The energy of the thermodynamic system is called internal energy. It includes all the possible forms of energy of the system. The sum of different forms of energy associated with the molecule is called internal energy.
Internal energy = Translational energy + rotational energy + vibrational energy + electrical energy + nuclear energy + bonding energy
The internal energy of the system depends upon the state of the system but not upon the path on which the system attains another state. So, internal energy is a state function. Therefore, the absolute value internal energy cannot be determined. However, the change in internal energy can be calculated as:
ΔE = E2 – E1
In a chemical reaction,
ΔE = Eproduct – Ereactant
If Eproduct > Ereactant, heat is absorbed by the system and hence reaction is endothermic.
If Eproduct < Ereactant, heat is released by the system and hence reaction is exothermic.