The threshold energy that must be overcome to initiate a reaction is called

• A. Activated Complex
• B. Spectator Ion
• C. Redox
• D. Activation Energy

Answer: (D)

Activation energy is the minimum amount of energy required to start a reaction—the energy barrier that reactants must overcome to reach the products. On an energy vs. progress diagram, reactants sit at their own energy level, and to transform into products they must be raised up to the top of the barrier, which corresponds to the transition state or activated complex. The difference between the reactants’ energy and that peak is the activation energy, and it explains why higher temperatures or catalysts often speed up reactions: more molecules have enough energy to get over the barrier, or the barrier is lowered by a catalyst.

The term for the transient arrangement at the top of the barrier is the activated complex; it’s part of the reaction pathway, not the energy hurdle itself. A spectator ion is an ion that doesn’t participate in the chemical change, so it doesn’t define the energy needed to start the reaction. Redox is a type of reaction involving electron transfer, not the energy required to initiate the reaction. Activation energy best captures the idea of the threshold energy to initiate the process.