I know that: Change in Internal Energy = Heat change - (pressure X change in volume) and at constant volume PV is negligable. But i heard that ENTHALPY is like internal energy but at constant PRESSURE, so PV becomes negligable and enthalpy = heat change - (PV) PLUS (PV) = heat change. How come at constant pressure, we just use enthalpy (heat change)?? I hope you understood the question, help is very much appreciated!!! thankyou!!!