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Which of the following statements is INCORRECT regarding order and molecularity of a reaction?

1. Order of a reaction is an experimental quantity; it can be zero or a fraction, but molecularity cannot be zero or a non-integer.
2. Order is applicable to both elementary and complex reactions, whereas molecularity is meaningful only for elementary steps.
3. For a complex reaction, the overall order may be the same as the molecularity of the rate-determining step.
4. For a complex reaction, order is determined by the overall balanced equation, and the molecularity of the equilibrium step must equal the overall order.

Correct answer: For a complex reaction, order is determined by the overall balanced equation, and the molecularity of the equilibrium step must equal the overall order.

Solution

For complex reactions, the order is determined by the rate-determining step (RDS) and experimental data, NOT by the overall balanced equation. The molecularity of an equilibrium step (pre-equilibrium) is unrelated to the overall order. Statement (D) is incorrect on both counts.

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