Activation energy is the energy difference between the reactants and which point on the energy diagram?

• A. The products
• B. The activated complex
• C. The final state
• D. The transition state energy equals the energy of the products

Answer: (C)

Activation energy is the energy needed to climb to the highest point along the reaction path—the peak of the energy diagram. That peak is the transition state, also called the activated complex, a fleeting arrangement of atoms as old bonds break and new ones form. So the activation energy is the difference in energy between the reactants and that top point.

The final state corresponds to the products after the reaction, which is not what you measure for activation energy. And the transition state isn’t generally at the same energy as the products; it sits higher than both reactants and products, representing the barrier that must be overcome to proceed.