Use the formula: H = U + PV
Identify the system’s internal energy, U
Measure or determine the pressure, P
Measure or determine the volume, V
Multiply pressure by volume, PV
Add PV to internal energy, U
For a process, calculate enthalpy change: ΔH = ΔU + Δ(PV)
At constant pressure, use: ΔH = q_p
For reactions, use: ΔH = ΣH(products) – ΣH(reactants)
Use standard enthalpies of formation when available: ΔH°rxn = ΣΔH°f(products) – ΣΔH°f(reactants)
Apply Hess’s law if the reaction is split into steps
Use bond enthalpies if estimating from bonds broken and formed: ΔH ≈ Σ(bonds broken) – Σ(bonds formed)