Partition coefficients

Summary and Exam Tips for Partition coefficients

Partition coefficients is a subtopic of Equilibria (A-Level Physical Chemistry), which falls under the subject Chemistry in the Cambridge International A Levels curriculum.

In a separating funnel, two immiscible liquids will form two distinct layers, with the less dense liquid on top. A substance, such as X, may dissolve in both liquids but will have varying solubility in each. At the interface of these two liquids, a dynamic equilibrium is established, represented by the equation:

$X(\text{in more dense liquid}) \rightleftharpoons X(\text{in less dense liquid})$

The partition coefficient ($K_{pc}$) is a measure of the ratio of concentrations of a compound in the two phases at equilibrium. It is dimensionless because all units cancel out. For example, if 1.00 g of X is distributed between 100 cm$^3$ of water and 5 cm$^3$ of ether, and $K_{pc} = 40$, the concentration of X in ether is given by $\frac{m}{5} \, \text{g cm}^{-3}$, and in water by $\frac{1.00 - m}{100} \, \text{g cm}^{-3}$.

Exam Tips

• Understand the concept of dynamic equilibrium between two immiscible liquids and how it relates to partition coefficients.
• Practice calculations involving partition coefficients, ensuring you can rearrange equations to solve for unknowns.
• Remember that the partition coefficient is dimensionless, which simplifies calculations.
• Visualize the separation process in a funnel to better grasp the concept of solubility differences.
• Review examples and practice problems to reinforce your understanding of how substances distribute between two phases.