The mechanism of a forward reaction is the reverse of the backward mechanism at equilibrium.
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The principle of microscopic reversibility states that at equilibrium every elementary process occurring in one direction is exactly balanced by the same process occurring in reverse. It follows that the mechanism of a reaction in the reverse direction must be the same as the forward mechanism traversed backwards, passing through the same intermediates and transition states. This principle follows from the time reversal symmetry of the underlying molecular dynamics. It explains why catalysts accelerate both directions of a reversible reaction through the same pathway. The principle is valuable for deducing mechanisms, since a mechanism proposed for one direction constrains the other.
Key ideas: Practical use: if the forward mechanism through a particular intermediate is known, the reverse takes the same path, so the same catalyst, same rate-determining step reversed and same transition state apply. The principle gives a rigorous basis for detailed balance, in which each elementary step is balanced at equilibrium, not just the overall cycle.
Common mistakes and connections: Students think the reverse reaction may use a different pathway with a lower barrier, which is not allowed at the same conditions, or that the principle only says forward and reverse rates are equal overall. It is more specific. A related mistake is expecting that a catalyst lowers only the forward barrier. Links to catalysts, relationships of k and K, and reversible reactions.
Ester hydrolysis and esterification go through the same tetrahedral intermediate; one is the exact reverse of the other. The interactive lesson shows this as a live animation or chart.
The hydrolysis of an ester and its esterification share the same tetrahedral intermediate, traversed in opposite directions.
The mechanism of a forward reaction is the reverse of the backward mechanism at equilibrium.
Principle of Microscopic Reversibility is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Kinetics & Equilibrium first: Equilibrium in Gas Phase, Degree of Dissociation, Relationship of Rate Constants and K.
The next topics in Kinetics & Equilibrium are Kinetic vs Thermodynamic Product and Hammond Postulate.
Yes. The interactive lesson includes a self-check question on Principle of Microscopic Reversibility, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Principle of Microscopic Reversibility is topic 70 of 91 in Kinetics & Equilibrium.