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Question
Why is it not possible to obtain pure ethanol by fractional distillation? What general name is given to binary mixtures which show deviation from Raoult’s law and whose components cannot be separated by fractional distillation. How many types of such mixtures are there?
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Solution
The general name given to binary mixtures which show deviation from Rault's law and whose components can not be separated by fractional distillation is "aziotropes".
It is not possible to obtain oure ethanol by fractional distillation because of following reasons, Azeotropes are binary solutions (liquid mixtures) having the same composition in liquid and vapour phase and it is not possible to separate the components of an azeotrope by fractional distillation. Ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation gives a solution of approx. 95% ethanol by volume of ethanol. This has the same composition in liquid and vapour phase and hence it is not possible to separate them.
There are two types of such binary mixtures termed as:
(1) Minimum boiling azeotropes: The non-ideal solutions showing large positive deviation from minimum boiling azeotrope at a specific composition. For example, 95% ethanoal and 5% water (by volume): The boiling points of pure athanoal, water and its azeotrope is given below, Ethanoal = 351.3 K, Water = 373 K, Azeotrope = 351.1 K.
(2) Maximum boiling azeotropes: The non-ideal solutions showing large negative deviation from Rault's law form maximum boiling azeotrope at a specific composition, viz, Nitric acid and water with approximate composition, 68% nitric acid and 32% water by mass. Boiling point of such HNO3 - H20 Azeotropic mixture is 393.5 K.
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