Problem 109 - General Chemistry: Principles and Modern Applications

Question

Consider the following redox reaction: $$\begin{array}{r}4 \mathrm{NO}(\mathrm{g})+3 \mathrm{O}_{2}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \longrightarrow 4 \mathrm{NO}_{3}^{-}(\mathrm{aq})+4 \mathrm{H}^{+}(\mathrm{aq})\end{array} $$ (a) Which species is oxidized? (b) Which species is reduced? (c) Which species is the oxidizing agent? (d) Which species is the reducing agent? (e) Which species gains electrons? (f) Which species loses electrons?

Step-by-Step Solution

Answer

a) NO is oxidized. b) O2 is reduced. c) O2 is the oxidizing agent. d) NO is the reducing agent. e) O2 gains electrons. f) NO loses electrons.

Step 1: Identification of oxidized species

To find which species is oxidized, compare the oxidation numbers. NO gas has oxidation number of +2, and NO3- ion (nitrate) has nitrogen with oxidation number of +5. Hence, Nitrogen in NO gas is oxidized since it increases its oxidation number.

Step 2: Identification of reduced species

To find which species is reduced, compare the oxidation numbers. The oxidation number for oxygen in O2 gas is 0, and in NO3- ion (nitrate) is -2. Hence, Oxygen in O2 gas is reduced as it decreases its oxidation number.

Step 3: Identification of oxidizing agent

The species that is reduced is the oxidizing agent. From step 2, it can be seen that O2 was reduced, thus it is the oxidizing agent.

Step 4: Identification of reducing agent

The species that is oxidized is the reducing agent. From step 1, it can be seen that NO was oxidized, thus it is the reducing agent.

Step 5: Identification of the species that gains electrons

The process of gaining electrons corresponds to reduction - the species that is reduced gains electrons. From step 2, it can be seen that O2 was reduced, thus it gains electrons.

Step 6: Identification of the species that loses electrons

The process of losing electrons corresponds to oxidation - the species that is oxidized loses electrons. From step 1, it can be seen that NO was oxidized, thus it loses electrons.

Key Concepts

Oxidation Numbers

Oxidation numbers are essential for identifying how electrons are transferred in chemical reactions. They help us determine whether a reaction involves oxidation or reduction. Oxidation numbers are hypothetical charges assigned to atoms based on established rules. This helps in tracking electron movement in reactions.

For our exercise with the reaction: \(4 \mathrm{NO} + 3 \mathrm{O}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow 4 \mathrm{NO}_{3}^{-} + 4 \mathrm{H}^{+}\), we assign oxidation numbers to each element in reactants and products:

• Nitrogen (N) in \(NO\) starts with an oxidation number of +2.
• In \(\mathrm{NO}_{3}^{-}\), nitrogen's oxidation number is +5.
• Oxygen (O) in \(\mathrm{O}_{2}\) starts at 0 and changes to -2 in the product \(\mathrm{NO}_{3}^{-}\).

Tracking these changes helps us determine which species are oxidized or reduced.

Oxidizing Agent

The oxidizing agent in a redox reaction is a substance that gains electrons. This gain is part of the reduction process. The oxidizing agent, therefore, is reduced during the reaction.

In the exercise, the molecule \(\mathrm{O}_{2}\) acts as the oxidizing agent. Here's why:

• Initially, each oxygen atom in \(\mathrm{O}_{2}\) has an oxidation number of 0.
• During the reaction, it changes to -2 in \(\mathrm{NO}_{3}^{-}\).

This decrease in oxidation number indicates that oxygen atoms are gaining electrons, confirming \(\mathrm{O}_{2}\) as the oxidizing agent in this reaction.

Reducing Agent

A reducing agent is a species that donates electrons to another species during a chemical reaction. By doing so, it becomes oxidized. This means its oxidation number increases as it loses electrons.

In the given reaction, \(\mathrm{NO}\) is identified as the reducing agent. Here's the detailed explanation:

• Nitrogen in \(\mathrm{NO}\) has an initial oxidation number of +2.
• In \(\mathrm{NO}_{3}^{-}\), the oxidation number increases to +5.

The increase in nitrogen's oxidation number signifies the loss of electrons, affirming \(\mathrm{NO}\) as the reducing agent in this scenario.

Electron Transfer

Electron transfer is the core of redox reactions, involving shifts of electrons between species. Understanding which species lose and gain electrons highlights oxidation and reduction processes.

Let's dissect the electron transfer in this reaction:

• Oxygen (\(\mathrm{O}_{2}\)) gains electrons because its oxidation number decreases from 0 to -2, indicating reduction.
• Nitrogen (in \(\mathrm{NO}\)) loses electrons, as its oxidation number jumps from +2 to +5, indicating oxidation.

These changes emphasize the fundamental aspect of redox reactions, depicting how one element donates electrons while another accepts them, completing the cycle of oxidation and reduction.