What is activation energy?

• A. The energy required to initiate a chemical reaction
• B. The energy released during the reaction
• C. The energy stored in the products
• D. The energy absorbed from the environment

Answer: (A)

Activation energy is the energy barrier that must be overcome to start a chemical reaction. It represents the minimum energy needed for reactants to reach a high-energy transition state where bonds are broken and new bonds form. This barrier is not the energy released or absorbed by the overall reaction—that’s the enthalpy change—and it’s not the energy stored in the products. Instead, it’s the energy that must be supplied from the surroundings (or supplied by a catalyst, which provides a lower-energy pathway) to get the reaction going. Temperature can raise the number of molecules with enough energy to overcome this barrier, increasing the reaction rate, while a catalyst lowers the barrier without changing the overall energy change of the reaction.