Activation energy is defined as

Activation energy is defined as

• A. The energy difference between reactants and products.
• B. The minimum energy required to start a reaction.
• C. The energy released when products form.
• D. The rate constant at equilibrium.

Answer: (B)

Activation energy is the energy barrier that must be overcome for reactants to be converted into products. It’s the minimum extra energy the reacting molecules need to reach the transition state along the reaction pathway. On an energy diagram, it’s the height from the reactants up to the peak corresponding to the transition state. This barrier, and how easily molecules can surmount it, governs reaction rates: higher barriers slow reactions, while higher temperatures help more molecules exceed the barrier, increasing the rate. The other descriptions describe different concepts: the energy difference between reactants and products is the reaction enthalpy (ΔH), indicating overall energy change, not the barrier; the energy released when products form refers to energy changes during bond formation and overall energetics rather than the starting hurdle; and a rate constant at equilibrium isn’t a single barrier-focused quantity, since equilibrium involves both forward and reverse processes with their own constants.

Activation energy is the energy barrier that must be overcome for reactants to be converted into products. It’s the minimum extra energy the reacting molecules need to reach the transition state along the reaction pathway. On an energy diagram, it’s the height from the reactants up to the peak corresponding to the transition state. This barrier, and how easily molecules can surmount it, governs reaction rates: higher barriers slow reactions, while higher temperatures help more molecules exceed the barrier, increasing the rate. The other descriptions describe different concepts: the energy difference between reactants and products is the reaction enthalpy (ΔH), indicating overall energy change, not the barrier; the energy released when products form refers to energy changes during bond formation and overall energetics rather than the starting hurdle; and a rate constant at equilibrium isn’t a single barrier-focused quantity, since equilibrium involves both forward and reverse processes with their own constants.