Using Standard Enthalpies of Formation to Calculate a Reaction’s Enthalphy

Using ΔH_f⁰ to Calculate ΔH_rxn

Last Update: April 13, 2011

Enthalpies for chemical species cannot be measured directly.  However, we can use standard enthalpies of formation,ΔH_f⁰, as functional equivalents of a substance’s enthalpy.  This is because enthalpy is a state function and we define Δi of an element to be zero when it is in its standard state.

To calculate ΔH for a reaction (ΔH_rxn⁰), we simply need to use Eqn. 1.

So, for example ΔH_rxn⁰ for the reaction HCl (aq) + NaOH (aq) ® H₂O (l) + NaCl (aq) would be Eqn. 2.

We could also use the net ionic equation to calculate ΔH_rxn⁰. The ΔH_f⁰ for NaCl (aq), HCl (aq) and NaOH (aq) includeΔH_f⁰ for the ions that they dissociate into in aqueous solution.  But since the Cl^– and Na⁺ are spectator ions, their ΔH_f⁰do not need to be included in the overall calculation of ΔH_rxn⁰ .  So, ΔH_rxn⁰ for the reaction HCl (aq) + NaOH (aq) ®H₂O (l) + NaCl (aq) is the same as for the reaction H⁺ (aq) + OH^– (aq) ® H₂O (l), which is shown in Eqn. 3.