Use the formula ΔH = Hproducts − Hreactants
Determine the enthalpy of the reactants and products
Subtract the total enthalpy of the reactants from the total enthalpy of the products
For reactions at constant pressure, use q = ΔH
Use standard enthalpies of formation: ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants)
Use bond energies: ΔHrxn = Σ(bonds broken) − Σ(bonds formed)
Use Hess’s law by adding known reaction enthalpies to get the target reaction
Use calorimetry data: ΔH = −q / n for the reacting substance when appropriate
Convert all quantities to consistent units before calculating
Report the sign of ΔH as positive for endothermic and negative for exothermic reactions