Topic 5 - Energetics - 5.2 Hess's Law

Nature of science: Hypotheses-based on the conservation of energy and atomic theory, scientists can test the hypothesis that if the same products are formed from the same initial reactants then the energy change should be the same regardless of the number of steps. (2.4)


Essential idea: In chemical transformations energy can neither be created nor destroyed (the first law of thermodynamics).

The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps.

Applications and skills

Application of Hess's Law to calculate enthalpy changes.

Calculation of ΔH reactions using ΔHf° data.

Determination of the enthalpy change of a reaction that is the sum of multiple reactions with known enthalpy changes.


Enthalpy of formation data can be found in the data booklet in section 12.

An application of Hess's Law is:

ΔH reaction = Σ(ΔHfo products) - Σ(ΔHfo reactants)