Enthalpy

Measure of energy in a thermodynamic system

Enthalpy is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant external pressure, which is conveniently provided by Earth's ambient atmosphere. The pressure–volume term expresses the work W {\displaystyle W} that was done against constant external pressure P ext {\displaystyle P_{\text{ext}}} to establish the system's physical dimensions from V system, initial = 0 {\displaystyle V_{\text{system, initial}}=0} to some final volume V system, final {\displaystyle V_{\text{system, final}}} (as W = P ext Δ V {\displaystyle W=P_{\text{ext}}\Delta V}), i.e.

From Wikipedia, under CC BY-SA. More on occurri.