Advanced Placement (AP) Biology Practice Exam

When the reactants reach the activation energy summit, they achieve a transition state. This transition state is a high-energy, unstable configuration that represents the point at which old bonds are breaking and new bonds are forming. It is a critical stage in a chemical reaction because it allows the molecules involved to react and rearrange into the final products. At this summit, the energy required to reach the transition state is the activation energy, which is necessary for the reaction to proceed. Once this energy threshold is surpassed, the reaction can continue towards forming products, often leading to a release of energy as the system moves from a high-energy state to a more stable state with lower energy—this is where the transition from reactants to products occurs. In contrast, becoming stable suggests that the reactants no longer have the energy to react, which takes place after the products are formed. The idea of reverting to reactants does not occur at the activation energy summit but instead relates to the equilibrium of a reversible reaction, which can happen under different conditions.