entalpie

Enthalpie, symbol H, is a thermodynamic state function that represents the total heat content of a system. It is defined as H = U + pV, where U is the internal energy, p is pressure, and V is volume. The concept is particularly useful because, for many processes, the heat transferred at constant pressure equals the change in enthalpy: ΔH = Qp. This makes enthalpy a convenient measure of heat effects in chemical reactions and phase changes conducted at approximately constant pressure.

Enthalpy is an extensive property and is quantified in joules in the SI system, or kilojoules per mole in chemical thermodynamics. For ideal gases, enthalpy depends only on temperature, with dH = Cp dT; thus H(T) = ∫ Cp(T) dT + H0. For liquids and solids, Cp can also vary with temperature, but the relation remains valid locally.

The specific enthalpy h is H per unit mass, h = u + p v, where u is specific internal energy and v is specific volume; cp is the specific heat at constant pressure, defined as (∂h/∂T)p. Standard enthalpies, such as the standard enthalpy of formation ΔH°f, describe the heat released or absorbed when substances form from elements in their reference states. Enthalpy of reaction, phase changes (fusion ΔHfus, vaporization ΔHvap), and calorimetry are common topics in chemistry.

In engineering, enthalpy is essential in energy balances for flowing systems and heat exchangers; in those contexts, specific enthalpy is used to account for energy transport with mass flow.