(a) Using the bond enthalpies H−H $= 436$ kJ mol$^{-1}$, Cl−Cl $= 242$ kJ mol$^{-1}$ and H−Cl $= 431$ kJ mol$^{-1}$, calculate the enthalpy change for the reaction H$_2$(g) + Cl$_2$(g) → 2HCl(g), and state whether it is exothermic or endothermic. (5 marks) (b) Sketch a labelled energy profile diagram for an exothermic reaction, showing the reactants, products, activation energy and enthalpy change. (4 marks) (c) 50 cm$^3$ of 1.0 mol dm$^{-3}$ hydrochloric acid was mixed with 50 cm$^3$ of 1.0 mol dm$^{-3}$ sodium hydroxide in a plastic cup and the temperature rose by 6.8 °C. Calculate the enthalpy of neutralisation. (Assume the density of the solution is 1 g cm$^{-3}$ and its specific heat capacity is 4.2 J g$^{-1}$ K$^{-1}$.) (6 marks)