Problem 102 - Chemistry The Molecular Nature of Matter

Question

A reaction has the following mechanism: $$\begin{aligned}2 \mathrm{NO} \longrightarrow & \mathrm{N}_{2} \mathrm{O}_{2} \\\\\mathrm{~N}_{2} \mathrm{O}_{2}+\mathrm{H}_{2} & \longrightarrow \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{O} \\ \mathrm{N}_{2} \mathrm{O}+\mathrm{H}_{2} \longrightarrow & \mathrm{N}_{2}+\mathrm{H}_{2} \mathrm{O}\end{aligned}$$ What is the net overall change that occurs in this reaction? Identify any intermediates in the reaction.

Step-by-Step Solution

Answer

The net overall change in the reaction is \(2 NO + 2 H_2 \rightarrow N_2 + 2 H_2O\). The intermediates in the reaction are \(N_2O_2\) and \(N_2O\).

Step 1: Identify Intermediates

An intermediate is a species that is formed in one step of a reaction mechanism and consumed in a subsequent step. Look through the reaction steps and identify species that are both formed and then later consumed within the mechanism.

Step 2: Write Down Given Steps

Transcribe the individual steps of the reaction mechanism to better visualize the entire process:Step 1: \(2 NO \rightarrow N_2O_2\)Step 2: \(N_2O_2 + H_2 \rightarrow N_2O + H_2O\)Step 3: \(N_2O + H_2 \rightarrow N_2 + H_2O\)

Step 3: Combine Steps to Find Overall Reaction

Add the individual steps together, cancelling out intermediates and species that appear on both sides of the reaction arrows, to find the net overall reaction. Remember that the intermediates should not appear in the final balanced equation.

Step 4: Cancel Intermediates and Simplify

Upon adding the equations, the intermediates \(N_2O_2\) and \(N_2O\) should cancel out as they appear on both the product side and the reactant side across the steps, leading to a net change.

Step 5: State Net Overall Change

Write the final balanced net overall reaction after simplifying and removing intermediates.

Key Concepts

Reaction Intermediates

Understanding reaction intermediates is crucial for grasping how complex chemical processes occur in a series of simpler steps. These are species that appear in an intermediary stage of a mechanism but do not exist in the final chemical equation. They are pivotal in linking together the stepwise transformation of reactants into products. For instance, in our given exercise, the intermediate species are formed when nitrogen monoxide (\r\(NO\)) reacts to form dinitrogen tetroxide (\r\(N_2O_2\)) and then the \r\(N_2O_2\) reacts further to form dinitrogen monoxide (\r\(N_2O\)). These intermediates do not appear in the final net reaction as they are consumed as they are produced in subsequent steps. Identifying intermediates is like solving a mystery; one must follow the clues (the chemical species) to understand the complete story of the reaction journey.

Type of intermediates vary widely in different reactions, including ions, radicals, or neutral molecules, and their stability can range from fleeting moments to relatively long-lived species. Hence, understanding their role often provides insight into the kinetic and thermodynamic aspects of the reactions they participate in. This underscores the importance of identifying reaction intermediates when trying to deduce the pathway from reactants to products.

Net Overall Reaction

The net overall reaction represents the summarized outcome of all individual steps in a chemical reaction. It's the 'big picture' that shows the direct transformation of reactants to products without the intricacies of the pathway taken. In the context of our exercise, after identifying and removing the intermediates, we are left with the net change, which provides a streamlined view of what happens from the start of our reaction to its conclusion. This net reaction is what you would observe externally when these reactants are mixed under appropriate conditions.

Constructing the net overall reaction is rather like assembling a puzzle without the intermediate pieces: while the details of their connections are lost, the overall image is revealed. It is the culmination of balancing the species and ensuring that the law of conservation of mass is upheld. By evaluating the overall change, scientists can identify the stoichiometry of the reaction as well as predict the amounts of products formed from given quantities of reactants.

Chemical Reaction Balancing

Chemical reaction balancing is a fundamental skill in chemistry that ensures the chemical equation adheres to the Law of Conservation of Mass, stating that matter cannot be created or destroyed in a chemical reaction. It involves verifying that there are equal numbers of each type of atom on both sides of the equation. The steps in the provided exercise involve manipulating coefficients and canceling out intermediates to achieve this balance. When the process is complete, the balanced equation will accurately relate the quantities of reactants and products involved in the net overall reaction.

For a successful chemical reaction balancing, one generally follows a methodical approach. Starting with the most complex molecule can often simplify the process, and adjustments made to balance one element may require re-balancing of others previously adjusted. The balancing of reactions is both a science and an art. It's scientific in its requirement for precision and accuracy, but it's artistic in the sense that several paths can be taken to achieve the same balanced equation. When successful, it enables chemists to quantify relationships in reactions and craft precise recipes for generating desired chemical products.