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 behaviour between carbon dioxide and sulphur dioxide at 300K. [ 1 ] SO 2 deviates more from ideal gas behaviour as compared to CO2. SO2 is a polar molecule while CO2 is a non-polar molecule Hence, there is stronger permanent dipole-permanent dipole attractions between SO 2 molecules as compared to induced dipole- induced dipole attractions between CO2 molecules. (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] At lower temperat ure of 100K, the SO 2 molecules possess less kinetic energy and they move slower. ⇒ Thus, intermolecular forces of attraction between SO 2 molecules becomes significant. 3

2 (a) One method that is used to determine th e 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] O3 + 2I- + 2H+ → H2O + O2 + I2 (ii) Calculate the percentage of ozone in this sample of air. [2] [I2] = 1.61 x 10-4 mol dm-3 No. of mole of I2 = 2.415 x 10-6 No of mole of O3 = 2.415 x 10-6 Volume of O 3 = 2.47 x 10-7 m3 Percentage of ozone = 0.0247 % 4

(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] Eθ cell = + 0.26 V Electron flow + - 298K 298K Iodine (aq) = 1 moldm-3 (iv) 20 cm 3 of 0.1 mol dm -3 sodium iodide solution is added into the standard platinum/iodine(aq)/iodide electrode ce ll, describe what would happen to E θ cell value measured. [2] I2 (aq) + 2e 2I- (aq) Eoxid (1) By adding in sodium iodide solution, the concentration of iodide increases. By Le Chatelier’s principle, the position of equilibrium (1) shifts to the left to decrease the amount of iodide. E oxid will be reduced Thus Eθ cell > + 0. 26 V [AgNO3 -3(aq)] = 1 moldm 5

(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] ∆H rxn = - 391 kJ mol-1 (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 the reaction is spontaneous at -273 oC, and predict how ΔGθ will change with increasing temperature. [2] ∆G = ∆H - T∆S = - 391 kJ mol-1 Since ∆G is negative, the reaction is thermodynamically feasible at -273 oC Since ΔS is positive, -TΔS is negative. and ∆H is negative, ∆G = ∆H - T∆S is negative at all temperature. 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 6

(i) Briefly relate these boiling points to the structure of, and bonding in, each of these chlorides. [3] MgCl2 has giant ionic lattice structure. As large amount of energy are required to overcome the extensive and strong electrostatic forces of attraction betw een oppositely charged ions present in the crystal lattice. Thus MgCl 2 has the highest boiling point. Both AlCl3 and SiCl4 have simple molecular structures. Hence, AlCl 3 and SiCl 4 have low boiling point as a small amount of energy is required to overcome the weak van der Waals’ forces of attraction between m