Equilibrium between immiscible liquids

Distribution Law:

States that a solute distributes itself between immiscible solvents such that a constant temperature and pressure the ratio of its concentrations in the two solvents is constant at a given temperature

That is,

If C is the solute, and A and B are the immiscible solvents, them

 Limitations

• Solute should be in the same molecular state in both solvents
• Temperature should be constant
• None of the solvent should be saturated / solutions should be dilute.

Definition

Partition Coefficient or distribution constant is ratio of the concentration of the solute in immiscible solvents at equilibrium.

Experiment to determine partition coefficient

• By finding the ration of concentrations of a solute in one solvent over the concentration a solute in another immiscible solvent at equilibrium.
• The concentration of a solute in a solvent may be obtained by titration or other reliable method.

Application of distribution laws

1. Extraction of organic compound

Example 1

A solution of 6g of a substance X in 50cm³ of aqueous is in equilibrium at room temperature with an ethereal solution of X containing 108g of x in 100cm³.

Calculate the mass of x that could be extracted by shaking 100cm³ of an aqueous solution containing 10g of X with:

• 100cm³ of ether
• 50cm³ of ether twice at room temperature.

Solution

First determine the partition constant Kd

 

• Study of complexes
• The moles of a ligand that forms a complex with a metal ion = to the total moles of ligand in aqueous solution – moles of ligand that did not form a complex
• The mole of a ligand that did not a complex can be found partition coefficient, Kd
• The moles of ligand that form a complex per mole of a metal cation is obtained by division of the moles that formed a complex by the moles of metal ions

Example 2

Excess ammonia was shaken with equal volume of trichloromethane

And a 0.05M aqueous solution of copper (II) ions to form a complex (Cu(NH₃)_n]²⁺. At equilibrium, the concnentrations of ammonia in the trichloromethane and in the aqueous layer were 0.021moll^-1 and 0.725moll^-1 respectively.

(the partition coefficient, KD, of ammonia between water and trichloromethane is 25)

Calculate:

(i) The concentration of free ammonia in aqueous layer

(ii) The concentration of ammonia that formed the complex with copper ions

(iii) The values of n in the complex

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PARTITION LAW

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Compiled by Dr. Bbosa Science