Chemistry Reaction Kinetics MDCAT 2014
PMDC Verified Question 72 of 80
For the reaction, \( 2\text{NO} + \text{O}_{2} \rightleftharpoons 2\text{NO}_{2} \), the rate equation for the forward reaction is:
A
Rate = \( k[\text{NO}][\text{O}_{2}] \)
B
Rate = \( k[\text{NO}_{2}]^{2} \)
C
Rate = \( k[\text{NO}_{2}] \)
D
Rate = \( k[\text{NO}]^{2}[\text{O}_{2}] \)
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Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: Rate = \( k[\text{NO}]^{2}[\text{O}_{2}] \)
Concept:

The rate law is determined experimentally. For the gas-phase oxidation of nitric oxide, it follows third-order kinetics overall.

Formula:

$$ \text{Rate} = k[\text{NO}]^{2}[\text{O}_{2}] $$

Solution:

  • Experimental data shows the reaction is second order with respect to \( \text{NO} \).
  • It is first order with respect to \( \text{O}_{2} \).
  • Combining these gives the overall rate law: \( \text{Rate} = k[\text{NO}]^{2}[\text{O}_{2}] \).


Why other options are incorrect:

The other expressions do not match the experimentally determined orders of the reactants. Option B represents the reverse reaction rate, not the forward.

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