Standardenthalpie

Standardenthalpie, or standard enthalpy, is the enthalpy change of a chemical process calculated under standard conditions, defined as a pressure of 1 bar and a specified temperature (commonly 298.15 K). The symbol ΔH° denotes a standard enthalpy change for a reaction, while ΔHf° denotes the standard enthalpy of formation for a substance. The standard state of elements is their most stable form at 1 bar and 298 K, and the standard enthalpy of formation for elements in their standard states is defined as zero.

The standard enthalpy of formation ΔHf° plays a central role in thermochemistry. It is the enthalpy change when one mole of a compound is formed from its elements in their standard states. Values for many substances are tabulated, and the enthalpy change of any reaction at standard conditions can be estimated by Hess’s law: ΔH°rxn = sum νi ΔHf°(products) − sum νi ΔHf°(reactants). In this way, formation enthalpies enable calculation of reaction enthalpies without direct calorimetry for every reaction.

Measurements are typically carried out using calorimetry or inferred from related data. All ΔH° values are defined at 298.15 K and 1 bar, and they can vary with temperature due to heat capacities, Cp, of the reactants and products. Exothermic reactions have negative ΔH°; endothermic reactions have positive ΔH°. Standard enthalpies are widely used in chemistry to predict reaction feasibility, compare energies, and balance energy requirements in synthesis and industrial processes.