What can slow down a reaction even if it is thermodynamically favorable?

The activation energy barrier is a crucial concept in understanding chemical reactions. Even when a reaction is thermodynamically favorable—meaning that the products are lower in energy than the reactants, making it likely for the reaction to occur—there can still be a significant amount of energy required to initiate the reaction. This energy requirement is termed the activation energy.

The activation energy is the minimum energy threshold that must be overcome for the reactants to transform into products. This often involves breaking existing bonds and forming new ones, which can be energetically demanding. If the activation energy barrier is high, it means that the reaction may proceed very slowly or not at all, despite being energetically favorable once started.

This barrier can be influenced by various factors, such as temperature and the presence of catalysts (like enzymes), which can lower the activation energy and increase the rate of the reaction. However, without these catalysts, even reactions that are spontaneous at standard conditions can take an impractically long time to occur due to a high activation energy barrier. Therefore, the activation energy barrier is a fundamental reason why a reaction can be slow despite being thermodynamically favorable.