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3.3.2Physical Chemistry

Dynamic Equilibrium — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 9 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Part 1 of 5Reversible Reactions

3.3.2 Dynamic Equilibrium

Some reactions do not go to completion because products can react to re-form the reactants.

Meaning
  • A reversible reaction can go in both directions, shown by the symbol ⇌ in equations.
Forward and reverse
  • Reactants form products in the forward reaction, and products change back to reactants in the reverse reaction.
Example
  • In the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), nitrogen and hydrogen make ammonia and ammonia also decomposes.
Heating hydrated copper(II) sulfate
  • Blue CuSO₄·5H₂O loses water to form white CuSO₄, and adding water reverses the change.

Part 2 of 5Reaching Equilibrium

The graph of rates shows the two lines converging.

1
At the start only reactants are present, so the forward reaction has its highest rate and the reverse rate is zero.
2
As reactants are used up, their concentrations fall and the forward reaction slows.
3
As products build up, the reverse reaction speeds up.
4
Eventually the forward and reverse rates become equal, and this is dynamic equilibrium.
5
From then on the concentrations of reactants and products stay constant.

Part 3 of 5Features of Dynamic Equilibrium

Features of Dynamic Equilibrium
Equal rates
  • The rate of the forward reaction equals the rate of the reverse reaction.
Constant concentrations
  • The concentrations of reactants and products stay constant, though not necessarily equal.
Dynamic
  • Both reactions are still happening all the time, so molecules keep changing in both directions.
Closed system
  • Equilibrium is reached only in a closed system where nothing can escape or enter.
SystemEquilibrium possible?Reason
Sealed containerYesNo substances escape
Open beakerNoGaseous product escapes

Part 4 of 5Common Misunderstandings

Common Misunderstandings
Reaction stopped
  • At equilibrium the reactions have not stopped; they continue at the same rate in both directions.
Equal amounts
  • There is no requirement for equal concentrations of reactants and products.
Time to reach
  • Reaching equilibrium takes time, and a catalyst makes it quicker without changing the final balance.

Part 5 of 5Exam Checklist & Tips

3.3.2 – Key Points for the Exam

  • Reversible reaction A reaction that can proceed in both the forward and reverse directions.
  • Dynamic equilibrium A state in which forward and reverse reactions occur at equal rates.
  • Closed system A system in which no substances enter or leave.
  • Equilibrium sign The ⇌ symbol that shows a reaction is reversible.

Common Mistakes

  • Saying that nothing happens at equilibrium; the reactions continue in both directions.
  • Claiming the concentrations of reactants and products must be equal.
  • Forgetting to mention that the system must be closed.
  • Writing a single arrow for a reversible reaction.

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3 & IGCSE —

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