The Enthalpy reference article from the English Wikipedia on 24-Apr-2004
(provided by Fixed Reference: snapshots of Wikipedia from


Watch videos on African life
In thermodynamics, enthalpy, or heat content, (H) is a state function closely related to internal energy. As with internal energy, the total enthalpy of a system cannot be measured directly, so thermodynamicists always speak of the enthalpy change of a system.

Enthalpy is defined by the following expression:

H = U + PV,
where H is enthalpy, U is the internal energy, P is the pressure of the system, and V is the volume.

Enthalpy is most useful when pressure is held constant through exposure to the surroundings, to analyse reactions that increase the volume of the system, causing it to do work on the surroundings and lose energy. Conversely, reactions that cause a decrease in volume cause the surroundings to do work on the system, and an increase in the energy of the system. In this case, enthalpy change may be expressed as:

DH = DU + P DV,
where D may indicate an infinitesimal change (often denoted "d") or a finite difference (often denoted "Δ"). Regardless of whether the external pressure is constant, infinitesimal enthalpy change obeys:
dH = T dS + V dP,
so long as the only work done is through volume change. Since the expression T dS always represents transfer of heat, it makes sense to treat the enthalpy as a measure of the total heat in the system, so long as the pressure is held constant; this explains the term heat content.

For an exothermic reaction at constant pressure, the system's change in enthalpy is equal to the energy released in the reaction, including the energy retained in the system and that lost through expansion against the surroundings. Similarly, for an endothermic reaction, the system's change in enthalpy is equal to the energy absorbed in the reaction, including the energy lost by the system and that gained through expansion against the surroundings.

The standard enthalpy change (denoted H0 or HO) of a reaction is the enthalpy change that occurs when the reaction is conducted at the standard state. The standard enthalpy change of formation has been determined for a vast number of substances. The enthalpy change of any reaction under any conditions can be computed from the standard enthalpy changes of formation of all of the reactants and products.

Further types of standard enthalpy change include: