Enthalpy defines the sum of a system’s internal energy (U) and the mathematical product of its pressure (P) and volume (V):

H = U + PV

Enthalpy is also a state function. Enthalpy values for specific substances cannot be measured directly; only enthalpy changes for chemical or physical processes determined. For processes that take place at constant pressure (a common condition for many chemical and physical changes), the enthalpy change (ΔH) is:

ΔH = ΔU + PΔV

The mathematical product PΔV represents work (w), namely, expansion or pressure-volume work, as noted. By their definitions, the arithmetic signs of ΔV and w will always be opposite:

PΔV = −w

Substituting this equation and the definition of internal energy into the enthalpy-change equation yields:

ΔH = ΔU + PΔV

= qp + (w−w)

= qp

Where qp is the heat of reaction under conditions of constant pressure.