# Explain the change of internal energy in a chemical reaction and prove that internal energy is a definite quantity

Change of internal energy in a chemical reaction:
The energy associated with the random, disordered motion of molecules is called Internal energy. Internal energy is a state property i.e. its value depends only upon the state of the substance but does not depend upon how that state is achieved. At a given temperature, change of internal energy change in a chemical reaction is a definite property. Let us take a system which undergoes a chemical change having internal energy U1. Due to chemical change the system give rise to another chemical system having internal energy U2.

Hence, the change of internal energy will be give as the difference between their internal energies i.e.:
∆U = U 2 – U 1

As U2 and U1 are definite quantities, hence the change in internal energy ∆U is also a definite quantity.
Now what will be the internal change in chemical reaction if it is taking place at constant temperature and at constant volume? From the first law of equation i.e. ∆U = q + w we can calculate the internal energy change and it will be as follows:

As temperature and volume is constant, hence w = 0
Therefore,
Using first law of equation:
∆U = q v
Where q v is the heat exchanged at constant volume.

Category: Thermochemistry