Question

NO catalyzes the decomposition of ${\mathrm{N}}_2\mathrm{O}$, possibly by the following mechanism: $$\begin{array}{r}\mathrm{NO}(\mathrm{g})+{\mathrm{N}}_2\mathrm{O}(\mathrm{g})⟶{\mathrm{N}}_2(g)+{\mathrm{NO}}_2(g)\\ 2{\mathrm{NO}}_2(g)⟶2\mathrm{NO}(g)+{\mathrm{O}}_2(g)\end{array}$$ (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.

Short answer

The overall reaction is $N_{2}O(g)\to N_2(g)+{\displaystyle \frac{1}{2}}{O}_2(g)$. NO is considered a catalyst because it is involved in the reaction, helps to accelerate it, and remains unaltered after the reaction is completed. The fact that NO₂ does not accumulate in measurable quantities during the decomposition of N₂O does not necessarily rule out the proposed mechanism. It is possible that the second step of the reaction is much faster than the first step, meaning that as soon as NO₂ is produced, it is quickly consumed to produce NO and O₂. This would result in a low concentration of NO₂ throughout the reaction and make it difficult to detect experimentally.

Step-by-step solution

(a) Determine the overall reaction

To determine the overall reaction, combine the two steps provided: $NO(g)+N_{2}O(g)\to N_2(g)+N{O}_2(g)$ $2N{O}_2(g)\to 2NO(g)+O_2(g)$ When these two steps are added, the NO₂ is canceled out on both sides, as well as the NO. The resulting overall reaction is: $N_{2}O(g)\to N_2(g)+{\displaystyle \frac{1}{2}}{O}_2(g)$

(b) Explain why NO is considered a catalyst and not an intermediate

A catalyst is a substance that is involved in a reaction, helps to accelerate it, and remains unaltered after the reaction is completed. An intermediate, on the other hand, is a short-lived species that is produced and consumed during the course of a reaction. In the given mechanism, NO first reacts with N₂O to form N₂ and NO₂. Then, NO₂ is transformed back into NO and O₂ in the second step. Because NO is initially involved in the reaction but is regenerated in the end, it is considered a catalyst rather than an intermediate.

(c) Discuss if the fact that NO₂ does not accumulate in measurable quantities rules out the proposed mechanism

The fact that NO₂ does not accumulate in measurable quantities during the decomposition of N₂O does not necessarily rule out the proposed mechanism. It is possible that the second step of the reaction is much faster than the first step, meaning that as soon as NO₂ is produced, it is quickly consumed to produce NO and O₂. This would result in a low concentration of NO₂ throughout the reaction and make it difficult to detect experimentally. If the proposed mechanism is indeed correct, it would be expected that increasing the concentration of NO would increase the rate of the overall reaction. This is because NO plays a key role in the mechanism, acting as a catalyst. Experimentally, one could verify the proposed mechanism by investigating the rate dependence of the reaction on the concentration of NO.

Key concepts

Chemical Reaction Mechanism

Understanding a chemical reaction mechanism is akin to following a step-by-step recipe for a cooking process. It’s the detailed account of the molecular-level transformations and energy changes that occur from the moment reactants come together until products are formed.

When chemists study a reaction mechanism, they are looking for details such as the order in which bonds break and form, the role of various species involved, and the speed or rate at which these changes happen. This insight helps predict how changes in conditions can affect the reaction’s progress and outcomes.

For example, in the decomposition of dinitrogen monoxide ((N_{2}O)), through the steps outlined in the provided exercise, we see the involvement of nitric oxide (NO) in a sequence of steps, each representing a part of the reaction's mechanism. By examining such interactions, chemists can offer explanations for the behavior of species during the reaction, like why NO₂ does not accumulate significantly, suggesting it is rapidly consumed as it forms, which is an important clue to understanding the overall mechanism.

Role of Catalysts

Catalysts are the unsung heroes of many chemical reactions, prized for their ability to make things happen faster without being consumed in the process. Their primary function is to lower the activation energy required for the reaction to proceed, which can amazingly transform a slow and impractical reaction into one that is readily usable and efficient.

In our exercise, NO acts as a catalyst for the decomposition of (N_{2}O); it enters the reaction, momentarily forms products, and re-emerges ready to assist again without any net change to its quantity. This recycling of NO exemplifies the defining trait of a catalyst – it participates but is ultimately not consumed by the reaction.

Even though a catalyst does not appear in the final reaction equation because it is not a reactant or a product, its role is crucial. Without it, many industrial and biological processes would proceed at an impractical rate or require much higher energy inputs. Hence, the role of catalysts like NO is to make chemical reactions more conducive to the conditions under which they occur in the real world.

Reaction Intermediates

Imagine reaction intermediates as temporary pit stops on the highway of a chemical reaction. They are species that appear in the reaction mechanism but not in the overall balanced equation for the reaction because they are formed and used up during the process.

Intermediates are often very reactive, with lifespans that can be as fleeting as a few milliseconds. Their instability and transitory nature mean that they might not be directly observable. However, they are critical to our understanding of how a reaction progresses because they provide information about the sequence and pathways through which reactants convert into products.

The fact that NO₂, in our exercise, does not build up to measurable quantities might suggest that it plays the role of an intermediate; it quickly transforms into other products, playing a pivotal part in the reaction pathway. When NO₂'s brief existence facilitates the transformation of (N_{2}O) into N₂ and O₂, it exemplifies the role of reaction intermediates in steering the course of a reaction towards its final products.