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5.3.1Additional lessons

The Characteristics of Dynamic Equilibrium — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 109 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 4What Equilibrium Means

5.3.1 The Characteristics of Dynamic Equilibrium

A dynamic equilibrium is a state in which change is still happening at the particle level but nothing changes overall.

Reversible reaction
  • Some reactions can run in both directions, shown by the symbol ⇌, so products can turn back into reactants.
Equal rates
  • At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.
Constant concentrations
  • Because the two rates match, the concentrations of reactants and products stay constant, even though both reactions continue.
Dynamic
  • Particles keep reacting in both directions all the time, so there is no change that can be seen at the macroscopic level.

Part 2 of 4Conditions Needed

Conditions Needed
Closed system
  • No reactant or product can enter or leave, for example a sealed flask;
    • in an open system the substances escape and equilibrium is never reached.
Constant temperature
  • The equilibrium position depends on temperature, so the temperature must stay fixed for the concentrations to stay constant.
Either direction
  • The same equilibrium mixture forms whether you start with only reactants or only products, if the conditions are the same.
Physical examples
  • A liquid and its vapour in a sealed container,
    • such as H₂O(l) ⇌ H₂O(g), reach an equilibrium in which evaporation and condensation rates are equal.

Part 3 of 4Position of Equilibrium

1
At the start of a reaction the forward rate is high because the reactant concentrations are high, while the reverse rate is zero.
2
As products build up the reverse rate increases, and as reactants are used up the forward rate decreases.
3
Eventually the two rates become equal, and from this moment the concentrations stop changing.
4
If the equilibrium mixture contains mostly products the position lies to the right and the equilibrium constant is large, much greater than 1.
5
If it contains mostly reactants the position lies to the left and the constant is small, much less than 1.

Part 4 of 4Exam Checklist & Tips

5.3.1 – Key Points for the Exam

  • Dynamic equilibrium A state in a closed system where forward and reverse rates are equal and concentrations are constant.
  • Closed system A system that does not exchange matter with its surroundings.
  • Position of equilibrium The relative amounts of reactants and products in the equilibrium mixture.
  • Reversible reaction A reaction in which products can react to re-form the reactants.

Common Mistakes

  • Saying that the concentrations of reactants and products are equal at equilibrium;
    • they are constant, not necessarily equal.
  • Thinking that the reactions stop when equilibrium is reached.
  • Describing equilibrium in an open container where gases can escape.
  • Believing that a catalyst changes the position of equilibrium;
    • it only helps equilibrium to be reached faster.

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3 & IGCSE —

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