Activation energy is defined as the energy difference between which two entities?

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

Answer: (C)

Activation energy is the energy barrier that must be overcome for a reaction to proceed. It corresponds to the energy difference between the reactants and the activated complex (the transition state), the high-energy arrangement that forms as bonds break and new bonds begin to form. In other words, Ea is how much energy the system needs to reach the transition state from the reactants. The energy difference between products and reactants is the overall enthalpy change of the reaction, not the barrier. The energy difference between the activated complex and the products would describe the energy remaining to reach products from the transition state (the reverse activation energy), not the forward barrier.