PPT Slide

Many reactions involve little or no PV work, so most (or all) of the energy change occurs as heat.

Case 1. Reactions that do not involve gases.

Case 2. The total moles of gaseous reactants equals the total moles of gaseous products. Example:

Here V is constant, so _V = 0, P_V = 0, and _H = _E.

Case 3. Number of gas moles changes in the reaction, so P_V _ 0. But q P is usually larger than P_V, most of _E is heat and _H _ _E.