Question

Consider the reaction, $2A+B\to$ Products When concentration of B alone was doubled, the half life did not change. When the concentration of A alone was doubled, the rate increased by two times. The unit of rate constant for this reaction is

• A. no unit
• B. $mol{L}^{-1}{s}^{-1}$
• C. $s^{-1}$
• D. $Lmo{l}^{-1}{s}^{-1}$

Answer 1

The correct option is D $Lmo{l}^{-1}{s}^{-1}$

Explanation : Since the half life does not change on changing the concentration of B, so it is first order wrt B.

i.e. $r\propto [B{]}^1$

When the concentration of A was doubled, the rate becomes double $\therefore r\propto [A{]}^1$

The rate equation is $r=k\left[A{]}^{\prime }\right[B{]}^1$

Order of reaction = $1+1=2$

Unit of $k=\frac{1}{tme}.\frac{1}{[conc.{]}^{n-1}}$ where n is order

$k=\frac{1}{s}.\frac{1}{[mol{L}^{-1}{]}^{2-1}}=\frac{1}{S}\times \frac{1}{\left(mol{L}^{-1}\right)}$

$=Lmo{l}^{-1}{s}^{-1}$