Reaction Mechanisms

Practice Problems

Problem 1

Given the following information, determine if the reaction mechanism is plausible as written. If it isn’t, derive a rate law that agrees with experiment.

Overall reaction: 2H2(g) + 2NO(g) ⟶ 2H2O(g) + N2(g)

Step 1: 2NO(g) $$\underset{k_{-1}}{\stackrel{k_1}{\rightleftharpoons}}$$ N2O2(g)

Step 2: H2(g) + N2O2(g) $$\overset{k_2}{\longrightarrow}$$ H2O(g) + N2O(g)

Step 3: N2O(g) + H2(g) $$\overset{k_3}{\longrightarrow}$$ N2(g) + H2O(g)

Experimental rate law: rate = k[H2][NO]2

Predicted rate law: rate = k2[H2][N2O2]

Solution

Step 1 reaches equilibrium: rate$$_{\mathrm{f}}$$ = rate$$_{\mathrm{r}}$$

Set forward and reverse rates equal: \begin{align*} \mathrm{rate} = k_1[\mathrm{NO}]^2 \qquad \mathrm{rate} = k_{-1}[\mathrm{N_2O_2}] ~~~~~~ \longrightarrow ~~~~~~ k_1[\mathrm{NO}]^2 = k_{-1}[\mathrm{N_2O_2}] \end{align*} Isolate the intermediate, N$$_2$$O$$_2$$: \begin{align*} \mathrm{N_2O_2} = \dfrac{k_1}{k_{-1}}[\mathrm{NO}]^2 \end{align*} Substitute the previous expression into the predicted rate law: \begin{align*} \mathrm{rate} &= k_2[\mathrm{H_2}][\mathrm{N_2O_2}] \\ &= k_2[\mathrm{H_2}]\dfrac{k_1}{k_{-1}}[\mathrm{NO}]^2 \\ &= \dfrac{k_2k_1}{k_{-1}} [\mathrm{H_2}][\mathrm{NO}]^2 \\ &= k[\mathrm{H_2}][\mathrm{NO}]^2 \end{align*}

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