2.2.6 (HL)—Reaction mechanisms

A mechanism is a sequence of elementary steps. The slowest step is the rate-determining step; an intermediate is formed in one step and consumed in a later step, whereas a transition state is the high-energy configuration at a barrier.

A proposed mechanism must be compared with the experimental rate equation and stoichiometry. Matching a rate equation can support a mechanism but does not prove it, because different mechanisms may give the same expression.

Add all elementary steps and cancel intermediates to recover the overall equation. Then derive the rate dependence expected from the slow step, eliminating an intermediate when necessary, and compare with experiment. A catalyst consumed early and regenerated later cancels from the overall equation but is not an intermediate.

Representative question

[Maximum number: 1]

Suggest why experimental confirmation of the rate equation would not prove that the mechanism is correct.

Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.

Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.

2.2.6 (HL)—Reaction mechanisms

Reaction Mechanisms

Evaluating a Proposed Mechanism

Assessment in practice

Rate and Mechanisms Summary

Question 1