Explain reversible reactions and dynamic equilibrium
Distinguish reversible change from dynamic equilibrium and explain why equilibrium requires a closed system.
Section 7 · Equilibria
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At dynamic equilibrium, how do the forward and reverse rates compare?
What you will learn
- Define a reversible reaction
- Describe dynamic equilibrium by rate and concentration
- Explain the need for a closed system
A reversible reaction proceeds in both directions
In a reversible reaction, reactants form products and products can react to reform the reactants. The double arrow shows both directions are possible under the stated conditions.
A reversible reaction does not automatically mean it has reached equilibrium. The forward and reverse reactions may initially occur at different rates.
Dynamic equilibrium balances rates in a closed system
At dynamic equilibrium, the forward and reverse reactions continue at equal rates. The concentrations of reactants and products remain constant, although they are not necessarily equal.
A closed system prevents substances from entering or escaping. If a product continually leaves, the reverse reaction cannot build to the rate needed for dynamic equilibrium.
Pause and explain
What stays constant at dynamic equilibrium?
Worked example
Explain what is happening in a closed vessel once a reversible reaction reaches dynamic equilibrium.
Show the worked solution
- Both forward and reverse reactions continue.
- Their rates are equal.
- Reactant and product concentrations therefore remain constant.
Answer The reaction is dynamic because both directions continue, but concentrations stay constant because the forward and reverse rates are equal in the closed vessel.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define dynamic equilibrium using rate and concentration language.
Give me a hint
State what the two rates do and what happens to concentrations.
Compare my reasoning
- The system is closed.
- Forward and reverse reactions continue at equal rates.
- Reactant and product concentrations remain constant.
Dynamic equilibrium is reached in a closed system when forward and reverse reactions continue at equal rates, so concentrations remain constant.
Look for these in your work
- Closed system included
- Equal rates included
- Constant concentrations included
In a sealed vessel, A ⇌ B. At one moment the forward rate is 0.40 mol dm⁻³ s⁻¹ and the reverse rate is 0.40 mol dm⁻³ s⁻¹. Is the system at dynamic equilibrium?
Give me a hint
Compare rates and check whether the vessel is closed.
Compare my reasoning
- The rates are equal.
- The vessel is sealed, so material cannot escape.
- The definition is met, assuming rates and concentrations are steady.
Yes. In the sealed vessel the forward and reverse rates are equal, so concentrations remain constant while both reactions continue.
Look for these in your work
- Rates compared
- Closed-system condition checked
- Dynamic nature stated
A product is continuously removed from A ⇌ B. Explain why the original closed-system equilibrium condition no longer applies.
Give me a hint
Track whether material can leave and how that affects the reverse reaction.
Compare my reasoning
- The vessel is no longer a closed system.
- Removing B reduces the material available for the reverse process.
- The system responds until its new conditions are established; the original dynamic-equilibrium state is disturbed.
The system is open because B leaves. The reverse reaction is affected, so the original closed-system equilibrium is disturbed and concentrations need not remain at their former values.
Look for these in your work
- Closed-system limitation identified
- Reverse reaction linked
- Original equilibrium distinguished from a new state
Common mistakes
- Saying the reaction has stopped
- Saying equilibrium means reactant and product concentrations are equal
- Using equal concentrations instead of equal rates
- Forgetting that the system must be closed
What can you explain now?
At dynamic equilibrium, how do the forward and reverse rates compare?
Compare with the explanation
They are equal
Equal rates keep concentrations constant.
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Make it stick
How can a reaction be dynamic while its concentrations stay constant?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.