Most liquids follow Trouton’s rule (see Exercise), which states that the molar entropy of vaporization is approximately 88 ± 5 J/mol-K. The normal boiling points and enthalpies of vaporization of several organic liquids are as follows:
Substance | Normal Boiling Point (ºC) | ΔHvap(kJ/mol) |
Acetone, (CH3)2CO | 56.1 | 29.1 |
Dimethyl ether, (CH3)2O | -24.8 | 21.5 |
Ethanol, C2H5OH | 78.4 | 38.6 |
Octane, C8H18 | 125.6 | 34.4 |
Pyridine, C5H5N | 115.3 | 35.1 |
(a) Calculate ΔSvap for each of the liquids. Do all the liquids obey Trouton’s rule? (b) With reference to intermolecular forces (Section), can you explain any exceptions to the rule? (c) Would you expect water to obey Trouton’s rule? By using data in Appendix B, check the accuracy of your conclusion. (d) Chlorobenzene (C6H5Cl) boils at 131.8 °C. Use Trouton’s rule to estimate ΔHvap for this substance.
Exercise
Trouton’s rule states that for many liquids at their normal boiling points, the standard molar entropy of vaporization is about 88 J/mol-K. (a) Estimate the normal boiling point of bromine, Br2, by determining ΔH°vap for Br2 using data from Appendix C. Assume that ΔH°vap remains constant with temperature and that Trouton’s rule holds. (b) Look up the normal boiling point of Br2 in a chemistry handbook or at the Web Elements Web site (http://www.webelements.com) and compare it to your calculation. What are the possible sources of error, or incorrect assumptions, in the calculation?
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