4 2 Phosphorus(V) chloride, PCl5 , can be thermally decomposed to phosphorus(III) chloride, PCl3 , and chlorine, Cl2 . The equation for this reaction is PCl5(g) → PCl3(g) + Cl2(g) The enthalpy change for this reaction cannot be measured directly. (a) (i) Complete the Hess’s Law cycle to include the enthalpy change of formation of both phosphorus chlorides. Include the labels of the missing enthalpy changes. ΔvH is the enthalpy change for the vaporisation of the substance from the state shown to the gaseous state. (3) PCl5(g) PCl5(s) ΔvH [PCl5(s)] ΔrH ................................... ................................... ΔvH [PCl3(l)] PCl3(g) + Cl2(g) PCl3(l) + Cl2(g) Turn over 5 (ii) Calculate the enthalpy change for the thermal decomposition of PCl5(g) to PCl3(g) and Cl2(g), using the data given in the table. Include a sign and units in your answer. (2) Enthalpy change / kJ mol–1 ΔfH [PCl5(s)] –443.5 ΔfH [PCl3(l)] –319.7 ΔvH [PCl5(s)] +64.9 ΔvH [PCl3(l)] +30.5 (b) Another source gave a different value for the enthalpy change of this reaction. PCl5(g)  PCl3(g) + Cl2(g) ΔrH = +87.9 kJ mol–1 Explain the effect, if any, of increasing the temperature on the position of the equilibrium at constant volume. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 2 = 7 marks)