Steve Marsden’s

So with a sufficiently ambitious catalog of reaction enthalpy changes it is sometimes possible to calculate--rather than measure--the enthalpy change for a new reaction.

  • Examples

This additivity of heats of reaction (or reaction enthalpies) is generally known as Hess's Law. But it is also possible to state the equivalent of Hess's Law in purely mathematical terms with the introduction of an additional concept, standard enthalpy of formation.

If elements in their standard states (normal atmospheric pressure) and 25oC are defined as having no enthalpy of formation--i.e., it takes no energy to get an element the way it would normally be--then all compounds will have some enthalpy change associated with their formation from those elements.