ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants)
Enthalpy (H) is a thermodynamic property representing the total heat content of a system at constant pressure. The enthalpy change ΔH measures heat absorbed or released: negative ΔH = exothermic (heat released), positive ΔH = endothermic (heat absorbed). Hess's law states that ΔH for a reaction is independent of the pathway — it depends only on the initial and final states. Using standard enthalpies of formation (ΔH°f): ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants). Elements in their standard states have ΔH°f = 0 by definition. Bond enthalpies offer another approach: ΔH ≈ Σ(bonds broken) − Σ(bonds formed). Enthalpy calculations are essential in chemical engineering, environmental science, and materials design for predicting energy requirements and heat management.