Problem 51
What is an elementary step? What is the molecularity of a reaction?
Problem 52
Classify the following elementary reactions as unimolecular, bimolecular, or termolecular: (a) \(2 \mathrm{NO}+\mathrm{Br}_{2} \longrightarrow 2 \mathrm{NOBr}\) (b) \(\mathrm{CH}_{3} \mathrm{NC} \longrightarrow \mathrm{CH}_{3} \mathrm{CN}\) (c) \(\mathrm{SO}+\mathrm{O}_{2} \longrightarrow \mathrm{SO}_{2}+\mathrm{O}\)
Problem 53
Reactions can be classified as unimolecular, bimolecular, and so on. Why are there no zero-molecular reactions? Explain why termolecular reactions are rare.
Problem 54
Determine the molecularity, and write the rate law for each of the following elementary steps: (a) \(\mathrm{X} \longrightarrow\) products (b) \(\mathrm{X}+\mathrm{Y} \longrightarrow\) products (c) \(\mathrm{X}+\mathrm{Y}+\mathrm{Z} \longrightarrow\) products (d) \(\mathrm{X}+\mathrm{X} \longrightarrow\) products (e) \(\mathrm{X}+2 \mathrm{Y} \longrightarrow\) products
Problem 55
What is the rate-determining step of a reaction? Give an everyday analogy to illustrate the meaning of rate determining.
Problem 56
The equation for the combustion of ethane \(\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)\) is: $$ 2 \mathrm{C}_{2} \mathrm{H}_{6}(g)+7 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) $$ Explain why it is unlikely that this equation also represents the elementary step for the reaction.
Problem 57
Specify which of the following species cannot be isolated in a reaction: activated complex, product, intermediate.
Problem 59
The rate law for the reaction: $$ 2 \mathrm{NO}(g)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{NOCl}(g) $$ is given by rate \(=k[\mathrm{NO}]\left[\mathrm{Cl}_{2}\right]\). (a) What is the order of the reaction? (b) A mechanism involving the following steps has been proposed for the reaction: $$ \begin{aligned} \mathrm{NO}(g)+\mathrm{Cl}_{2}(g) & \longrightarrow \mathrm{NOCl}_{2}(g) \\ \mathrm{NOCl}_{2}(g)+\mathrm{NO}(g) & \longrightarrow 2 \mathrm{NOCl}(g) \end{aligned} $$ If this mechanism is correct, what does it imply about the relative rates of these two steps?
Problem 60
For the reaction \(\mathrm{X}_{2}+\mathrm{Y}+\mathrm{Z} \longrightarrow \mathrm{XY}+\mathrm{XZ},\) it is found that doubling the concentration of \(\mathrm{X}_{2}\) doubles the reaction rate, tripling the concentration of \(Y\) triples the rate, and doubling the concentration of \(Z\) has no effect. (a) What is the rate law for this reaction? (b) Why is it that the change in the concentration of \(Z\) has no effect on the rate? (c) Suggest a mechanism for the reaction that is consistent with the rate law.
Problem 61
The rate law for the reaction: $$ 2 \mathrm{H}_{2}(g)+2 \mathrm{NO}(g) \longrightarrow \mathrm{N}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g) $$ is rate \(=k\left[\mathrm{H}_{2}\right][\mathrm{NO}]^{2}\). Which of the following mechanism can be ruled out on the basis of the observed rate expression? Mechanism I $$ \begin{array}{cc} \mathrm{H}_{2}+\mathrm{NO} \longrightarrow & \mathrm{H}_{2} \mathrm{O}+\mathrm{N} & (\text { slow }) \\ \mathrm{N}+\mathrm{NO} \longrightarrow & \mathrm{N}_{2}+\mathrm{O} & \text { (fast) } \\ \mathrm{O}+\mathrm{H}_{2} \longrightarrow & \mathrm{H}_{2} \mathrm{O} & \text { (fast) } \end{array} $$ Mechanism II $$ \begin{array}{ll} \mathrm{H}_{2}+2 \mathrm{NO} \longrightarrow \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{O} & \text { (slow) } \\ \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \longrightarrow \mathrm{N}_{2}+\mathrm{H}_{2} \mathrm{O} & \text { (fast) } \end{array} $$ Mechanism III $$ 2 \mathrm{NO} \rightleftarrows \mathrm{N}_{2} \mathrm{O}_{2} $$ (fast equilibrium) $$ \begin{array}{c} \mathrm{N}_{2} \mathrm{O}_{2}+\mathrm{H}_{2} \longrightarrow \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{O} & \text { (slow) } \\ \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \longrightarrow \mathrm{N}_{2}+\mathrm{H}_{2} \mathrm{O} & \text { (fast) } \end{array} $$
