AS & A-Level Chemistry 04 — Enthalpy, chemical energy and redox

Chemical reactions involve changes in enthalpy $(\Delta H)$, representing energy transfers between the system and its surroundings. If a system releases heat, the reaction is exothermic, resulting in a negative $\Delta H$. If a system absorbs heat, the reaction is endothermic, yielding a positive $\Delta H$. Reaction pathway diagrams illustrate these energy changes, plotting the enthalpy of reactants against products as the reaction progresses. The initial energy barrier that must be overcome for successful collisions is the activation energy (Ea), depicted as the difference in energy between the reactants and the peak of the curve.

To ensure enthalpy changes are directly comparable, standard conditions are defined: a temperature of $298 \mathrm{K}$, a pressure of $101 \mathrm{kPa}$, and substances in their standard physical states. These are indicated by the symbol ⦵. Several specific standard enthalpy changes are defined. The standard enthalpy change of reaction $(\Delta H_{r}^{\ominus})$ is the enthalpy change when molar quantities of reactants as stated in the balanced equation react. The standard enthalpy change of formation $(\Delta H_{f}^{\ominus})$ is the enthalpy change when one mole of a compound is formed from its elements in their standard states. The standard enthalpy change of combustion $(\Delta H_{c}^{\ominus})$ occurs when one mole of a substance is completely burned in oxygen. The standard enthalpy change of neutralisation $(\Delta H_{\mathrm{neut}}^{\ominus})$ is the energy released when one mole of water is formed from the reaction between an acid and a base.