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Activation energy

In an energy profile, the chemical energy must reach at least the energy needed to break all the bonds apart.

This required energy is identified as the peak on the energy profile. Energy decreases after this point because new bonds start to form, releasing energy from the system.

The difference between the peak energy and the energy of the reactants is called the activation energy ($E_A$). The activation energy has to be overcome for the reaction to occur.

After the peak, the energy decreases until all the new bonds are formed. When all products are formed, the reaction ends.

At this point, the energy remains constant and is equal to the energy of the products.

Energy profile of the exothermic combustion of glucose. The lower activation energy peak represents the alternative pathway observed when a catalyst is used.