Question

Define zero order reaction. A reaction takes place in two steps : (i) NO(g) + Cl\(_2\)(g) \( \rightarrow \) NOCl\(_2\)(g) (ii) NOCl\(_2\)(g) + NO(g) \( \rightarrow \) 2NOCl(g). Write the overall reaction and identify the reaction intermediate.

Hint:

Intermediates are transient species that are part of the reaction mechanism but are not final products. Catalysts, on the other hand, are consumed in one step and regenerated in a later step, so they also don't appear in the overall equation but are present at the beginning and end.

Answer 1

Definition: Zero-Order Reaction

A zero-order reaction is a chemical reaction wherein the rate of reaction does not depend on the concentration of the reactants. The rate remains constant over time.

\[ \text{Rate} = k[\text{Reactant}]^0 = k \] where \(k\) is the rate constant.

Reaction Analysis

Step 1: Write the overall reaction.

To get the overall reaction, add the elementary steps and cancel out species that appear on both sides.

\[ \begin{aligned} \text{Step 1: } & NO(g) + Cl_2(g) \;\;\rightarrow\;\; NOCl_2(g) \\ \text{Step 2: } & NOCl_2(g) + NO(g) \;\;\rightarrow\;\; 2NOCl(g) \\ \hline \text{Overall: } & 2NO(g) + Cl_2(g) \;\;\rightarrow\;\; 2NOCl(g) \end{aligned} \]

Step 2: Identify the reaction intermediate.

A reaction intermediate is a species that is formed in one step of a reaction and consumed in a subsequent step. It does not appear in the overall balanced equation.

In this mechanism, NOCl\(_2\)(g) is formed in Step 1 and consumed in Step 2. Therefore, NOCl\(_2\) is the reaction intermediate.