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Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, the amount of energy required to create a system, and the amount of energy required to make room for it by displacing its environment and stabling its volume and pressure.

Enthalpy is a thermodynamic potential. It is a state function and an extensive quantity. It is a state function and extensive quantity. The unit of measurement for enthalpy is joule J. Enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements because it simplifies certain descriptions of energy transfer. Enthalpy change accounts for energy transferred to the environment at constant pressure through expansion or heating.

The enthalpy of a homogeneous system is defined as

H = U + pV


H is the enthalpy of the system

U is the internal energy of the system

p is the pressure of the system

V is the volume of the system

Enthalpy is an extensive property. This means that for homogeneous systems, the enthalpy is proportional to the size of the system.

For inhomogeneous systems the enthalpy is the sum of the enthalpies of the composing subsystems

H = ∑kHk

Where the label k refers to the various subsystems

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