Which expression correctly defines activation energy Ea?

Which expression correctly defines activation energy Ea?

• A. Ea = Er - Eac
• B. Ea = Eac - Er
• C. Ea = Ep - Er
• D. Ea = Eac + Er

Answer: (B)

Activation energy is the energy required to reach the transition state from the reactants. The transition state (activated complex) sits higher in energy than the reactants, so the barrier height is the difference between the energy of the activated complex and the energy of the reactants. In this notation, that means Ea = Eac − Er. This keeps Ea a positive quantity, reflecting the energy you must supply to get over the barrier. If you subtract in the opposite order, you’d get a negative value, which doesn’t make physical sense for a barrier. Adding energies or using the product energy would not describe the forward reaction barrier correctly, since product energy is not part of the barrier height.

Activation energy is the energy required to reach the transition state from the reactants. The transition state (activated complex) sits higher in energy than the reactants, so the barrier height is the difference between the energy of the activated complex and the energy of the reactants. In this notation, that means Ea = Eac − Er. This keeps Ea a positive quantity, reflecting the energy you must supply to get over the barrier. If you subtract in the opposite order, you’d get a negative value, which doesn’t make physical sense for a barrier. Adding energies or using the product energy would not describe the forward reaction barrier correctly, since product energy is not part of the barrier height.