Rate law
Rate law is the expression in which reaction rate is given in terms of molar concentration of reactants, with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation. The sum of powers of the concentration of the reactants, in the rate law expression is called the order of that chemical reaction. The number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction, is called molecularity of a reaction.
The conversion of X to Y follows second order kinetics. If concentration of X is increased to three times, how much the rate of formation of Y increases?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 9 times
The rate law shows how the rate depends on concentration. For a reaction of order n with respect to X:
\( \displaystyle \text{Rate}=k[X]^{n} \)
If [X] is multiplied by a factor a, the rate is multiplied by an. With a = 3:
Zero order (n = 0): 30 = 1, the rate does not change.
First order (n = 1): 31 = 3, the rate becomes 3 times.
Second order (n = 2): 32 = 9, the rate becomes 9 times.
The conversion of X to Y is second order:
\( \displaystyle \text{Rate}_1=k[X]^2 \)
\( \displaystyle \text{Rate}_2=k(3[X])^2=9\,k[X]^2 \)
\( \displaystyle \frac{\text{Rate}_2}{\text{Rate}_1}=9 \)
The rate of formation of Y equals the rate of the reaction, so it increases 9 times.
Note 1: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27 edition):
Molecularity of a Reaction – “… The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y?”
Note 2: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 25-05 S2 | 14-05 S2 | – | – | – |