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2.6.5Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Dynamic Equilibrium in Reversible Reactions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 41 of 53 Unit quiz

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Part 1 of 4Reversible Reactions

2.6.5 Dynamic Equilibrium in Reversible Reactions

In a reversible reaction the products can react to re-form the reactants.

Reversible Reactions
Notation
  • The symbol ⇌ shows that both the forward and the backward reactions can occur.
Complete reactions
  • In many reactions the reactants are used up and the reaction stops, shown by a single arrow.
Examples
  • N₂O₄(g) ⇌ 2NO₂(g) and H₂(g) + I₂(g) ⇌ 2HI(g) are reversible reactions.
Open systems
  • Where a gas can escape, the reaction may run to completion because products are removed.
Open flasks
  • Reactions entirely in solution can reach equilibrium in an open flask, because very little material escapes.

Part 2 of 4Reaching Equilibrium

1
At the start only reactants are present, so the forward reaction is fast and the backward reaction is zero.
2
As products form, the backward reaction starts and its rate rises.
3
As reactants are used up, the forward reaction slows down.
4
Eventually the rates of forward and backward reactions become equal.
5
The concentrations of reactants and products stay constant from then on, as long as conditions are unchanged.
6
After this point nothing seems to change on the macroscopic scale, although molecules continue to react in both directions.

Part 3 of 4Conditions for Dynamic Equilibrium

Closed system
  • No reactants or products can enter or leave, so matter is not lost to the surroundings.
Equal rates
  • The forward and backward reaction rates are the same, which is what makes the equilibrium dynamic.
Constant concentrations
  • The concentrations stay constant but are not necessarily equal.
Macroscopic properties
  • Observable properties such as colour and pressure stay constant.
Approach
  • The same equilibrium can be reached from reactants or from products under the same conditions.
Colour change
  • In a sealed tube of N₂O₄ ⇌ 2NO₂ the brown colour stops changing once equilibrium has been reached.

Part 4 of 4Exam Checklist & Tips

2.6.5 – Key Points for the Exam

  • Reversible reaction A reaction in which products can react to re-form the reactants.
  • Dynamic equilibrium A state in which forward and backward reactions occur at equal rates in a closed system.
  • Closed system A system in which no matter enters or leaves.
  • Position of equilibrium The relative amounts of reactants and products at equilibrium.

Common Mistakes

  • Saying that the reaction stops at equilibrium; both reactions continue at equal rates.
  • Writing that concentrations of reactants and products are equal at equilibrium.
  • Forgetting that equilibrium needs a closed system when gases are involved.
  • Describing the rates as constant but different.

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3 & IGCSE —

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