The First Law of Thermodynamics is Simply the Case of:
The law of conservation of energy
The First Law of Thermodynamices is simply the case of the law of conservation of energy. Energy can be neither nor be destroyed, but it transforms from one form to another, for instance, chemical energy can be converted to kinetic energy in the explosion of a stick of dynamite.
Energy can be defined as the capacity for doing work. It may exist in a variety of forms and may be transformed from one type of energy to another.