Britannica Sources used in this answer

Why does a reaction usually follow the pathway with the lowest activation energy?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

dissociation of E–N — reaction mechanism Possible energy diagram for the dissociation of a covalent molecule, E–N, into its ions E⁺ and N⁻ (see text).

In chemistry, most reactions are activated processes, meaning they require a supply of energy to overcome an energy barrier before they can proceed to form products. This barrier is defined by the transition state, which represents the configuration of atoms or molecules at the peak of the potential energy surface along the reaction coordinate.123

The reason a reaction typically follows the pathway with the lowest activation energy is rooted in the statistical distribution of molecular energies. According to the Maxwell-Boltzmann distribution, only a fraction of molecules possess sufficient energy to overcome a given barrier. The rate of a reaction is exponentially dependent on the activation energy, as shown in the Arrhenius equation:4

$k = A e^{(- E_{a} / R T)}$

Because the rate constant $k$ decreases exponentially as the activation energy $E_{a}$ increases, pathways with higher barriers occur at significantly slower rates. Consequently, the pathway requiring the least energy is the most accessible and proceeds most rapidly, effectively dominating the overall reaction course.561