Activation energy is one of the cornerstone ideas in chemical kinetics, and you will encounter it repeatedly in IB Chemistry—especially in Topic 6 and HL extension material. The concept seems simple at first, but many students misunderstand what activation energy actually represents, how to express it in exam language, and how it connects to potential energy profiles. This guide gives you a clear, IB-ready understanding.
Quick Start Checklist
Activation energy is:
- the minimum energy required for particles to react
- the energy needed to break bonds in reactants
- the “energy barrier” that must be overcome for a reaction to proceed
- represented as Ea on energy profile diagrams
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The IB Definition of Activation Energy
In IB Chemistry, activation energy is defined as:
“The minimum energy required for a successful collision to occur between reactant particles.”
This means that only particles with energy equal to or greater than Ea can form products.
Anything less results in an ineffective collision.
Why Reactions Need Activation Energy
1. Bonds must be broken first
Even exothermic reactions—those that release energy—still require an initial input of energy to break existing bonds.
2. It ensures only high-energy collisions react
Most collisions do not lead to a reaction.
Only collisions with enough kinetic energy overcome the barrier.
3. It determines reaction speed
A lower activation energy → more particles have enough energy → faster reaction.
A higher activation energy → fewer successful collisions → slower reaction.
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Activation Energy on an Energy Profile Diagram
On a potential energy diagram:
- Reactants start at a certain energy level.
- The curve rises to the transition state (highest point).
- The height difference between reactants and the transition state is the activation energy.
This diagram helps visualize why reactions don't happen instantly, even if they are thermodynamically favorable.
Factors That Influence Activation Energy
1. Catalysts
Catalysts provide an alternative reaction pathway with a lower activation energy.
This increases the number of successful collisions and speeds up the reaction without being consumed.
2. Temperature
Higher temperature gives more particles kinetic energy, but temperature does not change the activation energy itself. It simply increases the fraction of particles with energy ≥ Ea.
3. Molecular orientation
Collisions must have correct orientation and sufficient energy.
IB exam questions sometimes test both simultaneously.
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Common Misconceptions in IB Chemistry
- “Exothermic reactions don’t need activation energy.”
All reactions require energy to break initial bonds. - “Catalysts increase the energy of particles.”
They don’t. They simply lower the activation energy. - “If temperature increases, activation energy decreases.”
No — the curve stays the same, but more particles pass it.
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Frequently Asked Questions
1. Why do some reactions have very high activation energies?
Because breaking strong bonds (like C≡C or N≡N) requires a large energy input. Reactions involving stable molecules naturally have higher activation energies. IB HL questions often explore the relationship between bond strength and activation energy.
2. Does activation energy determine whether a reaction is endothermic or exothermic?
No. Activation energy relates to kinetics (speed), not thermodynamics (energy change). Even highly exothermic reactions can have large activation barriers.
3. How does activation energy relate to the Maxwell–Boltzmann distribution?
The distribution shows the range of particle energies. The activation energy line marks the threshold for successful collisions. Increasing temperature shifts the curve, allowing more particles to surpass Ea.
Conclusion
Activation energy is the minimum energy particles must have to react. It represents the energy barrier that reactants must overcome to form products and determines how fast reactions occur. Understanding activation energy is essential for kinetics, catalysis, equilibrium, and energetics in IB Chemistry. With RevisionDojo’s chemistry-focused resources, you can build strong conceptual foundations across the entire course.
