AL partition coefficient (best answer 30!)

Question

50cm3 of a solution of I2 in CCl4 were mixed and shaken with 50cm3 of H2O in a separating funnel until equilibrium was attained. If 50cm3 of H2O contain some dissolved KI, will this affect the value of the partition coefficient? The suggested answer is: No, the value of Kp is only temperature dependent.

But I think if the H2O contains some dissolved KI , it will react with I2 added to give I3- and this reduce the [free I2] in water, resulting in decrease in Kp. Why am I wrong?

I mean resulting in decrease in Kp of I2 between H2O and CCl4

Answer 1

The partition coefficient (Kp) keeps constant at constant temperature.

In your deduction, the conclusion has been drawn too early. It should be :
"If the H2O contains some dissolved KI , it will react with I2 added to give I3- and this reduce the [free I2] in water ......
Thus, the rate of migration of I2 from the aqueous layer to the CCl4 is thus decreased. Consequently, more I2 will migrate from the CCl4 layer to the aqueous layer to establish a state of equilibrium with the partition coefficient at this temperature."


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The following example may help you to understand it. However, the figures used in this example are arbitrary chosen, and they are NOT real figures.


Suppose Kp at temperature ToC = [I2(aq)]/[I2(CCl4)] = 0.25
and there is a 50 cm3 CCl4 solution containing 3 g of I2.

Case I :
The CCl4 solution is shaken with 50 cm3 of water.
Mass of I2 in aq + Mass of I2 in CCl4 = 3 g
At equilibrium,
[I2(aq)] = (0.6 g) / (50 cm3)
[I2(CCl4)] = (2.4 g) / (50 cm3)
Kp = [I2(aq)] / [I2(CCl4)] = 0.25

Case II:
The CCl4 solution is shaken with 50 cm3 of KI solution.
Suppose 1 g of I2 reacts with I-(aq) to give I3-(aq).
Mass of I2 in aq + Mass of I2 in CCl4 = 2 g
At equilibrium,
[I2(aq)] = (0.4 g) / (50 cm3)
[I2(CCl4)] = (1.6 g) / (50 cm3)
Kp = [I2(aq)] / [I2(CCl4)] = 0.25