Enthalpy

A thermodynamic state function given by the following expression:

     H = U + PV

where H indicates enthalpy, U indicates internal energy, and P and V
indicate pressure and volume, respectively.

Enthalpy is a useful function for two reasons. First, the expression U + PV
occurs often in thermodynamics, and it is convenient to define this
quantity. Second, if a process begins and ends at the same pressure, the
amount of energy exchanged with the environment as heat is given by the
change in the system's enthalpy.

An enthalpy change (ΔH) in chemistry describes the heat energy
exchange with the surroundings at constant pressure. It can provide a useful
guide to the feasibility of a reaction taking place.

There are a number of different types of standard enthalpy changes:

   * standard enthalpy change of reaction
   * standard enthalpy change of combustion
   * standard enthalpy change of formation
   * standard enthalpy change of neutralisation

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