Standard enthalpy of vaporisation ΔvapH° for water at 100°C is 40.66 kJ mol⁻¹. The internal energy of vaporisation of water at 100°C (in kJ mol⁻¹) is:

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+37.56. The relationship between enthalpy (ΔH) and internal energy (ΔU) is given by ΔH = ΔU + ΔnRT. For vaporization of water, Δn = 1 (1 mole of gas is formed from liquid), R = 8.314 J mol⁻¹ K⁻¹, and T = 373 K. Substituting values, ΔU = ΔH - ΔnRT = 40.66 - (1 × 8.314 × 373 / 1000) = 37.56 kJ mol⁻¹.

Standard enthalpy of vaporisation ΔvapH° for water at 100°C is 40.66 kJ mol⁻¹. The internal energy of vaporisation of water at 100°C (in kJ mol⁻¹) is:

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