Question

A complex reaction takes place in following steps:
$NO_{2}Cl_{(g)} \longrightarrow NO_{2(g)} + Cl_{(g)}$ (slow)
$NO_{2}Cl_{(g)} + Cl_{(g)} \longrightarrow NO_{2(g)} + Cl_{2(g)}$ (fast)
Identify rate law equation for this reaction.

• A. $r=k[NO_{2}Cl]$
• B. $r=k[NO_{2}][Cl]$
• C. $r=k[NO_{2}]^{2}$
• D. $r=k[NO_{2}Cl]^{2}$

Hint:

Rate is always determined by the slowest step of the mechanism.

Answer 1

The Correct Option is A

Step 1: Concept
In a multi-step (complex) reaction, the rate-determining step is the slowest step.

Step 2: Meaning
The rate law is derived directly from the stoichiometry of the reactants in the slow step.

Step 3: Analysis
- Slow step: $NO_{2}Cl \longrightarrow NO_{2} + Cl$. - The reactant is $NO_{2}Cl$ with a coefficient of 1.

Step 4: Conclusion
Hence, the rate law is $r = k[NO_{2}Cl]$. Final Answer: (A)