---
title: "A proposed mechanism for the gas-phase reaction \\(2\\text{NO}(g) + \\text{O}_2(g) \\rightarrow 2\\text{NO}_2(g)\\) consists of an initial fast equilibrium followed by a slow elementary step:  \\[ \\begin{aligned} \\text{Step 1: } & 2\\text{NO}(g) \\rightleftharpoons \\text{N}_2\\text{O}_2(g) & & (\\text{fast equilibrium, } \\Delta H_1^\\circ < 0) \\\\ \\text{Step 2: } & \\text{N}_2\\text{O}_2(g) + \\text{O}_2(g) \\rightarrow 2\\text{NO}_2(g) & & (\\text{slow, rate constant } k_2) \\end{aligned} \\]  Under constant initial concentrations of \\(\\text{NO}(g)\\) and \\(\\text{O}_2(g)\\), the initial rate of formation of \\(\\text{NO}_2(g)\\) is observed to decrease as the temperature is increased. Which of the following best accounts for this observed outcome in terms of the elementary rate constant \\(k_2\\) and the equilibrium intermediate concentration \\([\\text{N}_2\\text{O}_2]\\)?"
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url: "https://nerd-notes.com/ubq/123663/"
date_modified: "2026-09-28T12:01:56+00:00"
---

# A proposed mechanism for the gas-phase reaction \(2\text{NO}(g) + \text{O}_2(g) \rightarrow 2\text{NO}_2(g)\) consists of an initial fast equilibrium followed by a slow elementary step:

\[
\begin{aligned}
\text{Step 1: } & 2\text{NO}(g) \rightleftharpoons \text{N}_2\text{O}_2(g) & & (\text{fast equilibrium, } \Delta H_1^\circ < 0) \\
\text{Step 2: } & \text{N}_2\text{O}_2(g) + \text{O}_2(g) \rightarrow 2\text{NO}_2(g) & & (\text{slow, rate constant } k_2)
\end{aligned}
\]

Under constant initial concentrations of \(\text{NO}(g)\) and \(\text{O}_2(g)\), the initial rate of formation of \(\text{NO}_2(g)\) is observed to decrease as the temperature is increased. Which of the following best accounts for this observed outcome in terms of the elementary rate constant \(k_2\) and the equilibrium intermediate concentration \([\text{N}_2\text{O}_2]\)?

A proposed mechanism for the gas-phase reaction \(2\text{NO}(g) + \text{O}_2(g) \rightarrow 2\text{NO}_2(g)\) consists of an initial fast equilibrium followed by a slow elementary step:

\[
\begin{aligned}
\text{Step 1: } & 2\text{NO}(g) \rightleftharpoons \text{N}_2\text{O}_2(g) & & (\text{fast equilibrium, } \Delta H_1^\circ < 0) \\
\text{Step 2: } & \text{N}_2\text{O}_2(g) + \text{O}_2(g) \rightarrow 2\text{NO}_2(g) & & (\text{slow, rate constant } k_2)
\end{aligned}
\]

Under constant initial concentrations of \(\text{NO}(g)\) and \(\text{O}_2(g)\), the initial rate of formation of \(\text{NO}_2(g)\) is observed to decrease as the temperature is increased. Which of the following best accounts for this observed outcome in terms of the elementary rate constant \(k_2\) and the equilibrium intermediate concentration \([\text{N}_2\text{O}_2]\)?

- **A.** \(k_2\) decreases because higher thermal speeds reduce the fraction of collisions with favorable orientation, while \([\text{N}_2\text{O}_2]\) remains unchanged.
- **B.** \(k_2\) decreases because the overall reaction is exothermic, while \([\text{N}_2\text{O}_2]\) increases as the forward reaction in Step 1 is favored.
- **C.** \(k_2\) increases because a larger fraction of collisions possess sufficient energy, but \([\text{N}_2\text{O}_2]\) increases by a smaller factor that fails to accelerate the rate.
- **D.** \(k_2\) increases because a larger fraction of collisions possess sufficient energy, but \([\text{N}_2\text{O}_2]\) decreases by a factor that outweighs the increase in \(k_2\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123663/*