5.1 Enthalpy change, ΔH

1. Thermochemical equation

1) Coefficient before reactants and products
The unit of enthalpy change is kJ mol^-1. Mol is the unit of the amount of substance, so that it should be used to describe the amount of a specific reactant or product. Different enthalpy change has different meaning in mol. For example,
enthalpy change of combustion: the enthalpy change when one mole of fuel burnt;
enthalpy change of neutralisation: the enthalpy change when one mole of water produced in acid–base reaction.

For chemical equation which is used to describe enthalpy change, coefficient before reactants and products are important. For example,
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
means that

2) Physical state for each chemical