumes of FA 1, VFA 1, at point 2, while keeping the volume of FA 2, VFA 2, the same at point 3. In each case, you will need to ensure that the same total volume of reaction mixture is used by adding an appropriate volume of deionised water, Vwater, as required. Record all required volumes, time taken and calculated values in your table.

5 © EJC 9729/04/J2PE/23 [Turn Over expt VFA 1 / cm3 VFA 2 / cm3 Vwater / cm3 t / s 1000 t / s–1  lg VFA 1 1000lg t    1 50.00 5.0 0.0 18 55.6 1.70 1.75 2 10.00 5.0 40.0 100 10.0 1.00 1.00 3 40.00 5.0 10.0 23 43.5 1.60 1.64 4 30.00 5.0 20.0 28 35.7 1.48 1.55 5 20.00 5.0 30.0 43 23.3 1.30 1.37 Results [6]

6 © EJC 9729/04/J2PE/23 (b) (i) Plot a graph of 1000lg t    on the y-axis against  lg VFA 1 on the x-axis in Fig. 1.1. Draw the best-fit straight line taking into account all of your plotted points. Fig. 1.1 [3]

7 © EJC 9729/04/J2PE/23 [Turn Over The rate equation of the reaction is given by rate = k[H+]y[S2O2–3]x When H+ is present in large excess, [H+] is essentially a constant, and we have rate = k′[S2O2–3]x where k′ = k[H+]y. The initial rate of the reaction can be represented by 1000 t , while [S 2O2–3] can represented by the VFA 1. Hence, in order to determine the order of reaction with respect to [S 2O2–3], x, the following equation is used.  1000lg lg x V ct    FA 1 (ii) Use the graph in Fig 1.1 to determine the order of reaction with respect t o [S2O2–3]. order = [3] (c) Besides ensuring that the concentration of S 2O2–3 is proportional to VFA 1, suggest why it is critical to keep the total volume of reaction mixture constant in experiments 1 to 5. [1] (d) In the last steps of both procedures given, you were instructed to wash and drain the conical flask before using it again. State and explain the likely effect on t of not draining the conical flask before reuse. [1] 1 This is to ensure that depth of the solution is the same so that same amount of sulfur is required to when the solution becomes opaque for all 5 experiments. The t will increase. There will be residual water in the conical flask if it is not drained, hence diluting the mixture/causing the [reactants] to decrease . The rate of reaction will decrease, causing a larger t value to be recorded.

8 © EJC 9729/04/J2PE/23 (e) In the experiments, the burette and the measuring cylinders were used to measure volume of various solutions. Calculate the total percentage error for the volumes measured in experiment 2. [2] (f) Determination of order of reaction with respect to hydrogen ions The experimental procedure stated in 1(a) can be modified to enable the order with respect to the concentration of hydrogen ions, [H+], to be determined. Taking reference from the information provided for experiment 6 in Table 1.1, complete the table to show the volumes of FA 1, FA 2, and deionsied water you will use for experiment 7, that will enable you to determine the order with respect to [H+]. Carry out both experiments similar to how you did so for