It is the energy associated with a system by virtue of its molecular constitution and the motion of its molecules. The contribution of energy due to molecular constitution is known as Internal Potential Energy and the contribution of energy due to motion of molecules is called Internal Kinetic Energy. Hence the total internal energy of a system is given by the sum of two types of energies. i.e.
Total Internal Energy = Internal Potential Energy + Internal Kinetic Energy
Internal energy is represented by E or u. It is to be remembered that the absolute value of internal energy cannot be obtained. It is the change in internal energy accompanying a chemical or physical process, when the system is taken from one state to another, which is determined experimentally.
A fixed quantity of any substance, under a given set of conditions, has got a definite amount of internal energy associated with it. This amount will be different for different substances.
When a system undergoes a thermodynamic change from state A to B, then, the change in internal energy (DE) of a system is given as,
DE = EB – EA
Where EA is internal energy of system in the initial state A and EB is internal energy of system in the final state B.
When a chemical system having internal energy ER (internal energy of reactants) undergoes a chemical change giving rise to another chemical system having internal energy EP (internal energy of products), then change in internal energy is given by:
DE = EP – ER
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