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Chapter 14: Problem 66

(a) Can an intermediate appear as a reactant in the first step of a reaction mechanism? (b) On a reaction energy profile diagram, is an intermediate represented as a peak or a valley? (c) If a molecule like \(\mathrm{Cl}_{2}\) falls apart in an elementary reaction, what is the molecularity of the reaction?

Short Answer

Expert verified
(a) No, an intermediate cannot appear as a reactant in the first step of a reaction mechanism, as it is formed and consumed during the reaction process. (b) An intermediate is represented as a valley on a reaction energy profile diagram. (c) The molecularity of the elementary reaction where \(\mathrm{Cl}_{2}\) falls apart is 1, making it a unimolecular reaction.

Step by step solution

01

Answer to part (a)

In a reaction mechanism, an intermediate is a species that is formed and consumed during the sequence of steps. It is important to note that intermediates are not present at the beginning or the end of the reaction. As such, it is not possible for an intermediate to appear as a reactant in the first step of a reaction mechanism because intermediate species must be formed from the reactants during the reaction process.
02

Answer to part (b)

On a reaction energy profile diagram, the progress of the reaction is represented along the horizontal axis, and the reaction energy is represented along the vertical axis. Transition states, which are the highest energy states along the reaction pathway, are represented as peaks, while reactants, products, and intermediates are represented as valleys. Therefore, an intermediate is represented as a valley on a reaction energy profile diagram.
03

Answer to part (c)

The molecularity of a reaction refers to the number of species involved in an elementary reaction step. In the case of a molecule like \(\mathrm{Cl}_{2}\) falling apart in an elementary reaction, the reaction can be represented as: \[ \mathrm{Cl}_{2} \rightarrow 2\,\mathrm{Cl} \] In this elementary reaction, only one molecule, \(\mathrm{Cl}_{2}\), is involved in the reaction process. As such, the molecularity of this reaction is 1, which means it is a unimolecular reaction.

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Most popular questions from this chapter

The following mechanism has been proposed for the gasphase reaction of \(\mathrm{H}_{2}\) with ICl: $$ \begin{array}{l} \mathrm{H}_{2}(g)+\mathrm{ICl}(g) \longrightarrow \mathrm{HI}(g)+\mathrm{HCl}(g) \\ \mathrm{HI}(g)+\mathrm{ICl}(g) \longrightarrow \mathrm{I}_{2}(g)+\mathrm{HCl}(g) \end{array} $$ (a) Write the balanced equation for the overall reaction. (b) Identify any intermediates in the mechanism. (c) If the first step is slow and the second one is fast, which rate law do you expect to be observed for the overall reaction?

The human body is characterized by an extremely complex system of interrelated chemical reactions. A large number of enzymes are necessary for many of these reactions to occur at suitable rates. Enzymes are very selective in the reactions they catalyze, and some are absolutely specific. Use the lock-and- key model to account for the specificity of an enzyme.

Cobalt-60 is used in radiation therapy to treat cancer. It has a first-order rate constant for radioactive decay of $k=1.31 \times 10^{-1} \mathrm{yr}^{-1}$. Another radioactive isotope, iron59, which is used as a tracer in the study of iron metabolism, has a rate constant of $k=1.55 \times 10^{-2}\( day \)^{-1}$. (a) What are the half-lives of these two isotopes? (b) Which one decays at a faster rate? (c) How much of a 1.00-mg sample of each isotope remains after three half-lives? How much of a \(1.00-\mathrm{mg}\) sample of each isotope remains after five days?

The \(\mathrm{NO}_{x}\) waste stream from automobile exhaust includes species such as \(\mathrm{NO}\) and \(\mathrm{NO}_{2}\). Catalysts that convert these species to \(\mathrm{N}_{2}\) are desirable to reduce air pollution. (a) Draw the Lewis dot and VSEPR structures of \(\mathrm{NO}, \mathrm{NO}_{2}\), and \(\mathrm{N}_{2} .(\mathbf{b})\) Using a resource such as Table 8.3 , look up the energies of the bonds in these molecules. In what region of the electromagnetic spectrum are these energies? \((\mathbf{c})\) Design a spectroscopic experiment to monitor the conversion of \(\mathrm{NO}_{x}\) into \(\mathrm{N}_{2}\), describing what wavelengths of light need to be monitored as a function of time.

The iodide ion reacts with hypochlorite ion (the active ingredient in chlorine bleaches) in the following way: $\mathrm{OCl}^{-}+\mathrm{I}^{-} \longrightarrow \mathrm{OI}^{-}+\mathrm{Cl}^{-} .$ This rapid reaction gives the following rate data: $$ \begin{array}{ccc} \hline\left[\mathrm{OCI}^{-}\right](M) & {\left[\mathrm{I}^{-}\right](M)} & \text { Initial Rate }(\mathrm{M} / \mathrm{s}) \\ \hline 1.5 \times 10^{-3} & 1.5 \times 10^{-3} & 1.36 \times 10^{-4} \\ 3.0 \times 10^{-3} & 1.5 \times 10^{-3} & 2.72 \times 10^{-4} \\ 1.5 \times 10^{-3} & 3.0 \times 10^{-3} & 2.72 \times 10^{-4} \\ \hline \end{array} $$ (a) Write the rate law for this reaction. (b) Calculate the rate constant with proper units. (c) Calculate the rate when $\left[\mathrm{OCl}^{-}\right]=2.0 \times 10^{-3} \mathrm{M}\( and \)\left[\mathrm{I}^{-}\right]=5.0 \times 10^{-4} \mathrm{M}$
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