Question

The units of first order reaction :

• A. s$^{-1}$
• B. s
• C. mol L$^{-1}$
• D. L$^{-1}$ s

Hint:

The unit of the rate constant for a first-order reaction is strictly time$^{-1}$ and is independent of concentration units.

Answer 1

The Correct Option is A

For a reaction of $n$-th order, the rate law is Rate $= k[A]^n$.
The units of the rate constant $k$ are given by the general formula $(\text{mol L}^{-1})^{1-n} \text{s}^{-1}$.
For a first order reaction, $n = 1$.
Substituting $n=1$ into the formula: $(\text{mol L}^{-1})^{1-1} \text{s}^{-1} = (\text{mol L}^{-1})^0 \text{s}^{-1} = \text{s}^{-1}$.
Alternatively, since Rate (mol L$^{-1}$ s$^{-1}$) $= k \times$ Concentration (mol L$^{-1}$), $k$ must have units of s$^{-1}$.