chemistry

Partition equilibrium

Partition equilibrium is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Partition equilibrium rather than just read about it. In short: Partition equilibrium is a special case of chemical equilibrium wherein one or more solutes are in equilibrium between two immiscible solvents. The most common chemical equilibrium systems involve reactants and products in the same phase - either all gases or all solutions.

Key takeaways

Partition equilibrium belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Partition equilibrium to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Partition equilibrium from memory before moving on to harder problems.

Reference excerpt

Partition equilibrium is a special case of chemical equilibrium wherein one or more solutes are in equilibrium between two immiscible solvents. The most common chemical equilibrium systems involve reactants and products in the same phase - either all gases or all solutions. However, it is also possible to get equilibria between substances in different phases, such a liquid and gas that do not mix (are immiscible). One example is gas-liquid partition equilibrium chromatography, where an analyte equilibrates between a gas and liquid phase. Partition equilibria are described by Nernst's distribution law. Partition equilibrium are most commonly seen and used for Liquid–liquid extraction. The time until a partition equilibrium emerges is influenced by many factors, such as: temperature, relative concentrations, surface area of interface, degree of stirring, and the nature of the solvents and solute.

Example

For example, ammonia (NH3) is soluble in both water (aq) and the organic solvent trichloromethane (CHCl3) - two immiscible solvents. If ammonia is first dissolved in water, and then an equal volume of trichloromethane is added, and the two liquids shaken together, the following equilibrium is established:

Kc = [NH3 (CHCl3)]/[NH3 (aq)] (where Kc is the equilibrium constant) The equilibrium concentrations of ammonia in each layer can be established by titration with standard acid solution. It can thus be determined that Kc remains constant, with a value of 0.4 in this case.

Partition coefficient

This kind of equilibrium constant measures how a substance distributes or partitions itself between two immiscible solvents. It is called the partition coefficient or distribution coefficient.

Partition equilibrium chromatography See: Partition chromatography, Gas chromatography Partition equilibrium chromatography is a type of chromatography that is typically used in gas chromatography (GC) and high performance liquid chromatography (HPLC). The stationary phase in GC is a high boiling liquid bonded to solid surface and the mobile phase is a gas. In gas-liquid chromatography, analyte from the mobile gas phase equilibrates with the liquid phase. Molecules more soluble in the liquid phase will remain longer in the column, allowing for separation using partition equilibriums.

Worked examples

Example 1 — a first encounter with Partition equilibrium

Start with the simplest possible case. Write down what Partition equilibrium claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Partition equilibrium before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Partition equilibrium ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Partition equilibrium

In research: Partition equilibrium appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Partition equilibrium in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Partition equilibrium is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chromatography, Equilibrium chemistry, Gas chromatography, so understanding it makes those chapters shorter.
In everyday life: Look for Partition equilibrium outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Partition equilibrium in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Partition equilibrium means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Partition equilibrium out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Partition equilibrium in simple terms?

Why does Partition equilibrium matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Partition equilibrium?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Partition equilibrium.