At dynamic equilibrium, the forward and reverse reactions continue at the same rate, so the amounts of reactants and products stay constant. The reactions have not stopped. The examiner’s word is usually “dynamic”, and your job is to show you know why.
This lesson follows representing a reversible reaction and prepares the ground for predicting shifts in reversible changes and equilibrium.
What is actually going on?
Start with a reversible reaction in a sealed container. Only reactant is present at the start, so only the forward reaction can occur, and it runs fastest. As product builds up, the reverse reaction begins and speeds up, while the forward reaction slows because reactant is being used.
Eventually the two rates become equal. From then on, every particle of product formed in one moment is matched by one being broken down. The mixture looks unchanged, but it is busy.
A picture that helps
Think of a two-way escalator between two floors with the same number of people stepping on at each end each minute. The number of people on each floor stays constant, yet people are moving all the time.
The picture is useful, but it is an analogy. The true statement is about rates of reaction and concentrations.
Conditions for dynamic equilibrium
- The system is closed: nothing enters or leaves.
- The reaction is reversible.
- The temperature is steady (and other conditions too).
- The rates of forward and reverse reactions are equal.
- As a result, the concentrations stay constant (not equal).
Worked example
Nitrogen dioxide and dinitrogen tetroxide are in a sealed syringe at constant temperature. After some minutes the colour stops changing. A student writes: “The reaction has stopped because the colour is constant.” Write a correct explanation.
Step 1, name the state: the mixture has reached dynamic equilibrium.
Step 2, say what is constant: the concentrations of N₂O₄ and NO₂ stay constant, which is why the colour is steady.
Step 3, say why: the rate of the forward reaction, N₂O₄ → 2NO₂, equals the rate of the reverse reaction, 2NO₂ → N₂O₄.
Step 4, name the condition: this happens because the syringe is sealed, so the gases cannot escape.
Model answer: “The system is at dynamic equilibrium. The forward and reverse reactions are still occurring at the same rate, so the concentrations of NO₂ and N₂O₄ do not change and the colour stays constant. The system is closed.”
The mistake to watch for
The mistake is the phrase “the reaction stops”, and a close cousin: “the amounts are equal.”
Mistaken answer: “At equilibrium there is the same amount of reactant and product, because the reaction stops.”
Both parts are wrong. The amounts are constant, not necessarily equal, and the reactions continue.
Correction: “At equilibrium the rates of the forward and reverse reactions are equal, so the amounts of reactant and product stay constant.” A quick test is to ask which word describes the rates (equal) and which word describes the amounts (constant).
Check yourself
1. State two conditions needed for a reversible reaction to reach dynamic equilibrium.
Show answer
Any two of: the system is closed; the reaction is reversible; temperature and pressure are held steady. Some answers also accept that the forward and reverse rates become equal, but that is the result rather than a condition.
2. Explain why the colour in a sealed N₂O₄/NO₂ mixture can be constant while reactions are still occurring.
Show answer
The forward and reverse reactions happen at the same rate, so NO₂ is formed as fast as it is used up. The concentration of NO₂ therefore stays constant and so does the colour.
3. An open beaker is used for a reaction that releases a gas. Why can this never reach dynamic equilibrium?
Show answer
The gas escapes to the surroundings, so the reverse reaction cannot take place at the same rate. The system is open, so the amounts keep changing instead of settling.
Where this leads next
With the meaning secure, go on to predicting a shift under a stated change. Reasoning about rates also links back to rates of reaction.
If you understand equilibrium but your written explanations still miss marks, our teachers in online one-to-one Chemistry tuition can work through your wording with you.