standardentalpia

Standard enthalpy, denoted ΔH°, is the heat content change of a chemical process carried out under defined standard conditions. In chemistry, the standard state is typically 1 bar (often approximated to 1 atm) and a reference temperature of 298.15 K. The standard enthalpy change of a reaction is independent of the pathway taken, by Hess's law, and reflects the net heat absorbed or released when reactants in their standard states form products in their standard states.

The standard enthalpy of formation, ΔH°f, is a related quantity: it is the enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 bar. By convention, elements in their standard states have ΔH°f = 0. For example, ΔH°f for liquid water is approximately -285.8 kJ/mol, while that for water vapor is about -241.8 kJ/mol.

Enthalpies of reaction can be computed from tabulated ΔH°f values: ΔH°rxn = sum of coefficients × ΔH°f(products) − sum of coefficients × ΔH°f(reactants). Standard enthalpies are obtained from calorimetric measurements and are compiled in data tables; they can differ for phases (gas, liquid, solid) and for different temperatures or pressures.

Standard enthalpy values are widely used in chemical thermodynamics to predict reaction feasibility, balance energy budgets in processes, and perform Hess’s-law calculations. They are temperature-specific, so using them at temperatures far from 298 K requires corrections or integrating calorimetry data.