Mention three characteristics of ideal solutions. What cause deviation from ideal behaviour ? Discuss the positive deviations from ideal behaviour.

Explanation

Characteristics of an ideal solution
(1) It must obey Raoult.s law (2) `Delta `H (mix) = 0
(3)`Delta `V (mix) = 0
Cause of positive deviations from Raoult.s law. Consider a binary solution having components A and B. If the A B interactions in the solution are weaker than the A-A and B-B interactions in the two liquids forming the solution, then the escaping tendency of A and B types of molecules from the solution becomes

more than pure liquids. As a result each component of the solution has a partial vapour pressure greater than expected on the basis of Raoult.s law. The total vapour pressure will be greater than the corresponding vapour pressure expected in case of ideal solution of the same composition. This type of behaviour of solution is described as positive deviations from Raoult.s law. This may be expressed as
`P_A gt P_(A)^(@)x_A`
or ` P_B gt P_(B)^(@) x_(B)`
The positive deviations have been shown in Fig. In the figure, dotted lines show the ideal behaviour. Explanation for positive deviations. Let us consider a solution of ethanol and cyclohexane. In ethanol, the molecules are held together due to hydrogen bonding as shown below.

On adding cyclohexane, the cyclohexane molecules come in between the molecules of ethanol, and break the hydrogen bonds. As the resulting solution has weaker intermolecular attractions decreases, therefore the escaping tendency of alcohol and cyclohexane molecules from the solution increases. Hence the vapour pressure of the solution is greater than the vapour pressure as expected from Raoult.s law