Chemical energetics

Enthalpy changes and Hess's law

Enthalpy change $\Delta H$ is heat transferred at constant pressure. Standard conditions: 100 kPa, 298 K, 1 mol dm⁻³. Key definitions: formation ($\Delta_fH^\ominus$: 1 mol of compound from its elements in standard states), combustion ($\Delta_cH^\ominus$: 1 mol burned completely in oxygen), neutralisation (forming 1 mol of water). Calorimetry: $q=mc\Delta T$, $\Delta H=-\frac{q}{n}$. Hess's law: the enthalpy change is independent of route, so $\Delta_rH=\sum\Delta_fH(\text{products})-\sum\Delta_fH(\text{reactants})$, or $\sum\Delta_cH(\text{reactants})-\sum\Delta_cH(\text{products})$. Mean bond enthalpies give approximate values.