Le Châtelier’s Principle Study Pack
Kibin's free study pack on Le Châtelier’s Principle includes a 6-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Le Châtelier’s Principle Study Guide
Master Le Châtelier's Principle by examining how concentration, pressure, and temperature shifts drive equilibrium toward products or reactants — and why catalysts speed up equilibrium without changing its position or Kc.
Key Takeaways
- •Le Châtelier's Principle states that when a system at chemical equilibrium is disturbed by an external stress, the system will shift in the direction that partially counteracts that stress and re-establishes equilibrium.
- •Adding a reactant or removing a product drives the equilibrium position to the right (toward products), while removing a reactant or adding a product drives it to the left (toward reactants).
- •Increasing the concentration of any species shifts equilibrium away from that species; decreasing concentration shifts equilibrium toward that species.
- •For reactions involving gases, increasing total pressure by decreasing volume shifts equilibrium toward the side with fewer moles of gas, while decreasing pressure shifts it toward the side with more moles of gas.
- •Temperature changes alter both the equilibrium position and the value of the equilibrium constant Kc: raising temperature favors the endothermic direction, while lowering temperature favors the exothermic direction.
- •Adding a catalyst does not shift the equilibrium position or change Kc; it only increases the rate at which equilibrium is reached by lowering the activation energy equally for both the forward and reverse reactions.
The Core Principle: How Equilibrium Responds to Disturbance
Chemical equilibrium is a dynamic state in which the forward and reverse reactions occur at equal rates, keeping the concentrations of reactants and products constant. Le Châtelier's Principle provides a qualitative framework for predicting how that balance shifts when outside conditions change.
Definition and Scope of Le Châtelier's Principle
- •Le Châtelier's Principle applies to any system in a state of dynamic equilibrium — chemical, physical, or biological — when subjected to a stress such as a change in concentration, pressure, or temperature.
- •A 'stress' is any change that disturbs the ratio of product to reactant concentrations and therefore throws the reaction quotient Q out of alignment with the equilibrium constant Kc.
Equilibrium Position vs. Equilibrium Constant
- •The equilibrium position refers to the relative concentrations of reactants and products at equilibrium; this can shift without changing Kc.
- •The equilibrium constant Kc is fixed for a given reaction at a given temperature and only changes when temperature changes — not when concentration or pressure is altered.
Direction of Shift: Comparing Q to Kc
- •When a stress makes Q < Kc, the system shifts right (forward reaction is favored) to produce more products until Q again equals Kc.
- •When a stress makes Q > Kc, the system shifts left (reverse reaction is favored) to consume products and regenerate reactants.
Concentration Changes and Equilibrium Shifts
Changing the amount of any reactant or product in solution is the most straightforward way to stress an equilibrium, and the system's response follows directly from comparing the new Q to Kc.
Adding or Removing a Reactant
- •Adding a reactant increases its concentration, raising Q above nothing — wait, it lowers Q below Kc — so the system shifts right to consume the added substance and restore balance.
- •Removing a reactant decreases its concentration, making Q > Kc, so the system shifts left to replenish that reactant.
Adding or Removing a Product
- •Adding a product raises Q above Kc, driving the system left toward reactants.
- •Removing a product lowers Q below Kc, driving the system right — a principle exploited industrially to push reactions to near-complete conversion of reactants.
Industrial Application: Haber–Bosch Process
- •In the synthesis of ammonia (N₂ + 3 H₂ ⇌ 2 NH₃), continuous removal of NH₃ from the reaction chamber keeps Q below Kc, sustaining forward reaction and maximizing yield.
- •This same logic applies to any reversible industrial synthesis where one product can be selectively separated from the equilibrium mixture.
Pressure and Volume Effects on Gas-Phase Equilibria
For reactions that involve gases, changing the total pressure of the system — typically by compressing or expanding the reaction vessel — shifts equilibrium toward whichever side has fewer or more moles of gas.
Mole Count as the Key Variable
- •The direction of an equilibrium shift under pressure change depends on comparing the total moles of gaseous reactants to the total moles of gaseous products, as written in the balanced equation.
- •A reaction such as N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g) has 4 moles of gas on the left and 2 on the right, so increasing pressure favors the right side (fewer gas moles).
Increasing Pressure (Decreasing Volume)
- •Compressing the vessel increases the partial pressures of all gas species, making Q ≠ Kc; the system shifts toward the side that reduces total moles of gas, relieving the pressure increase.
- •If both sides have equal moles of gas (e.g., H₂ + I₂ ⇌ 2 HI), pressure changes do not shift the equilibrium position.
Decreasing Pressure (Increasing Volume)
- •Expanding the vessel lowers partial pressures of all gases; the system shifts toward the side with more moles of gas to partially restore pressure.
Adding an Inert Gas at Constant Volume
- •Introducing a non-reacting gas (such as argon) at constant volume raises total pressure but does not change the partial pressures of the reacting species, so Q is unchanged and no equilibrium shift occurs.
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What does Le Châtelier's Principle state about a system at chemical equilibrium when subjected to an external stress?
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Le Châtelier's Principle
Explain Le Châtelier's Principle in your own words. What does it mean for a system to be 'stressed,' and how does the system respond to restore equilibrium?
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