7.3Equilibrium
Le Châtelier’s Principle — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 9 parts Lesson 10 of 19 1 quiz
Lesson summary
7.3 Le Châtelier's Principle — AP CHEMISTRY notes: Predict equilibrium shifts for concentration, volume, pressure and temperature changes and relate them to Q and K.Statement — a system at equilibrium shifts to partially counteract an applied stressStresses — addition or removal …
- Temperature rise
- Temperature fall
- After a stress
- Pressure doubled
- Equilibrium position
- Reaction quotient
Section 1 of 9Principle
Statement
- Statement a system at equilibrium shifts to partially counteract an applied stress
Stresses
- Stresses addition or removal of a species, dilution, volume or pressure change in gas systems, and temperature change
Section 2 of 9Effect of adding or removing a species
| Stress | Shift | Effect |
|---|---|---|
| Add reactant | Forward | Reactants converted to products |
| Add product | Reverse | Products converted to reactants |
| Remove reactant | Reverse | Products converted to reactants |
| Remove product | Forward | Reactants converted to products |
Section 3 of 9Gases and dilution
| Stress | Shift |
|---|---|
| Higher pressure (smaller volume) | Fewer moles of gas |
| Lower pressure (larger volume) | More moles of gas |
| Dilution of aqueous system | More moles of aqueous particles |
| Pure solid or liquid added or removed | None |
Section 4 of 9Temperature
Temperature rise
- Temperature rise endothermic direction favoured
Temperature fall
- Temperature fall exothermic direction favoured
K changes
- K changes only a temperature change alters K
Section 5 of 9Using Q
After a stress
- Q is compared with KQ < K shifts forward and Q > K shifts in reverse
Catalyst
- Catalyst no shift and no change in K because both rates increase equally
Section 6 of 9Example: PCl₃ + Cl₂ ⇌ PCl₅
Pressure doubled
- Pressure doubled shift toward the fewer moles of gas (PCl5), so P(PCl5) increases
Kp
- Kp unchanged at constant temperature
Section 7 of 9More from the specification
More from the specification
Ionic dissociation in electrolyte solution
- Ionic dissociation in electrolyte solution polar water molecules surround dissociated ions, forming a conducting aqueous electrolyte solution.
Reaction quotient
- The ratio of product to reactant concentrations at any moment.
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+]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Le Châtelier's principle | Systems at equilibrium oppose applied stresses. |
| Equilibrium position | Relative amounts of products and reactants. |
| Stress | Change applied to a system at equilibrium. |
| Reaction quotient | Q, compared with K to find the direction of shift. |
Section 9 of 9Exam technique
Avoid these mistakes
Believing a shift undoes the stress completely — it only partly counteracts it.
Saying volume changes affect aqueous systems.
Saying adding a catalyst increases the yield.
Marked practiceQuizzes for this lesson
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