Energy of activation is defined as what?

• A. The energy released by a reaction
• B. The total energy of the reactants
• C. The energy needed to start a reaction
• D. The energy stored in ATP

Answer: (C)

Activation energy is the energy required to start a reaction—the energy barrier reactants must surpass to reach the high-energy transition state and reorganize bonds into products. This initial input is what determines how fast a reaction can proceed at a given temperature: higher activation energy means slower reactions, while catalysts or enzymes lower that barrier to speed things up. It's not the energy released by the reaction (the overall energy change) nor the total energy of the reactants, and it's not energy stored in ATP.