1. For any thermodynamic process or a chemical reaction at constant volume,
ΔV = 0.
2. Since ΔH = ΔU + PΔV, at constant volume ΔH = ΔU.
3. In the reactions, involving only solids and liquids, Δ V is negligibly small, hence ΔH = ΔH.
4. In a chemical reaction, in which number of moles of gaseous reactants and gaseous products are equal, then change in number of moles,
Δn = n – n = 0.
Since ΔH = ΔU + ΔnRT, as Δn = 0, ΔH = ΔU.
