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# Enthalpy

In thermodynamics, enthalpy (denoted by the capital letter H)is a concept closely related to that of 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. Like energy, enthalpy is a state function.

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 surroundings, and V is the volume. Thus, the much more useful quantity of enthalpy change is defined, for a constant external pressure, as:
ΔH = ΔU + PΔV

Reactions that take place in surroundings under pressure and cause an increase in volume cause the system 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.

For an exothermic reaction, 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.