Dynamic equilibrium

Busy, but nothing seems to change

Start a reversible reaction by mixing the reactants. At first it runs forward only. Once enough product has built up, the products begin reacting to give back reactants.

Both directions keep going until the rate of the forward reaction equals the rate of the backward reaction. Now reactant molecules are used up exactly as fast as they are made again. This is chemical equilibrium.

The concentrations of reactants and products stay constant, so it looks as though nothing is happening. But the reaction has not stopped: both directions are still running. That is why it is called dynamic equilibrium.

How long it takes depends on the reaction and the conditions. A very slow reaction may take years. Ammonia formation at about 400 °C with a catalyst reaches equilibrium within minutes.

Constant is not the same as stopped: the amounts stay fixed because both directions run at the same rate.

Key terms

Forward reaction

The direction that starts first.

Backward reaction

Starts once products build up.

Chemical equilibrium

The point where concentrations stop changing.

Dynamic

Why equilibrium is not a stop.

Short questions with model answers

1. Q1. N₂O₄ (colourless) ⇌ 2NO₂ (brown) is kept in a sealed flask at 100 °C. Describe the colour over time. (Exercise 2, p. 86)

   • Start: colourless
   • Brown colour deepens
   • Colour stops changing

   It turns brown, then stays a steady shade at equilibrium.

2. Q2. What happens to product concentrations if a reaction at equilibrium is not disturbed? (MCQ x, p. 91)

   • Forward rate = backward rate
   • No net change

   They remain constant.

Common mistakes

• ✗ At equilibrium the reaction has stopped.

  ✓ Both directions keep running at equal rates: dynamic.

• ✗ At equilibrium, reactants and products are present in equal amounts.

  ✓ The rates are equal; the concentrations are constant, not necessarily equal.

• ✗ Equilibrium is reached the moment the reactants are mixed.

  ✓ It takes time, from seconds to years, depending on the reaction and conditions.

MCQs

1. 1. A reversible reaction reaches equilibrium when:

   1. (a) All reactants become products
   2. (b) 50% of reactants become products
   3. (c) Concentrations of all species become constant
   4. (d) A product is removed
   Show answer

   (c) MCQ (v), p. 90; key point 5.

2. 2. Why is chemical equilibrium called dynamic?

   1. (a) Because the reaction stops
   2. (b) Because both reactions keep going
   3. (c) Because the colour keeps changing
   4. (d) Because it is very fast
   Show answer

   (b) p. 85.

3. 3. If the rates of forward and reverse reactions are very high, equilibrium is reached:

   1. (a) Very soon
   2. (b) Very late
   3. (c) Never
   4. (d) Only if heated
   Show answer

   (a) MCQ (i), p. 90: fast reactions settle quickly.

4. 4. At equilibrium, the rate of the forward reaction is:

   1. (a) Zero
   2. (b) Equal to the rate of the backward reaction
   3. (c) Greater than the backward rate
   4. (d) Less than the backward rate
   Show answer

   (b) Key point 4.

5. 5. Ammonia formation at about 400 °C with a catalyst reaches equilibrium in:

   1. (a) Minutes
   2. (b) Years
   3. (c) Never
   4. (d) Exactly one day
   Show answer

   (a) p. 86.

Quick revision

• Equilibrium: forward rate equals backward rate.
• Concentrations stay constant, but both reactions keep running: dynamic equilibrium.
• The time to reach it depends on the reaction and the conditions.