MI Prelim 9647 P2 Answers

Class Adm No Candidate Name: This question paper consists of 11 printed pages and 1 blank page. 2017 Preliminary Examination II Pre-university 3 H2 CHEMISTRY 9647/02 Paper 2 Structured Questions 12th Sept 2017 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. Write in dark 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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. Question 1 2 3 4 5 Total Marks 12 14 15 11 20 72

2 1 Planning (P) Hydrogen peroxide undergoes a redox reaction with acidified KMnO4 as follows: 2MnO4-(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 5O2(aq) + 8H2O(l) By itself, hydrogen peroxide decomposes slowly in accordance to the following equation: 2H2O2(aq) → 2H2O(l) + O2(g) It is found that the decomposition reaction is first order with respect to H 2O2. The reaction can be accelerated by using solid manganese(IV) oxide as the catalyst. (a) Write the rate equation for the decomposition of hydrogen peroxide. ………………………………………………………………………………………………………… [1] Rate = k[H2O2] (b) Using the information given, you are required to write a plan to determine the rate constant for the decomposition of hydrogen peroxide using the continuous titration method. You may also assume that you are provided with:  250 cm3 of 0.0100 mol dm-3 acidified KMnO4;  250 cm3 of 0.0250 mol dm-3 H2O2 solution;  Solid manganese(IV) oxide;  25.0 cm3 pipette;  Stopwatch;  the apparatus normally found in a school or college laboratory. Your plan should include:  practical details of how you would  determine if dilution of the reaction mixture is needed for titration against H2O2;  ensure the reaction is complete;  carry out the titration;  a sketch of the graph you would expect to obtain;  brief, but specific, details of how the results would then be used to obtain  the initial rate of reaction of decomposition in mol dm-3 min-1,  the rate constant for the reaction

3 [Turn over ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………….. [11] [Total: 12 marks] 2MnO4-(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 5O2(aq) + 8H2O(l) Assuming that 25 cm3 of 0.0250 mol dm-3 of H2O2 is used, Amount of H2O2 present = 25/1000 x 0.025 = 0.000625 mol Amount of MnO4- needed = 2/5 x 0.000625 = 0.00025 mol Vol. of KMnO4 needed = 0.00025/0.01 = 0.025 dm3 ;

4 Since two volumes used are within the capacity of the apparatus used , no di lution of the solutions needed; Procedure: 1. From the 250 cm 3 of 0.0250 mol dm -3 H2O2 solution, pipette 25.0 cm3 into another conical flask. 2. Fill up the 50.00 cm3 burette with the standard acidified KMnO4 solution. 3. Take the initial burette reading. 4. Titrate the solution with acidified KMnO 4. Swirl continuously during the addition of the titrant. 5. Toward the end-point, add the KMnO4 solution dropwise and swirl. Stop the addition of the titrant when one drop of titrant causes the solution in the conical flask to change from colourless to pale pink. 6. Record the final burette reading. Calculate the volume of acidified KMnO 4 solution needed to react with the hydrogen peroxide solution. (This tit re volume would give us the concentration of the H2O2 solution at that instant of decomposition). 7. Add some solid manganese(IV) oxide into the 500 cm3 conical flask containing the H2O2 solution. Start the stopwatch. Swirl gently. 8. At every 1 min (student can put a range from 30 s to 10 min) interval, pipette 25.0 cm3 aliquots of the solution containing 20 cm3 of cold water. 9. Repeat steps 2 to 7 at 1st, 2nd, 3rd, 4th and 5th min. Table of results Time/min 0 1 2 3 4 5 Initial burette reading/ cm3 Final burette reading/ cm3 Titre volume/ cm3 Sketch of graph Since the gradient of the graph at t = 0 is x cm3 min-1, Volume of KMnO4 used/ cm3 time/ min

5 [Turn over Initial rate of reaction = (x/1000 x 0.01 x 5/2) ÷ 25/1000 mol dm-3 min-1 Since rate = k [H2O2], k = [(x/1000 x 0.01 x 5/2) ÷ 25/1000] / 0.025

6 2 Hydrogen iodide can undergo decomposition to give a mixture of hydrogen gas and iodine gas. 2HI(g) ⇌ H2(g) + I2(g) ∆H = +12.0 kJ mol-1 (a) State Le Chatelier’s Principle. ……………………………………………………………………………………………………………. ………………………………………………………………………………………………………… [1] Le Chatelier’s Principle states that when a system in equilibrium is disturbed, the position of the equilibrium will shift in a direction that tends to reduce that change so as to re-establish the equilibrium. (b) Predict and explain the effect of the following changes on the position of the above equilibrium, if any. (i) Increasing the temperature ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. …………………………………………………………………………………………………… [2] When temperature of the system is increased, by Le Chatelier’s Principle, the equilibrium position shifts to the right towards the endothermic reaction to remove the excess heat (ii) Reducing the pressure ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. …………………………………………………………………………………………………… [1] When pressure is reduced, by Le Chatelier’s Principle, the equilibrium position does not shift as the number of gaseous particles on both the reactants and products are the same. (iii) Addition of catalyst ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. …………………………………………………………………………………………………… [1]

7 [Turn over Catalyst catalyses both forward and backward reactions to the same extent in a reversible reaction, hence there is no shift in equilibrium position. (c) When 1.4 mol of hydrogen iodide is hea ted in a closed vessel at 550 K, the total pressure at equilibrium was 6 atm. Given that the mole ratio of hydrogen iodide to iodine gas at equilibrium is 9:4, calculate the equilibrium constant, Kp, at 550 K. [3] PH2 = PI2 = (4/17) x 6 = 1.41 atm ; PHI = (9/17) x 6 = 3.18 atm ; Kp = (1.411)2 / (3.176)2 = 0.198 ; (d) Hydrogen chloride and hydrogen bromide also undergoes a similar decomposition to give hydrogen gas and its respective halogens. Describe the trend in the volatility and colours of the halogens. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………