For an exothermic reaction, how do the forward and reverse activation energies compare?

For an exothermic reaction, how do the forward and reverse activation energies compare?

• A. Ea reverse is larger than Ea forward
• B. Ea forward is larger than Ea reverse
• C. Ea forward equals Ea reverse
• D. Ea reverse equals zero

Answer: (A)

Activation energy is the energy difference between the starting state and the transition state. In an exothermic reaction, the products end up at a lower energy than the reactants. The transition state sits at the same energy level for both directions, so the energy gap from reactants up to the transition state (forward activation energy) is smaller than the gap from products up to the transition state (reverse activation energy). Put simply, you have to climb a larger energy barrier when going from products back to the transition state than when going from reactants to it. A quick way to see it is Ea,r = Ea,f + |ΔHrxn|, and since ΔHrxn is negative for an exothermic process, Ea,r is larger than Ea,f.

Activation energy is the energy difference between the starting state and the transition state. In an exothermic reaction, the products end up at a lower energy than the reactants. The transition state sits at the same energy level for both directions, so the energy gap from reactants up to the transition state (forward activation energy) is smaller than the gap from products up to the transition state (reverse activation energy). Put simply, you have to climb a larger energy barrier when going from products back to the transition state than when going from reactants to it. A quick way to see it is Ea,r = Ea,f + |ΔHrxn|, and since ΔHrxn is negative for an exothermic process, Ea,r is larger than Ea,f.