The order of a certain reaction with respect to \( Cl^- \) ion is -1. The \( Cl^- \) is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → An inhibitor
The order with respect to a species is the power of its concentration in the rate law. An order of −1 for \( \ce{Cl-} \) means (A stands for the other reactants)
\( \displaystyle \text{Rate} = k\,[\mathrm{A}]^{x}\,[\ce{Cl-}]^{-1} = \frac{k\,[\mathrm{A}]^{x}}{[\ce{Cl-}]} \)
So the rate is inversely proportional to the chloride ion concentration: doubling \( [\ce{Cl-}] \) halves the rate. A substance whose presence slows the reaction down is an inhibitor, so \( \ce{Cl-} \) acts as an inhibitor.
Why the other options are incorrect
Option (1) A catalyst — Incorrect. A catalyst speeds a reaction up; when it appears in the rate law, it has a positive order.
Option (2) A promotor — Incorrect. A promoter increases the activity of a catalyst, which again raises the rate, the opposite of a negative order.
Option (4) Always a product molecule — Incorrect. A negative order says only that the species slows the reaction; it may or may not be a product, so “always” does not follow.