Exergonic reactions are spontaneous, while endergonic reactions are not. However, it is not the free energy change that determines the rate of a reaction. The rate of a reaction is determined by the activation energy instead. An exergonic reaction can be slow, while an endergonic reaction is fast. The activation energy is the energy needed to reach the transition state between reactants and products, and all reactions require activation energy, even exergonic reactions. Reactions with smaller activation energies have faster rates than those with larger activation energies. With a high activation energy, fewer moleules have enough energy to reach the transition state, causing the reaction to proceed slower. When the activation energy is lower, more molecules can easily reach the transition state, accelerating the reaction. This is precisely how enzymes work to speed up a reaction: They lower the activation energy of the reaction. They do so by stabilizing the structure of the transition state, which then requires less energy to be reached. Enzymes do not affect the free energies of the substrates or products, and they do not alter the equilibrium of a reaction. They simply allow equilibrium to be reached quicker. Enzymes can enhance the rate of a reaction in many ways. They can form favorable interactions with the transition state in its active site. It can also orient two subtrates to react easier. An enzyme can also directly participate in the reaction, or it can strain the bonds in the reactant(s). Enzymes usually use more than one of these strategies to stabilize the transition state, lower the activation energy, and speed up a mechanism.