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Chapter 13: Problem 94

Consider the following mechanism for a reaction in aqueous solution and indicate the species acting as a catalyst: $$\begin{aligned} \mathrm{NH}_{2} \mathrm{NO}_{2}+\mathrm{OH}^{-} & \rightleftharpoons \mathrm{H}_{2} \mathrm{O}+\mathrm{NHNO}_{2}^{-} \\ \mathrm{NHNO}_{2}^{-} \longrightarrow \mathrm{N}_{2} \mathrm{O}(g)+\mathrm{OH}^{-} \end{aligned}$$ Explain why you believe this species is a catalyst. What is the overall reaction? What substance might be added to the reaction mixture to give the catalytic activity?

Short Answer

Expert verified
The catalyst is \( \mathrm{OH}^{-} \). The overall reaction is \( \mathrm{NH}_2 \mathrm{NO}_2 \rightarrow \mathrm{H}_2 \mathrm{O} + \mathrm{N}_2\mathrm{O}(g) \). Adding NaOH can enhance the catalytic activity.

Step by step solution

01

Identify the Catalytic Step

Examine the given mechanism to identify which species might be acting as a catalyst based on its appearance and how it is regenerated.In the mechanism, \( \mathrm{OH}^{-} \) appears as a reactant in the first equation and is regenerated as a product in the second equation. This suggests that \( \mathrm{OH}^{-} \) is acting as a catalyst.
02

Rationalize Catalyst Function

Identify why \( \mathrm{OH}^{-} \) is considered a catalyst.A catalyst is a substance that increases the rate of reaction without being consumed in the process. In this mechanism, \( \mathrm{OH}^{-} \) is neither consumed nor altered after the reactions occur. It is used at the start and regenerated at the end, which is characteristic of a catalyst.
03

Determine the Overall Reaction

Combine the two steps of the mechanism to find the overall chemical reaction.The mechanism gives:1. \( \mathrm{NH}_2 \mathrm{NO}_2 + \mathrm{OH}^{-} \rightarrow \mathrm{H}_2 \mathrm{O} + \mathrm{NHNO}_2^{-} \)2. \( \mathrm{NHNO}_2^{-} \rightarrow \mathrm{N}_2\mathrm{O}(g) + \mathrm{OH}^{-} \)In the overall reaction, \( \mathrm{OH}^{-} \) is not present on the reactant side but does not appear as a net product, confirming its role as a catalyst:\( \mathrm{NH}_2 \mathrm{NO}_2 \rightarrow \mathrm{H}_2 \mathrm{O} + \mathrm{N}_2\mathrm{O}(g) \).
04

Suggest an Addition for Catalysis

Identify what substance can be added to enhance the reaction by acting as or producing more of the catalyst.Since \( \mathrm{OH}^{-} \) is the catalyst, adding a substance that provides OH\(^-\), such as \( \mathrm{NaOH} \), could enhance the catalytic process. NaOH dissolves in water to provide OH\(^-\), thus increasing the reaction rate.

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Key Concepts

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

Catalyst Identification
In chemical reactions, identifying a catalyst involves examining the reactants and products carefully. A catalyst is a substance that is involved in a reaction but is not consumed in the process. It typically appears in the initial steps of a reaction and is regenerated by the end of the reaction.
In the given mechanism, \[\mathrm{OH}^-\] acts as the catalyst because it appears on both sides of the reactions. It starts as a reactant in the first equation and is regenerated in the second equation.
This makes \[\mathrm{OH}^-\] an ideal example of a catalyst since it facilitates the reaction without being permanently altered.
Reaction Mechanism
The concept of a reaction mechanism is crucial for understanding how reactions proceed at a molecular level. A mechanism is a series of proposed steps that depict the transformation of reactants into products.
For the reaction described, the mechanism involves two main steps:
  • The conversion of \[\mathrm{NH}_2\mathrm{NO}_2 + \mathrm{OH}^-\] into \[\mathrm{H}_2\mathrm{O} + \mathrm{NHNO}_2^-\]
  • The decomposition of \[\mathrm{NHNO}_2^-\] into \[\mathrm{N}_2\mathrm{O}(g) + \mathrm{OH}^-\]
These steps show how the reaction proceeds through intermediate states, which helps explain the role of intermediate species and the catalyst.
Overall Reaction
The overall reaction is derived by combining the individual steps of a reaction mechanism, excluding the catalyst since it remains unchanged.
For the reaction described in the mechanism, the overall reaction is: \[\mathrm{NH}_2\mathrm{NO}_2 \rightarrow \mathrm{H}_2\mathrm{O} + \mathrm{N}_2\mathrm{O}(g)\].
This equation represents the net transformation, summarizing the reaction process without including the catalytic role of \[\mathrm{OH}^-\], as it is not consumed in the overall process.
Catalytic Activity Enhancement
Enhancing catalytic activity in a reaction involves increasing the availability of the catalyst to improve the reaction rate. In this scenario, the catalyst \[\mathrm{OH}^-\] can be increased by adding substances that release it.
One common method to enhance the catalytic activity in this reaction is by introducing \[\mathrm{NaOH}\].
NaOH dissolves in water to release \[\mathrm{OH}^-\], ensuring there is ample \[\mathrm{OH}^-\] present to quickly react with \[\mathrm{NH}_2\mathrm{NO}_2\].
By providing more catalyst, the reaction can proceed more efficiently and at a faster rate.

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Most popular questions from this chapter

Methyl acetate, \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\), reacts in basic solution to give acetate ion, \(\mathrm{CH}_{3} \mathrm{COO}^{-}\), and methanol, \(\mathrm{CH}_{3} \mathrm{OH}\). $$\mathrm{CH}_{3} \mathrm{COOCH}_{3}(a q)+\mathrm{OH}^{-}(a q) \underset{\mathrm{CH}_{3} \mathrm{COO}^{-}(a q)+\mathrm{CH}_{3} \mathrm{OH}(a q)}{\longrightarrow}$$ The overall order of the reaction was determined by starting with methyl acetate and hydroxide ion at the same concentrations, so $$\begin{aligned}&{\left[\mathrm{CH}_{3} \mathrm{COOCH}_{3}\right]=\left[\mathrm{OH}^{-}\right]=x \text { . Then }} \\ &\qquad \text { Rate }=k\left[\mathrm{CH}_{3} \mathrm{COOCH}_{3}\right]^{m}\left[\mathrm{OH}^{-}\right]^{n}=k x^{m+n}\end{aligned}$$ Determine the overall order and the value of the rate constant by plotting the following data assuming first- and then second-order kinetics.

Nitrogen monoxide, \(\mathrm{NO}\), is believed to react with chlorine according to the following mechanism: $$\begin{aligned}\mathrm{NO}+\mathrm{Cl}_{2} & \rightleftharpoons \mathrm{NOCl}_{2} & & \text { (elementary reaction) } \\ \mathrm{NOCl}_{2}+\mathrm{NO} & \longrightarrow 2 \mathrm{NOCl} & &(\text { elementary reaction }) \end{aligned}$$ Identify any reaction intermediate. What is the overall equation?

In the presence of excess thiocyanate ion, \(\mathrm{SCN}^{-}\), the following reaction is first order in iron(III) ion, \(\mathrm{Fe}^{3+}\); the rate constant is \(1.27 / \mathrm{s}\). $$\mathrm{Fe}^{3+}(a q)+\mathrm{SCN}^{-}(a q) \longrightarrow \mathrm{Fe}(\mathrm{SCN})^{2+}(a q)$$ What is the half-life in seconds? How many seconds would be required for the initial concentration of \(\mathrm{Fe}^{3+}\) to decrease to each of the following values: \(25.0 \%\) left, \(12.5 \%\) left, \(6.25 \%\) left, \(3.125 \%\) left? What is the relationship between these times and the half-life?

To obtain the rate of the reaction $$3 \mathrm{I}^{-}(a q)+\mathrm{H}_{3} \mathrm{AsO}_{4}(a q)+2 \mathrm{H}^{+}(a q) \longrightarrow$$ you might follow the \(\mathrm{I}^{-}\) concentration or the \(\mathrm{I}_{3}^{-}\) concentration. How are the rates in terms of these species related?

Draw and label the potential-energy curve for the reaction $$\mathrm{N}_{2} \mathrm{O}_{4}(g) \rightleftharpoons 2 \mathrm{NO}_{2}(g) ; \Delta H=57 \mathrm{~kJ}$$ The activation energy for the reverse reaction is \(23 \mathrm{~kJ} .\) Note \(\Delta H\) and \(E_{a}\) on the diagram. What is the activation energy for the forward reaction? For which reaction (forward or reverse) will the reaction rate be most sensitive to a temperature increase? Explain.
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