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Chapter 14: Problem 65

(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? (d) What is meant by the term rate determining step?

Short Answer

Expert verified
(a) An elementary reaction is a single molecular event occurring during a chemical reaction, typically in a single step. (b) Unimolecular and bimolecular elementary reactions differ based on the number of molecules involved. Unimolecular reactions involve one molecule transitioning, while bimolecular reactions involve two molecules colliding and reacting. (c) A reaction mechanism is a sequence of elementary reactions or steps describing the pathway of reactants converting into products at the molecular level. (d) The rate-determining step is the slowest elementary step in a reaction mechanism, determining the overall reaction rate and influencing factors affecting reaction rates.

Step by step solution

01

(a) Elementary Reaction

An elementary reaction is a single molecular event that occurs during a chemical reaction. It represents the most basic process at the molecular level, involving the rearrangement or interaction of atoms or molecules, and typically occurs within a single step.
02

(b) Unimolecular and Bimolecular Elementary Reactions

Unimolecular and bimolecular elementary reactions are categories of elementary reactions based on the number of molecules involved in the reaction event. A unimolecular elementary reaction involves the transition of a single molecule from one chemical species to another, either through decomposing or rearranging. A bimolecular elementary reaction, on the other hand, involves two molecules colliding and reacting to form new chemical species. In general, the two types can be represented by the following equations: - Unimolecular: \(A \rightarrow products\) - Bimolecular: \(A + B \rightarrow products\)
03

(c) Reaction Mechanism

A reaction mechanism is a sequence of elementary reactions or steps that describes the pathway by which reactants are converted into products at the molecular level. It provides detailed information about the intermediate species formed during the reaction, as well as the energy changes and activation energies associated with each elementary step. The reaction mechanism helps explain the observed rate law and overall reaction kinetics.
04

(d) Rate-Determining Step

The rate-determining step (RDS) is the slowest elementary step in a reaction mechanism, which effectively determines the overall reaction rate. Since all other steps in the mechanism occur relatively faster than the RDS, the progress of the reaction is limited by the rate at which the RDS occurs. By understanding the RDS, chemists can gain insights into the factors that influence the reaction rates and predict how the reaction conditions can be optimized to improve reaction efficiency.

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Most popular questions from this chapter

Urea \(\left(\mathrm{NH}_{2} \mathrm{CONH}_{2}\right)\) is the end product in protein metabolism in animals. The decomposition of urea in $0.1 \mathrm{M} \mathrm{HCl}$ occurs according to the reaction $$ \mathrm{NH}_{2} \mathrm{CONH}_{2}(a q)+\mathrm{H}^{+}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{NH}_{4}^{+}(a q)+\mathrm{HCO}_{3}^{-}(a q) $$ The reaction is first order in urea and first order overall. When \(\left[\mathrm{NH}_{2} \mathrm{CONH}_{2}\right]=0.200 \mathrm{M},\) the rate at \(61.05^{\circ} \mathrm{C}\) $$ \text { is } 8.56 \times 10^{-5} \mathrm{M} / \mathrm{s} $$ (a) What is the rate constant, \(k\) ? (b) What is the concentration of urea in this solution after $4.00 \times 10^{3} \mathrm{~s}\( if the starting concentration is \)0.500 \mathrm{M}$ ? (c) What is the half-life for this reaction at \(61.05^{\circ} \mathrm{C}\) ?

For the elementary process $\mathrm{N}_{2} \mathrm{O}_{5}(g) \longrightarrow \mathrm{NO}_{2}(g)+\mathrm{NO}_{3}(g)$ the activation energy \(\left(E_{a}\right)\) and overall \(\Delta E\) are $154 \mathrm{~kJ} / \mathrm{mol}\( and \)136 \mathrm{~kJ} / \mathrm{mol}$, respectively. (a) Sketch the energy profile for this reaction, and label \(E_{a}\) and \(\Delta E\). (b) What is the activation energy for the reverse 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.

As described in Exercise 14.41 , the decomposition of sulfuryl chloride \(\left(\mathrm{SO}_{2} \mathrm{Cl}_{2}\right)\) is a first-order process. The rate constant for the decomposition at \(660 \mathrm{~K}\) is $4.5 \times 10^{-2} \mathrm{~s}^{-1}\(. (a) If we begin with an initial \)\mathrm{SO}_{2} \mathrm{Cl}_{2}\( pressure of \)60 \mathrm{kPa}$, what is the partial pressure of this substance after 60 s? (b) At what time will the partial pressure of \(\mathrm{SO}_{2} \mathrm{Cl}_{2}\) decline to one-tenth its initial value?

The enzyme urease catalyzes the reaction of urea, $\left(\mathrm{NH}_{2} \mathrm{CONH}_{2}\right)$, with water to produce carbon dioxide and ammonia. In water, without the enzyme, the reaction proceeds with a first-order rate constant of \(4.15 \times 10^{-5} \mathrm{~s}^{-1}\) at $100^{\circ} \mathrm{C}$. In the presence of the enzyme in water, the reaction proceeds with a rate constant of \(3.4 \times 10^{4} \mathrm{~s}^{-1}\) at $21^{\circ} \mathrm{C}$. (a) Write out the balanced equation for the reaction catalyzed by urease. \((\mathbf{b})\) If the rate of the catalyzed reaction were the same at \(100^{\circ} \mathrm{C}\) as it is at \(21^{\circ} \mathrm{C}\), what would be the difference in the activation energy between the catalyzed and uncatalyzed reactions? (c) In actuality, what would you expect for the rate of the catalyzed reaction at \(100^{\circ} \mathrm{C}\) as compared to that at \(21^{\circ} \mathrm{C} ?(\mathbf{d})\) On the basis of parts (c) and (d), what can you conclude about the difference in activation energies for the catalyzed and uncatalyzed reactions?
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