Dynamic equilibrium is one of the most important concepts in IB Chemistry. You’ll encounter it in reversible reactions, Le Chatelier’s Principle, equilibrium constants, acids and bases, and even electrochemistry. Because it forms the foundation of multiple topics, examiners expect you not only to define it correctly but also to explain its behavior in molecular terms.
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Quick Start Checklist
Before diving in, make sure you understand:
- Dynamic equilibrium occurs in a closed system.
- The forward and reverse reactions occur at the same rate.
- Concentrations of reactants and products remain constant, not equal.
- Particles are continuously reacting.
- The system is stable but still active on the particle level.
These points appear frequently in Paper 2 questions, especially in “explain,” “distinguish,” or “state” prompts.
What Is Dynamic Equilibrium?
Dynamic equilibrium describes a state in a reversible reaction where the forward and reverse reactions occur at the same rate, so the concentrations of all reactants and products remain constant over time.
The word “dynamic” is essential. It means:
- Reaction particles continue reacting.
- Products are still turning back into reactants.
- There is continuous molecular activity.
However, because the rates are equal, there is no net change in concentration.
A familiar example is:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
At equilibrium, nitrogen and hydrogen still form ammonia, and ammonia still decomposes—but the overall amounts don’t change.
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Conditions for Dynamic Equilibrium
Dynamic equilibrium can only be established if:
- The reaction is reversible.
- The system is closed, meaning no substances enter or leave.
- Sufficient time has passed for both forward and reverse reactions to balance.
Changing any condition—like temperature, pressure, or concentration—will disturb equilibrium, allowing Le Chatelier’s Principle to predict the shift.
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Concentrations Are Constant, Not Equal
One of the most common IB misconceptions is thinking that at equilibrium the concentrations of reactants and products are equal. They are not.
Instead:
- Concentrations remain constant
- But their values depend on the reaction’s equilibrium constant, Kc
For example, in many reactions the product concentration is much higher at equilibrium; in others, reactant concentration is higher.
Understanding this distinction helps you avoid common mistakes in Paper 1 multiple-choice questions.
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Molecular Behavior at Equilibrium
Even though concentrations appear unchanged, molecular behavior is active. At equilibrium:
- Collisions continue
- Bonds form and break
- Particles interchange constantly
Because the forward and reverse processes occur at the same rate, the macroscopic (visible) properties appear stable. This particle-level explanation is one examiners love to see in high-scoring answers.
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Disturbing Equilibrium
Once equilibrium is established, changes in conditions will disturb it. Le Chatelier’s Principle predicts how the system will respond:
- Adding reactants shifts toward products
- Increasing temperature affects exo-/endothermic reactions
- Changing pressure affects gases
- Removing products shifts the equilibrium to replace them
These ideas form the basis of many equilibrium essays and structured questions.
Frequently Asked Questions
Why do reactions not stop once equilibrium is reached?
Because particles still have kinetic energy and continue to collide. The system remains dynamic even though concentrations stay constant.
Can equilibrium be reached in an open system?
No. Substances entering or leaving prevent the balance of forward and reverse reactions.
Does equilibrium mean equal amounts of products and reactants?
No. It means equal rates, not equal amounts.
Conclusion
Dynamic equilibrium is a balance of forward and reverse reaction rates in a closed system, producing constant but not equal concentrations. Understanding this concept is essential for predicting shifts, solving equilibrium problems, and writing high-quality explanations in IB Chemistry.
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