According to Raoult, in a binary solution, the vapour pressure of a component at a given temperature is equal to the mole fraction of that component in the solution multiplied by the vapour pressure of that component in the pure state.
PA = XAP°A
PB = XBP°B
Thus for solution, total pressure becomes PT
PA = XAP°A
PB = XBP°B
Thus for solution, total pressure becomes PT
PT = PA + PB
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where PA and PB are vapour pressure of solvent and solute respectively. XA and XB are mole fraction of solvent and solute respectively. P°A and P°B are vapour pressure of pure solvent and pure solute respectively.
If solute is non-volatile,
PT = PA
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Ideal and non-ideal solutions
Ideal solution: The solution which obeys Raoult's law at all compositions of solute and solvent and at all .emperature is called an ideal solution. For an ideal solution, solvent-solvent intermolecular attractions and z - olute-solute intermolecular attractions are almost same as solvent-solute intermolecular attractions.
Characteristics of an ideal solution
(i) PA = XAP°A and PB = XBP°B [Raoult's law is obeyed]
(ii) ΔH mixing = 0, i.e., no heat should be absorbed or evolved during mixing.
(iii) ΔVmixing = 0, i.e., no change in volume (expansion or contraction) on mixing.
(ii) ΔH mixing = 0, i.e., no heat should be absorbed or evolved during mixing.
(iii) ΔVmixing = 0, i.e., no change in volume (expansion or contraction) on mixing.
Examples:
Substances having similar structures and polarities form nearly ideal solutions.
(i) Benzene and Toluene
(ii) n-hexane and n-heptane
(iii) Chlorobenzene and bromobenzene
(iv) CCI 4 and SiCl4
(v) Ethyl chloride and ethyl bromide
(ii) n-hexane and n-heptane
(iii) Chlorobenzene and bromobenzene
(iv) CCI 4 and SiCl4
(v) Ethyl chloride and ethyl bromide
Non-ideal solutions: The solutions which deviate from ideal behaviour are called non-ideal solutions or real solutions. For a real solution, solvent-solvent intermolecular attractions and solute-solute intermolecular attractions are different from solute-solvent intermolecular attractions.
Characteristics:
(i) PA ≠ P°AXA and PB ≠ P°BXB [Raoult's law is not obeyed]
(ii) ΔH mixing ≠ 0 i.e., solution may absorb or release heat.
(iii) ΔV mixing ≠ 0 i.e., solution may expand or contract on mixing of solute and solvent.
(ii) ΔH mixing ≠ 0 i.e., solution may absorb or release heat.
(iii) ΔV mixing ≠ 0 i.e., solution may expand or contract on mixing of solute and solvent.
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