A compound itself has a definite amount of energy which includes kinetic energy, potential energy, energy of atom, constituting molecules and chemical bond energy. The sum of these different forms of energy (molecules) is called internal energy denoted by E. Internal energy is the state function so, it depends upon initial and final state and also its absolute value cannot be determined however change can be measured.
Now, change in internal energy is given by:
Where, ∆E= change in Internal energy.
Ep= Internal energy of product.
Er= Internal energy of reactant.