Question

In the simple distillation of a mixture of toluene and cyclohexane, the average highest temperature recorded was in 95 degrees Celsius. Explain why in the fractional distillation of the same mixture, the average highest temperature was 108 degrees Celsius.

Answer 1

In the simple distillation each time a distillation is run, material is lost. Some evaporates into the air and some is left behind, stuck to the apparatus. But, in a fractional distillation it is possible to separate compounds with closer boiling points. Condensing of the higher-boiling vapor releases heat causing the vaporization of lower-boiling liquid on the packing so that the lower boiling component moves up while the higher boiling component moves down, some of it running back into the distilling flask. Each successive condensation/vaporization cycle affords a vapor that is richer in the more volatile fraction. As the temperature is raised the lower boiling fractions get enriched in the vapor. Consider a 60:40 mixture of cyclohexane (BP 81 °C) and toluene (BP 110 °C). This will boil at 88 °C. The same thing happen with your mixture that the average highest temperature was 108 °C

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