Question

The addition of NO accelerates the decomposition of \(\mathrm{N}_{2} \mathrm{O},\) possibly by the following mechanism: $$\begin{array}{c}{\mathrm{NO}(g)+\mathrm{N}_{2} \mathrm{O}(g) \longrightarrow \mathrm{N}_{2}(g)+\mathrm{NO}_{2}(g)} \\ {2 \mathrm{NO}_{2}(g) \longrightarrow 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g)}\end{array}$$ (a) What is the chemical equation for the overall reaction? Show how the two steps can be added to give the overall equation. (b) Is NO serving as a catalyst or an intermediate in this reaction? (c) If experiments show that during the decomposition of \(N_{2} \mathrm{O}, \mathrm{NO}_{2}\) does not accumulate in measurable quantities, does this rule out the proposed mechanism?

Short answer

a) The overall chemical equation for the reaction can be obtained by adding the two elementary steps and canceling out any species that appear on both sides: N2O(g) -> N2(g) + O2(g). b) NO is a catalyst in this reaction, as it is consumed in the first elementary step and regenerated in the second elementary step, remaining unchanged. c) The absence of accumulating NO2 during the decomposition of N2O in experiments does not rule out the proposed mechanism. It could mean that NO2 is formed and then quickly converted back to NO and O2 in the second elementary step.

Step-by-step solution

a) Overall chemical equation

To find the overall chemical equation, we first need to add the two given elementary steps: 1) NO(g) + N2O(g) -> N2(g) + NO2(g) 2) 2 NO2(g) -> 2 NO(g) + O2(g) Next, we'll cancel any species that appear on both sides of the reaction equation. We notice that 2 NO2(g) is formed in step 1 and completely consumed in step 2, so it will be canceled out. Similarly, NO is consumed in step 1 and regenerated in step 2, so it will also cancel out: Overall reaction: N2O(g) -> N2(g) + O2(g)

b) NO as a catalyst or an intermediate

To determine whether NO is a catalyst or an intermediate, we need to check if it is consumed and regenerated during the course of the reaction. From our previous step, we can see that NO is consumed in the first elementary step and regenerated in the second elementary step. Since NO is regenerated during the reaction and is not consumed, it remains unchanged. Therefore, NO is serving as a catalyst in this reaction.

c) Accumulation of NO2 and the validity of the proposed mechanism

The absence of accumulating NO2 during the decomposition of N2O in experiments does not rule out the proposed mechanism. The proposed mechanism suggests that NO2 is formed and then quickly converted back to NO and O2 in the second elementary step. The rapid conversion of NO2 could explain why it does not accumulate in measurable quantities during the experiment.

Key concepts

Catalysts in Reactions

In chemical reactions, catalysts play a crucial role. They are substances that increase the rate of a reaction without being consumed by it. In simpler terms, a catalyst helps the reaction occur faster or under milder conditions, but it's still there at the end.

Let's connect this to our exercise: here, nitrogen monoxide (NO) acts as a catalyst. It's used up in the initial step, then regenerated in the subsequent one. This means NO doesn't vanish; rather, it facilitates the breakdown of another substance without being part of the end product.

• Catalysts lower the activation energy needed for the reaction, which speeds up the process.
• They're crucial in many industrial processes because they can reduce energy requirements and costs.

Notice that though NO shows up in reactions, it's always found afterwards intact. This quality distinguishes it from intermediates, which are typically consumed during the reaction.

Elementary Reaction Steps

Elementary reaction steps are the individual stages that make up the overall reaction mechanism. Each step involves a specific molecular action, and they often occur in sequence, leading to the overall chemical reaction.

Consider the two steps given in the problem:

• The first step is: NO + N₂O → N₂ + NO₂. This is an individual event that forms part of the total process.
• The second step is: 2 NO₂ → 2 NO + O₂, following straight after and depending on the previous step's products.

These steps should be thought of like gears in a machine; they work together efficiently, leading to the final rundown of the whole reaction. Each elementary step is essential for achieving the final result when combined they give a comprehensive mechanism of how a particular chemical reaction proceeds.

Chemical Equation Balancing

Balancing chemical equations is fundamental in understanding reactions. It shows the conservation of mass in a chemical process. Every atom that appears at the start must also appear at the end in equal numbers to satisfy the law of conservation of mass.

Take our given reactions: by adding the individual step equations, we aim to balance the equation overall. You start by writing each involved substance and then "cancel" out any repeated appearances, on both sides across different steps.

• NO initially reacts and gets regenerated; it cancels out in the total equation.
• NO₂ formed in step one is entirely consumed in step two, so it cancels out too.

After managing these cancellations, you're left with a balanced equation: N₂O → N₂ + O₂. This balancing ensures the equation mirrors the actual number of involved atoms and accurately represents the reaction.