Questions: Reaction Coordinate Diagrams

Question 1

A chemist adds a platinum catalyst to a reaction that is strongly exothermic (ΔH = −200 kJ/mol) but proceeds negligibly slowly at room temperature. Which statement best describes the effect of the catalyst?

• A. The reaction becomes more exothermic because the catalyst lowers the energy of the products
• B. The equilibrium position shifts toward products, increasing the theoretical yield
• C. The activation energy decreases, speeding up both forward and reverse reactions equally without changing ΔH
• D. The transition state becomes a stable reaction intermediate that can be isolated

Answer: (C)

A catalyst lowers activation energy by providing an alternative mechanistic pathway, but it does not change the energies of reactants or products. Therefore ΔH, ΔG, and the equilibrium position are all unchanged. Because Ea is lowered for both directions equally, both forward and reverse rates increase by the same factor, so equilibrium is reached faster but not shifted. The reaction was slow because of a high kinetic barrier, not thermodynamic unfavorability.

Question 2

In the energy profile of a two-step reaction, what does a valley (local energy minimum) between two peaks represent?

• A. A transition state — the highest-energy configuration on that segment of the pathway
• B. A reaction intermediate — a real, transiently stable species that forms and then reacts further
• C. The activation energy of the rate-determining step
• D. The point at which the forward and reverse reaction rates become equal

Answer: (B)

In a multi-step energy profile, peaks are transition states (maximum-energy configurations that cannot be isolated) and valleys between peaks are reaction intermediates (real chemical species in local energy minima with finite, if brief, lifetimes). This distinction is critical: transition states exist for only a molecular vibration (~femtoseconds), while intermediates can sometimes be detected spectroscopically or even isolated under the right conditions.

Question 3

A reaction with a large negative ΔH (highly exothermic) should have a small activation energy, since releasing a large amount of energy implies the reactants are already close to the transition state.

• T. True
• F. False

Answer: False

Activation energy (Ea) and reaction enthalpy (ΔH) are independent quantities. Ea measures the energy barrier from reactants to the transition state; ΔH measures the energy difference between reactants and products. A reaction can be strongly exothermic (large negative ΔH) yet still have a high Ea — meaning it is thermodynamically favorable but kinetically slow. Combustion of diamond is a classic example: highly exothermic but effectively does not occur at room temperature due to an enormous kinetic barrier.

Question 4

Adding a catalyst to a reaction lowers the activation energy for both the forward and reverse reactions by the same amount.

• T. True
• F. False

Answer: True

A catalyst works by providing an alternative reaction pathway. Because it does not change the energies of reactants or products (the endpoints), the difference between the new Ea,forward and Ea,reverse must still equal ΔH — which is unchanged. Therefore if the forward barrier is lowered by some amount ΔEa, the reverse barrier is also lowered by the same ΔEa. This is why catalysts accelerate both directions and cannot shift equilibrium.

Question 5

Why can a highly exothermic reaction still proceed very slowly at room temperature? Use the features of a reaction coordinate diagram to explain the difference between thermodynamic favorability and kinetic accessibility.

Model answer: Thermodynamic favorability (ΔH or ΔG) describes the energy difference between reactants and products — whether the reaction releases or absorbs energy overall. Kinetic accessibility describes how easily the system can get from reactants to products, which depends on the activation energy (the height of the peak in the energy diagram). A reaction coordinate diagram for a slow exothermic reaction shows a deep drop from reactants to products (large negative ΔH), but a tall peak in between (large Ea). Molecules at room temperature lack sufficient thermal energy to surmount this barrier, so the reaction is negligible even though products are much lower in energy. The two quantities are independent: ΔH tells you where you end up; Ea tells you how hard it is to get there.