Enthalpy is a measure of the chemical energy in a substance.

A reaction or process that releases heat energy is described as exothermic.

In industry, exothermic reactions may require heat to be removed to prevent the temperature rising.

A reaction or process that takes in heat energy is described as endothermic.

In industry, endothermic reactions may incur costs in supplying heat energy in order to maintain the reaction rate.

The formula $E_h=cmΔT$ links the heat energy released by a fuel, the specific heat capacity of the substance being heated, the temperature rise, and the mass of the substance being heated. This can be used in calculations to find any one of the quantities $E_h$, $c$, $m$ or $ΔT$, in the correct units, given relevant data. This can also be used, with the mass of fuel used, to find the enthalpy of combustion.

The enthalpy of combustion of a substance is the enthalpy change when one mole of the substance burns completely in oxygen.

Hess’s law states that the enthalpy change for a chemical reaction is independent of the route taken.

The enthalpy change for a reaction can be calculated using Hess’s law, given appropriate data.

The molar bond enthalpy is the energy required to break one mole of bonds in a diatomic molecule.

A mean molar bond enthalpy is the average energy required to break one mole of bonds, for a bond that occurs in a number of compounds.

Bond enthalpies can be used to estimate the enthalpy change occurring for a gas phase reaction, by calculating the energy required to break bonds in the reactants and the energy released when new bonds are formed in the products.

All substances must be in the gas state to use bond enthalpies as the bond enthalpies stated in the data booklet were measured with substances in the gaseous state.