1.8.2Physical Chemistry
Le Chatelier’s principle — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 46 of 102
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.
Part 1 of 4The Principle
Statement
- Statement If a system at equilibrium is subjected to a change, the position of equilibrium shifts to oppose that change.
Dynamic equilibrium
- Dynamic equilibrium At equilibrium the forward and reverse reactions occur at the same rate and concentrations remain constant in a closed system.
Concentration
- Concentration Adding a reactant shifts the position to the right, to use it up, while removing a product also shifts it to the right.
Pressure
- Pressure Increasing pressure shifts the position to the side with fewer moles of gas, and no effect occurs when moles are equal.
Temperature
- Temperature Increasing temperature shifts the position in the endothermic direction, so the equilibrium moves left for an exothermic forward reaction.
Part 2 of 4Catalysts and Industry
Catalysts and Industry
Catalyst
- Catalyst A catalyst changes the rate at which equilibrium is reached but not the position of equilibrium or the value of Kc.
Haber process
- Haber process N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹, uses a high pressure, about 450 °C and an iron catalyst.
Compromise
- Compromise A low temperature favours a high yield but is too slow, so a moderate temperature gives an acceptable rate and yield.
Contact process
- Contact process 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) is exothermic and uses a vanadium(V) oxide catalyst at about 450 °C and a low pressure.
| Change | Shift for N₂ + 3H₂ ⇌ 2NH₃ (ΔH negative) |
|---|---|
| Increase pressure | Right (fewer gas moles) |
| Increase temperature | Left (endothermic direction) |
| Add catalyst | No shift |
Part 3 of 4Answering Questions
1
State the change imposed on the system and the principle: the equilibrium opposes the change.
2
Decide which direction reduces the effect of the change, such as using up added reactant.
3
Say that the position of equilibrium moves to the right or left, giving more or less product.
4
Mention that the rate is affected separately, so a high temperature may speed up the reaction even when it reduces yield.
5
For industrial questions, balance yield, rate, safety and cost to justify the chosen conditions.
Part 4 of 4Exam Checklist & Tips
1.8.2 – Key Points for the Exam
- Le Chatelier's principle When a change is made to a system at equilibrium, the system shifts to counteract the change.
- Dynamic equilibrium A state in which forward and reverse reactions proceed at equal rates and concentrations are constant.
- Position of equilibrium The relative amounts of reactants and products at equilibrium.
- Closed system A system in which no substances enter or leave.
Common Mistakes
- Saying that a catalyst increases the yield; it only helps equilibrium to be reached sooner.
- Stating that raising the pressure always favours the products, ignoring the number of gas moles.
- Describing equilibrium as a stage in which the reaction has stopped.
- Mixing up the effects on rate and on position of equilibrium when temperature changes.
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3 & IGCSE —
Kc
- : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
- : ( ).
- : Kc .