Activation Energy Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Activation Energy.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The minimum kinetic energy that reactant particles must possess upon collision in order to break existing bonds and initiate a chemical reaction, represented as the energy barrier on a reaction energy diagram.

The 'hill' reactants must climb over before the reaction can proceed.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Even exothermic reactions need activation energy to begin — the energy barrier must be crossed.

Common stuck point: Catalysts lower activation energy without changing the overall energy release.

Sense of Study hint: When working with activation energy, read the energy diagram carefully. First identify the energy level of reactants and the peak of the energy barrier. Then calculate E_a as the difference between the peak and reactant energy. Finally, note that a catalyst lowers this peak but does not change the reactant or product energy levels.

Worked Examples

Example 1

easy
Explain why a match must be struck before it ignites, even though combustion is exothermic.

Solution

  1. 1
    Combustion is exothermic overall, but it requires an initial input of energy to start — this is the activation energy (E_a).
  2. 2
    Striking the match provides friction-generated heat that overcomes the activation energy barrier.
  3. 3
    Once started, the exothermic reaction sustains itself by providing energy to activate neighboring molecules.

Answer

E_a\text{ must be overcome to initiate the reaction}
Activation energy is the minimum energy that reactant molecules must possess for a successful collision. Even highly exothermic reactions need this initial push.

Example 2

medium
On an energy diagram, Reaction A has E_a = 50\,\text{kJ/mol} and Reaction B has E_a = 120\,\text{kJ/mol}. Both occur at the same temperature. Which reaction is faster and why?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
Does a catalyst change the activation energy of a reaction? How?

Example 2

medium
Paper burns exothermically, but a sheet of paper can sit at room temperature for days without catching fire. Why?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

chemical reactionenergy