2020 YIJC Chem Prelim P2 QP

©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CG INDEX NO CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9729/02 1 September 2020 2 hours This document consists of 24 printed pages and 4 blank pages. For Examiner’s Use Paper 1 /30 Paper 2 1 /14 2 /13 3 /15 4 /13 5 /20 Penalty /75 Paper 3 /80 Paper 4 /55 Overall Percentage (%) READ THESE INSTRUCTIONS FIRST Write your name, class and index number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. 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. 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.

2 ©YIJC Answer all the questions in this section in the spaces provided. 1 Sodium chlorate(I), NaClO, found in household bleach produces chlorine upon addition of acidified dilute hydrochloric acid according to the equation below. ClO–(aq) + 2H+(aq) + Cl–(aq)  Cl2(aq) + H2O(l) A 15.0 cm 3 sample of a household bleach is diluted to 100 cm 3. 25.0 cm 3 of this solution is then pipetted out into a conical flask containing dilute hydrochloric acid. Excess potassium iodide solution is added which will react with the chlorine present in the sample according to the equation below. Cl2(aq) + 2I–(aq)  I2(aq) + 2Cl–(aq) This resultant solution is then titrated against 0.150 mol dm –3 sodium thiosulfate solution and required 17.20 cm 3 for complete reaction. Sodium thiosulfate reacts with iodine according to the following equation. I2(aq) + 2S2O32–(aq)  2I–(aq) + S4O62–(aq) (a) (i) Calculate the amount of iodine produced in this reaction. [1] (ii) Calculate the amount of sodium chlorate(I) present in the 15.0 cm3 sample. [1] (iii) Calculate the concentration of the sodium chlorate(I) in the household bleach in g dm–3. [2]

3 ©YIJC [Turn over (b) In a thiosulfateacid reaction, a pale yellow precipitate of sulfur is formed slowly. S2O32–(aq) + 2H+(aq)  S(s) + SO2(g) + H2O(l) The rate of the reaction can be studied by following the time taken for the reaction mixture to turn opaque. Outline a suitable experiment for determining the order of reaction with respect to S2O32–. You may assume that you are provided with:  100 cm3 of 0.100 mol dm3 sodium thiosulfate, Na2S2O3 and  100 cm3 of 2.0 mol dm-3 hydrochloric acid, HCl No details regarding the use of specific glassware are required. [3]

4 ©YIJC (c) Thiosulfate, S2O32 and tetrathionate, S4O62 are oxoanions of sulfur. Peroxodisulfate(VI), S2O82, is another oxonaion of sulfur. S2O82 is able to oxidise the tartrate ion, [CH(OH)(CO2)]2 in the following reaction. (i) With the aid of a Boltzmann distribution diagram, explain how an increase in temperature will lead to an increase in the rate of the reaction. [4]

5 ©YIJC [Turn over (ii) The reaction between S2O82 and [CH(OH)(CO2)]2 is very slow, even at elevated temperature. Suggest an explanation for this observation. [1] (iii) Steps 1 and 2 represent a possible mechanism for the reaction between S2O82 and the tartrate ion, [CH(OH)(CO2)]2 involving Mn3+ cations. step 1: 6Mn3+ + [CH(OH)CO2]2 + 2H2O  6Mn2+ + 2CO2 + 2HCO2 + 6H+ step 2: 6Mn2+ + 3S2O82  6Mn3+ + 6SO42 Explain the role of the Mn3+ ions in this reaction with reference to the mechanism given. [2] [Total: 14]

6 ©YIJC 2 Nitrosyl chloride, NOC l, is a yellow gas that is most commonly encountered as a decomposition product of aqua regia, a mixture of hydrochloric acid and nitric acid. At 250 oC, NOCl readily dissociates into NO and Cl2. 2NOCl(g) ⇌ 2NO(g) + Cl2(g) (a) (i) In NOCl, the central atom is nitrogen. Draw a dot-and-cross diagram to show the bonding present within a NOCl molecule. You should distinguish carefully between electrons originating from the central atom and those from the other atoms. [1] (ii) State and explain, with reference to the Valence Shell Electron Pair Repulsion theory, the shape of the nitrosyl chloride molecule. [2]

7 ©YIJC [Turn over (iii) Table 2.1 shows the electronegativity values of the atoms in nitrosyl chloride. Table 2.1 atom electronegativity / Pauling units nitrogen 3.04 oxygen 3.44 chlorine 3.16 Predict whether nitrosyl chloride is a polar molecule. Explain your reasoning. [2]

8 ©YIJC (b) The graph of pV/RT against p for 1 mole of O2 at various pressures are plotted below in Fig 2.1. Fig. 2.1 Sketch the pV/RT graph for NOCl in Fig 2.1 and explain your reasoning. [2] pV/RT 1.0 p O2 ideal gas

9 ©YIJC [Turn over (c) The dissociation was investigated by adding 5 moles of NOCl into a 5.0 dm3 sealed vessel, and equilibrium was established at 250 °C under a pressure of 5.80 106 Pa. (i) Assuming ideal gas behaviour, determine the total amount of gas in moles, at equilibrium. [1] (ii) Write the expression for the equilibrium constant, Kp, for the dissociation of NOC l, giving its units. [2] (iii) The equilibrium amounts of NO and C l2 was found to be 3.346 mol and 1.673 mol respectively. Calculate the partial pressures of the gases in the equilibrium mixture and hence calculate a value for Kp under the stated conditions. [3] [Total: 13]

10 ©YIJC 3 Group 2 carbonates undergo thermal decomposition to form the corresponding metal oxide and carbon dioxide according to the equation below. MCO3(s)  MO(s) + CO2(g) Magnesium carbonate decomposes at 350C whereas barium carbonate decomposes at 1100C. (a) Draw a dot-and-cross diagram of magnesium carbonate. [2] (b) Explain the difference in the decomposition temperature of magnesium carbonate and barium carbonate. [2]