Enthalpies of Formation

The following reaction describes the formation of a compound from the constituent elements in their most stable forms at 1 atm, 25°C, the standard reference states.

The subscript "f" on ΔH° reminds us that this is a formation reaction. In a formation reaction, we are specifically concerned with the product and so write the reaction so that the product has unit stoichiometric coefficient even though we often encounter fractional coefficients for reactants; ΔHf° values have units of kJ/ mol to emphasize this point.

Table 1.1 lists some values of ΔHf° at 25°C. Note the absence of entries for H₂(g), I₂(s), and C(graphite), and all other elements in their standard reference states. By definition ΔHf° = 0 for all elements in their most stable states. By a special convention, ΔHf° = 0 for H+(aq).

A table of standard enthalpies of formation can be used, following rule 2 above, to compute the enthalpy change for any reaction involving species for which values of ΔHf° can be found.

Example 3

Compute the standard enthalpy change for the reaction

Table 1.1. Standard Enthalpies of Formation at 25°C (kJ/mol)

We can imagine carrying out this reaction by first dissociating the reactant molecules to their constituent elements and then recombining the elements to form the products:

Adding the four equations and the ΔH's, we have

We can generalize the procedure used in Example 3 as follows:

This can be expressed by the equation (p = products, r = reactants):

The calculation of ΔH° from enthalpies of formation applies, strictly speaking, only to the reaction of reactants in standard states to form products in standard states. Nonetheless, calculations of ΔH° are very useful in the general case because ΔH is usually not very sensitive to conditions so that ΔH° provides us with a good semi-quantitative estimate of the reaction energy in most situations.