Standard Enthalpy of Reaction

Computes a reaction's enthalpy change from standard enthalpies of formation: products minus reactants, weighted by coefficients.

What it means

Because enthalpy is a state function, you never need to measure a reaction's heat directly: you can compute it from tabulated formation enthalpies. Conceptually you tear the reactants apart into their elements and rebuild the products from elements — the formation enthalpy of each compound is the heat of that rebuild step — and the difference is the reaction's enthalpy. Elements in their standard states get zero formation enthalpy by definition. When to use it: whenever you need ΔH°rxn for a reaction with known products and reactants and tabulated ΔH°f values — combustion, neutralization, formation, and decomposition problems all start here.

Variables

Assumptions

• The process occurs at constant pressure (the definition of enthalpy).
• All tabulated ΔH°f values refer to standard states (25°C, 1 atm), so the result is a standard enthalpy change.
• Stoichiometric coefficients multiply each formation enthalpy — they are not optional.

Worked example

Given:

• CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
• ΔH°f: CH4 = −74.8, O2 = 0, CO2 = −393.5, H2O(l) = −285.8 kJ/mol

1. List formation enthalpies: ΔH°f(CH4) = −74.8, ΔH°f(O2) = 0 (element in standard state), ΔH°f(CO2) = −393.5, ΔH°f(H2O, l) = −285.8 kJ/mol.

2. Products sum: 1 × (−393.5) + 2 × (−285.8) = −393.5 − 571.6 = −965.1 kJ/mol.

3. Reactants sum: 1 × (−74.8) + 2 × 0 = −74.8 kJ/mol.

4. Subtract: ΔH°rxn = −965.1 − (−74.8) = −965.1 + 74.8 = −890.3 kJ/mol, which rounds to −890 kJ/mol.