The quantity of 25 kJ/mol represents the energy required for the phase transition of water from liquid to gas at a constant temperature and pressure. In this specific case, when water transitions from liquid (H₂O(l)) to vapor (H₂O(g)), the enthalpy change (ΔH) is greater than the internal energy change (ΔE) by this amount.
This difference arises because when water vaporizes, it not only requires energy to break the intermolecular forces holding the liquid molecules together (which contributes to ΔH), but there is also a contribution from the work done against the atmospheric pressure as the gas expands. This work done is accounted for in the ΔH value but not in ΔE, resulting in the difference of 25 kJ/mol. Therefore, this quantity essentially quantifies the additional energy needed for the expansion work during the vaporization process under the given conditions.