Enthalpy, H is the thermodynamic function that accounts for heat flow in processes occurring at constant pressure when no forms of work are performed other than P-V work.
H = E + PV
At constant pressure, a change in enthalpy equals the change in internal energy plus the product of the constant pressure times the change in volume.
ΔH = ΔE + P ΔV
- When ΔH is positive, the system has gained heat from the surroundings - endothermic.
- When ΔH is negative, the system has lost heat to the surroundings - exothermic.