Hi, I know enthalpy is used a lot to calculate isobaric processes, but what about isochoric ones? For example, if I have gas in a piston and I add heat at constant VOLUME. I know that [itex]Q=\Delta U[/itex] But since [itex]H=U+PV[/itex] or [itex]\Delta H=\Delta U + \Delta PV[/itex] I see that although V is constant P is not, it increases and so my entropy increase is bigger than my internal energy increase. That is ok, I've searched this forum and found the same results, but my question is. Since enthalpy is energy, and energy is conserved from where does this energy come from? Since the heat supplied to the system is smaller than the change in enthalpy. Let's say I give 1000 Joules to the system in form of heat, the internal energy increases by 1000J, but the Enthalpy increases more... Any help appreciated.