9701 · 25.2
Partition coefficients flashcards
Revision flashcards for Cambridge 9701 Partition coefficients (syllabus 25.2). Flip, recall, then mark a real past-paper question.
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What is a partition coefficient, $K_{pc}$?
The ratio of the concentrations of a solute in two immiscible solvents at equilibrium and at a constant temperature.
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What does 'immiscible' mean?
Describes liquids that do not mix to form a single phase, such as oil and water. They form separate layers.
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Write the formula for the partition coefficient.
$K_{pc} = \frac{[\text{solute in solvent 1}]}{[\text{solute in solvent 2}]}$. The solvents are specified in the question, e.g., organic/aqueous.
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What does a large value of $K_{pc}$ (e.g., > 10) indicate?
The solute is significantly more soluble in the solvent in the numerator of the expression than in the solvent in the denominator.
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What does a small value of $K_{pc}$ (e.g., < 0.1) indicate?
The solute is significantly less soluble in the numerator solvent and prefers the denominator solvent.
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What is the key principle governing the magnitude of $K_{pc}$?
The principle of 'like dissolves like'. Non-polar solutes have high solubility in non-polar solvents, and polar solutes have high solubility in polar solvents.
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Why is solvent extraction more efficient when performed with several small portions of solvent compared to one large portion?
Each extraction removes a fraction of the solute according to the equilibrium established by $K_{pc}$. Multiple extractions compound this removal, extracting a greater total mass of solute than a single extraction with the same total volume of solvent.
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What effect does temperature have on the partition coefficient?
Solubility is temperature-dependent, so $K_{pc}$ is also temperature-dependent. An increase in temperature can increase or decrease $K_{pc}$ depending on how the solubilities in both solvents change. Therefore, $K_{pc}$ is always quoted at a specific temperature.
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What are the units of $K_{pc}$?
$K_{pc}$ is dimensionless (has no units) because it is a ratio of two concentrations, and the units (e.g., mol dm⁻³ or g cm⁻³) cancel out.
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Common exam trap: When calculating concentrations for the $K_{pc}$ formula, what must you ensure?
You must use the equilibrium concentrations/masses, not the initial ones. You often need to calculate the mass transferred to one solvent by subtracting the mass remaining in the other from the initial total mass.
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In a separating funnel containing ethoxyethane (ether) and water, which layer is on top?
The ethoxyethane (ether) layer is on top because it is less dense than water. The less dense liquid always forms the upper layer.