Question

For a reaction: $H_2+C{l}_2\stackrel{hv}{\to }2HClRate=k$
(i) Write the order and molecularity of this reaction.
(ii) Write the unit of k.

Answer 1

Order of a reaction: Order of a reaction is sum of the powers to which the concentration (pressure) terms are raised in the rate- law expression.

Molecularity of a reaction: It is defined as the number of molecules involved in the step leading to the chemical reaction.

Given reaction is:-

$H_2+C{l}_2\underrightarrow{hv}2HCl$

& $Rate=K$

Since the rate of the reaction is independent of the concentration of reactants, so it is a zero order reaction.

Now, No. of molecules involved in the reaction is=$2$

(One molecule of each of the reactants i.e. $H_2$ & $C{l}_2$)

So, as per definition,

Molecularity of the reaction is $2$.

Now, $[\because ]$ Rate=K (given)

So, a unit of $K$ will be the same as the rate of the reaction, i.e., a unit of $K$ is $mol{L}^{-1}{s}^{-1}$