What term describes the maximum-energy arrangement of atoms at the top of the energy barrier?

• A. Catalyst
• B. Activated complex
• C. Rate Law
• D. Half-life

Answer: (B)

The concept being tested is the high-energy arrangement that occurs along the reaction path—the activated complex, also called the transition state. This is the moment at the top of the energy barrier where bonds are in the process of breaking and forming, giving the system its maximum potential energy during the reaction. Such a configuration is fleeting and not isolable, existing only long enough for the reaction to proceed toward products. The energy of this arrangement defines the activation energy, the barrier that must be overcome for the reaction to occur. A catalyst influences the rate by providing an alternative pathway with a lower activation energy, not by creating a stable, resting form at the barrier. In contrast, the rate law describes how the rate depends on concentrations, and half-life is simply the time required for the amount of reactant to drop by half; neither describes the actual high-energy arrangement at the barrier.