Q.What is the rate determining step of a reaction?

Complex reactions occur via a multi-step mechanism, in contrast to simple/elementary reactions which occur in exactly one step as written.

Most real chemical reactions with more than 2-3 reacting species do NOT occur exactly as their overall balanced equation suggests. Instead they proceed through a series of simpler elementary steps, each involving only 1, 2 (or rarely 3) molecules colliding - this sequence of steps is called the reaction mechanism, and such a multi-step reaction is called a complex reaction.

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The slow (rate-determining) step gives Rate = k[O][O3]; substituting the O-atom concentration from the fast pre-equilibrium gives Rate = K[O3]^2[O2]^-1.

Mechanism:

• Step 1 (fast, equilibrium): O3 ⇌ O2 + O
• Step 2 (slow, rate-determining): O + O3 → 2O2

The overall rate is set by the slow step:

Rate = k2 [O][O3]

But [O] is a reactive intermediate; express it from the fast equilibrium. For step 1: …

The slowest step of a multistep reaction mechanism controls the overall rate and is called the rate-determining (rate-limiting) step.

A reaction usually proceeds through a sequence of elementary steps. The overall rate cannot be faster than its slowest step.

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In a multi-step reaction, the overall rate is governed by whichever elementary step is the slowest — this is the rate-determining step.

Most reactions occur through a sequence of elementary steps (the mechanism), not in a single step. Since the steps occur one after another, the overall reaction can proceed no faster than its slowest step. This slo …

When a reaction occurs via several elementary steps, the overall rate is dictated entirely by whichever single step is slowest.

Most real chemical reactions do not occur in a single elementary step but proceed through a sequence (mechanism) of several elementary steps, each with its own rate.

Among these steps, one is invariably much slower than all the others — this step acts as a 'bottleneck': no matter how fast the other steps are, the overall reaction cannot proceed any faster than this slowest step allows.

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