Enthalpy Changes, Energy Profiles, and Calorimetry

Chemical reactions almost always involve a change in energy, most commonly observed as a change in temperature. In thermochemistry we study these energy changes systematically. When a reaction occurs at constant pressure, the heat exchanged with the surroundings is called the enthalpy change, symbolized $\Delta H$. The enthalpy of a system is its internal energy plus the product of pressure and volume, but because we almost never measure absolute enthalpy, we focus on the change between products and reactants: $\Delta H = H_{\text{products}} - H_{\text{reactants}}$.

Reactions fall into two categories. An exothermic reaction releases heat to the surroundings, so the products store less chemical energy than the reactants and $\Delta H$ is negative. Combustion, neutralization, and most oxidat