Although the enthalpy of a substance under standard conditions is a most useful concept, it would be even more useful if a numeric value could be assigned to it. The scale of enthalpies must have a known, and preferably an experimaentally accessible, zero point. The first requirement, a unit of enthalpy, was once filled by the calorie and is now filled by the joule. The second requirement, the zero point, is met by realizing that all pure substances are either compounds or one of the chemical elements.
Any compound is made up of two or more elements.
At least in principle, and often also in practice, a compound can be prepared by reaction of its constituent elements or decomposed to its constituent elements in a calorimeter. The heat of the formation or decomposition reaction can then be measured. Many chemical reactions between compounds involve heat and the enthalpy changes which occur in them are of real chemical interest. Only a small minority of these reactions are reactions in which compounds are formed from their constituent elements. The general term reaction enthalpies or enthalpies of reaction, DHR, is used to describe them all, and is identical to the older term heats of reaction.