Match List-I with List-II
| List-I Reaction/ Constant | List-II Unit |
| (A) Zero order reaction | (I) mol-1 L s-1 |
| (B) First order reaction | (II) cal K-1 mol-1 |
| (C) R (Gas constant) | (III) mol L-1 s-1 |
| (D) Second Order reaction | (IV) s-1 |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) - (III), (B) - (IV), (C) - (II), (D) - (I)
The rate of a reaction is always in mol L−1 s−1. For a reaction of order n, the unit of the rate constant follows from the rate law:
\( \displaystyle k=\frac{\text{Rate}}{[A]^n} \quad\Rightarrow\quad \text{Unit of }k=\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{(\mathrm{mol\,L^{-1}})^n}=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)
(A) Zero order reaction — This matches (III) mol L-1 s-1. With n = 0, k has the same unit as the rate.
\( \displaystyle (\mathrm{mol\,L^{-1}})^{1}\,\mathrm{s^{-1}}=\mathrm{mol\,L^{-1}\,s^{-1}} \)
(B) First order reaction — This matches (IV) s-1. With n = 1, the concentration unit cancels.
\( \displaystyle (\mathrm{mol\,L^{-1}})^{0}\,\mathrm{s^{-1}}=\mathrm{s^{-1}} \)
(C) R (Gas constant) — This matches (II) cal K-1 mol-1. R is energy per kelvin per mole: R = 8.314 J K−1 mol−1 = 1.987 cal K−1 mol−1. It appears in the Arrhenius equation, k = A e−Ea/RT.
(D) Second Order reaction — This matches (I) mol-1 L s-1. With n = 2:
\( \displaystyle (\mathrm{mol\,L^{-1}})^{-1}\,\mathrm{s^{-1}}=\mathrm{mol^{-1}\,L\,s^{-1}} \)
Shortcut: The pair (C)–(II) appears in Options (2) and (4), and (A)–(III) then leaves only Option (2).