Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Equilibrium

Le Châtelier’s Principle — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 11 parts Lesson 1 of 19 Unit quiz

📚 AP CHEMISTRY Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

Le Châtelier's Principle — AP CHEMISTRY notes: Predict how an equilibrium shifts when concentration, volume or pressure, temperature or dilution is changed, using Q and K.Le Châtelier's principle — when a stress is applied to a system at equilibrium, it shifts in the direction th…

  • Pure solids and liquids
  • Aqueous systems
  • Heat as a reagent
  • Temperature increase
  • Temperature decrease
  • Only temperature changes K

Part 1 of 9The principle

  • Le Châtelier's principle
    • when a stress is applied to a system at equilibrium, it shifts in the direction that counteracts the stress
    • Stressesadding or removing a species, dilution, change in volume or pressure (gases), and change in temperature.

Part 2 of 9Adding or removing a species

StressShiftEffect
Add reactantForwardNet conversion of reactants to products
Add productReverseNet conversion of products to reactants
Remove reactantReverseNet conversion of products to reactants
Remove productForwardNet conversion of reactants to products

Part 3 of 9Solids and liquids

  • Pure solids and liquids
    • changing their amount has no effect because they do not appear in the equilibrium expression

Part 4 of 9Dilution

  • Aqueous systems
    • adding water lowers the concentration of all aqueous species
    • The system shifts toward the side with more moles of aqueous particles.

Part 5 of 9Volume and pressure (gases)

ChangeShift
Pressure increase (volume decrease)Side with fewer moles of gas
Pressure decrease (volume increase)Side with more moles of gas

Part 6 of 9Temperature

1Heat as a reagent
  • Heat as a reagent heat is a product in exothermic reactions and a reactant in endothermic reactions
2Temperature increase
  • Temperature increase the system shifts in the endothermic direction
3Temperature decrease
  • Temperature decrease the system shifts in the exothermic direction
4Only temperature changes K
  • Only temperature changes K concentration, pressure and catalyst changes do not change K

Part 7 of 9Catalysts

  • Effect
    • a catalyst speeds forward and reverse reactions equally, so equilibrium is reached faster
    • It does not shift the equilibrium or change K.

Part 8 of 9Link to observable properties

pH
  • pH adding a common ion to Sr(OH)2(aq) shifts the reaction to reduce [OH−], so the pH decreases while K stays constant
Colour
  • CoCl42− (blue) + 6H2O ⇌ Co(H2O)62+ (pink) + 4Cl−, forward exothermic
  • Heating gives blue; cooling or dilution gives pink; changing the volume has no effect.

Part 9 of 9More from the specification

More from the specification
Solute dissolution in aqueous solution
  • Solute dissolution in aqueous solution polar water dipoles hydrate dissociated ions, forming a homogenous conducting electrolyte solution.
pH
-log[H+]
What the symbols mean: pH has no unit · [H+] = concentration of hydrogen ions (mol dm−3); each 1 pH unit is a ten-fold change in [H+]
Reaction quotient
  • The ratio of product to reactant concentrations at any moment.

Key terms

TermMeaning
Le Châtelier's principleA system at equilibrium shifts to oppose an applied stress.
StressA change in conditions applied to a system at equilibrium.
Equilibrium positionRelative amounts of reactants and products at equilibrium.
Equilibrium constant KValue of the equilibrium expression, changing only with temperature.

Avoid these mistakes

1
Saying removing a pure solid shifts the equilibrium — solids are not in the expression.
2
Saying a pressure change affects aqueous equilibria — it affects only systems with gases.
3
Saying a catalyst changes K or the yield — it affects only the time to reach equilibrium.
4
Saying adding a reactant changes K — K changes only with temperature.

Last updated:

\n