Problem: Find ΔH for the change in state of 1.0 mol H2O(l) at 81 oC to H2O(g) at 115 oC. The heat capacity of H2O(l) = 75.3 J/mol K, heat capacity of H2O(g) = 25.0 J/mol K, and the heat of vaporization of H2O is 40.7 103/mol at 100 oC.

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We are asked to calculate ΔH for the change in state of 1.0 mol H2O(l) at 81 oC to H2O(g) at 115 oC.


For a constant pressure system q =ΔH


H2O(l)81 °C q1H2O(l)100 °C q2H2O(g)100 °Cq3H2O(g)115 °C

 q1 =mCT = (1 mol)(75.3 J mol-K)(100-81)K = 1430.7 J×1 kJ103 J =1.431 kJq2= nHvap = (1  mol)( 40.67 kJ mol ) = 40.67 kJ q3 =mCT = (1 mol)(25.0 J mol-K)(115 -100)K = 375  J×1 kJ103 J = 0.375 kJ


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Problem Details

Find ΔH for the change in state of 1.0 mol H2O(l) at 81 oC to H2O(g) at 115 oC. The heat capacity of H2O(l) = 75.3 J/mol K, heat capacity of H2O(g) = 25.0 J/mol K, and the heat of vaporization of H2O is 40.7 103/mol at 100 oC.

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