Read the passage carefully and answer the questions based on the passage:
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. In the rate equation Rate = \( k [A]^x [B]^y \)
\( x \) and \( y \) indicate how sensitive the rate is to the change in concentration of A and B, respectively. Sum of these exponents, i.e., \( x + y \) gives the overall order of a reaction where \( x \) and \( y \) represent the order with respect to the reactants A and B, respectively. Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. For a first order reaction, the concentration of the reactant varies as \( [R] = [R]_0e^{-kt} \)
The units of first and zero order rate constant of reactions are respectively:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(s^{-1}, Ms^{-1}\)
The unit of a rate constant follows from the rate law. The rate has the unit mol L−1 s−1 (M s−1) for every reaction, so for a reaction of order n:
\( \displaystyle k = \frac{\text{rate}}{[\mathrm{R}]^{n}} \)
For a first order reaction, \( n = 1 \):
\( \displaystyle \text{unit of } k = \frac{\mathrm{M\,s^{-1}}}{\mathrm{M}} = \mathrm{s^{-1}} \)
For a zero order reaction, \( n = 0 \):
\( \displaystyle \text{unit of } k = \frac{\mathrm{M\,s^{-1}}}{\mathrm{M}^{0}} = \mathrm{M\,s^{-1}} \)
So the units are s−1 and M s−1, in that order.
Shortcut: The unit of k for order n is \( \mathrm{M^{1-n}\,s^{-1}} \): \( \mathrm{s^{-1}} \) for n = 1 and \( \mathrm{M\,s^{-1}} \) for n = 0. Every rate constant has s−1 in it, which rules out Options (3) and (4) at once.
NCERT (2026-27 edition):
Order of a Reaction – …
