Question

The rate law for the reaction
$2{{H}_{2(g)}}+2N{{O}_{(g)}}\to {{N}_{2(g)}}+2{{H}_{2}}{{O}_{(g)}}$ is given by rate=$K=[{{H}_{2}}]{{[NO]}^{2}}$
The reaction occurs in the following two steps:
(a) $2{{H}_{2(g)}}+2N{{O}_{(g)}}\to {{N}_{2}}{{O}_{(g)}}+{{H}_{2}}{{O}_{(g)}}$ (b) ${{H}_{2(g)}}+{{N}_{2}}{{O}_{(g)}}\to {{N}_{2}}_{(g)}+{{N}_{2}}{{O}_{(g)}}$
What is the rate of ${{N}_{2}}O$ in the mechanism? What is the molecularity of the elementary steps?

Answer 1

Hint: By the rate law we mean the molar concentrations of the reactants raised to the power of their stoichiometric coefficient and depends on the slowest and the rate determining step and the molecularity includes the total number of species reacting with each other to bring about the chemical reaction. Now answer the statement.

Complete step by step solution0:
First of all, let’s discuss what rate law is. By the rate law we mean the rate of reaction in terms of molar concentrations of reactants with each term raised to some power which may or may not be the same as the stoichiometric coefficient of the reacting species in a balanced chemical equation.
The given reaction i.e. $2{{H}_{2(g)}}+2N{{O}_{(g)}}\to {{N}_{2(g)}}+2{{H}_{2}}{{O}_{(g)}}$ occurs in two steps, out of which ;the reaction (a) $2{{H}_{2(g)}}+2N{{O}_{(g)}}\to {{N}_{2}}{{O}_{(g)}}+{{H}_{2}}{{O}_{(g)}}$is the slowest and rate determining step and reaction (b) ${{H}_{2(g)}}+{{N}_{2}}{{O}_{(g)}}\to {{N}_{2}}_{(g)}+{{N}_{2}}{{O}_{(g)}}$is a fast step.
So, the rate of ${{N}_{2}}O$ in the reaction (a) is as;
$Rate\text{ }of\text{ }{{N}_{2}}O={{[{{H}_{2}}]}^{2}}{{[NO]}^{2}}$
Since, it is the slowest step , so the rate of reaction depends on both the reactants.
On the other hand, molecularity of the reaction may be defined as the number of the reacting species i.e. Molecules or atoms or ions which collide simultaneously to bring about the collisions to bring out a chemical reaction is called molecularity of the reaction.
Now considering the statement;
The molecularity of the reaction is (a) i.e. $2{{H}_{2(g)}}+2N{{O}_{(g)}}\to {{N}_{2}}{{O}_{(g)}}+{{H}_{2}}{{O}_{(g)}}$ is three since it involves three species which are colliding with each other and hence, the reaction is also called as the trimolecular reaction.
On the other hand, the molecularity of reaction (b) i.e. ${{H}_{2(g)}}+{{N}_{2}}{{O}_{(g)}}\to {{N}_{2}}_{(g)}+{{N}_{2}}{{O}_{(g)}}$ is two since it involves two species reacting with each other and hence, the reaction is also called as the bimolecular reaction.

Note: Don’t get confused in the order and molecularity of the reaction. By the order of the reaction we mean the sum of the powers to which the concentration terms must be raised in the rate law to express the rate of reaction . On the other hand, by the term molecularity we mean the number of species involved that collide with each other to undergo the chemical reaction.