Exothermic and endothermic reactions and their uses, reaction profiles, activation energy, and calculating the overall energy change from bond energies.

A C3.2 structured question. Reward: an exothermic reaction transfers energy to the surroundings, usually by heating, so the temperature of the surroundings rises (the products have less energy than the reactants). An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings falls (the products have more energy than the reactants). A use of an exothermic reaction is in hand warmers or self-heating cans (combustion and oxidation give out heat). A use of an endothermic reaction is in instant cold packs for sports injuries (the reaction absorbs heat and cools the area). Markers credit energy released to the surroundings (temperature rise) for exothermic, energy taken in (temperature fall) for endothermic, and a sensible use of each.

A Higher tier bond-energy calculation. Reward: energy to break bonds (in the reactants) $= 436 + 243 = 679$ kJ/mol. Energy released making bonds (in the products) $= 2 \times 432 = 864$ kJ/mol (there are two H-Cl bonds). Overall energy change $=$ energy in to break bonds $-$ energy out making bonds $= 679 - 864 = -185$ kJ/mol. The value is negative, so more energy is released than absorbed, and the reaction is exothermic. Markers credit summing the bond energies of the bonds broken, summing those of the bonds made (remembering the 2 for two H-Cl bonds), the subtraction (broken minus made), and the conclusion that a negative value means exothermic.