Boiling Point and Molecular Weight Ronald Rich
Bethel College, North Newton,
Kansas
This note is prompted by the statements in several textbooks to the effect that an increase in molecular weight causes an increase in boiling point for non-polar substances. This correlation is sometimes used as a basis for comparisons between as well as within homologous series. Explanations have been elaborated for exceptions to this rule, e.g., in the case of fluorocarbons versus hydrocarbons. The purpose of the accompanying graph is to show that molecular weight, by itself, has negligible influence on boiling points. The solid lines connect substances who% outer atoms have similar polarizabilities and whose inner atoms are more or less well-shielded or identical. The dashed lines connect molecules in series of the types usually cited to illustrak t,he accepted rule. Even in these latter series, however, the increases in boiling points can be correlated more meaningfully with increases in molecular size and polarisability than with molecular weight.
454 / Journal o f Chemical Ecucation
- 100 25
75
125
175
225
275
Molecular welqht
325
375 425
