Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

1.8.5Physical Chemistry

Changes Which Affect the Equilibrium — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 49 of 102

📚 AQA AS 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

Part 1 of 5Le Chatelier's Principle

If a system at equilibrium is disturbed, the position shifts to oppose the change and partly cancel its effect.

Le Chatelier's Principle
Concentration
  • Concentration Adding a reactant shifts the position towards products, while removing a product has the same effect by pulling the reaction forward.
Pressure
  • Pressure Raising pressure shifts the position towards the side with fewer gas moles, which reduces the pressure again.
Temperature
  • Temperature Raising temperature shifts the position in the endothermic direction, absorbing the added heat energy.
Catalyst
  • Catalyst A catalyst speeds up forward and reverse rates equally, so equilibrium is reached sooner but the position is unchanged.

Part 2 of 5Effects on Position and on Kc

Only temperature alters Kc
  • Only temperature alters Kc Concentration and pressure changes move the position, but the ratio of concentrations restores to the same Kc.
Why Kc changes with temperature
  • Why Kc changes with temperature The new equilibrium has different proportions of products and reactants, so the ratio defining Kc is different.
Equal gas moles
  • Equal gas moles If both sides have the same number of gas moles, pressure changes do not shift the position at all.
ChangeEffect on positionEffect on Kc
Increase concentration of a reactantMoves to the rightNone
Increase total pressureMoves to side with fewer gas molesNone
Increase temperature (exothermic forward reaction)Moves to the leftDecreases
Increase temperature (endothermic forward reaction)Moves to the rightIncreases
Add a catalystNo changeNone

Part 3 of 5Industrial Compromise

Industry balances yield against rate and cost, as in the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹.

1
The forward reaction is exothermic, so a low temperature would give a higher equilibrium yield of ammonia.
2
A low temperature makes the reaction far too slow, so a compromise of around 450 °C is used to achieve a reasonable rate.
3
Four gas moles become two, so high pressure favours ammonia, but very high pressure needs expensive plant, so about 200 atm is used.
4
An iron catalyst increases the rate at the chosen temperature without changing the equilibrium yield, saving energy and cost.
5
Unreacted nitrogen and hydrogen are recycled after ammonia is liquefied and removed, which raises the overall conversion.

Part 4 of 5Worked Example

Question

  • CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹.
  • State and explain the effect on the equilibrium yield of methanol of (a) raising the pressure and (b) raising the temperature.

  1. Count gas molesthe left side has 1 + 2 = 3 mol and the right side has 1 mol.
  2. (a) Higher pressure favours the side with fewer gas moles, so the position shifts right and the yield of methanol increases.
  3. The forward reaction is exothermic (ΔH negative), so the reverse reaction is endothermic.
  4. (b) Raising the temperature favours the endothermic direction, so the position shifts left, the yield falls and Kc decreases.

Answer

  • (a) yield increases;
    • (b) yield decreases and Kc decreases.

Part 5 of 5Exam Checklist & Tips

1.8.5 – Key Points for the Exam

  • Le Chatelier's principle When a change is applied to a system at equilibrium, the position shifts to oppose that change.
  • Position of equilibrium The relative proportions of reactants and products present at equilibrium.
  • Exothermic A reaction that releases heat energy to the surroundings and has a negative ΔH.
  • Catalyst A substance that increases reaction rate by providing an alternative pathway with lower activation energy, without being used up.

Common Mistakes

  • Saying a catalyst increases the yield; it only helps equilibrium to be reached faster.
  • Claiming that a change in pressure or concentration alters Kc, when only temperature does.
  • Forgetting to count only gaseous moles when deciding which side pressure favours.
  • Writing that the equilibrium shifts to cancel the change completely, rather than only partly oppose it.

Last updated:

3 & IGCSE —

Kc

  • : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
  • : ( ).
  • : Kc .
\n