In simple terms
A friendly intro before the formal notes — no formulas yet.
The Solvent Tug-of-War
A partition coefficient describes how a substance 'chooses' to divide itself between two liquids that don't mix, like oil and water. This 'choice' depends on how soluble the substance is in each liquid, reaching a predictable balance.
Imagine you're at a food court with two stalls: one selling pizza (a non-polar solvent) and one selling salad (a polar solvent). If you love pizza (you're a non-polar solute), most of your time will be spent at the pizza stall, but you might briefly visit the salad stall. The ratio of time you spend at the pizza stall versus the salad stall is your personal 'partition coefficient'. It's a stable equilibrium based on your preference.
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= [substance in solvent A] / [substance in solvent B] at equilibrium.
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Non-polar solute prefers non-polar solvent (like dissolves like).
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Extraction uses repeated partitioning to isolate product.
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Temperature affects — state assumptions in questions.
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Key formulas
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Full topic notes
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Defining the Partition Coefficient ($K_{pc}$)
When a solute is added to a system of two immiscible liquids, it will dissolve in both. After shaking the mixture and allowing it to settle, a dynamic equilibrium is established. At this point, the rate at which solute molecules move from solvent 1 to solvent 2 is equal to the rate at which they move from solvent 2 to solvent 1. The ratio of the solute's concentration in each solvent at this equilibrium is a constant, known as the partition coefficient, .
The system must be at equilibrium.
The two solvents must be immiscible.
The temperature must be constant, as solubility changes with temperature.
The concentration units must be the same for both solvents so they cancel out, making a dimensionless quantity.
Applications in Solvent Extraction
Solvent extraction is a common and powerful technique used in organic chemistry to separate a desired product from a reaction mixture. For instance, if an organic product is formed in an aqueous solution, we can add an immiscible organic solvent (like ether or ethyl acetate) in which the product is much more soluble. By shaking the mixture in a separating funnel, the product partitions preferentially into the organic layer, which can then be physically separated. This process isolates the product from water-soluble impurities.
The Efficiency of Multiple Extractions
A crucial concept in practical chemistry is that performing multiple extractions with smaller volumes of solvent is more effective than a single extraction with a large volume. Each time fresh solvent is added, a new equilibrium is established, and more solute is drawn out of the original layer. While you will never extract 100% of the solute, multiple sequential extractions can remove a very high percentage, leading to a much better yield.
Worked examples
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A solution of 6.0 g of an organic compound X in 100 cm³ of water was shaken with 20 cm³ of ether. After allowing the layers to separate, the aqueous layer was found to contain 1.2 g of compound X. Calculate the partition coefficient of X between ether and water.
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Find the mass of X transferred to the ether layer.
The partition coefficient for the distribution of iodine between trichloromethane () and water is 130. (). Calculate the mass of iodine remaining in 100 cm³ of aqueous solution, initially containing 1.00 g of iodine, after it is shaken with one 20 cm³ portion of .
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Define variables.
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Glossary
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Revision flashcards
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What is a partition coefficient, ?
The ratio of the concentrations of a solute in two immiscible solvents at equilibrium and at a constant temperature.
Key takeaways
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The system must be at equilibrium.
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The two solvents must be immiscible.
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The temperature must be constant, as solubility changes with temperature.
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The concentration units must be the same for both solvents so they cancel out, making a dimensionless quantity.
Practice — then mark it
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Test Your Knowledge on Partition Coefficients
Test Your Knowledge on Partition Coefficients
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