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Chapter 12: Problem 11

Define what is meant by unimolecular and bimolecular steps. Why are termolecular steps infrequently seen in chemical reactions?

Short Answer

Expert verified
A unimolecular step is a single-step chemical reaction involving one reactant molecule in the rate-determining step, such as isomerization or dissociation. A bimolecular step involves the collision of two reactant molecules in the rate-determining step, resulting in an exchange of atoms between them. Termolecular steps are infrequent because they require the simultaneous collision of three reactant molecules with appropriate orientation and energy, which has a significantly lower probability compared to unimolecular or bimolecular steps. Additionally, the reaction is more likely to proceed through simpler steps.

Step by step solution

01

Define Unimolecular Step

A unimolecular step is a single-step chemical reaction in which only one reactant molecule is involved in the rate-determining step. These reactions typically involve isomerization, dissociation, or a rearrangement of atoms within a molecule. An example of a unimolecular reaction is the isomerization of cis-butene to trans-butene.
02

Define Bimolecular Step

A bimolecular step is a single-step chemical reaction in which the rate-determining step involves the collision of two reactant molecules. These reactions typically involve an exchange of atoms between the two reactants, resulting in two new products. An example of a bimolecular reaction is the reaction between hydrogen and iodine molecules to form hydrogen iodide (\(H_2 + I_2 \rightarrow 2HI\)).
03

Explain Infrequency of Termolecular Steps

Termolecular steps are infrequently seen in chemical reactions because they require the simultaneous collision of three reactant molecules with the appropriate orientation and energy. The probability of such collisions occurring is significantly lower than for unimolecular or bimolecular steps, making termolecular steps much less frequent in most chemical reactions. In addition, due to the increased complexity of the termolecular step, it is more likely that the reaction proceeds through a series of unimolecular and bimolecular steps instead.

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Most popular questions from this chapter

Consider the hypothetical reaction $$ \mathrm{A}+\mathrm{B}+2 \mathrm{C} \longrightarrow 2 \mathrm{D}+3 \mathrm{E} $$ where the rate law is $$ \text { Rate }=-\frac{\Delta[\mathrm{A}]}{\Delta t}=k[\mathrm{~A}][\mathrm{B}]^{2} $$ An experiment is carried out where \([\mathrm{A}]_{0}=1.0 \times 10^{-2} \mathrm{M}\), \([\mathrm{B}]_{0}=3.0 M\), and \([\mathrm{C}]_{0}=2.0 M .\) The reaction is started, and after \(8.0\) seconds, the concentration of \(\mathrm{A}\) is \(3.8 \times 10^{-3} \mathrm{M}\). a. Calculate the value of \(k\) for this reaction. b. Calculate the half-life for this experiment. c. Calculate the concentration of A after \(13.0\) seconds. d. Calculate the concentration of \(\mathrm{C}\) after \(13.0\) seconds.

Define stability from both a kinetic and thermodynamic perspective. Give examples to show the differences in these concepts.

Hydrogen reacts explosively with oxygen. However, a mixture of \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) can exist indefinitely at room temperature. Explain why \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) do not react under these conditions.

Consider the general reaction $$ \mathrm{aA}+\mathrm{bB} \longrightarrow \mathrm{cC} $$ and the following average rate data over some time period \(\Delta t\) : $$ \begin{aligned} -\frac{\Delta \mathrm{A}}{\Delta t} &=0.0080 \mathrm{~mol} / \mathrm{L} \cdot \mathrm{s} \\ -\frac{\Delta \mathrm{B}}{\Delta t} &=0.0120 \mathrm{~mol} / \mathrm{L} \cdot \mathrm{s} \\ \frac{\Delta \mathrm{C}}{\Delta t} &=0.0160 \mathrm{~mol} / \mathrm{L} \cdot \mathrm{s} \end{aligned} $$ Determine a set of possible coefficients to balance this general reaction.

The mechanism for the gas-phase reaction of nitrogen dioxide with carbon monoxide to form nitric oxide and carbon dioxide is thought to be $$ \begin{aligned} &\mathrm{NO}_{2}+\mathrm{NO}_{2} \longrightarrow \mathrm{NO}_{3}+\mathrm{NO} \\\ &\mathrm{NO}_{3}+\mathrm{CO} \longrightarrow \mathrm{NO}_{2}+\mathrm{CO}_{2} \end{aligned} $$ Write the rate law expected for this mechanism. What is the overall balanced equation for the reaction?
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