Enthalpy, H, is a thermodynamic state function equal to internal energy plus the product of pressure times volume or H = E + PV, i.e., the heat content of the system. For example, the change in enthalpy, ΔH, is the heat involved, either released (exothermic) or absorbed (endothermic), in a system for a constant pressure process (e.g., a reaction at a fixed pressure). Exothermic reactions have a negative enthalpy change, whereas endothermic reactions have a positive enthalpy change.
See state function, entropy