Chemistry Ideal, Non-ideal Solutions and Azeotropes

Topics Covered :

● Ideal and Non-ideal Solution
● Positive Deviation
● Negative Deviation
● Azeotropes

Ideal and Nonideal Solutions :

Liquid-liquid solutions can be classified into ideal and non-ideal solutions on the basis of Raoult’s law.

Ideal Solutions :

`color{green}("Definition") :` The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions.

`=>` The enthalpy of mixing of the pure components to form the solution is zero. It means that no heat is absorbed or evolved when the components are mixed, i.e., `Delta_text(mix) H = 0`

`=>` The volume of mixing is zero. The volume of solution would be equal to the sum of volumes of the two components, i.e., `Delta_text(mix)V = 0`

`=>` Ideal behaviour of the solutions can be explained by considering two components `A` and `B` at molecular level.

`=>` In pure components, the intermolecular attractive interactions will be of types `A-A` and `B-B`, but in the binary solutions `A-B` type of interactions will also be present.

`=>` If the intermolecular attractive forces between the `A-A` and `B-B` are nearly equal to those between `A-B`, this leads to the formation of ideal solution.

`=>` A perfectly ideal solution is rare. Solution of n-hexane and n-heptane, bromoethane and chloroethane, benzene and toluene, etc. fall into nearly ideal solutions category.

Non-ideal Solutions :

`color{green}("Definition") :` When a solution does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution.

`=>` The vapour pressure of such a solution is either higher or lower than that predicted by Raoult’s law (equation 2.16). The plots of vapour pressure as a function of mole fractions for such solutions are shown in Fig. 2.6.

`color{green}("Positive Deviation") :` If vapour pressure of a solution is higher than that predicted by Raoult’s law, the solution exhibits positive deviation.

`->` In case of positive deviation from Raoult’s law, `A-B` interactions are weaker than those between `A-A` or `B-B`, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules.

`->` In such solutions, molecules of `A` (or `B`) will find it easier to escape than in pure state. This will increase the vapour pressure and result in positive deviation.

`color{red}("Examples") :` (i) Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the ethanol molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult’s law [Fig. 2.6 (a)].

(ii) A solution formed by adding carbon disulphide to acetone, the dipolar interactions between solute-solvent molecules are weaker than the respective interactions among the solute-solute and solvent-solvent molecules. This solution also shows positive deviation.

`color{green}("Negative Deviation") :` If vapour pressure of a solution is lower than that predicted by Raoult’s law, it exhibits negative deviation.

`->` In case of negative deviations from Raoult’s law, the intermolecular attractive forces between `A-A` and `B-B` are weaker than those between `A-B` and leads to decrease in vapour pressure resulting in negative deviations.

`color{red}("Examples") :` (i) Mixture of phenol and aniline. In this case, the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules.

(ii) A mixture of chloroform and acetone forms a solution with negative deviation from Raoult’s law. This is because chloroform molecule is able to form hydrogen bond with acetone molecule as shown. This decreases the escaping tendency of molecules for each component and as a result the vapour pressure decreases resulting in negative deviation from Raoult’s law [Fig. 2.6. (b)].

Azeotropes :

`color{green}("Definition") :` Azeotropes are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature.

`=>` It is not possible to separate the components by fractional distillation.

`=>` There are two types of azeotropes : (i) Minimum Boiling Azeotrope (ii) Maximum Boiling Azeotrope

`color{green}("Minimum Boiling Azeotrope") :` The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition.

● `color{red}("Example") :` Ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation gives a solution containing approximately `95%` by volume of ethanol. At this composition, the liquid and vapour have the same composition, and no further separation occurs.

`color{green}("Maximum Boiling Azeotrope") :` The solutions that show large negative deviation from Raoult’s law form maximum boiling azeotrope at a specific composition.

● `color{red}("Example") :` Nitric acid and water. This azeotrope has the approximate composition, `68%` nitric acid and `32%` water by mass, with a boiling point of `393.5 K`.

 
SiteLock