A closed flask contains the following reversible reaction: dinitrogen tetroxide ⇌ nitrogen dioxide. At a certain temperature, the mixture reaches dynamic equilibrium. Explain what happens to the rates of the forward and backward reactions from the moment the dinitrogen tetroxide is first added to the flask until dynamic equilibrium is established.

Edexcel GCSE Chemistry (1CH0) — 4.2 Reversible reactions and equilibria · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Dinitrogen tetroxide is a colourless gas. Nitrogen dioxide is a brown gas. When dinitrogen tetroxide is sealed in a flask, it begins to decompose to form nitrogen dioxide. Both gases are present in the flask at equilibrium.

Model answer (4 marks)

Initially, only the forward reaction occurs, so the rate of the forward reaction is high. As dinitrogen tetroxide decomposes, its concentration falls, causing the forward rate to decrease. Meanwhile, nitrogen dioxide builds up, increasing the rate of the backward reaction. When the forward and backward rates become equal, dynamic equilibrium is achieved and the concentrations of both gases remain constant.

Examiner tips

  • State the initial high forward rate and that the backward rate is initially zero.
  • Explain how decreasing reactant concentration lowers the forward rate and increasing product concentration raises the backward rate.
  • Show that equilibrium is when the two rates are equal and concentrations are constant.
  • Use correct terminology: forward reaction, backward reaction, dynamic equilibrium.

Common mistakes

  • Confusing the initial state with the equilibrium state; claiming both reactions start immediately.
  • Using vague terms like ‘slow down’ without linking to concentration changes.
  • Failing to mention that the backward reaction rate is zero at the very start.

Mark scheme (4 marks)

  1. Initially, only the forward reaction occurs / the rate of the forward reaction is high at the start
  2. As the concentration of dinitrogen tetroxide decreases, the rate of the forward reaction decreases
  3. As the concentration of nitrogen dioxide increases, the rate of the backward reaction increases
  4. Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the backward reaction, so the concentrations of reactants and products remain constant

Key terms in this question

reversible reaction · dynamic equilibrium · backward reaction

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