Ideal and Non-Ideal Solutions.
Ideal Solutions– Solutions that obey Raoult’s law at all concentrations are called ideal solutions.
Ideal solutions have the following properties.
1. The enthalpy change for an ideal solution is zero.
[H = 0]
That is, there is neither absorption nor emission of heat in the formation of an ideal solution.
2. The volume change for an ideal solution is zero.
[V = 0]
That is, there is no change in volume in the formation of an ideal solution.
3. Three types of intermolecular interactions occur in solutions.
Between solute and solute molecules (A-A)
Between solvent and solvent molecules (B-B)
Between solute and solvent molecules (A-B)
All three intermolecular interactions are equal in ideal solutions.
(A-A) = (B-B) = (A-B)
Examples of positive deviation.
- n-hexane + n-heptane
- chloroethane + bromoethane
- benzene + toluene
Non-ideal solution- “Solutions that do not obey Raoult’s law at all concentrations are called non-ideal solutions.”
→ The vapor pressure of these solutions is either lower or higher than the vapor pressure determined by Raoult’s law.
There are two types of steady-quantity mixtures.
Solutions with positive deviation
Solutions with negative deviation
1. Solutions with positive deviation– The vapor pressure of these solutions is higher than the vapor pressure determined by Raoult’s law.
Solutions that show positive deviation have the following properties:
1. The enthalpy change is positive.
[H > 0 ]
This means it is an endothermic reaction.
2. The volume change is positive.
[V > 0]
3. In such solutions, the intermolecular attraction between the solute and solvent (A-B) is weaker than that between the molecules (A-A) and (B-B), i.e., (solute-solute) and (solvent-solvent).
Examples of positive deviation:
- Ethanol + acetone
- Carbon disulfide + acetone
2. Solutions with negative deviation – Solutions whose vapor pressure is less than the vapor pressure determined by Raoult’s law.
→ Solutions showing negative deviation have the following properties:
1. The enthalpy change is negative.
[H < 0 ]
This means the reaction is exothermic.
2. The volume change is negative.
[V < 0 ]
3. In this, the intermolecular force of attraction between the solute and solvent, i.e., (A-B), is stronger than (A-A), (B-B).
Examples of negative deviation.
- Phenol + Aniline
- Chloroform + Acetone
Explanation [Chloroform + Acetone] –
H-bond between chloroform and acetone –
Due to the presence of intermolecular H-bonds in a mixture of chloroform and acetone, the solute-solvent attraction is greater than the solute-solute or solvent-solvent attraction. Therefore, this mixture shows a negative deviation from Raoult’s law.
Constant Quantity Mixtures – A bicomponent mixture that has the same composition in its liquid and vapor phases and boils at a fixed temperature is called a constant-quantity mixture. These cannot be separated by fractional distillation.
There are two types of constant-quantity mixtures.
Minimum boiling point constant-quantity mixture
Maximum boiling point constant-quantity mixture
1. Minimum boiling point is a constant-quantity mixture– In this type of mixture, the boiling point of the mixture is lower than the boiling point of each component.
Solutions that show a positive deviation from Raoult’s law are called minimum boiling point constant-quantity mixtures.
Example– Ethanol (95%) + Water (5%)
2. Maximum boiling point is a constant-quantity mixture– In this type of mixture, the boiling point of the mixture is higher than the boiling point of both components.
Solutions that show a negative deviation from Raoult’s law are called maximum boiling point constant-quantity mixtures.
Example– Nitric acid (68%) + Water (32%)
[Boiling point = 393.5 K]
Que. A constant-quantity mixture cannot be separated by fractional distillation. Why?
Both components of a constant-quantity mixture boil at the same temperature, so they cannot be separated by fractional distillation.
