Activation energy is the energy difference between the reactants and the activated complex. Which option correctly describes this concept?

• A. The energy difference between the reactants and the products
• B. The energy difference between the products and the activated complex
• C. The energy difference between the reactants and the activated complex
• D. The energy difference between the activated complex and the products

Answer: (C)

Activation energy is the energy needed to reach the activated complex, or transition state, from the reactants. On an energy diagram, the reactants sit at a lower energy, then the system must climb up to the peak—the activated complex—before settling into the products. That energy difference from reactants up to the peak is what we call the activation energy. The activated complex is a fleeting arrangement of atoms at that peak, not a stable species.

So the option describing the energy gap between reactants and the activated complex matches this idea exactly. The overall energy change from reactants to products is a different quantity (the enthalpy change of the reaction). And the energy differences involving the activated complex and the products describe other parts of the pathway, not the barrier that must be overcome to start the reaction.