Problem 79 - Chemistry Matter and Change

Question

A two-step mechanism has been proposed for the decomposition of nitryl chloride \(\left(\mathrm{NO}_{2} \mathrm{CL}\right) .\) \begin{equation} \begin{array}{l}{\text { Step } 1 : \mathrm{NO}_{2} \mathrm{Cl}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g})+\mathrm{Cl}(\mathrm{g})} \\ {\text { Step } 2 : \mathrm{NO}_{2} \mathrm{Cl}(\mathrm{g})+\mathrm{Cl}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g})+\mathrm{Cl}_{2}(\mathrm{g})}\end{array} \end{equation} What is the overall reaction? Identify any intermediates in the reaction sequence, and explain why they are called intermediates.

Step-by-Step Solution

Answer

The overall reaction is \( 2 \text{NO}_2 \text{Cl (g)} \rightarrow 2 \text{NO}_2 \text{ (g)} + \text{Cl}_2 \text{ (g)} \). The intermediate is \( \text{Cl (g)} \).

Step 1: Write Out Given Mechanism Steps

The given reaction mechanism has two steps: \( \text{Step 1: NO}_2 \text{Cl (g)} \rightarrow \text{NO}_2 \text{ (g)} + \text{Cl (g)} \) and \( \text{Step 2: NO}_2 \text{Cl (g)} + \text{Cl (g)} \rightarrow \text{NO}_2 \text{ (g)} + \text{Cl}_2 \text{ (g)} \). These steps represent the proposed pathway for the decomposition of nitryl chloride.

Step 2: Identify and Summarize the Overall Reaction

To find the overall reaction, sum the reactants and products from the two steps. Cancel out any species that appear on both sides. \( \text{Step 1 produces Cl (g) which is consumed in Step 2; thus, cancel } \text{Cl (g)} \). The overall reaction is \( 2 \text{NO}_2 \text{Cl (g)} \rightarrow 2 \text{NO}_2 \text{ (g)} + \text{Cl}_2 \text{ (g)} \).

Step 3: Identify the Intermediates

An intermediate is a species that is formed in one step and consumed in a subsequent step, not appearing in the overall reaction. Here, \( \text{Cl (g)} \) is produced in Step 1 and consumed in Step 2. Therefore, \( \text{Cl (g)} \) is the intermediate.

Key Concepts

Chemical Decomposition

Chemical decomposition refers to the process where a single compound breaks down into two or more simpler substances. This process is often influenced by factors such as heat, light, or a catalyst.
There are several key points to remember about chemical decomposition:

• It involves a chemical change, as new substances are formed.
• The original compound, or reactant, is separated into smaller molecules, atoms, or ions.
• Energy often needs to be absorbed to break the bonds in the original compound.

To give you a chemistry angle, the decomposition of nitryl chloride, represented by the formula \( \text{NO}_2\text{Cl} \), is a good example. This reaction involves the breakdown of nitryl chloride gas to form nitrite (\( \text{NO}_2 \)) and chlorine gas (\( \text{Cl}_2 \)). These resulting products are simpler than the original compound, illustrating the core concept of decomposition.

Intermediate Species

In a chemical reaction, intermediate species play a crucial role but are not seen in the final reaction's equation. They are quite fascinating because they only appear in the early part of a reaction and are used up before the reaction finishes.
To understand intermediates better, consider the following characteristics:

• They are produced in one step and consumed in another.
• Intermediates are typically unstable or have a short lifetime.
• They don't appear in the overall chemical equation.

In the decomposition of nitryl chloride, \( \text{Cl} \) gas serves as the intermediate. It is generated in the first step and then used in the second to form chlorine gas. Despite playing a critical role in the reaction mechanism, it does not appear in the final, summed-up reaction equation.

Overall Reaction Equation

The overall reaction equation provides a snapshot of the net change from reactants to products for a chemical process. It summarizes the entire process, excluding any intermediate species.
Here's how you can determine the overall reaction equation:

• Combine all individual reaction steps, listing out reactants and products.
• Cancel out intermediate species, which appear on both sides of the equation.
• Rearrange and simplify the reaction components to reflect the complete journey from start to finish.

Taking the example of the nitryl chloride decomposition, after summing the two steps and omitting \( \text{Cl} \) as an intermediate, the overall reaction equation simplifies to: \[ 2 \text{NO}_2\text{Cl (g)} \rightarrow 2 \text{NO}_2\text{ (g)} + \text{Cl}_2\text{ (g)} \] This equation perfectly encapsulates the chemical transformation without intermediate distractions.

Nitryl Chloride Decomposition

The decomposition of nitryl chloride showcases an intriguing chemical process where one compound breaks down into others via a specific pathway. This decomposition is neatly explained by a two-step mechanism:

• Step 1: \( \text{NO}_2 \text{Cl (g)} \) breaks into \( \text{NO}_2 \text{ (g)} \) and \( \text{Cl (g)} \)
• Step 2: \( \text{NO}_2 \text{Cl (g)} \) combines with \( \text{Cl (g)} \) to form \( \text{NO}_2 \text{ (g)} \) and \( \text{Cl}_2 \text{ (g)} \)

Each step in the sequence highlights how molecules interact and transform through chemical bonds breaking and forming. The reaction pathway includes the production and consumption of \( \text{Cl} \) as an intermediate. Ultimately, understanding this mechanism provides insight into the behavior and structure of nitryl chloride as it decomposes. It's a clear illustration of how intricate and detailed the processes underlying chemical reactions can be, even when they start with something as simple as decomposition.