In a reaction coordinate diagram, what does the activation energy correspond to?

• A. It shows energy versus time for the reaction progress, with no energy axis.
• B. It shows energy along the reaction path, with a peak representing the transition state; the height from reactants to the peak is the activation energy.
• C. It shows concentration versus time during the reaction.
• D. It shows the rate constant as a function of temperature.

Answer: (B)

Activation energy is the energy barrier that must be overcome for reactants to be converted into products. In a reaction coordinate diagram, energy is plotted against the progress of the reaction, and the highest point along the path represents the transition state—the most energetic arrangement of atoms during the transformation. The activation energy is simply the difference in energy between the reactants and that peak. This energy gap determines how fast the reaction can proceed at a given temperature: larger barriers require more energy and slow the rate, while catalysts lower the barrier by stabilizing the transition state and speeding up the reaction. Other ways of plotting a reaction—like energy versus time with no energy axis, concentration versus time, or rate constant versus temperature—do not depict this energy barrier to reach the transition state.