According to transition state theory, which description best matches the 'transition state'?

• A. They are always products
• B. They are intermediates that are closer to reactants
• C. Species form that are neither a product nor reactant, but something in between
• D. They are catalysts

Answer: (C)

In transition state theory, the transition state is the activated complex—a fleeting, high-energy arrangement of atoms at the peak of the energy barrier along the reaction path. It is not a reactant or a product, but a momentary configuration that exists between them as bonds are breaking and forming. This species is not stable and is in rapid, quasi-equilibrium with the reactants before it proceeds to products. It represents the maximum on the potential energy surface along the reaction coordinate, so it largely governs how fast the reaction occurs. It is not a catalyst, and it is not a stable intermediate you could isolate. Because the transition state is neither reactant nor product but something in between, it best matches the description in the correct choice.