Advanced Topic 508 of 1102 Kinetics & Equilibrium
Distribution Law
At constant temperature a solute distributes between two solvents in a constant ratio.
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In-depth explanation of Distribution Law
The distribution law, also called Nernst's distribution law, states that when a solute distributes itself between two immiscible solvents at a fixed temperature, the ratio of its concentrations in the two solvents is constant. The law applies provided the solute has the same molecular form in both solvents, so that no association or dissociation complicates the ratio. This constancy makes it possible to predict how much solute remains after an extraction and to design efficient separation procedures. Repeated extractions with small portions of solvent remove more solute than a single extraction using the same total volume, because each portion comes into equilibrium with fresh solvent.
Going deeper: Distribution Law
Key ideas: Associated solute case: if the solute forms n-mers in the organic phase, C_water / (C_org)^(1/n) is constant. For ionizing solute, only the neutral form partitions, so use the molecular concentration. Total concentration ratios change with pH for acids and bases, which is exploited in acid base extractions in the lab.
Common mistakes and connections: Students use total concentration in both phases despite association or ionization, getting inconsistent constants. Another is the belief that extracting once with all the solvent is as good as several smaller portions; the latter is more efficient. Equilibrium is reached only after shaking and settling. Related to the partition coefficient and to extraction and chromatography design.
Visualization of Distribution Law
With KD = 3 and 50 mL of aqueous solution, three small extractions leave 24 percent of the solute while one large one leaves 36 percent. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Three extractions with 10 mL of solvent each remove more solute than a single extraction with 30 mL.
Frequently asked questions about Distribution Law
What is Distribution Law?
At constant temperature a solute distributes between two solvents in a constant ratio.
Is Distribution Law a beginner, intermediate or advanced topic?
Distribution Law is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Distribution Law?
Study these earlier topics in Kinetics & Equilibrium first: Buffer Equilibria, Heterogeneous Equilibrium Vapor, Partition Coefficient.
What comes after Distribution Law?
The next topics in Kinetics & Equilibrium are Equilibrium Constant of Water and Steady State vs Equilibrium.
Can I practise Distribution Law with quizzes?
Yes. The interactive lesson includes a self-check question on Distribution Law, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Distribution Law fits in Kinetics & Equilibrium
Distribution Law is topic 89 of 91 in Kinetics & Equilibrium.