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title: "A proposed mechanism for the gas-phase reaction between \\(\\text{NO}(g)\\) and \\(\\text{Br}_2(g)\\) is shown below.  \\[ \\text{Step 1: } \\text{NO}(g) + \\text{Br}_2(g) \\rightleftharpoons \\text{NOBr}_2(g) \\quad (\\text{fast equilibrium}) \\] \\[ \\text{Step 2: } \\text{NOBr}_2(g) + \\text{NO}(g) \\rightarrow 2\\,\\text{NOBr}(g) \\quad (\\text{slow}) \\]  A mixture of \\(\\text{NO}(g)\\) and \\(\\text{Br}_2(g)\\) is placed in a reaction vessel fitted with a movable piston at a constant temperature. If the piston is pushed down so that the volume of the gas mixture decreases to \\(\\dfrac{1}{3}\\) of its initial volume, which of the following best predicts the effect on the initial rate of the reaction, and why?"
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url: "https://nerd-notes.com/ubq/123637/"
date_modified: "2026-09-28T12:01:52+00:00"
---

# A proposed mechanism for the gas-phase reaction between \(\text{NO}(g)\) and \(\text{Br}_2(g)\) is shown below.

\[ \text{Step 1: } \text{NO}(g) + \text{Br}_2(g) \rightleftharpoons \text{NOBr}_2(g) \quad (\text{fast equilibrium}) \]
\[ \text{Step 2: } \text{NOBr}_2(g) + \text{NO}(g) \rightarrow 2\,\text{NOBr}(g) \quad (\text{slow}) \]

A mixture of \(\text{NO}(g)\) and \(\text{Br}_2(g)\) is placed in a reaction vessel fitted with a movable piston at a constant temperature. If the piston is pushed down so that the volume of the gas mixture decreases to \(\dfrac{1}{3}\) of its initial volume, which of the following best predicts the effect on the initial rate of the reaction, and why?

A proposed mechanism for the gas-phase reaction between \(\text{NO}(g)\) and \(\text{Br}_2(g)\) is shown below.

\[ \text{Step 1: } \text{NO}(g) + \text{Br}_2(g) \rightleftharpoons \text{NOBr}_2(g) \quad (\text{fast equilibrium}) \]
\[ \text{Step 2: } \text{NOBr}_2(g) + \text{NO}(g) \rightarrow 2\,\text{NOBr}(g) \quad (\text{slow}) \]

A mixture of \(\text{NO}(g)\) and \(\text{Br}_2(g)\) is placed in a reaction vessel fitted with a movable piston at a constant temperature. If the piston is pushed down so that the volume of the gas mixture decreases to \(\dfrac{1}{3}\) of its initial volume, which of the following best predicts the effect on the initial rate of the reaction, and why?

- **A.** The rate will increase by a factor of \(27\) because the concentration of each gaseous reactant increases by a factor of \(3\) and the reaction is third order overall.
- **B.** The rate will increase by a factor of \(9\) because the concentration of each gaseous reactant increases by a factor of \(3\) and the reaction is second order overall.
- **C.** The rate will increase by a factor of \(3\) because decreasing the volume triples the total pressure, which increases the collision frequency between reactant molecules by a factor of \(3\).
- **D.** The rate will remain unchanged because the rate constant \(k\) depends only on temperature, which remains constant.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123637/*