In a chemical reaction, the activation energy, or Ea, is the difference in free energy between the substrates in their ground state and in their high energy transition state, an unstable transient structure. These reactants must have sufficient kinetic energy to collide frequently and undergo chemical modifications in order to form products. Due to the activation energy for a reaction, the transition state has a higher energy than either the reactants or the products. It can quickly lose that energy and return to the reactants or convert into the products. When substrates bind to their enzymes, the activation energy decreases, allowing more reactants to reach the transition state. This allows more molecules to be transformed into products and increases the rate of an enzyme-catalyzed reaction compared to an uncatalyzed one. Enzymes speed up reaction rates in several ways, including stabilizing the transition state by positioning the substrates in the proper orientation or providing appropriate chemical environments, such as charge or pH.