The energy associated with the disordered, random motion of molecules is called Internal Energy. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. Internal energy is independent of the path.
Internal energy is a state property i.e. its value depends only upon the state of the substance but does not depend upon how that state is achieved. The absolute value of internal energy of a substance cannot be determined. However determining the absolute values of internal energies is neither necessary nor required. It is the change in internal energy accompanying a chemical or a physical process that is of interest and this is a measurable quantity.
EXAMPLE
A very simple example of this can be a glass of water at room temperature which is kept on a shelf or table. This glass of water has no apparent energy(potential or kinetic). But when we analyze it on the microscopic scale, we can see high speed molecules. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole.
Another example, when oil is reacted with oxygen, potential energy is released, since new chemical bonds are formed in the products which are more powerful than those in the oil and oxygen. The energy resulting from this process may be converted directly to electricity (as in a fuel cell), or into thermal energy (if the oil is simply burned).
