Enthalpy of Reaction Calculator

Using Hess's Law and Standard Enthalpies of Formation (ΔH°f)

ΔH°rxn = ΣΔH°f (Products) - ΣΔH°f (Reactants)

Reactants

Products

Worked Examples: ΔH°rxn via Standard Enthalpies of Formation

Example 1: Combustion of Methane

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

  1. ΔH°f (kJ/mol): CH4 = −74.8; O2 = 0; CO2 = −393.5; H2O(l) = −285.8.
  2. ΣΔH°f(products) = [−393.5] + 2×[−285.8] = −965.1.
  3. ΣΔH°f(reactants) = [−74.8] + 2×0 = −74.8.
  4. ΔH°rxn = (−965.1) − (−74.8) = −890.3 kJ/mol (exothermic).

Example 2: Formation of Ammonia (Haber)

N2(g) + 3 H2(g) → 2 NH3(g)

  1. ΔH°f (kJ/mol): N2 = 0; H2 = 0; NH3(g) = −46.1.
  2. Products: 2×(−46.1) = −92.2; Reactants: 0.
  3. ΔH°rxn = −92.2 − 0 = −92.2 kJ/mol (exothermic).

Example 3: Decomposition of Calcium Carbonate

CaCO3(s) → CaO(s) + CO2(g)

  1. ΔH°f (kJ/mol): CaCO3 = −1206.9; CaO = −635.1; CO2 = −393.5.
  2. Products: (−635.1) + (−393.5) = −1028.6; Reactants: −1206.9.
  3. ΔH°rxn = (−1028.6) − (−1206.9) = +178.3 kJ/mol (endothermic).