A derived overall rate law contains the concentration of a product species in its denominator. What does this inverse dependence indicate about the mechanism?
AAn equilibrium step precedes the rate determining step
BThe product catalyses its own formation in the mechanism
CThe overall reaction is zero order in every reactant
DThe rate determining step must be the first step listed
Answer & Solution
Correct answer: A. An equilibrium step precedes the rate determining step
1. When a fast equilibrium comes before the slow step, its intermediates cannot stay in the final rate law.
2. Each intermediate concentration is replaced by an algebraic expression built from the equilibrium.
3. Those expressions carry reverse reaction terms, and product concentrations can land in the denominator.
4. An inverse dependence on a product is therefore the fingerprint of an equilibrium step ahead of the rate determining step.
5. Extra product then slows the net forward rate by pushing the early equilibrium backwards.
6. If the slow step came first, the rate law would copy that step directly and no product term could appear.
7. A product that catalysed the reaction would speed it up, giving a positive dependence, not an inverse one.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 12 "Kinetics", section 12.6 Reaction Mechanisms_
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