In the rate law Rate = k[A]x[B]y for a reaction between A and B, what do the exponents x and y indicate?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → How sensitive the rate is to the change in concentration of A and B
The exponents in a rate law are found by experiment. Each exponent tells how strongly the rate responds when that concentration is changed: with x = 2, doubling [A] makes the rate four times; with y = 0, changing [B] has no effect. x and y are the orders with respect to A and B.
Why the other options are incorrect
Option (1) The stoichiometric coefficients of A and B in the balanced chemical equation — Incorrect. The exponents may or may not equal the coefficients; in CHCl3 + Cl2 the power of [Cl2] is 1/2.
Option (2) The numbers of molecules of A and B that collide in the slowest step — Incorrect. The exponents come from measured rates, not from counting colliding molecules; they can even be fractions.
Option (3) The molecularity of the reaction with respect to A and B — Incorrect. Molecularity is the number of reacting species in an elementary reaction; the exponents of the rate law give the order.
NCERT (2026-27 edition):
3.2.3 Order of a Reaction – “In the rate equation (3.4) Rate = k [A]x [B]y x and y indicate how sensitive the rate is to the change in concentration of A and B.”