Consider the following statements about the units of the rate constant, k, when concentration is expressed in mol L−1 and time in seconds.
(A) For a zero order reaction, the units of k are the same as the units of the rate of the reaction.
(B) For a first order reaction, the units of k are s−1.
(C) For a reaction of order n, the units of k are (mol L−1)n−1 s−1.
(D) For a second order reaction, the units of k are mol−1 L s−1.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (D) only
For a reaction of order n, Rate = k(concentration)n, so the units of k are found by dividing the units of rate by (mol L−1)n:
\( \displaystyle k = \frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{(\mathrm{mol\,L^{-1}})^{n}} = (\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)
(A) For a zero order reaction, the units of k are the same as the units of the rate of the reaction. — True. With n = 0, k has the units mol L−1 s−1, which are the units of rate; for a zero order reaction, rate = k.
(B) For a first order reaction, the units of k are s−1. — True. With n = 1, the concentration units cancel and only s−1 is left.
(C) For a reaction of order n, the units of k are (mol L−1)n−1 s−1. — False. The power is 1 − n, not n − 1. For a zero order reaction the false form would give mol−1 L s−1 instead of mol L−1 s−1.
(D) For a second order reaction, the units of k are mol−1 L s−1. — True. With n = 2, (mol L−1)−1 s−1 = mol−1 L s−1.
NCERT (2026-27 edition):
3.1 Rate of a Chemical Reaction – “…if concentration is in mol L–1 and time is in seconds then the units will be mol L-1s–1.”
3.2.3 Order of a Reaction – “Taking SI units of concentration, mol L–1 and time, s, the units of k for different reaction order are listed in Table 3.3”
