Exercises: Endothermic and exothermic reactions

Classify each process as endothermic or exothermic and justify your answer according to whether the system absorbs or releases energy: a) the combustion of LP gas on a stove; b) an instant cold pack (the kind used for bruises), which turns cold when activated; c) the oxidation of iron (the formation of rust); d) the evaporation of sweat from the skin, which cools you down.

The approximate bond energies are: $\text{H}-\text{H}$: $436\ \text{kJ}/\text{mol}$, $\text{Cl}-\text{Cl}$: $243\ \text{kJ}/\text{mol}$ and $\text{H}-\text{Cl}$: $432\ \text{kJ}/\text{mol}$. For the reaction $\text{H}_2 + \text{Cl}_2 \rightarrow 2\ \text{HCl}$: a) calculate the energy needed to break the bonds in the reactants; b) calculate the energy released when the bonds of the product are formed; c) find the $\Delta H$ of the reaction and state whether it is endothermic or exothermic.