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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 17.48

The normal boiling point of Br2(l) is 59.47 oC. Estimate Hovapn of bromine. Compare your result with a value based on data from Appendix C.


Trouton's Rule suggests that Hovapn = 87 J/mol•K×Tbp = (87)(273.15 + 59.47) = 29000 J/mol = 29 kJ/mol.

To use the data from the Table of Thermodynamic Quantities, use the reaction:

Br2(l) Br2(g)

with Hfo (Br2(l)) = 0 kJ/mol and Hfo (Br2(l)) = 30.91 kJ/mol so Ho = Hovapn = [30.91] – [0] = 30.91 kJ/mol.

Given the approximate nature of Trouton's Rule, the estimate and the experimental value are remarkably similar.