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Chapter 14: Problem 62

(a) What is meant by the term molecularity? (b) Why are termolecular elementary reactions so rare? (c) What is an intermediate in a mechanism?

Short Answer

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(a) Molecularity refers to the number of reacting species involved in a single step of a chemical reaction and can be unimolecular, bimolecular, or termolecular. (b) Termolecular elementary reactions are rare due to the low probability of three reacting species simultaneously colliding with the correct orientation and sufficient energy to react, as well as their relatively slow reaction rate. (c) An intermediate is a short-lived and highly reactive species formed during a reaction mechanism but is not present in the reactants or products of the overall reaction, playing an essential role in understanding the step-by-step process of a chemical reaction.

Step by step solution

01

(a) Definition of Molecularity

Molecularity refers to the number of reacting species (atoms, molecules, or ions) that are involved in a single step of a chemical reaction. It is an important concept as it helps in understanding the mechanism of a reaction and predicting its rate. Molecularity can be unimolecular, bimolecular, or termolecular, depending on whether it involves one, two, or three reacting species, respectively.
02

(b) Rarity of Termolecular Elementary Reactions

Termolecular elementary reactions are rare, mainly due to the low probability of three reacting species simultaneously colliding with the correct orientation and sufficient energy to react. For this to occur, a precise alignment of the molecules is necessary, which is statistically unlikely. Furthermore, termolecular reactions are also relatively slow because the concentration of reactants is usually low, making it less likely for the three reactants to come together and react. As a result, most elementary reactions are unimolecular or bimolecular, as the probability of the required simultaneous collision is much higher in these cases.
03

(c) Intermediates in Reaction Mechanisms

An intermediate is a species that is formed during the course of a reaction mechanism but is not present in either the reactants or products of the overall chemical reaction. Intermediates are generally short-lived and highly reactive species, often participating in subsequent steps of the mechanism before being consumed. They play an essential role in understanding the step-by-step process of a chemical reaction and can provide valuable information on the reaction's kinetics and potential energy surfaces. Identification of intermediates can help optimize reaction conditions or identify potential catalysts for a given reaction.

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Most popular questions from this chapter

(a) Define the following symbols that are encountered in rate equations: \([\mathrm{A}]_{0}, t_{1 / 2}[\mathrm{~A}]_{t}, k .(\mathrm{b})\) What quantity, when graphed versus time, will yield a straight line for a firstorder reaction?

Zinc metal dissolves in hydrochloric acid according to the reaction $$ \mathrm{Zn}(s)+2 \mathrm{HCl}(a q)-\ldots \rightarrow \mathrm{ZnCl}_{2}(a q)+\mathrm{H}_{2}(g) $$ Suppose you are asked to study the kinetics of this reaction by monitoring the rate of production of \(\mathrm{H}_{2}(g)\). (a) By using a reaction flask, a manometer, and any other common laboratory equipment, design an experimental apparatus that would allow you to monitor the partial pressure of \(\mathrm{H}_{2}(g)\) produced as a function of time. (b) Explain how you would use the apparatus to determine the rate law of the reaction. (c) Explain how you would use the apparatus to determine the reaction order for \(\left[\mathrm{H}^{+}\right]\) for the reaction. (d) How could you use the apparatus to determine the activation energy of the reaction? (e) Explain how you would use the apparatus to determine the effects of changing the form of \(\mathrm{Zn}(s)\) from metal strips to granules.

(a) Two reactions have identical values for \(E_{a}\). Does this ensure that they will have the same rate constant if run at the same temperature? Explain. (b) Two similar reactions have the same rate constant at \(25^{\circ} \mathrm{C}\), but at \(35^{\circ} \mathrm{C}\) one of the reactions has a higher rate constant than the other. Account for these observations.

One of the many remarkable enzymes in the human body is carbonic anhydrase, which catalyzes the interconversion of carbonic acid with carbon dioxide and water. If it were not for this enzyme, the body could not rid itself rapidly enough of the \(\mathrm{CO}_{2}\) accumulated by cell metabolism. The enzyme catalyzes the dehydration (release to air) of up to \(10^{7} \mathrm{CO}_{2}\) molecules per second. Which components of this description correspond to the terms enzyme, substrate, and turnover number?

The rate of the reaction \(\mathrm{CH}_{3} \mathrm{COOC}_{2} \mathrm{H}_{5}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow\) \(\mathrm{CH}_{3} \mathrm{COO}^{-}(a q)+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(a q)\) was measured at several temperatures, and the following data were collected: $$ \begin{array}{ll} \hline \text { Temperature }\left({ }^{\circ} \mathrm{C}\right) & k\left(\mathrm{M}^{-1} \mathrm{~s}^{-1}\right) \\ \hline 15 & 0.0521 \\ 25 & 0.101 \\ 35 & 0.184 \\ 45 & 0.332 \end{array} $$ Using these data, graph \(\ln k\) versus \(1 / T\). Using your graph, determine the value of \(E_{g}\)
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