Hello Everyone,
You know Internal Energy (U) that is the total energy of a particle or a substance in chemistry.
Its a fact that "Change in heat at constant volume is equal to the difference between internal energy of the reactant and the internal energy of the product." But I can't get it why at constant volume. Why there is need of constant volume ? Can someone please explain me ? Is my question clear to you all ?

Constant volume is required because otherwise the system does work as its volume expands, which would reduce the internal energy of the final product.