Question

Explain why phenol and aniline show negative deviation?Also why does Ethanol and Carbon disulphide show positive deviation ?
Is there any method to know which mixture shows negative or positive deviation ?
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Answer 1

(i) The reason for negative deviation of phenol (A) and aniline(B) is explained as below.

Delta H mixing and Delta V mixing is less than zero. A-B attractive force should be greater than A-A and B-B attractive forces. 'A' and 'B' have different shape, size and character. Escaping tendency of both components 'A' and 'B' is lowered showing lower vapour pressure than expected ideally.

(ii) Molecules of ethanol in pure state are hydrogen bonded. ... Because of this mixture of ethanol and acetone shows positive deviation in vapour pressure from Raoult's Law. (b) Mixture of carbon disulphide to the acetone shows positive deviation from Raoult's Law.

or When attractive forces between constituents of different components of the mixture are weaker than those between constituents of like ones.

(iii)  Deviations from Raoult's law can either be positive or negative. A positive deviation means that there is a higher than expected vapor pressure above the solution. A negative deviation, conversely, means that we find a lower than expected vapor pressure for the solution.

hope this works