CIE Jun 2017 v2 Paper 2 Q3

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The hydrogen halides, HCl, HBr and HI, can undergo thermal decomposition. In a sealed container an equilibrium is established according to the equation shown. PMT (a) Some bond energies are shown in the table. 2HX(g) H2(g) + X2(g) (where X = Cl, Br or I) bond energy / kJ mol1 HBr HH BrBr 366 436 193 Use these data to calculate a value for the enthalpy change, H, for the thermal decomposition of hydrogen bromide, HBr, according to the equation shown. H =kJ mol1 [1] (b) At a temperature of 700 K a sample of HBr is approximately 10% decomposed. Changing the temperature affects both the rate of decomposition of HBr and the percentage that decomposes. The Boltzmann distribution for a sample of HBr at 700 K is shown. Ea represents the activation energy for the reaction. proportion of molecules with a given energy Ea molecular energy (i) Using the same axes, sketch a second curve to indicate the Boltzmann distribution at a [2] higher temperature.UCLES 2017 9701/22/M/J/17<br /> PMT (ii) With reference to the curves, state and explain the effect of increasing temperature on the rate of decomposition of HBr.[3] (iii) The decomposition of HBr is endothermic. State the effect of increasing temperature on the percentage of HBr that decomposes. Use Le Chateliers principle to explain your answer.[3] (iv) At 700 K HBr is approximately 10% decomposed but hydrogen iodide, HI, is approximately 20% decomposed. Explain this difference with reference to bond strengths and the factors that affect them.[3]UCLES 2017 9701/22/M/J/17 [Turn over<br /> PMT (c) At temperatures above 1500 K, HCl will decompose. A sample of 0.300 mol of HCl decomposed in a sealed container. The resulting equilibrium mixture was found to contain 1.50102 mol of Cl 2. (i) Calculate the amounts, in mol, of H2 and HCl present in the equilibrium mixture. (ii) Calculate the mole fraction of each gas in the equilibrium mixture. H2 =mol HCl =mol [2] mole fraction of HCl =mole fraction of H2 =mole fraction of Cl 2 =[1] (d) In another experiment under different conditions, an equilibrium mixture was produced with mole fractions for each species as shown. species mole fraction HCl H2 Cl 2 0.88 0.06 0.06 (i) Write the expression for the equilibrium constant, Kp, for the decomposition of HCl. 2HCl (g) H2(g) + Cl 2(g) Kp = [1]UCLES 2017 9701/22/M/J/17<br /> PMT (ii) Explain why the total pressure of the system does not need to be known for Kp to be calculated for this experiment.[1] (iii) Calculate the value of Kp for this experiment. Kp =[1] [Total: 18]UCLES 2017 9701/22/M/J/17 [Turn over<br />

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