6.8 Enthalpy of Formation

What is enthalpy of formation?

The standard enthalpy of formation (ΔHf°) is the enthalpy change when 1 mole of a compound forms from its elements in their standard states (most stable form, 25°C, 1 atm). By definition, ΔHf° of any element in its standard state = 0. Values are usually given in kJ/mol and can be negative (exothermic formation) or positive (endothermic formation).

Using ΔHf° to find ΔHrxn°

Because enthalpy is a state function, you can calculate a reaction's enthalpy using Hess's Law: ΔH°rxn = Σ n·ΔHf°(products) − Σ n·ΔHf°(reactants). Multiply each ΔHf° by its stoichiometric coefficient in the balanced equation, sum products, subtract sum of reactants. This works regardless of the actual reaction pathway.

Bond energies vs. formation data

ΔHrxn° can also be estimated using average bond enthalpies: ΔH°rxn ≈ Σ(bonds broken) − Σ(bonds formed), since breaking bonds requires energy (endothermic) and forming bonds releases energy (exothermic). This method gives approximate values because bond energies are averages across many compounds, unlike the more precise ΔHf° tabulated data.

Key terms

Standard enthalpy of formation (ΔHf°)

Enthalpy change when 1 mole of a compound forms from its elements in their standard states.

ΔHf° of an element in standard state

Equal to 0 kJ/mol by definition.

Hess's Law formula using ΔHf°

ΔH°rxn = Σ n·ΔHf°(products) − Σ n·ΔHf°(reactants)

State function

A property (like enthalpy) that depends only on initial and final states, not the path taken.

Bond enthalpy method estimate

ΔH°rxn ≈ Σ(bonds broken) − Σ(bonds formed); approximate since bond energies are averages.

Try an AP-style question

What is the standard enthalpy of formation of O₂(g)?

1. It depends on temperature only
2. −286 kJ/mol
3. 0 kJ/mol
4. +249 kJ/mol