13619.06R [Turn over 12 The thermal decomposition of potassium hydrogencarbonate establishes a dynamic equilibrium in a closed vessel at 200 °C. The enthalpy change for the decomposition reaction is difficult to determine directly. 2KHCO3(s) ⇌ K2CO3(s) + CO2(g) + H2O(g) (a) (i) The equilibrium is heterogeneous. Define the term heterogeneous. [1] (ii) Suggest why the water is present as a gas in the equilibrium mixture. [1] (b) The enthalpy change for this thermal decomposition reaction can be calculated from the enthalpy changes of neutralisation of both potassium carbonate and potassium hydrogencarbonate. K2CO3 + 2HCI → 2KCI + H2O + CO2 KHCO3 + HCI → KCI + H2O + CO2 Both of these enthalpy changes may be determined using the following procedure:  Using a measuring cylinder, place 25 cm3 of 2.0 mol dm-3 hydrochloric acid into a polystyrene cup.  Measure the temperature of the acid with a thermometer.  Add a known mass of either potassium carbonate or potassium hydrogencarbonate to the acid in the polystyrene cup, stir, and measure the highest or lowest temperature reached. (i) Define the term standard enthalpy of neutralisation. [2] 13619.06R (ii) When the procedure was carried out using 2.5 g of potassium carbonate, a temperature rise of 5.5 °C was recorded. Calculate the enthalpy change for the reaction per mole of potassium carbonate. Give your answer in kJ mol-1 to 1 decimal place. (c = 4.2 J g-1 °C-1, density of solution = 1 g cm-3) Answer                         kJ mol-1 [3] (iii) When the procedure was repeated using potassium hydrogencarbonate, the enthalpy change for the reaction was determined to be +29.1 kJ mol-1 (per mole of potassium hydrogencarbonate). Use this value and the value calculated in (b)(ii) to calculate the enthalpy change for the thermal decomposition of potassium hydrogencarbonate. 2KHCO3(s) ⇌ K2CO3(s) + CO2(g) + H2O(g) Answer                         [2]