Determination of Equilibrium Constant from Distribution Coefficient

Determination of Equilibrium Constant from Distribution Coefficient

Determination of Equilibrium Constant from Distribution Coefficient

Determination of Equilibrium Constant from Distribution Coefficient

Distribution law helps in the determination of equilibrium constant of a reaction when one of the reactants is soluble in two immiscible solvents.
Example: When KI reacts with I2 to form KI3
KI + I2 ⇌ KI3
This reaction can be carried out in water, while I2 is soluble in both water and benzene.
Procedure
To find Distribution Coefficient of I2
I2 is shaken with water and benzene in a bottol. The concentration of I2 in the two layers is then determined by titration against standard thiosulphate solution.
So, (Conc. I2 in water/Conc. I2 in benzene) = K
To find Equilibrium Constant using the value of K
Determination of Equilibrium Constant  from Distribution Coefficient
A solution of KI of concentration 'a' is shaken with I2 in a bottol, some benzene is also added and shaken. On standing, the mixture seperates into two layers.
The concentration of I2 is determined in the two layers by titration against standard thiosulphate solution.
Let 'b' the concentration of I2 in benzene layer, 'c' be the concentration of I2 in water layer which is really the total of the concentration of free I2 and KI3.
K is the value of Distribution coefficient of I2 between water and I2.
So, (Conc. I2 in water/Conc. I2 in benzene) = K
∴ Concentration of free I2 in water layer = K X b
Hence, Concentration of KI3 in water layer = c-Kb
∴ Concentration of KI in water layer = a-(c-Kb) = a-(c+Kb)
Now, we can write the equilibrium constant of the reaction-
KI + I2 ⇌ KI3
so, Kc = [KI3]/[KI ] [I2]
or, Kc = (c-Kb)/(a-(c+Kb))Kb
The square brackets denote concentrations in water layer.
Since all the quantities on RHS are known, the equilibrium constant may be determined.

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