Specification Point 3.19C (Paper 2C Only):
Know that a reversible reaction can reach dynamic equilibrium in a sealed container

Dynamic Equilibrium

Reversible reaction: A reaction that is occurring in both directions.

Dynamic equilibrium: The condition that exists in a sealed container when the rate of the forward and backward reactions in a reversible reaction mixture are equal

Example: The reaction between hydrogen gas and nitrogen gas to make ammonia:

  • When only Nitrogen and Hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of Hydrogen and Nitrogen are at their highest.
  • As the reaction proceeds, the concentrations of Hydrogen and Nitrogen gradually decreases, so the rate of forward reaction will decrease. However, the concentration of Ammonia is gradually increasing and so the rate of the backward reaction will increase (Ammonia will decompose to reform hydrogen and nitrogen).
  • Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal:

3H2 (g)      +      N2 (g)           2NH3 (g)

Specification Point 3.20C (Paper 2C Only):
Know the characteristics of a reaction at dynamic equilibrium are: the forward and reverse reactions occur at the same rate, the concentrations of reactants and products remain constant

Conditions of Dynamic Equilibrium

  • Forward and backward reactions occur at the same rate
  • Concentrations of reactants and products remain constant
  • Both forward and backward reactions are occurring

Diagram:

Dynamic equilibrium, CIE IGCSE Chemistry

When the rates of forward and backward reactions become equal

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.