D20.4 Standard Formation Enthalpy

Data derived from calorimetric measurements for many different chemical reactions, that is from many thermochemical expressions, can be summarized in a table of standard formation enthalpies and used to calculate accurate enthalpy changes for a variety of chemical reactions. A standard formation enthalpy, Δ_fH°, is the enthalpy change for a reaction in which exactly one mole of a pure substance in a specified state (s, l, or g) is formed from free elements in their most stable states under standard-state conditions. Δ_fH° is also referred to as the standard heat of formation.

For example, Δ_fH° of CO₂(g) at 25 °C is −393.5 kJ/mol. This is the enthalpy change for the exothermic reaction:

The gaseous reactant and product are at a pressure of 1 bar, the carbon is present as solid graphite, which is the most stable form of carbon under standard-state conditions.

For nitrogen dioxide, NO₂(g), Δ_fH° is 33.2 kJ/mol at 25 °C:

A reaction equation with ½ mol of N₂ and 1 mol of O₂ is appropriate in this case because the standard enthalpy of formation always refers to formation of 1 mol of the substance; here, it is 1 mol NO₂(g).

By definition, the Δ_fH° of an element in its most stable form under standard conditions is 0 kJ/mol. A table of Δ_fH° values for many common substances can be found in the Appendix.

A simple calculation can be used to determine the Δ_rH° of any reaction if the Δ_fH° of the reactants and products are available. The Δ_rH° of the overall reaction is equal to the sum of all the standard formation enthalpies of the products minus the sum of all the standard formation enthalpies of the reactants: