Statement of the law
"Energy can be neither created nor destroyed; it can only be transformed from one form of energy to another".
This emerged from experimental observations. It is called a law and was assumed to be always obeyed by physical and chemical systems. In the 20th century nuclear physicists 'broke' the law by carrying out nuclear transformations in which matter was converted into energy and vice versa. From these experiments the equation E=mc2 defined the relationship between energy and matter in these reactions. In terms of chemical reactions the law still applies to a very close approximation.
As energy can be neither created nor destroyed, it follows that any reaction that produces a new substance must have the same overall energy change regardless of the route taken.
Similarly, it means that the energy change of any process must be exactly the opposite for the reverse process.
For example, if 100 kJ of energy are needed to turn a mass of water into steam, then when the same amount of steam is condensed into water exactly the same amount of energy must be released.
These two consequences are essential for the calculation of energy changes in chemical and physical systems.