8.1: Energy and Its Units Energy is the ability to do work. Heat is the transfer of energy due to temperature differences. Energy and heat are expressed in units of joules. 8.2: Heat Transfer Heat transfer is related to temperature change. Heat is equal to the product of the mass, the change in temperature, and a proportionality constant called the specific heat. 8.3: Energy Associated with Phase Changes There is an energy change associated with any phase change. There is an energy change associated with any phase change.