2024 ASRJC H2 Chemistry Prelim P4 (Ans)

ASRJC JC2 PRELIM 20248 9729/04/H2 [Turn Over ANDERSON SERANGOON JUNIOR COLLEGE 2024 JC 2 PRELIMINARY EXAMINATION NAME:________________________________ ( ) CLASS: 24 /____ CHEMISTRY 9729/04 Paper 4 Practical SUGGESTED SOLUTIONS

2 ASRJC JC2 PRELIM 20248 9729/04/H2 Answer all the questions in the spaces provided. 1 Investigation of the kinetics of the catalysed decomposition of hydrogen peroxide Hydrogen peroxide decomposes very slowly to form water and oxygen gas as shown in equation 1. equation 1 2H2O2 2H2O + O2 Many transition element ions are able to catalyse the decomposition of hydrogen peroxide. Iron(III) nitrate, Fe(NO3)3, is an effective catalyst for this reaction. FA 1 is 0.170 mol dm-3 aqueous hydrogen peroxide, H2O2 FA 2 is 0.2 mol dm-3 sulfuric acid, H2SO4 FA 3 is 0.020 mol dm-3 potassium manganate(VII), KMnO4 You are also provided with iron(III) nitrate, Fe(NO3)3 You will add a measured volume of iron(III) nitrate to a measured volume of FA 1 and, at timed intervals, transfer aliquots (portions) of the reaction mixture to titrate remaining H 2O2 against KMnO4 in FA 3. Acidified KMnO4 and H2O2 react as shown in equation 2. equation 2 2MnO4–(aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g) (a) (i) Preparation and titration of the reaction mixture Notes: You will perform each titration once only. Great care must be taken that you do not overshoot the end-point. Once you have started the stopwatch, it must continue running for the duration of the experiment. You must not stop it until you have finished this experiment. You should aim to transfer your first aliquot within the first three minutes of starting the reaction. You should aim not to exceed a maximum reaction time of 25 minutes for this experiment. In an appropriate format in the space provided, prepare a table to record for each aliquot • the time of transfer, t, in minutes and seconds, • the decimal time, td, in minutes, to 0.1 min, for example, if t = 4 min 33 s then td = 4 min + 33/60 min = 4.6 min, • the burette readings and the volume of FA 3 added.

3 ASRJC JC2 PRELIM 2024 9729/04/H2 [Turn Over 1. Fill a burette with FA 3. 2. Using a measuring cylinder, add 100.0 cm 3 of FA 1 to the conical flask labelled reaction mixture. 3. Using a measuring cylinder, add 2.0 cm3 of iron(III) nitrate , in one portion, to the same conical flask. Start the stopwatch and swirl the mixture thoroughly to mix its contents. 4. Using a measuring cylinder, add 50.0 cm3 of FA 2 to a second conical flask. 5. Transfer a 10.0 cm 3 aliquot (portion) of the reaction mixture to a 10 cm 3 measuring cylinder, using another dropping pipette. 6. Immediately transfer this aliquot into the second conical flask and vigorously swirl the mixture. Read and record the time of transfer in minutes and seconds, to the nearest second, when the aliquot is added. 7. Immediately titrate the H 2O2 in the second conical flask with FA 3. The end -point is reached when a permanent pale pink colour is obtained. Record your titration results. 8. Wash out the second conical flask with water. 9. Repeat steps 4 to 8 until a total of five aliquots have been titrated and their results recorded. Results Aliquot 1 2 3 4 5 Time of transfer, t 1 min 1 s 5 min 2 s 10 min 1s 15 min 2 s 20 min 2s Decimal time, td / min 1.0 5.0 10.0 15.0 20.0 Final burette reading / cm3 29.10 24.20 19.50 16.30 34.00 Initial burette reading / cm3 0.10 0.00 0.00 0.00 20.00 Volume of FA3 used / cm3 29.00 24.20 19.50 16.30 14.00 [4] [1] correct header and units [1] – record transfer time in minutes and seconds, to 1s – calculate decimal time 1 dp – record all initial / final burette readings to the nearest 0.05 cm3 [1] – collect a total of 5 readings at regular interval (3-6 minutes) – first aliquot withdrawn within 3 min, last not exceeding 25 min from start [1] correctly calculate the volume of FA 3 used and VFA3 decreases with time

4 ASRJC JC2 PRELIM 20248 9729/04/H2 (ii) Plot a graph of the volume of FA 3 added, on the y-axis, against decimal time, x-axis on the grid in Fig. 1.1. Draw the most appropriate best-fit curve taking into account of all your plotted points. Extrapolate (extend) this curve to td = 0.0 min. Fig. 1.1 [3] [1] Axes correct way round with correct labels and units, with appropriate scale (x axis starts at 0 min and plotted points to occupy at least half the graph grid in both x and y directions. Do not allow awkward scales, eg 3:10) [1] All points plotted within ±½ small square [1] Draw a best-fit curve line graph, extrapolated to include the point at 0 min 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 0.0 5.0 10.0 15.0 20.0 25.0 30.0 decimal time, td / min Volume of FA 3 used / cm3 (21.25, 0) (30.5 , 0)

5 ASRJC JC2 PRELIM 2024 9729/04/H2 [Turn Over (b) The initial rate of change of the concentration of hydrogen peroxide, FA 1 , [H 2O2], can be determined from the gradient of the tangent to the graph in Fig. 1.1 at time td = 0.0 min. (i) Draw a tangent to your graph in Fig. 1.1 at time td = 0.0 min. Determine the gradient of this line, showing clearly how you did this. Gradient = 30.5 − 0 0 −21.25 = −1.435 = −1.44 [1] triangle drawn should be at least occupy half the grid in the x and y direction of line drawn and coordinates of points chosen correctly read off the grid to ±½ small square [1] calculate gradient correctly, negative sign shown Gradient = …………………………. cm3 min–1 [2] (ii) Use your answer in (b)(i) to determine the rate of change of the amount of MnO 4– ions required in mol min–1. [MnO4–] = 0.02010–3 mol cm-3 Rate of change of the amount of MnO4– ions = (−1.435  (0.02010–3) mol min–1 = −2.87 10–5 mol min–1 [1] ignore negative sign rate of change of the amount of MnO4– ions required = ………………………….. mol min–1 [1] (iii) With reference to equation 2, determine the amount of H2O2 decomposed per minute in the 10.0 cm3 aliquot. 2MnO4– ≡ 5H2O2 Amount of H2O2 decomposed per minute = 5 2  (2.87 10–5) = 7.18 10–5 mol min–1 [1] amount of H2O2 decomposed per minute = ………………………….. mol min–1 [1] (iv) Hence, deduce the rate of change of [H2O2] at td = 0.0 min, in mol dm–3 min–1. Rate of change of [H2O2] = (7.18 10–5 0.010) = 0.00718 mol dm–3 min–1 [1] rate of change of [H2O2] at td = 0.0 min = …………………… mol dm–3 min–1 [1]

6 ASRJC JC2 PRELIM 20248 9729/04/H2 (v) The procedure that you followed in 1(a)(i) can be modified to determine the order of reaction with respect to H2O2. Outline how you would verify if decomposition of hydrogen peroxide is first order with respect to H2O2 using the initial rates method. Your plan should include: • the further experiments to conduct and data to collect, • suggestion of a suitable graph to explain of how the data obtained can be used to determine order of reaction from initial rates. No details regarding the use of specific glassware are required. [2] • Repeat the procedure using H2O2 of different concentrations • For each concentration, plot the corresponding graph of the volume of FA 3 added against decimal time, and determine the initial rate of reaction • Plot a graph of the initial rate against concentration of H2O2, and a first order reaction will yield a straight-line graph passing through the origin [1]1st and 2nd point [1] 3rd point (vi) By considering suitable half-equations in the table below, write two equations to show how Fe3+(aq) acts as a homogeneous catalyst in the decomposition of hydrogen peroxide. Electrode reaction Fe3+ + 3e Fe Fe3+ + e Fe2+ H2O2 + 2H+ + 2e 2H2O O2 + 2H+ + 2e H2O2