Introduction to Le Châtelier’s Principle
Separate the immediate physical disturbance from the subsequent net reaction and measurement response.
AP Chemistry Topic 7.9 · CED printed p. 137 / PDF p. 144. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Dynamic equilibrium
- Particle counts
- Endothermic and exothermic processes
Le Châtelier’s principle predicts the response to a disturbance of an equilibrium. Add a dissolved or gaseous reactant and the subsequent reaction often consumes part of the added material; remove a product and more product forms. The system does not usually undo the entire addition. Adding or removing part of a pure solid does not change its activity while the same macroscopic phase remains. If dissolved concentrations, temperature and solution volume stay unchanged, this amount change alone gives no quotient or saturated-composition change. Removing the phase entirely is a different boundary condition. Changing the volume of a gas mixture at constant temperature favors the direction with fewer gas particles on compression and more on expansion. Equal total gas coefficients give no composition response to uniform volume change in the ideal model. Adding an inert gas at constant volume does not change reacting species’ partial pressures; adding one at constant total pressure expands the volume, which is a different stress.
Temperature is special because it changes K. Heating favors the endothermic direction; cooling favors the exothermic direction. Write heat on the appropriate side as a reasoning aid, but do not put a heat term in the quotient. Dilution lowers every dissolved concentration immediately; the later response often favors the side with more dissolved particles. Use Q if the answer is not obvious. A catalyst changes the time needed to reach equilibrium, not its composition or K.
Connect the predicted response to what is measured. If a colored complex forms, a net increase of that complex can deepen its color at a fixed optical path. If a weak acid releases more hydronium, pH falls. But identify the immediate physical change first: dilution itself weakens concentration-dependent color, and compression immediately raises gas concentrations. A reaction’s subsequent response is only part of the observation. Control temperature, volume, path length, and concentrations when interpreting a color or pH experiment.
Let's work through it
- For Fe3+(aq) + SCN−(aq) ⇌ FeSCN2+(aq), the complex is red. Consider a small addition of SCN− that changes volume negligibly.
- The immediate increase in SCN− lowers Q, so the subsequent net response forms more red complex.
- At fixed temperature K is unchanged. The new equilibrium has a different composition but the same equilibrium ratio.
- For a color comparison, hold the optical path fixed and compare against a dilution control; otherwise concentration and geometry can masquerade as a chemical response.
For dilute A(aq) + B(aq) ⇌ AB(aq), start at Qc = Kc and keep temperature fixed.
- A small selective removal of AB at unchanged volume lowers the numerator of Q. The subsequent net reaction forms more AB.
- Doubling the volume with pure water halves every concentration. Q becomes ([AB]/2)/(([A]/2)([B]/2)) = 2Kc, so the later net reaction instead consumes AB.
- For a separate saturated MX(s) solution, changing only solid amount does not change [M+][X−] while that phase remains. Check the immediate state and phase conditions before applying a direction slogan.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Does adding an inert gas at constant volume always shift equilibrium?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Does adding an inert gas at constant volume always shift equilibrium?
Worked answer and reason
No. Reacting gas partial pressures remain unchanged in the ideal model.
Their n, T, and V have not changed.
Retrieval 2
What does a catalyst change?
Worked answer and reason
It speeds the approach in both directions; equilibrium composition and K remain the same.
A pathway change does not change the equilibrium state.
Retrieval 3
A red complex forms by A(aq) + B(aq) ⇌ AB(aq). A small addition of B negligibly changes volume. Predict the response and color at a fixed light path.
Worked answer and reason
The net response forms more red AB, so the red color deepens at fixed path length. The immediate extra B lowers Q and the later reaction restores Q = K.
Measured color follows complex concentration when optical geometry and dilution are controlled.
Catch the tempting mistake
- The system completely cancels any stress.
- It re-establishes Q = K; the final concentrations need not return to their original values.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 7 problem 10 · ap synthesis · macroscopic
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For the teacher
- Demand an immediate-state statement and a later-reaction statement. Use a dilution control in a supplied-data discussion.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 7.9.A
- Identify the response of a system at equilibrium to an external stress, using Le Châtelier's principle.
- 7.9.A.1
- Le Châtelier’s principle can be used to predict the response of a system to stresses such as addition or removal of a chemical species, change in temperature, change in volume/pressure of a gas-phase system, or dilution of a reaction system.
- 7.9.A.2
- Le Châtelier’s principle can be used to predict the effect that a stress will have on experimentally measurable properties such as pH, temperature, and color of a solution.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §13.2, Equilibrium Constants — Pressure/concentration quotients, pure-phase activities, and the distinction between composition and rate.
Glossary and deeper reading · 8 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Endothermic process
An endothermic process absorbs heat into the chosen system. Under constant-pressure conditions with only pressure–volume work, its enthalpy change is positive. For example, melting ice absorbs heat while the phase change occurs.
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Exothermic process
An exothermic process transfers heat from the chosen system to its surroundings. Under constant-pressure conditions with only pressure–volume work, its enthalpy change is negative. For example, freezing water releases heat to the surroundings.
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Chemical equilibrium
Chemical equilibrium is a dynamic state in which forward and reverse reaction rates are equal. Macroscopic composition remains constant in a closed system under fixed conditions, but reactant and product concentrations need not be equal. For example, a reversible reaction can reach equilibrium with much more reactant than product.
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Mixture
A mixture contains two or more substances whose proportions can vary. Its components retain their chemical identities until a chemical reaction changes them. For example, water and ethanol can form mixtures with many different compositions.
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Dilution
Dilution lowers solute concentration by adding solvent while conserving the solute amount when no reaction or loss occurs. Concentration calculations use the final solution volume rather than simply the volume of solvent added. For example, increasing a solution's volume from 100 to 200 milliliters halves its concentration if its solute amount is unchanged.
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Catalyst
A catalyst participates in a reaction pathway and is regenerated overall. It can increase the reaction rate by providing a different pathway without changing the reaction's equilibrium constant at fixed temperature. For example, a catalyst can help a mixture reach equilibrium sooner without changing the final equilibrium composition.
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Weak acid or base
A weak acid or base reacts only partially with water and establishes an equilibrium. Its equilibrium constant describes strength, while analytical concentration describes how much was prepared per volume. For example, acetic acid leaves both acid molecules and its conjugate base in water.
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Hydronium
Hydronium is the H₃O⁺ ion used in the introductory model of proton behavior in water. Aqueous H⁺ notation is shorthand for a solvated proton rather than a bare proton moving independently in the liquid. For example, a Brønsted acid can transfer a proton to water and form hydronium.