MJC H2 Chem Prelim Paper 2 (Answers)

Answers for MJC 2008 H2 Chem Prelim Paper 2 1(a) Phosgene gas, COCl 2, can be prepared by allowing gaseous chlorine and carbon monoxide to react under pressure. A scientist 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] P13CO at 227 °C = ×+5.00 (273 227)(273+25) = 8.39 atm (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] Kp = () () 13 2 13 2 COCl ClCO P PP 13CO (g) + Cl2 (g) 13COCl2 (g) Initial partial pressure / atm 8.39 5.59 0 Δ in partial pressure / atm – 1.09 – 1.09 + 1.09 Eqm partial pressure / atm 7.30 4.50 + 1.09 Kp = 0.0332 atm-1 (iii) Calculate the average molecular mass of the gaseous mixture at equilibrium. [Ar of 13C: 13.0, O: 16.0, Cl: 35.5] [2] Total pressure, Ptotal = 12.89 atm Average M r = 1313 22Cl COCl13 13CO rr 2 r total total total PPP ×(M of CO) + ×(M of Cl ) + ×(M of COCl )PP P 2 = 49.7 1

(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] 0 13COCl2 13CO Cl2 13COCl2 Cl2 13CO Time t when additional 13CO (g) was added Partial Pressure Time (b)(i) State one of the assumptions of the kinetic theory of gases. [1] Any 1 of the following assumptions 1) The intermolecular forces of attracti on between gas particles are negligible. 2) The total volume of gas particles is negligible compared to the volume of the container. (i.e. gas particles are widely spread and far apart) 3) Gas particles are in continuous random linear motion. 4) All collisions between gas particles (and that of gas part icles with the walls of the container) are perfectly elastic so that no kinetic energy is lost on collision. 2

(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 SO2 (300 K) Ideal Gas (300K) CO2 (300 K) SO2 (100 K) 1.0 P (iii) Explain the difference in beh