Concept:The order of a chemical reaction is experimentally determined. Some classic gas-phase reactions are rigorously established in chemical literature as following third-order kinetics.
Formula:$$ 2\text{NO}_{(g)} + \text{O}_{2(g)} \longrightarrow 2\text{NO}_{2(g)} $$
Solution:- The gas-phase oxidation of nitric oxide (NO) by oxygen is a well-known example of a third-order reaction.
- Experimental data yields the rate law: \( \text{Rate} = k[\text{NO}]^2[\text{O}_2]^1 \).
- The sum of the exponents (2 + 1) equals 3, confirming it follows third-order kinetics overall.
Why other options are incorrect:Decomposition of \( \text{HI} \) and formation of \( \text{HI} \) are standard second-order reactions. Decomposition of \( \text{N}_2\text{O}_5 \) is first-order.
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