Energy associated with a body is its capacity to do work. The total energy stored in a system under a given set of conditions is called its internal energy. E.
A system may be composed of quite a number of species or molecules. Therefore, its internal energy will be the sum of all sorts of energies possessed by various constituents of molecules, such as electronic, translational, rotational and vibrational energy as well as their potential energy which is due to:
The intermolecular interaction, and
Positions of molecules in the system leading to change in pressure (P) and volume (V).
It is important to note that it is not possible to determine the absolute value of internal energy of a system, though there is a definite quantity of energy in a particular state of the system. However, if the state of a system changes, so will its internal energy and this change of energy can be determined,
E = E2 - E1
Where E1 and E2 are the initial and final states of energy of a system respectively and E is the change in internal energy.
Internal energy being a state function
allows the heat to pass in or out of the system
allows matter to enter in or leave out and does work on or by the system.
Similarly for a system representing a chemical action.
E = Ep - Er
Where Ep and Er are the internal energies of the products and reactants respectively. If a reaction is carried out at constant temperature and constant volume, then no work is done and the beat exchanged with the surroundings is equal to E. Thus, E is the heat evolved or absorbed in a reaction of constant temperature and volume.
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