Mention three characteristics of ideal solutions . What cause deviation from ideal behaviour ? Discuss the + ve deviation from ideal behaviour for ilquid pairs

Explanation

Characteristics of an ideal solution . a solution which obeys Raoult.s law exactly at all concentrations and all temperatures is called an ideal solution . For ideal heat change upon mixing `(Delta H_("mix"))` and volume change upon mixing `(Delta V _("mix"))` are equal t zero i.e., `Delta H_("mix") = 0 Delta V _("mix") =`
Cause of deviation from ideal solution behaviour :
1. Solutions of two components A (solute) and B (solvent) in which A . . . . B moleular interactions are different from A . . . . . A and B . . . . . B interactions
2. There is a change in volume when solute and solvent are mixed i.e. `Delta V ne 0 `
3. There is change in enthalpy i.e. `Delta H ne 0 `
4. The solution is not dilute i.e., the cone of solution is more than `5% `

Solutions showing positive deviations from Raoult.s law. In solution showing this type of deviations, the partial pressure of each component (say A and B) of solutions is greater than that as expected according
`P_(A) gt P_(A)^(@ x_(A) and P_(B) gt P_(B)^(@) x_(B)`
The positive deviations have been shows in figure in which the solid lines exhibit the actual behaviour on mixing , while the dotted lines show the ideal behaviour. This type of deviations are shown by solutions in which like molecules have greater attraction for each other than the unlike molecules .
Explanation for positive deviations. Let us explain by considering a solution of ethanol and cyclohexane. In ethanol, the molecules are held together due to hydrogen bonding as shown below .
`. . . . . overset(C_(2)H_(5))overset(|)(O)-H. . . . .overset(C_(2)H_(5))overset(|)(O)- H . . . . . overset(C_(2)H_(5))overset(|)(O)- H . . . . . overset(C_(2)H_(5))overset(|)(O)- H . . . . `
On adding cyclohexane, the cyclohexane molecules get in between the molecules of ethanol , thus breading the hydrogen bonds, As the resulting solution has weaker intermolecular attractions, the escaping tendency of alcohol and cyclohexane molecules from the solutions increases. Consequently, the vapour pressure of the solution is greater than the vapour pressure as expected from Raoult.s law