We said, on the previous page, that if the system is in a fixed volume, with no other way of doing work (like a battery), then the change in internal energy is the same as the heat supplied to the system. However, if we make that walls of this fixed volume container of an adiabatic substance, then there is no way of influencing the internal energy of the system with the surroundings. The system is closed.

This is a very useful situation, since we can now run chemical reactions inside the container and see how the internal energy of the system changes as a direct result of the reaction at a particular temperature.

The closest we can come to this situation is an adiabatic bomb calorimeter. The volume is fixed, but the container will change in temperature if the reaction does, however, we can measure the change in temperature of the container. The temperature change will be proportional to the 'heat of reaction' and so we can calculate ΔU.