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8.2.2The Periodic Table

The Effect of Changing Conditions on Equilibrium — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 7 of 23 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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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.
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Part 1 of 4Le Chatelier's Principle

The principle gives a qualitative way to predict the effect of a change on a system at equilibrium.

Principle
  • Principle When a change is made to a system at equilibrium, the system moves to oppose the change.
Origin
  • Origin Henri Le Chatelier, a French engineer of the 19th century, formulated the principle.
Shift right
  • Shift right A shift to the right means the forward reaction is favoured, giving more products.
Shift left
  • Shift left A shift to the left means the reverse reaction is favoured, giving more reactants.

Part 2 of 4Factors That Shift Equilibrium

Temperature, pressure and concentration can all move the position of equilibrium.

Factors That Shift Equilibrium
Temperature
  • Temperature The effect depends on whether the forward reaction is exothermic or endothermic.
Pressure
  • Pressure Changing pressure affects only reactions in which the number of gas molecules differs on each side.
Concentration
  • Concentration Adding a reactant shifts the position of equilibrium to the right, using up some of the added reactant.
Catalysts
  • Catalysts A catalyst does not change the position of equilibrium;
    • it only changes how quickly equilibrium is reached.

Part 3 of 4Making Predictions

A systematic approach gives a reliable prediction.

1
Identify whether the forward reaction is exothermic or endothermic, and count the gas molecules on each side.
2
Decide which change is being made, such as an increase in temperature or an increase in pressure.
3
Apply the principle: the system partly opposes the change by favouring the reaction that reduces its effect.
4
State the direction of the shift and explain its effect on the yield of the product in the question.

Part 4 of 4Exam Checklist & Tips

8.2.2 – Key Points for the Exam

  • Le Chatelier's principle A rule stating that a system at equilibrium opposes any change made to it.
  • Position of equilibrium The relative amounts of reactants and products present when equilibrium has been reached.
  • Yield The amount of product obtained from a reaction.
  • Shift A movement of the position of equilibrium to favour the forward or reverse reaction.

Common Mistakes

  • Stating that the system stops the change completely; it only partly opposes the change.
  • Saying a catalyst increases the yield at equilibrium; a catalyst does not change the position of equilibrium.
  • Confusing a shift in the position of equilibrium with a change in the rate of reaction.
  • Predicting pressure effects by counting atoms instead of gas molecules.

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3 & IGCSE —

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