What is the activated complex in a chemical reaction?

• A. A stable intermediate that remains after the reaction.
• B. An unstable arrangement of atoms that exists momentarily at the peak of the activation-energy barrier.
• C. A product formed at the end.
• D. A catalyst-substrate complex that never dissociates.

Answer: (B)

The activated complex is the temporary, high-energy arrangement of atoms that exists right at the peak of the activation-energy barrier along the reaction path. It represents the moment when bonds are being broken and new bonds are forming, so the system is at the highest potential energy along the route from reactants to products. Because this state sits at the energy maximum, it is incredibly unstable and cannot be isolated under normal conditions; it exists only for an instant as the reaction either proceeds to products or falls back to reactants. It’s not a stable intermediate, it’s not the final product, and it isn’t a forever-bound catalyst–substrate complex—catalysts merely provide a lower-energy route through a transition state, with the complex still dissociating after formation.