Problem 96 - Chemistry The Central Science

Question

The commercial production of nitric acid involves the following chemical reactions: $$ \begin{aligned} 4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) & \longrightarrow 4 \mathrm{NO}(g)+6 \mathrm{H}_{2} \mathrm{O}(g) \\ 2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) & \longrightarrow 2 \mathrm{NO}_{2}(g) \\ 3 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) & \longrightarrow 2 \mathrm{HNO}_{3}(a q)+\mathrm{NO}(g) \end{aligned} $$ (a) Which of these reactions are redox reactions? (b) In each redox reaction identify the element undergoing oxidation and the element undergoing reduction.

Step-by-Step Solution

Answer

(a) All three reactions are redox reactions. (b) In Reaction 1, \(N\) is oxidized and \(O\) is reduced. In Reaction 2, \(N\) is oxidized and \(O\) is reduced, and in Reaction 3, \(N\) undergoes both oxidation and reduction.

Step 1: Assign Oxidation Numbers

First, let's assign oxidation numbers to each element in each reaction. Reaction 1: \(4 \,\text{NH}_3(g) + 5 \,\text{O}_2(g) \longrightarrow 4\, \text{NO}(g) + 6 \,\text{H}_2\text{O}(g)\) \(N: -3, +2; H: +1; O: 0, -2\) Reaction 2: \(2\, \text{NO}(g) + \text{O}_2(g) \longrightarrow 2\, \text{NO}_2(g)\) \(N: +2, +4; O: 0, -2\) Reaction 3: \(3 \,\text{NO}_2(g) + \text{H}_2\text{O}(l) \longrightarrow 2 \,\text{HNO}_3(aq) + \text{NO}(g)\) \(N: +4, +5, +2; H: +1; O: -2\)

Step 2: Identify Redox Reactions

Now that we have assigned oxidation numbers, let's find out which of these reactions are redox reactions by looking for changes in oxidation numbers. Reaction 1: \(N\) changes from -3 to +2 (oxidation), and \(O\) changes from 0 to -2 (reduction). So, Reaction 1 is a redox reaction. Reaction 2: \(N\) changes from +2 to +4 (oxidation), and \(O\) changes from 0 to -2 (reduction). So, Reaction 2 is also a redox reaction. Reaction 3: \(N\) changes from +4 to +5 and +2 (both oxidation and reduction). So, Reaction 3 is a redox reaction as well. (a) All three reactions are redox reactions.

Step 3: Identify Elements Undergoing Oxidation and Reduction

Let's now identify the elements undergoing oxidation and reduction in each redox reaction. Reaction 1: Oxidation: \(N\) in NH3, as oxidation number increases from -3 to +2. Reduction: \(O\) in O2, as oxidation number decreases from 0 to -2. Reaction 2: Oxidation: \(N\) in NO, as oxidation number increases from +2 to +4. Reduction: \(O\) in O2, as oxidation number decreases from 0 to -2. Reaction 3: Oxidation: \(N\) in NO2, as oxidation number increases from +4 to +5 (in HNO3) Reduction: \(N\) in NO2, as oxidation number decreases from +4 to +2 (in NO) (b) In Reaction 1, \(N\) is oxidized and \(O\) is reduced. In Reaction 2, \(N\) is oxidized and \(O\) is reduced, and in Reaction 3, \(N\) undergoes both oxidation and reduction.

Key Concepts

Understanding Oxidation Numbers

Oxidation numbers are like scorekeepers in chemical reactions. They help us keep track of how many electrons are lost or gained by elements. For any element in its pure form, the oxidation number is zero. However, in compounds, this number can vary based on the rules of chemistry.

• For hydrogen, it's usually +1.
• Oxygen typically has an oxidation number of -2.
• For nitrogen, it can change depending on the compound, as seen in reactions.

Assigning oxidation numbers to each atom in the given reactions helps us identify which atoms are gaining or losing electrons. This process is key to understanding redox reactions and is the first step in figuring out who gets oxidized and who gets reduced.

Exploring Chemical Reactions

Chemical reactions involve the transformation of substances through breaking and forming chemical bonds. In the context of the commercial production of nitric acid, several chemical reactions occur, each with a specific role in transforming reactants into products.
In these reactions:

• Ammonia (NH₃) reacts with oxygen (O₂) to form nitrogen monoxide (NO) and water (H₂O).
• Nitrogen monoxide further reacts with more oxygen to produce nitrogen dioxide (NO₂).
• Finally, nitrogen dioxide reacts with water to produce nitric acid (HNO₃) and more nitrogen monoxide.

Each reaction is a step in the industrial synthesis of nitric acid and involves shifts in electron configurations, which are crucial to understanding their reactivity and behavior.

The Process of Oxidation and Reduction

Oxidation and reduction are two sides of the same chemical process, often referred to as "redox" reactions. In a redox reaction, one element's oxidation number increases, indicating oxidation, while another decreases, indicating reduction.
Let's break this down for each reaction:

• In the first reaction, nitrogen in ammonia undergoes oxidation (from -3 to +2), and oxygen undergoes reduction (from 0 to -2).
• In the second, nitrogen in NO is oxidized (from +2 to +4), with oxygen reduced (from 0 to -2).
• In the third, nitrogen in NO₂ is involved in both its oxidation (from +4 to +5) and reduction (from +4 to +2) as it forms HNO₃ and NO, respectively.

This interplay ensures that the total electron count remains constant in the reaction, adhering to the law of conservation of matter. Understanding these electron exchanges is foundational to mastering chemistry.