enthalpy

The heat added or lost by the system is measured as the change in enthalpy (ΔH), not the actual amount of heat. It is totally reliant on T, p, and U, the state functions. Enthalpy can also be written as: ΔH = ΔU + ΔPV. At constant temperature, the heat flow (q) for the process is equal to the change in enthalpy, which is expressed as.