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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

Expert verified
(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

Consider the following reaction: $$ \mathrm{CH}_{3} \mathrm{Br}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{CH}_{3} \mathrm{OH}(a q)+\mathrm{Br}^{-}(a q) $$ The rate law for this reaction is first order in \(\mathrm{CH}_{3} \mathrm{Br}\) and first order in \(\mathrm{OH}^{-}\). When \(\left[\mathrm{CH}_{3} \mathrm{Br}\right]\) is \(5.0 \times 10^{-3} \mathrm{M}\) and \(\left[\mathrm{OH}^{-}\right]\) is \(0.050 \mathrm{M}\), the reaction rate at \(298 \mathrm{~K}\) is \(0.0432 \mathrm{M} / \mathrm{s}\). (a) What is the value of the rate constant? (b) What are the units of the rate constant? (c) What would happen to the rate if the concentration of \(\mathrm{OH}^{-}\) were tripled?

You study the effect of temperature on the rate of two reactions and graph the natural logarithm of the rate constant for each reaction as a function of \(1 / T\). How do the two graphs compare (a) if the activation energy of the second reaction is higher than the activation energy of the first reaction but the two reactions have the same frequency factor, and (b) if the frequency factor of the second reaction is higher than the frequency factor of the first reaction but the two reactions have the same activation energy? [Section \(14.5\)

Consider the reaction of peroxydisulfate ion $\left(\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}\right)\( with iodide ion (I \)^{-}$ ) in aqueous solution: $$ \mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}(a q)+3 \mathrm{I}^{-}(a q) \longrightarrow 2 \mathrm{SO}_{4}{ }^{2-}(a q)+\mathrm{I}_{3}^{-}(a q) $$ At a particular temperature the rate of disappearance of $\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}$ varies with reactant concentrations in the following manner: \begin{tabular}{llll} \hline & & & Initial Rate \\ Experiment & {\(\left[\mathrm{S}_{2} \mathrm{O}_{8}^{2-}\right](M)\)} & {\(\left[\mathrm{I}^{-}\right](M)\)} & \((M / s)\) \\ \hline 1 & \(0.018\) & \(0.036\) & \(2.6 \times 10^{-6}\) \\ 2 & \(0.027\) & \(0.036\) & \(3.9 \times 10^{-6}\) \\ 3 & \(0.036\) & \(0.054\) & \(7.8 \times 10^{-6}\) \\ 4 & \(0.050\) & \(0.072\) & \(1.4 \times 10^{-5}\) \\ \hline \end{tabular} (a) Determine the rate law for the reaction. (b) What is the average value of the rate constant for the disappearance of $\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}$ based on the four sets of data? (c) How is the rate of disappearance of \(\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}\) related to the rate of disappearance of \(I^{-} ?(\mathrm{~d})\) What is the rate of disappearance of I when \(\left[\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}\right]=0.025 \mathrm{M}\) and \(\left[\mathrm{I}^{-}\right]=0.050 \mathrm{M} ?\)

(a) What factors determine whether a collision between two molecules will lead to a chemical reaction? (b) According to the collision model, why does temperature affect the value of the rate constant?

The enzyme invertase catalyzes the conversion of sucrose, a disaccharide, to invert sugar, a mixture of glucose and fructose When the concentration of invertase is \(4.2 \times 10^{-7} M\) and the concentration of sucrose is \(0.0077 M\), invert sugar is formed at the rate of \(1.5 \times 10^{-4} \mathrm{M} / \mathrm{s} .\) When the sucrose concentration is doubled, the rate of formation of invert sugar is doubled also. (a) Assuming that the enzyme- substrate model is operative, is the fraction of enzyme tied up as a complex large or small? Explain. (b) Addition of inositol, another sugar, decreases the rate of formation of invert sugar. Suggest a mechanism by which this occurs.
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