Question

One pollutant in smog is nitrogen dioxide, \(\mathrm{NO}_{2}\). The gas has a reddish brown color and is responsible for the redbrown color associated with this type of air pollution. \(\mathrm{Ni}\) trogen dioxide is also a contributor to acid rain because when rain passes through air contaminated with \(\mathrm{NO}_{2}\), it dissolves and undergoes the following reaction: \(\mathrm3{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{NO}(g)+2 \mathrm{H}^{+}(a q)+2 \mathrm{NO}_{3}^{-}(a q)\) In this reaction, which element is reduced and which is oxidized? Which is the oxidizing agent and which is the reducing agent?

Short answer

Nitrogen is reduced from +4 to +2 in NO, and oxidized from +4 to +5 in NO_{3}^{-}. NO_{2} acts as both the oxidizing and reducing agent.

Step-by-step solution

Identify the Oxidation States

First, determine the oxidation states of nitrogen in both the reactants and the products. In NO_{2}, nitrogen has an oxidation state of +4. In NO, nitrogen has an oxidation state of +2. The H^{+} and NO_{3}^{-} ions have nitrogen with an oxidation state of +5.

Determine Reduction and Oxidation

Reduction is the gain of electrons, which means a decrease in oxidation state. Oxidation is the loss of electrons, which means an increase in oxidation state. Nitrogen in NO_{2} goes from an oxidation state of +4 to +2 in NO, so it is reduced. Nitrogen in NO_{2} goes from +4 to +5 in NO_{3}^{-}, so it is oxidized.

Identify the Oxidizing and Reducing Agents

The oxidizing agent is the species that is reduced, while the reducing agent is the species that is oxidized. Therefore, NO_{2} is the oxidizing agent because it is reduced to NO. Also, NO_{2} is the reducing agent because it provides oxygen for the formation of NO_{3}^{-}, indicating it is oxidized.

Key concepts

Oxidation States

In chemistry, understanding oxidation states is crucial for analyzing redox reactions. An oxidation state indicates the degree of oxidation of an atom in a chemical compound. It is a hypothetical charge that an atom would have if all bonds to atoms of different elements were fully ionic. For instance, in the pollutant nitrogen dioxide \(\mathrm{NO}_2\), nitrogen has an oxidation state of +4. This is determined by knowing that oxygen typically has an oxidation state of -2, and since there are two oxygen atoms, nitrogen must balance this with a +4. By comparing the oxidation states of nitrogen in different compounds, like \(\mathrm{NO}_2\) to \(\mathrm{NO}\) and \(\mathrm{NO}_3^-\), we can determine which atoms are oxidized or reduced during a chemical reaction.

To maximize clarity for students, it's beneficial to emphasize the method of calculating oxidation states: subtracting the charge on the more electronegative atoms from the overall charge of the molecule or ion. By practicing with various examples, students can develop confidence in identifying the oxidation states in more complex molecules or reactions.

Reduction and Oxidation

Reduction and oxidation are two halves of the same coin in redox chemistry. They occur simultaneously during redox reactions. A helpful mnemonic to remember this is 'OIL RIG'—Oxidation Is Loss, Reduction Is Gain. It refers to electrons, which are the currency of chemical reactions. When an atom undergoes oxidation, it loses electrons, causing an increase in its oxidation state. Conversely, reduction involves an atom gaining electrons, leading to a decrease in its oxidation state.

In our \(\mathrm{NO}_2\) smog example, nitrogen's state decreases from +4 in \(\mathrm{NO}_2\) to +2 in \(\mathrm{NO}\). Hence, \(\mathrm{NO}_2\) gets reduced. Similarly, nitrogen in \(\mathrm{NO}_2\) is also oxidized as it changes to the +5 state in \(\mathrm{NO}_3^-\), forming a part of acid rain. Facilitating student comprehension here may involve associating the concept of electron 'ownership' shift with oxidation and reduction, using illustrations or animations to visualize the movement of electrons.

Oxidizing and Reducing Agents

Identifying the agents responsible for the transfer of electrons in reactions aids in understanding the mechanism of redox processes. The oxidizing agent, or oxidant, accepts electrons and gets reduced in the process. In contrast, the reducing agent, or reductant, donates electrons and is oxidized. Referring back to the smog problem involving \(\mathrm{NO}_2\), not only does it serve as a concerning environmental pollutant, but it also plays a dual role in its reaction with water. It acts as both an oxidizing and reducing agent, as it gets reduced to \(\mathrm{NO}\) on one hand and is oxidized to \(\mathrm{NO}_3^-\) on the other.

For educational clarity, demonstrating examples of common oxidizing and reducing agents can be illustrative. For instance, halogens like chlorine often act as oxidizing agents, while metals such as zinc can frequently serve as reducing agents. By learning the typical roles of different elements and compounds, students can predict and understand the behavior of substances in redox reactions.