Kpc

Chemistry: Cambridge International A Level, topic 1.8.1

The Partition Coefficient (K_pc)

Partition coefficients concern immiscible liquids. Immiscible liquids include things like oil and water.

Immiscible liquids

Immiscible liquids

When you mix immiscible liquids, they form two distinct layers.
Think about oil on top of water, or an unshaken bottle of your favourite vinaigrette.
If you then add a molecule that is soluble in liquids, the molecule will distribute between the two liquids. The distribution depends on which liquid the added molecule is more soluble in.
Eventually, a dynamic equilibrium is set up.

Example

Example

Water and oil are immiscible. Substance Y dissolves in both oil and water.
We can write an equation for the dissolution of substance Y:
Y_{dissolved in water} ⇌ Y_{solution in oil}
To calculate K_pc, we can set up an equilbrium expression:
K_pc = $\Large\frac {[Y_{in \ oil}]}{[Y_{in \ water}]}$
The same general principles that apply to other equilibrium constants apply to K_pc.

Image

$Units$

Units

The concentration for K_pc expressions is normally given in g cm^-3.
If there were 5 g cm^-3 of substance Y in oil, and 10 g cm^-3 in water, the K_pc would be:
K_pc = $\Large\frac {[Y_{in \ oil}]}{[Y_{in \ water}]}$ = 5 / 10 = 0.5.

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