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Chapter 14: Problem 65

\(\begin{array}{l}{\text { (a) What is meant by the term elementary reaction? }} \\ {\text { (b) What is the difference between a unimolecular }} \\\ {\text { and a bimolecular elementary reaction? (c) What is a }}\end{array}\) \(\begin{array}{l}{\text {reaction mechanism?}(\mathbf{d}) \text { What is meant by the term rate- }} \\ {\text { determining step? }}\end{array}\)

Short Answer

Expert verified
(a) Elementary reaction is a single molecular event in which reactants are transformed into products without the formation of any reaction intermediates. (b) Unimolecular and bimolecular elementary reactions are types of elementary reactions based on the number of molecules involved. Unimolecular reactions involve one molecule, while bimolecular reactions involve the collision of two reactant molecules. (c) A reaction mechanism is a series of elementary reactions that together describe the step-by-step process by which reactants are transformed into products, providing a detailed explanation of the molecular events and intermediates involved. (d) The rate-determining step is the slowest step in a reaction mechanism, which governs the overall rate of the reaction. It limits the speed at which reactants are transformed into products and is important in understanding factors controlling the reaction rate.

Step by step solution

01

Answer (a)

Elementary reaction refers to a single molecular event in which reactants are transformed into products. In an elementary reaction, the reactants directly convert into products through a single step, without the formation of any reaction intermediates.
02

Answer (b)

Unimolecular and bimolecular elementary reactions are types of elementary reactions based on the number of molecules involved in the reaction. - Unimolecular elementary reaction: A reaction in which only one molecule is involved, leading to a rearrangement of atoms or bonds within that molecule. An example is the decomposition of a molecule into two or more smaller molecules. - Bimolecular elementary reaction: A reaction involving the collision of two reactant molecules, resulting in a new product. An example is the reaction between two molecules, A and B, that combine to form a single product C (A + B -> C).
03

Answer (c)

A reaction mechanism is a series of elementary reactions that together describe the step-by-step process by which reactants are transformed into products. It provides a detailed explanation of the sequence of molecular events and the intermediates formed during the overall reaction. This includes information about the molecular species, their transformations, and the energy changes involved at each step of the reaction.
04

Answer (d)

The term "rate-determining step" refers to the slowest step in a reaction mechanism, which governs the overall rate of the reaction. Since the reaction can only proceed as fast as the slowest step, the rate-determining step limits the speed at which the reactants are transformed into products. By identifying the rate-determining step, we can better understand the factors controlling the reaction rate and suggest ways to optimize the reaction conditions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Unimolecular Reaction
Understanding a unimolecular reaction starts with the recognition that it involves only one molecule undergoing a change to form products. Imagine a single molecule, all by itself, deciding to rearrange its atoms without any influence from another molecule. This process might seem lonely, but it's an important concept in chemical reactions.

For example, consider a molecule of ozone (O_3) breaking down into a molecule of oxygen (O_2) and a single oxygen atom. This single-step reaction doesn't need another molecule to have a 'conversation' with; it proceeds when the molecule has enough energy to rearrange itself. This idea allows us to predict and understand how changes in conditions such as temperature can influence the rate at which the reaction proceeds, as more molecules gain the necessary energy to undergo this transformation.
Bimolecular Reaction
In contrast to the solitary nature of unimolecular reactions, bimolecular reactions require a dance between two molecules. They must 'meet' and collide with sufficient energy and in the correct orientation for a reaction to occur.

Think of it like a perfectly choreographed dance routine where two dance partners need to be in sync. By considering the factors that affect these molecular collisions, such as concentration and temperature, chemists can gain insights into how quickly the reaction will take place. For instance, the iconic reaction between hydrogen and iodine molecules to form hydrogen iodide (2HI) exemplifies a bimolecular reaction where two molecules interact directly to form a product.
Reaction Mechanism
A reaction mechanism is akin to a detailed recipe for a chemical reaction, outlining each step that reactant molecules take on their journey to becoming product molecules. Just as a cook needs to follow a series of steps to prepare a dish, molecules follow a series of elementary reactions in a specific sequence.

This concept enables chemists to visualize the transformation from reactants to products, discovering intermediate species and transition states that might be targets for controlling or altering the reaction's pathway. Overall, the reaction mechanism tells the full story of a chemical reaction, beyond the simple reactants-to-products summary.
Rate-Determining Step
The rate-determining step is often described as the bottleneck of a reaction mechanism, which dictates the reaction's pace much like the narrowest part of a funnel restricts the flow of liquid. This concept is critical in both understanding and optimizing reactions because, by identifying and addressing this slowest step, chemists can strategize ways to accelerate the overall reaction.

It's similar to addressing the slowest runner in a relay race to improve the team's total time. By focusing on the rate-determining step, we can apply various chemical principles to improve reaction efficiency, such as increasing the temperature or using a catalyst to lower the activation energy required for this crucial step.

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Most popular questions from this chapter

The oxidation of \(\mathrm{SO}_{2}\) to \(\mathrm{SO}_{3}\) is accelerated by \(\mathrm{NO}_{2} .\) The reaction proceeds according to: $$ \begin{array}{l}{\mathrm{NO}_{2}(g)+\mathrm{SO}_{2}(g) \longrightarrow \mathrm{NO}(g)+\mathrm{SO}_{3}(g)} \\ {2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}_{2}(g)}\end{array}$$ (a) Show that, with appropriate coefficients, the two reactions can be summed to give the overall oxidation of \(S O_{2}\) by \(\mathrm{O}_{2}\) to give \(S O_{3} .(\mathbf{b})\) Do we consider \(N O_{2}\) a catalyst or an intermediate in this reaction? (c) Would you classify NO as a catalyst or as an intermediate? { ( d ) } Is this an example of homogeneous catalysis or heterogeneous catalysis?

Indicate whether each statement is true or false. (a) If you compare two reactions with similar collision factors, the one with the larger activation energy will be faster. (b) A reaction that has a small rate constant must have a small frequency factor. (c) Increasing the reaction temperature increases the fraction of successful collisions between reactants.

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 reaction \(2 \mathrm{NO}_{2} \longrightarrow 2 \mathrm{NO}+\mathrm{O}_{2}\) has the rate constant \(k=0.63 M^{-1} \mathrm{s}^{-1}\) . (a) Based on the units for \(k,\) is the reaction first or second order in \(\mathrm{NO}_{2} ?\) ? (b) If the initial concentration of \(\mathrm{NO}_{2}\) is \(0.100 \mathrm{M},\) how would you determine how long it would take for the concentration to decrease to 0.025 \(\mathrm{M}\) ?

Platinum nanoparticles of diameter \(\sim 2 \mathrm{nm}\) are important catalysts in carbon monoxide oxidation to carbondioxide. Platinum crystallizes in a face-centered cubic arrangement with an edge length of 3.924 A. (a) Estimate how many platinum atoms would fit into a 2.0 -nm sphere; the volume of a sphere is \((4 / 3) \pi r^{3} .\) Recall that \(1 \hat{\mathrm{A}}=1 \times 10^{-10} \mathrm{m}\) and \(1 \mathrm{nm}=1 \times 10^{-9} \mathrm{m} .\) (b) Estimate how many platinum atoms are on the surface of a \(2.0-\mathrm{nm}\) Pt sphere, using the surface area of a sphere \(\left(4 \pi r^{2}\right)\) and assuming that the "footprint" of one Pt atom can be estimated from its atomic diameter of 2.8 A. (c) Using your results from (a) and (b), calculate the percentage of Pt atoms that are on the surface of a 2.0 -nm nanoparticle. (d) Repeat these calculations for a 5.0 -nm platinum nanoparticle. (e) Which size of nanoparticle would you expect to be more catalytically active and why?
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