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Chapter 14: Problem 69

(a) What is meant by the term elementary reaction? (b) What is the difference between a unimolecular and a bimolecular elementary reaction? (c) What is a reaction mechanism?

Short Answer

Expert verified
(a) An elementary reaction is a single-step chemical process that occurs as indicated by the balanced chemical equation, involving the collision and reactions between individual particles. (b) Unimolecular elementary reactions involve only one reacting species, typically in decomposition or isomerization reactions, with a rate law expression of Rate = k[A]. Bimolecular elementary reactions involve the simultaneous collision and reaction of two molecular species, with a rate law expression of Rate = k[A][B]. (c) A reaction mechanism is a series of elementary steps describing the detailed molecular events in a complex chemical reaction, providing information about intermediates, transition states, and the order of bond breaking and formation. The overall rate depends on the slowest step (rate-determining step).

Step by step solution

01

(a) Definition of Elementary Reaction)

An elementary reaction is a single-step chemical process that happens exactly as the balanced chemical equation indicates. It can involve the collision and react-ions between individual particles (atoms, molecules, or their fragments). Elementary reactions can be classified based on the number of molecular species involved, such as unimolecular, bimolecular, or termolecular reactions.
02

(b) Unimolecular vs. Bimolecular Elementary Reactions)

Unimolecular elementary reactions involve only one reacting species in the rate-determining step. Typically, they indicate decomposition or isomerization reactions. The rate law expression for a unimolecular reaction is: Rate = k[A], where k is the rate constant and [A] is the concentration of the reacting species. Bimolecular elementary reactions involve the simultaneous collision and reaction of two molecular species in the rate-determining step. Examples include reactions between two different molecules or two identical molecules. The rate law expression for a bimolecular reaction is: Rate = k[A][B], where k is the rate constant, and [A] and [B] are the concentrations of the two reacting species.
03

(c) Definition of Reaction Mechanism)

A reaction mechanism is a series of elementary steps or reactions that describe the detailed molecular events occurring in a complex chemical reaction. It provides information about the intermediates, transition states, and the order of bond breaking and bond formation throughout the reaction. The overall rate of the reaction depends on the slowest step in the reaction mechanism, which is known as the rate-determining step.

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Most popular questions from this chapter

NO catalyzes the decomposition of \(\mathrm{N}_{2} \mathrm{O},\) possibly by the following mechanism: $$ \begin{aligned} \mathrm{NO}(g)+\mathrm{N}_{2} \mathrm{O}(g) & \longrightarrow \mathrm{N}_{2}(g)+\mathrm{NO}_{2}(g) \\ 2 \mathrm{NO}_{2}(g) & \longrightarrow 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \end{aligned} $$ (a) What is the chemical equation for the overall reaction? Show how the two steps can be added to give the overall equation. (b) Why is NO considered a catalyst and not an intermediate? (c) If experiments show that during the decomposition of \(\mathrm{N}_{2} \mathrm{O}, \mathrm{NO}_{2}\) does not accumulate in measurable quantities, does this rule out the proposed mechanism? If you think not, suggest what might be going on.

Ozone in the upper atmosphere can be destroyed by the following two-step mechanism: $$ \begin{array}{l} \mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \\ \mathrm{ClO}(g)+\mathrm{O}(g) \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g) \end{array} $$ (a) What is the overall equation for this process? (b) What is the catalyst in the reaction? How do you know? (c) What is the intermediate in the reaction? How do you distinguish it from the catalyst?

The enzyme carbonic anhydrase catalyzes the reaction \(\mathrm{CO}_{2}(g)+\) \(\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{HCO}_{3}^{-}(a q)+\mathrm{H}^{+}(a q) .\) In water, without the enzyme, the reaction proceeds with a rate constant of \(0.039 \mathrm{~s}^{-1}\) at \(25^{\circ} \mathrm{C}\). In the presence of the enzyme in water, the reaction proceeds with a rate constant of \(1.0 \times 10^{6} \mathrm{~s}^{-1}\) at \(25^{\circ} \mathrm{C}\). Assuming the collision factor is the same for both situations, calculate the difference in activation energies for the uncatalyzed versus enzyme-catalyzed reaction.

The temperature dependence of the rate constant for a reaction is tabulated as follows: $$ \begin{array}{lc} \hline \text { Temperature (K) } & k\left(M^{-1} \mathrm{~s}^{-1}\right) \\ \hline 600 & 0.028 \\ 650 & 0.22 \\ 700 & 1.3 \\ 750 & 6.0 \\ 800 & 23 \\ \hline \end{array} $$ Calculate \(E_{a}\) and \(A\).

Enzymes are often described as following the two-step mechanism: $$ \begin{array}{l} \mathrm{E}+\mathrm{S} \rightleftharpoons \mathrm{ES} \quad(\text { fast }) \\ \mathrm{ES} \longrightarrow \mathrm{E}+\mathrm{P} \quad(\text { slow }) \end{array} $$ where \(\mathrm{E}=\) enzyme, \(\mathrm{S}=\) substrate, \(\mathrm{ES}=\) enzyme- substrate complex, and \(\mathrm{P}=\) product. (a) If an enzyme follows this mechanism, what rate law is expected for the reaction? (b) Molecules that can bind to the active site of an enzyme but are not converted into product are called enzyme inhibitors. Write an additional elementary step to add into the preceding mechanism to account for the reaction of \(\mathrm{E}\) with \(\mathrm{I}\), an inhibitor.
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