Hess's Law states that if a reaction is carried out in a series of steps, ΔH for the overall reaction will equal the sum of the enthalpy changes for the individual steps.

ΔH₁ = ΔH₂ + ΔH₃

The enthalpy diagram below demonstrates Hess's Law.

Enthalpy changes associated with the formation of a compound from its constituent elements is known as enthalpy of formation, (or heat of formation), ΔH_f.

To compare enthalpies of different reactions, a set of conditions must be defined at which the enthalpies will be tabulated.

These conditions are called the standard state:

• a pure substance
   
• at 1 atmosphere of pressure (1 atm)
   
• at 298 K (25 ^oC)