Most liquids follow Trouton’s rule (see Exercise 19.95), 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 (°C) | ΔHrap (kj/mol) |
Acetone, (CH3)2CO | 56.1 | 29.1 |
Dimethyl ether, (CH3)2O | -24.8 | 21.5 |
Ethanol, C2H5OH | 78.4 | 38.6 |
Octane, CSH|S | 125.6 | 34.4 |
Pyridine, QHSN | 115.3 | 35.1 |
(a) Calculate ΔSo for each of the liquids. Do all the liquids obey Trouton’s rule? (b) With reference to intermolecular forces (Section 11,2), 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 (C46H5Cl) boils at 131.8 °C. Use Trouton’s rule to estimate ΔHvap for this substance.
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