All the physical and chemical changes are accompanied by the energy changes.
The evolution or the absorption of the energy in the physical or the chemical change is possible only when a substance is associated with a definite amount of energy known as the internal energy.
Internal energy or intrinsic energy is the total amount of energy stored in a system in different forms such as potential energy, kinetic energy, nuclear energy and chemical energy under a given set of conditions. It is denoted by U or E.
Internal energy of a system depends upon the quantity and nature of the substance, temperature, pressure, etc. The internal energy is a state function and its absolute value cannot be determined.
The internal energy of a system changes when heat passes in or out of the system, work is done on or by the system and matter enters or leaves the system.
The internal energy depends upon the state of the system and is independent of the method by which this state has been attained. Hence it is a state function.
For an isochoric process, at constant volume, the quantity of heat supplied to the system is equal to the increase in its internal energy.
For a cyclic process, the change in the internal energy is zero.
For exothermic reactions, the sign of U is negative and for endothermic reactions the sign of U is positive.
Bomb calorimeter is used for the experimental determination of change in the internal energy.
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