Activation Energy – IGCSE Chemistry Definition

Activation energy is the energy barrier that must be overcome for a reaction to occur. It is a central concept in collision theory and appears on energy level diagrams. The 0620 syllabus requires you to define activation energy, show it on diagrams, and explain how catalysts lower it.

The 0620 definition

Activation energy is the minimum energy that colliding particles must possess for a reaction to take place.

Why activation energy exists

For a reaction to occur, bonds in the reactants must be broken before new bonds can form in the products. The activation energy is the energy needed to start breaking these bonds. Even exothermic reactions need activation energy to get started — for example, a fuel needs a spark or match to ignite.

Activation energy on energy level diagrams

On an energy level diagram:

• Activation energy is the height from the energy level of the reactants to the top of the energy “hump” (the transition state)
• It is always positive — there is always a barrier to overcome
• It is labelled with an upward arrow from the reactant level to the peak

For exothermic reactions, the hump is above the reactant level but the products end up below it. For endothermic reactions, the products end up above the reactants, and the hump is higher still.

Activation energy and reaction rate

The activation energy determines what fraction of collisions lead to a reaction:

Activation energy	Effect on rate
High Ea	Few particles have enough energy to react; slow rate
Low Ea	Many particles have enough energy to react; fast rate

Increasing temperature gives particles more kinetic energy, so a greater proportion exceed the activation energy, increasing the rate.

Catalysts and activation energy

A catalyst lowers the activation energy by providing an alternative pathway. On the energy level diagram, the hump is lower. The reactant and product energy levels do not change — only the barrier height is reduced. This means more collisions are successful, increasing the rate without changing the products or the overall energy change.

Worked exam question

Hydrogen peroxide decomposes slowly. Adding manganese(IV) oxide speeds up the reaction. (a) What term is used for the minimum energy needed for particles to react? (1) (b) Explain how the manganese(IV) oxide increases the rate. (2) (c) On an energy level diagram, show how the catalyst changes the reaction profile. (2)

Mark scheme

(a) Activation energy [1]

(b) The manganese(IV) oxide is a catalyst / provides an alternative reaction pathway [1]; with a lower activation energy, so more particles have enough energy to react / more successful collisions [1]

(c) A second curve with a lower hump drawn on the same diagram [1]; reactant and product levels unchanged [1]

Common exam mistakes

• Saying catalysts “give particles more energy”. Catalysts do not increase particle energy — they lower the activation energy barrier so that the existing particle energies are sufficient for more collisions to succeed.
• Forgetting that activation energy is always required. Even very exothermic reactions have an activation energy. A match provides the activation energy to start combustion.
• Drawing the catalyst curve with different product energy levels. The catalyst only changes the height of the hump, not the start or end energy levels.