AP Chemistry
5.9 Pre-Equilibrium Approximation
5.8 Reaction Mechanism and Rate Law
5.2 Introduction to Rate Law
A student investigates the kinetics of the reaction represented by the balanced equation below.
\[ 2\text{ NO}(g) + \text{Br}_2(g) \rightarrow 2\text{ NOBr}(g) \]
To determine the experimental rate law, the student measures the initial rate of formation of \(\text{NOBr}(g)\) across three trials at \(298\text{ K}\), collecting the data shown in the following table.
Three different reaction mechanisms are proposed:
Mechanism I
Step 1: \(\text{NO}(g) + \text{Br}_2(g) \rightarrow \text{NOBr}_2(g)\) (slow)
Step 2: \(\text{NOBr}_2(g) + \text{NO}(g) \rightarrow 2\text{ NOBr}(g)\) (fast)
Mechanism II
Step 1: \(\text{NO}(g) + \text{Br}_2(g) \rightleftharpoons \text{NOBr}_2(g)\) (fast equilibrium)
Step 2: \(\text{NOBr}_2(g) + \text{NO}(g) \rightarrow 2\text{ NOBr}(g)\) (slow)
Mechanism III
Step 1: \(\text{Br}_2(g) \rightleftharpoons 2\text{ Br}(g)\) (fast equilibrium)
Step 2: \(\text{NO}(g) + \text{Br}(g) \rightarrow \text{NOBr}(g)\) (slow)
Based on the experimental data, which mechanism is consistent with the observed rate law, and why?
\[ 2\text{ NO}(g) + \text{Br}_2(g) \rightarrow 2\text{ NOBr}(g) \]
To determine the experimental rate law, the student measures the initial rate of formation of \(\text{NOBr}(g)\) across three trials at \(298\text{ K}\), collecting the data shown in the following table.
| Trial | Initial \([\text{NO}]\) (\(\text{M}\)) | Initial \([\text{Br}_2]\) (\(\text{M}\)) | Initial Rate of Formation of \(\text{NOBr}\) (\(\text{M}\cdot\text{s}^{-1}\)) |
|---|---|---|---|
| \(1\) | \(0.10\) | \(0.10\) | \(1.2 \times 10^{-3}\) |
| \(2\) | \(0.20\) | \(0.10\) | \(4.8 \times 10^{-3}\) |
| \(3\) | \(0.20\) | \(0.20\) | \(9.6 \times 10^{-3}\) |
Three different reaction mechanisms are proposed:
Mechanism I
Step 1: \(\text{NO}(g) + \text{Br}_2(g) \rightarrow \text{NOBr}_2(g)\) (slow)
Step 2: \(\text{NOBr}_2(g) + \text{NO}(g) \rightarrow 2\text{ NOBr}(g)\) (fast)
Mechanism II
Step 1: \(\text{NO}(g) + \text{Br}_2(g) \rightleftharpoons \text{NOBr}_2(g)\) (fast equilibrium)
Step 2: \(\text{NOBr}_2(g) + \text{NO}(g) \rightarrow 2\text{ NOBr}(g)\) (slow)
Mechanism III
Step 1: \(\text{Br}_2(g) \rightleftharpoons 2\text{ Br}(g)\) (fast equilibrium)
Step 2: \(\text{NO}(g) + \text{Br}(g) \rightarrow \text{NOBr}(g)\) (slow)
Based on the experimental data, which mechanism is consistent with the observed rate law, and why?
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