MJC 2008 H2 Chem Prelim Paper 2 (Question Paper)

Reg Number Class Candidate Name _____________________________ Meridian Junior College JC2 Prelim Examination 2008 H2 Chemistry 9746 11 September 2008 1 hr 30 min Paper 2 STRUCTURED QUESTIONS Additional Materials Data Booklet INSTRUCTION TO CANDIDATES Write your name, class and register number in the spaces provided at the top of this page. Write in dark blue or black ink. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions in the spaces provided on the question paper. All working must be shown clearly. You are reminded on the need for good English and clear presentation of your answers. FOR EXAMINER’S USE INFORMATION FOR CANDIDATES Q1 / 12The number of marks is given in brackets [ ] at the end of each question or part of question. Q2 / 12 Q3 / 10 Q4 / 11 Q5 / 15 Total / 60 This question paper consists of 19 printed pages. 1

Answer all the questions in the space provided. 1(a) Phosgene gas, COCl 2, can be prepared by allowing gaseous chlorine and carbon monoxide to react under pressure. A sci entist needed to prepare isotopically- labeled phosgene for use in an organic synthesis. 13CO (g) + Cl2 (g) 13COCl2 (g) Some Cl 2 (g) and 5.00 atm of isotopically-labeled 13CO (g) were pumped into a steel cylinder set at 25 °C which was then sealed. The cylinder was subsequently heated to 227 °C and the partial pressure of Cl 2 (g) was found to be 5.59 atm at 227 °C. The cylinder was then maintained at 227 °C for an extended peri od of time to allow dynamic equilibrium to be reached. The new partial pressure of Cl 2 (g) was found to be 4.50 atm. (i) Calculate the partial pressure of 13CO (g) at 227 °C. [1] (ii) Write an expression for the equilibrium constant, K p. Hence, or otherwise, calculate the Kp value for the equilibrium at 227 °C, stating its units. [2] 2

(iii) Calculate the average molecular mass of the gaseous mixture at equilibrium. [Ar of 13C: 13.0, O: 16.0, Cl: 35.5] [2] (iv) Sketch on the graph below to show what the scientist should expect to observe if additional 13CO (g) was added at time t to the cylinder which was maintained at constant temperature of 227 °C. Label the graph extensions for each species till equilibrium is reached. [2] Partial Pressure 13CO Cl2 13COCl2 0 Time (b)(i) State one of the assumptions of the kinetic theory of gases. [1] 3

(ii) The plots of PV/RT against P for one mole of an ideal gas and one mole of SO 2 at 300K are given below. Show, on the same axes, how one mole of CO 2 will behave at the same temperature of 300K. Label your graph clearly. [1] PV/RT Ideal Gas (300K) SO2 (300 K) 1.0 P (iii) Explain the difference in behaviour between carbon dioxide and sulphur dioxide at 300K. [1] (iv) Explain what happens when sulphur dioxide gas is cooled to 100K. Illustrate your answer clearly on the same axes in b(ii). [2] [Total: 12] 4

2(a) One method that is used to determine the concentration of ozone in the ozone layer is to pass air through acidified potassium iodide and to measure the amount of iodine liberated. In the process O 2 is also being liberated as one of the products. The iodine liberated is measured usi ng a platinum/ aqueous iodine/aqueous iodide electrode against a standard silver /aqueous silver nitrate reference electrode. The e.m.f. of the system, Ecell, is given by the following equation. Ecell = 0.32 + 0.029 lg ([I2]) To determine ozone in the atmosphere above New Zeal and, a balloon was used to carry a sampling device. A 1.0 dm3 sample of the air, at room temperature and 0.24 atm pressure, was passed through 15 cm 3 of acidified potassium iodide. When the iodine liberated wa s measured using the above cell, the e.m.f of the system was 0.21 V. (i) Write a balanced equation for the reacti on of ozone and iodide. [1] (ii) Calculate the percentage of ozone in this sample of air. [2] 5

(iii) Draw a labeled cell diagram to show the set-up for measuring the e.m.f of the cell using silver/ aqueous silver nitrate and aqueous iodide/aqueous iodine half cells. In your diagram, show clearly the polarity of the electrode and the direction of the electron flow in the external cell. Calculate the Eθ cell for this set-up. [3] (iv) 20 cm 3 of 0.1 mol dm -3 sodium iodide solution is added into the standard platinum/iodine/iodide electrode cell, describe w hat would happen to E θ cell value measured. [2] 6

(b) Stratospheric ozone that protects the earth against harmful ultraviolet radiation is being depleted by the anthropogenic intro duction of various gases into the atmosphere. The most destructive ozone depletion processes are catalytic cycles in which trace amounts of gases are able to destroy large quantities of ozone. The overall reaction is shown below: O 3 (g) + O (g) → 2 O2 (g) (i) Given the standard enthalpy change of formation of O 3 (g) is + 142.67 kJ mol -1 and using relevant data from the Data Booklet, calculate the enthalpy change of the above reaction. [2] (ii) The standard entropy change of the reaction between O 3 (g) and O (g) is +10.17 kJ mol -1 K-1. Use the data to decide if th e reaction is s pontaneous at -273 oC, and predict how ΔGθ will change with increasing temperature. [2] [Total: 12] 7

3(a) The boiling points of three common chlorides are given in the following table. Compound Formula Boiling Point/ oC Magnesium chloride MgCl2 1412 Aluminum chloride AlCl3 178 Silicon tetrachloride SiCl4 58 (i) Briefly relate these boiling points to the structure of, and bonding in, each of these chlorides. [3] 8

(ii) State the pH of the solution formed w hen the each of the chlorides compound dissolved in water. Hence, predict the expected observation when these chlorides were added separately to test-tubes containing sodium carbonate solution. Give reasons to support your answers. [3] Compound pH of solution when the compound dissolves in water Observation on reaction of aqueous solution with Na2CO3 Reasons MgCl2 AlCl3 SiCl4 9

(b) The highest oxides of the Period 3 el ements (from sodium to sulphur) are separately added to water. (i) Using the grid provided belo w, sketch a graph to show the variation of pH of the resulting mixtures. [2] pH of oxides Oxides of Period III elements SO3 P4O10 SiO2 Na2O MgO Al2O3 (ii) Describe and explain the trend of the graph you have drawn in (b)(i). [2] [Total: 10] 10