AP Chemistry
5.8 Reaction Mechanism and Rate Law
5.7 Introduction to Reaction Mechanisms
A student investigates the gas-phase reaction represented by the following balanced equation:
\[ \text{NO}_2\text{(g)} + \text{CO(g)} \rightarrow \text{NO(g)} + \text{CO}_2\text{(g)} \]
Experimental initial-rate data show that the reaction is second order with respect to \(\text{NO}_2\) and zero order with respect to \(\text{CO}\), giving the rate law \(\text{Rate} = k[\text{NO}_2]^2\). Two proposed reaction mechanisms are shown below.
Mechanism I:
Step 1: \(\text{NO}_2\text{(g)} + \text{NO}_2\text{(g)} \rightarrow \text{NO}_3\text{(g)} + \text{NO(g)}\) (slow)
Step 2: \(\text{NO}_3\text{(g)} + \text{CO(g)} \rightarrow \text{NO}_2\text{(g)} + \text{CO}_2\text{(g)}\) (fast)
Mechanism II:
Step 1: \(\text{NO}_2\text{(g)} + \text{CO(g)} \rightarrow \text{NO}_2\text{CO(g)}\) (slow)
Step 2: \(\text{NO}_2\text{CO(g)} \rightarrow \text{NO(g)} + \text{CO}_2\text{(g)}\) (fast)
Which mechanism is consistent with the experimental rate law, and why?
\[ \text{NO}_2\text{(g)} + \text{CO(g)} \rightarrow \text{NO(g)} + \text{CO}_2\text{(g)} \]
Experimental initial-rate data show that the reaction is second order with respect to \(\text{NO}_2\) and zero order with respect to \(\text{CO}\), giving the rate law \(\text{Rate} = k[\text{NO}_2]^2\). Two proposed reaction mechanisms are shown below.
Mechanism I:
Step 1: \(\text{NO}_2\text{(g)} + \text{NO}_2\text{(g)} \rightarrow \text{NO}_3\text{(g)} + \text{NO(g)}\) (slow)
Step 2: \(\text{NO}_3\text{(g)} + \text{CO(g)} \rightarrow \text{NO}_2\text{(g)} + \text{CO}_2\text{(g)}\) (fast)
Mechanism II:
Step 1: \(\text{NO}_2\text{(g)} + \text{CO(g)} \rightarrow \text{NO}_2\text{CO(g)}\) (slow)
Step 2: \(\text{NO}_2\text{CO(g)} \rightarrow \text{NO(g)} + \text{CO}_2\text{(g)}\) (fast)
Which mechanism is consistent with the experimental rate law, and why?
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