When the concentration of $A$ in the reaction $A + B \rightleftharpoons AB$ is doubled,the rate of reaction will be

In the reaction sequence $A$ $\xrightarrow{K_1} B$ $\xrightarrow{K_2} C$ $\xrightarrow{K_3} D$,where $K_3 > K_2 > K_1$,which step determines the rate of the reaction?

For a reaction,$A + B \rightarrow$ products,the rate of the reaction at various concentrations is given below. The rate law for the above reaction is:

Expt. no.	$[A]$	$[B]$	Rate $(\text{mol} \ \text{dm}^{-3} \ \text{s}^{-1})$
$1$	$0.2$	$0.2$	$2$
$2$	$0.2$	$0.4$	$4$
$3$	$0.6$	$0.4$	$36$

The mechanism of the reaction,$2NO_{(g)} + 2H_{2(g)} \to N_{2(g)} + 2H_2O_{(g)}$ is:
Step $1$: $2NO_{(g)} + H_{2(g)} \xrightarrow{\text{slow}} N_2 + H_2O_2$
Step $2$: $H_2O_2 + H_2 \xrightarrow{\text{fast}} 2H_2O$
Then the correct statement is:

Why is molecularity applicable only for elementary reactions,while order is applicable for both elementary and complex reactions?

Identify the rate law expression for the reaction $2 NO_{(g)} + Cl_{2(g)} \rightarrow 2 NOCl_{(g)}$ if the reaction is second order in $NO$ and first order in $Cl_2$.