Activation Energy

Definition of Activation Energy

Activation energy is the minimum amount of energy that reacting particles must have for a chemical reaction to start. It acts as an energy barrier that the reactants need to overcome before they can be converted into products.

This concept applies to all chemical reactions, whether they release energy (exothermic) or absorb energy (endothermic). Without enough energy to overcome this barrier, the reaction will not proceed.

Energy Profile Diagrams

An energy profile diagram shows the energy changes during a chemical reaction. It plots the energy of the reactants and products against the progress of the reaction.

The activation energy is represented by the peak of the curve 6 the highest point on the diagram. This peak shows the energy barrier that must be overcome for the reaction to occur.

In an exothermic reaction, the products have less energy than the reactants, so the energy profile slopes downwards after the peak. In an endothermic reaction, the products have more energy than the reactants, so the profile slopes upwards after the peak.

The difference in height between the reactants and the peak is the activation energy, while the difference between reactants and products shows the overall energy change.

Role in Chemical Reactions

Activation energy determines how quickly a reaction happens. For particles to react, they must collide with enough energy to overcome the activation energy barrier.

If particles collide with less energy than the activation energy, they will bounce off without reacting.

This explains why some reactions need heating: increasing temperature gives particles more energy, so more collisions have enough energy to overcome the activation energy barrier, speeding up the reaction.

For instance, when lighting a match, the friction provides enough energy to overcome the activation energy and start the combustion reaction.

Example inline: If a reaction has an activation energy of 100 kJ/mol, particles must have at least this much energy to react. Increasing temperature increases the number of particles with energy  100 kJ/mol, thus speeding up the reaction.