Reaction Energy ProfileJEE Main

Concept

An energy profile plots energy along the reaction coordinate. Reactants must climb the activation barrier $E_a$ through a transition state before settling as products. The product level relative to reactants is $\Delta H$.

Key formula

Derivation

The forward barrier is $E_a$; the reverse barrier is measured from the product side, so $E_a^{rev} = E_a^{fwd} - \Delta H$. For an exothermic reaction ($\Delta H < 0$) this makes the reverse barrier larger than the forward one.

A catalyst provides an alternate path with a lower $E_a$ (here ~45% lower) but leaves $\Delta H$ untouched. A two-step mechanism (like SN1) shows two humps with a reactive intermediate (carbocation) in the dip between them; SN2 has a single hump.

Scenarios to explore

• Reaction Energy Profile — Activation energy, ΔH, catalysis & SN1/SN2 paths.

Real-world applications

• Comparing SN1 vs SN2 and E1 vs E2 mechanisms.
• Understanding enzyme and metal catalysis.
• Linking kinetics (barrier) to thermodynamics (ΔH).

JEE exam tips

• The rate-determining step is the one with the highest transition state.
• Exothermic + low $E_a$ ⇒ fast and favourable; the tallest peak controls the rate.

Common mistakes

• Thinking a catalyst changes $\Delta H$ or the equilibrium position — it does not.
• Confusing the transition state (a maximum) with an intermediate (a local minimum).
• Reading $E_a$ from the product side instead of the reactant side.

Exam traps to avoid

• A catalyst speeds up both forward and reverse rates equally.
• An intermediate (SN1 carbocation) can be isolated in principle; a transition state cannot.