Energy Diagrams and Reaction Coordinate Basics

Reaction coordinate diagrams plot energy (y-axis) vs progress from reactants to products (x-axis). They reveal kinetics (activation barriers, rate-determining step) and thermodynamics (overall energy change, exothermic vs endothermic).

• Transition States and Activation Energy
• Exothermic vs. Endothermic Profiles
• Summary

Transition States and Activation Energy

• The peak of the curve is the transition state (‡)—highest energy point where bonds are partly broken/formed.
• Activation energy (Eₐ) is the energy gap from reactants to the transition state; larger Eₐ = slower step.
• Multi-step reactions have multiple peaks (TS) and valleys (intermediates); the tallest barrier is rate-determining.

Exothermic vs. Endothermic Profiles

• Exothermic: products lower in energy than reactants; ΔH < 0 (energy released).
• Endothermic: products higher than reactants; ΔH > 0 (energy absorbed).

Summary

• Peaks = transition states; height to peak = activation energy (controls rate).
• Valleys (if present) = intermediates in multi-step mechanisms.
• Product energy vs reactant energy distinguishes exothermic vs endothermic outcomes.