The magnitudes of the heat effects in these two conditions are different. In the first case the volume of the system is kept constant during the course of the measurement by carrying out the reaction in a closed and rigid container, and as there is no change in the volume no work is involved. From the first law of thermodynamics, Δ Δ U = Q − − W , where W is the work done by the system. When only expansion work is possible for a process we have Δ Δ U = Q V ; this implies that the heat of reaction at constant volume is equal to the change in the internal energy Δ Δ U of the reacting system.