What is the term for the species intermediate between reactants and products that results from the collision of reactants and can proceed to products or revert to reactants?

• A. Catalyst
• B. Activated complex
• C. Rate Law
• D. Arrhenius

Answer: (B)

The idea being tested is the activated complex, also called the transition state. When reactants collide with enough energy and the right orientation, they form a short-lived, high-energy arrangement of atoms that sits at the top of the energy barrier between reactants and products. This activated complex is not inside either side; it’s an intermediate state that can proceed forward to form products or fall back to regenerate reactants. It's extremely unstable and cannot be isolated under normal conditions, existing only fleetingly as the system passes over the barrier.

A catalyst, in contrast, speeds up the reaction by providing an alternative, lower-energy pathway, but it isn’t this transient intermediate itself. The rate law describes how the rate depends on concentrations, not a species formed during the collision. The Arrhenius expression relates the rate constant to temperature and activation energy, again focusing on energy and temperature rather than a discrete intermediate species.