3.3.3Physical Chemistry
The Position of Equilibrium — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 9 of 9 Unit quiz
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 5Le Chatelier's Principle
The principle lets us predict the effect of changing conditions.
Statement
- Statement When a change is made to a system at equilibrium, the system responds to oppose that change.
Position
- Position The position of equilibrium describes the relative amounts of reactants and products when equilibrium is reached.
Shift right
- Shift right If the position moves right, the forward reaction is favoured and more product forms.
Shift left
- Shift left If the position moves left, the reverse reaction is favoured and more reactant forms.
Part 2 of 5Temperature and Pressure
Temperature and Pressure
Raising temperature
- Raising temperature The system opposes the rise by favouring the endothermic direction, which absorbs heat.
Lowering temperature
- Lowering temperature The system favours the exothermic direction, which releases heat.
Raising pressure
- Raising pressure The position shifts towards the side with fewer gas molecules, reducing the pressure.
Lowering pressure
- Lowering pressure The position shifts towards the side with more gas molecules.
Equal molecules
- Equal molecules If both sides have the same number of gas molecules, a pressure change does not alter the position.
| Change | Effect on position |
|---|---|
| Higher temperature | Favours endothermic direction |
| Higher pressure | Favours side with fewer gas molecules |
| Catalyst | No change in position |
Part 3 of 5Applying It to the Haber Process
Applying It to the Haber Process
Equation
- Equation N₂(g) + 3H₂(g) ⇌ 2NH₃(g), with the forward reaction exothermic.
Pressure
- Pressure There are 4 gas molecules on the left and 2 on the right, so high pressure favours ammonia.
Temperature
- Temperature Because the forward reaction is exothermic, a higher temperature lowers the yield of ammonia.
Compromise
- Compromise A moderate temperature, high pressure and an iron catalyst give a reasonable yield at an acceptable rate.
Part 4 of 5Catalysts and Examples
Catalysts and Examples
Catalyst
- Catalyst A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but the position is unchanged.
NO₂ and N₂O₄
- NO₂ and N₂O₄ In 2NO₂(g) ⇌ N₂O₄(g), higher pressure favours colourless N₂O₄, so the brown colour fades.
Exam approach
- Exam approach State the change, say which direction opposes it, and state the effect on the colour or yield.
Part 5 of 5Exam Checklist & Tips
3.3.3 – Key Points for the Exam
- Position of equilibrium The relative amounts of reactants and products in an equilibrium mixture.
- Le Chatelier's principle A system at equilibrium responds to oppose any change made to it.
- Catalyst A substance that speeds up a reaction without being used up or changing the position of equilibrium.
- Yield The amount of product obtained.
Common Mistakes
- Claiming a catalyst increases the yield at equilibrium; it only speeds up reaching it.
- Forgetting to count gas molecules only when explaining pressure effects.
- Saying a temperature rise always favours the forward reaction.
- Describing the effect without referring to the energy change or number of gas molecules.
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3 & IGCSE —
Kc
- : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
- : ( ).
- : Kc .