CIE AS/A Level Chemistry 25.2 Partition coefficients Study Notes- 2025-2027 Syllabus
CIE AS/A Level Chemistry 25.2 Partition coefficients Study Notes – New Syllabus
CIE AS/A Level Chemistry 25.2 Partition coefficients Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Chemistry latest syllabus with Candidates should be able to:
state what is meant by the term partition coefficient, Kpc
calculate and use a partition coefficient for a system in which the solute is in the same physical state in the two solvents
understand the factors affecting the numerical value of a partition coefficient in terms of the polarities of the solute and the solvents used
Partition Coefficient, \( \mathrm{K_{pc}} \)
The partition coefficient is used to describe how a substance distributes itself between two immiscible solvents.
Definition
The partition coefficient, \( \mathrm{K_{pc}} \), is defined as the ratio of the concentrations of a solute in two immiscible solvents when equilibrium has been established.
Mathematically:
\( \mathrm{K_{pc} = \dfrac{[\text{solute in solvent 1}]}{[\text{solute in solvent 2}]}} \)
The value of \( \mathrm{K_{pc}} \) is constant at a given temperature, provided the solute exists in the same molecular form in both solvents.
Important Exam Points
- The solvents must be immiscible (e.g. water and an organic solvent)
- Concentrations must be measured at equilibrium
- Units cancel, so \( \mathrm{K_{pc}} \) has no units
Example
A substance is shaken with water and hexane. At equilibrium, its concentration in hexane is \( \mathrm{0.80\ mol\,dm^{-3}} \) and in water is \( \mathrm{0.20\ mol\,dm^{-3}} \). State the partition coefficient.
▶️ Answer / Explanation
\( \mathrm{K_{pc} = \dfrac{0.80}{0.20} = 4.0} \)
Calculating and Using the Partition Coefficient, \( \mathrm{K_{pc}} \)
When a solute distributes itself between two immiscible solvents and remains in the same physical (molecular) state in both, the partition coefficient can be used quantitatively.
Definition Reminder
The partition coefficient is defined as:
\( \mathrm{K_{pc} = \dfrac{[\text{solute in solvent 1}]}{[\text{solute in solvent 2}]}} \)
This relationship applies only if the solute:
- is in the same molecular form in both solvents
- has reached equilibrium
- the temperature is constant
Using \( \mathrm{K_{pc}} \) in Calculations
Once \( \mathrm{K_{pc}} \) is known, it can be used to:
- calculate the concentration of solute in one solvent
- determine how much solute is extracted into each solvent
Example
A solute distributes between water and an organic solvent. At equilibrium, its concentration in the organic solvent is \( \mathrm{0.60\ mol\,dm^{-3}} \) and in water is \( \mathrm{0.15\ mol\,dm^{-3}} \).
Calculate the partition coefficient.
▶️ Answer / Explanation
Use the definition of the partition coefficient:
\( \mathrm{K_{pc} = \dfrac{0.60}{0.15}} \)
\( \mathrm{K_{pc} = 4.0} \)
This means the solute is four times more concentrated in the organic solvent.
Example
A solute has a partition coefficient of \( \mathrm{K_{pc} = 5.0} \) between hexane and water.
If the concentration of the solute in water at equilibrium is \( \mathrm{0.020\ mol\,dm^{-3}} \), calculate its concentration in hexane.
▶️ Answer / Explanation
Write the partition coefficient expression:
\( \mathrm{K_{pc} = \dfrac{[\text{hexane}]}{[\text{water}]}} \)
Substitute values:
\( \mathrm{5.0 = \dfrac{[\text{hexane}]}{0.020}} \)
Solve for the concentration in hexane:
\( \mathrm{[\text{hexane}] = 0.10\ mol\,dm^{-3}} \)
The solute is much more soluble in hexane than in water.
Factors Affecting the Partition Coefficient, \( \mathrm{K_{pc}} \)
The numerical value of the partition coefficient, \( \mathrm{K_{pc}} \), depends on how well a solute interacts with each of the two immiscible solvents. This is determined mainly by the relative polarities of the solute and the solvents.
Key Principle: Like Dissolves Like
A solute is more soluble in a solvent with a similar polarity.
As a result:
- Polar solutes dissolve preferentially in polar solvents
- Non-polar solutes dissolve preferentially in non-polar solvents
Effect of Solvent Polarity on \( \mathrm{K_{pc}} \)
If solvent 1 is more similar in polarity to the solute than solvent 2, then the concentration of the solute will be higher in solvent 1.
This leads to a larger partition coefficient:
\( \mathrm{K_{pc} = \dfrac{[\text{solute in solvent 1}]}{[\text{solute in solvent 2}]}} \)
Intermolecular Forces
The solubility of a solute in a solvent depends on the strength of intermolecular attractions between them.
- Polar solvents form dipole–dipole interactions and hydrogen bonds
- Non-polar solvents interact mainly via London dispersion forces
A solute will partition into the solvent where these interactions are strongest.
Important Exam Points
- The solute must be in the same molecular form in both solvents
- The value of \( \mathrm{K_{pc}} \) depends on solvent polarity, not volume
- A large \( \mathrm{K_{pc}} \) means the solute strongly prefers one solvent
Example
A non-polar organic compound has a high partition coefficient between hexane and water. Explain this observation.
▶️ Answer / Explanation
Hexane is non-polar, while water is polar.
The non-polar solute has stronger interactions with hexane.
As a result, the solute is much more soluble in hexane.
This gives a large value of the partition coefficient.
Example
A polar solute has a partition coefficient close to 1 between water and propanone. Explain what this indicates about the polarity of the solute and the solvents.
▶️ Answer / Explanation
Water and propanone are both polar solvents.
The polar solute interacts similarly with both solvents.
This results in similar solubility in each solvent.
Therefore, the partition coefficient is close to 1.