**QUESTION**

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4. The rate law for the reaction \[ 2 \mathrm{H}_{2}(\mathrm{~g})+2 \mathrm{NO}(\mathrm{g}) \rightarrow \mathrm{N}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(g) \] is rate $=k\left[\mathrm{H}_{2}\right][\mathrm{NO}]^{2}$. Which of the following mechanisms can be ruled out on the basis of the observed rate expression? $[\mathbf{4} \mathrm{pts}]$ Mechanism I \[ \begin{array}{l} \mathrm{H}_{2}+\mathrm{NO} \rightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{N} \\ \mathrm{N}+\mathrm{NO} \rightarrow \mathrm{N}_{2}+\mathrm{O} \\ \mathrm{O}+\mathrm{H}_{2} \rightarrow \mathrm{H}_{2} \mathrm{O} \end{array} \] (slow) (fast) (fast) Mechanism II \[ \begin{array}{l} \mathrm{H}_{2}+2 \mathrm{NO} \rightarrow \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{O} \\ \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \rightarrow \mathrm{N}_{2}+\mathrm{H}_{2} \mathrm{O} \end{array} \] (slow) (fast) Mechanism III \[ \begin{array}{l} 2 \mathrm{NO} \longrightarrow \mathrm{N}_{2} \mathrm{O}_{2} \\ \mathrm{~N}_{2} \mathrm{O}_{2}+\mathrm{H}_{2} \rightarrow \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{O} \\ \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \rightarrow \mathrm{N}_{2}+\mathrm{H}_{2} \mathrm{O} \end{array} \] (fast equilibrium) (slow) (fast)