[tex]\Delta[/tex] U = Q - W = Q - PV (where PV is the work done BY the system). [tex]\Delta[/tex] H = [tex]\Delta[/tex] U + PV (where PV is work done BY the system). 1) Above I defined PV in both equations as work done BY the system. I think this must be correct to show that when P is constant we can have [tex]\Delta[/tex] H = Q <proof>: H = [tex]\Delta[/tex] U + PV = Q - PV + PV = Q Surely both PV in the term above have to be defined equivalently as work done BY the system (in my case, as formulas written above) for them to cancel out? Can anyone confirm this? 2) I really do not understand WHY we need to enthalpy when we already have internal energy? Does the W in internal energy not already take into account any work done due to change in P or V? What is the point of inventing enthalpy when it is just internal energy added by a further PV? <example>: Say heat (Q) is added to a machine, and the piston expands (does work). So change in internal energy = Q - PV (heat added - work done by system) while change in enthalpy = Q + PV (heat added - work done by system + work done by system) So whats the point?