---
title: "At a fixed temperature, a student studies the gas-phase oxidation of \\(\\text{NO(g)}\\) in an atmospheric-monitoring chamber.  \\[ 2\\,\\text{NO(g)}+\\text{O}_2\\text{(g)}\\rightarrow 2\\,\\text{NO}_2\\text{(g)} \\]  Initial-rate experiments give \\(\\text{rate}=k[\\text{NO}]^2[\\text{O}_2]\\). The student claims that the reaction must occur as a single elementary termolecular step because the rate-law exponents match the coefficients in the overall equation. Another student proposes the following mechanism.  Step \\(1\\), fast equilibrium: \\[ 2\\,\\text{NO(g)}\\rightleftharpoons \\text{N}_2\\text{O}_2\\text{(g)} \\]  Step \\(2\\), slow: \\[ \\text{N}_2\\text{O}_2\\text{(g)}+\\text{O}_2\\text{(g)}\\rightarrow 2\\,\\text{NO}_2\\text{(g)} \\]  Which statement best evaluates the first student’s conclusion and explains which pathway is statistically more plausible?"
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url: "https://nerd-notes.com/ubq/120044/"
date_modified: "2026-08-21T08:40:55+00:00"
---

# At a fixed temperature, a student studies the gas-phase oxidation of \(\text{NO(g)}\) in an atmospheric-monitoring chamber.

\[
2\,\text{NO(g)}+\text{O}_2\text{(g)}\rightarrow 2\,\text{NO}_2\text{(g)}
\]

Initial-rate experiments give \(\text{rate}=k[\text{NO}]^2[\text{O}_2]\). The student claims that the reaction must occur as a single elementary termolecular step because the rate-law exponents match the coefficients in the overall equation. Another student proposes the following mechanism.

Step \(1\), fast equilibrium:
\[
2\,\text{NO(g)}\rightleftharpoons \text{N}_2\text{O}_2\text{(g)}
\]

Step \(2\), slow:
\[
\text{N}_2\text{O}_2\text{(g)}+\text{O}_2\text{(g)}\rightarrow 2\,\text{NO}_2\text{(g)}
\]

Which statement best evaluates the first student’s conclusion and explains which pathway is statistically more plausible?

At a fixed temperature, a student studies the gas-phase oxidation of \(\text{NO(g)}\) in an atmospheric-monitoring chamber.

\[
2\,\text{NO(g)}+\text{O}_2\text{(g)}\rightarrow 2\,\text{NO}_2\text{(g)}
\]

Initial-rate experiments give \(\text{rate}=k[\text{NO}]^2[\text{O}_2]\). The student claims that the reaction must occur as a single elementary termolecular step because the rate-law exponents match the coefficients in the overall equation. Another student proposes the following mechanism.

Step \(1\), fast equilibrium:
\[
2\,\text{NO(g)}\rightleftharpoons \text{N}_2\text{O}_2\text{(g)}
\]

Step \(2\), slow:
\[
\text{N}_2\text{O}_2\text{(g)}+\text{O}_2\text{(g)}\rightarrow 2\,\text{NO}_2\text{(g)}
\]

Which statement best evaluates the first student's conclusion and explains which pathway is statistically more plausible?

- **A.** The data establish a single termolecular elementary step because coefficients in any balanced equation must become rate-law exponents; therefore, the matching exponents rule out every multistep mechanism.
- **B.** The data establish a single termolecular elementary step because one collision event is statistically more probable than a sequence of bimolecular collisions; therefore, the one-step pathway is favored over the proposed mechanism.
- **C.** The data do not establish a single elementary step because molecularity applies only to elementary steps; however, the proposed mechanism is inconsistent with the rate law because neither elementary step contains three reacting particles.
- **D.** The data do not establish a single elementary step because the fast pre-equilibrium can produce the observed rate law; additionally, sequential bimolecular encounters are more probable than one simultaneous, properly oriented termolecular encounter.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120044/*