Standard enthalpy of vaporisation Δ_vapH^θ for water for water at 100^oC is 40.66 kJ mol^-1. The internal energy of vaporisation of water at 100^o C (in KJ mol^-1) is

(Assume water vapour to behave like an ideal gas.)

+37.56

-43.76

+43.76

+40.66

Solution

+37.56

straight H subscript 2 straight O space rightwards arrow with 100 to the power of straight o space straight C on top space straight H subscript 2 straight O space left parenthesis straight g right parenthesis increment subscript Vap straight H to the power of straight theta space equals increment subscript vap straight E to the power of straight theta space plus increment straight n subscript straight g RT For space the space above space reaction comma increment straight n subscript straight g space equals space straight n subscript straight p minus straight n subscript straight r space equals 1 minus 0 space equals 1 therefore comma 40.66 space KJ space mol to the power of negative 1 end exponent space equals space increment subscript vap straight E to the power of straight theta space plus 1 space straight x space 8.314 space straight x space 10 to the power of negative 3 end exponent space straight x space 373 increment subscript vap straight E to the power of straight theta space equals space 40.66 space KJ space mol to the power of negative 1 end exponent space minus 3.1 space KJ space mol to the power of negative 1 end exponent space equals plus 37.56 space KJ space mol to the power of negative 1 end exponent

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