Solutions
Deviation of Vapour Pressure of Non-Ideal Solutions from Raoult's Law
Cause of deviation of vapour pressure of non-ideal solutions from Raoult’s Law
The cause of deviation of vapour pressure of non-ideal solution lies in the nature of interactions of molecules of solute and solvent at molecular level.
Cause of positive deviation in vapour pressure than the Raoult’s Law
If intermolecular attractive forces between solute – solvent (A – B) are weaker than that of solute – solute (A – A) and / or solvent – solvent (B – B), then molecules of solute or solvent find easier to escape than in pure state. Because of this the vapour pressure increases and shows positive deviation than ideal vapour pressure in pure state.
Example –
(a) Mixture of ethanol and acetone shows positive deviations from Raoult’s Law.
Explanation:
Molecules of ethanol in pure state are hydrogen bonded. But when acetone is added to the ethanol, molecules of acetone get in between the molecules of ethanol and break some of the hydrogen bond, which weaken the intermolecular attractive forces resulting in increase in vapour pressure of mixture than ethanol in pure state.
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.
Explantion:
When carbon disulphide is added to the acetone, this weaken the dipolar interaction between molecule of solute solvent (A – B) than the respective interaction between solute-solute (A – B) and solvent – solvent (B – B), which leads to increase in vapour pressure. And mixture shows positive deviation in vapour pressure than Raoult’s Law.
Cause of negative deviation of vapour pressure of non-ideal solution than Raoult’s Law
If intermolecular interactive forces between solute – solute (A – A) and/or solvent – solvent (B – B) are weaker than that of solute – solvent (A – B), the molecules of solute and or solvent find it difficult to escape in compare to the pure state, resulting in decrease in vapour pressure. This shows negative deviation in vapour pressure from Raoult’s Law.
Example –
(a) Mixture of phenol and aniline
In the mixture the hydrogen bonding between proton of phenol and lone pair on nitrogen atom of anile becomes more stronger than that of pure state, i.e. solute – solvent (A – B) interactions become stronger than that of solute – solute (A – A) and solvent – solvent (B – B). And molecules of mixture find difficult to escape than in pure state. This leads to decrease in vapour pressure and given mixture shows negative deviation from Raoult’s Law.
(b) Mixture of chloroform and acetone
When chloroform is mixed with acetone, the molecules of chloroform form hydrogen bonding with the molecules of acetone, this increases the interactive molecular forces beween solute – solvent (A – B) than that of A – A and B – B, leads to decrease in vapour pressure of the mixture than the pure state. This is the cause that mixture of chloroform and acetone shows negative deviation than Raoult’s Law.
Azeotropes
Binary Liquid mixtures having same composition in liquid and vapour phase and boils at constant temperature are called Azeotropes.
As the mixture boils at constant temperature, so it is not possible to separate the components of such mixture by fractional distillation.
Types of Azeotropes
There are two types of Azeotropes – minimum boiling azeotropes and maximum boiling azeotropes.
Minimum boiling azeotrope
Solutions which show a large positive deviation from Raoult’s Law from minimum boiling azeotropes at a specific composition.
Example –
Solution of ethanol (approximately 95% by volume of ethanol) obtained from fractional distillation of mixture of ethanol-water (obtained by the fermentation of sugar) is known as azeotrope composition.
The liquid and vapour phase of this ethanol (95% by volume of ethanol) solution have the same composition and cannot be separated by further fractional distillation.
Maximum boiling azeotrope
Solutions that show large negative deviation of vapour pressure from Raoult’s Law are called maximum boiling azeotropes.
Example –
Solution of 68% nitric acid and 32% of water is a maximum boiling azeotrope. This boils constantly at 393.5 K and hence cannot be separated by fractional distillation. This solution of nitric acid (68%) and water (32%) shows large negative deviation from Raoult’s Law.
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