Question

Which of the following statement(s) about zero order reaction is/are not true?

• A. Unit of rate constant is $mol{L}^{-1}{s}^{-1}$
• B. The graph between log [A] versus time is a straight line.
  (where [A] is concentration of reactant)
• C. The rate of reaction increases with the increase in concentration of reactant.
• D. Rate of reaction is independent of concentration of reactant.

Answer 1

Answer: (B), (C)

The rate of reaction increases with the increase in concentration of reactant.

Let us consider a zero order reaction:

$A\to B$
Rate Law for Zero Order Reaction is given as:
$\text{Rate,}R=K[A{]}^0$
$-\frac{d\left[A\right]}{dt}=K[A{]}^0$
$-\frac{d\left[A\right]}{dt}=K\times 1$
$-d\left[A\right]=Kdt$
Taking limit on both side
$-\underset{[A{]}_0}{\overset{[A{]}_t}{\int }}d\left[A\right]=K\underset{0}{\overset{t}{\int }}dt$

when,
$time=0,A=[A{]}_o$
$time=t,A=[A{]}_t$

$-[A{]}_{[A{]}_o}^{[A{]}_t}=K[t{]}_0^t$

$[A{]}_0-[A{]}_t=Kt$

$[A{]}_t=[A{]}_0-Kt.......eqn\left(1\right)$
where $[A{]}_0$ is the initial concentration of Reactant A.
$K=\frac{[A{]}_0-[A{]}_t}{t}$

Unit of K for zero order reaction is,
$mol{L}^{-1}{s}^{-1}$

Taking log in the equation (1), we get,
$\log \left[A\right]=\log \left[A_0\right]-\log \left[kt\right]......\left(2\right)$
$\because$ Equation (2) is not in the form of
$log\left[A\right]=mt+c$.
$\therefore$ The graph between $log\left[\text{Reactant}\right]$ versus time (t) is not a straight line.

For zero order reaction,
$\text{Rate,}R=K\left(\text{Rate constant}\right)$
$\therefore$
Rate of Zero order reaction is equal to Rate constant of that reaction, which is independent of concentration. Hence, on increasing concentration of reactants, rate of reaction does not increase.
Plot of Rate Vs Concentration: