Identify the order of reaction from each of the following units of rate constant $(k)$ :
(a) $m o l$ $L^{- 1}$ ${s e c}^{- 1}$
(b) ${m o l}^{- 1}$ $L$ ${s e c}^{- 1}$

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Solution

(a) The unit of the rate constant is $m o l L^{- 1} {s e c}^{- 1}$ or $M {s e c}^{- 1}$
rate $= k [A]^{n}$
$M {s e c}^{- 1} = M {s e c}^{- 1} [A]^{n}$
$M^{0} = [A]^{n}$
$n = 0$
Note: Since, the unit of the rate constant is same as the unit of rate, the order of the reaction is 0.
(b) The unit of the rate constant is ${m o l}^{- 1} L {s e c}^{- 1}$ or $M^{- 1} {s e c}^{- 1}$
rate $= k [A]^{n}$
$M {s e c}^{- 1} = M^{- 1} {s e c}^{- 1} [A]^{n}$
$M^{2} = [A]^{n}$
$n = 2$
The order of the reaction is 2.

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What do you understand by the 'order of a reaction'? Identify the reaction order from each of the following units of reaction rate constant:

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