01Building from elements
ΔH°f is the enthalpy change to form 1 mol of a compound from its elements in standard states. ΔH°f of an element in its standard state = 0 (O₂(g), C(graphite)).
ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants).
02Notebook box
Multiply each ΔH°f by its coefficient.
Worked example: CH₄ + 2O₂ → CO₂ + 2H₂O(l): [−393.5 + 2(−285.8)] − [−74.8 + 0] = −890.3 kJ.