Recall that overall chemical reactions can be categorized as endothermic or exothermic based on energy transferred into or out of the system. The chemical reaction is the system in this case.

Positive ΔH = energy moves into system from surrounding = endothermic

Negative ΔH = energy moves out system to surrounding  = exothermic

When a chemical reaction occurs, there is a characteristic change in enthalpy. The enthalpy change for a reaction is based on the overall balanced equation and is expressed in kilojoules per mole. This can be calculated using the same equation used to calculate enthalpy (q = m c ∆T), then dividing by the number of moles of a substance utilized.

The enthalpy of an exothermic reaction is shown below:

2 H₂ (g) + O₂ (g) → 2 H₂O (g)   ΔH = -483.6 kJ/mol

This means that 483.6 kilojoules of energy are released for every two moles of hydrogen gas and one mole of oxygen gas that react to produce two moles of water vapor.