Question

The hydroxyl radical, \(\mathrm{OH}\), is formed at low altitudes via the reaction of excited oxygen atoms with water: $$ \mathrm{O}^{4}(g)+\mathrm{H}_{2} \mathrm{O}(g) \longrightarrow 2 \mathrm{OH}(g) $$ (a) Write the Lewis structure for the hydroxyl radical (Hint: It has one unpaired electron.) Once produced, the hydroxyl radical is very reactive. Explain why each of the following series of reactions affects the pollution in the troposphere: (b) \(\mathrm{OH}+\mathrm{NO}_{2} \longrightarrow \mathrm{HNO}_{3}\) (c) \(\mathrm{OH}+\mathrm{CO}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}+\mathrm{OOH}\) \(\mathrm{OOH}+\mathrm{NO} \longrightarrow \mathrm{OH}+\mathrm{NO}_{2}\) (d) \(\mathrm{OH}+\mathrm{CH}_{4} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{CH}_{3}\) \(\mathrm{CH}_{3}+\mathrm{O}_{2} \longrightarrow \mathrm{OOCH}_{3}\) \(\mathrm{OOCH}_{3}+\mathrm{NO} \longrightarrow \mathrm{OCH}_{3}+\mathrm{NO}_{2}\)

Short answer

The hydroxyl radical (OH) Lewis structure can be represented as O with two lone pairs and one unpaired electron bonded with H. The reactions given (OH with NO₂, OH with CO and O₂, and OH with CH₄) demonstrate how the hydroxyl radical participates in reducing air pollution in the troposphere by reacting with various pollutants and greenhouse gases such as NO₂, CO, and CH₄. These reactions reduce the concentration of these harmful substances, helping maintain the overall air quality.

Step-by-step solution

(a) Hydroxyl Radical Lewis Structure

To draw the Lewis structure of the hydroxyl radical (OH), we can start by determining the total number of valence electrons available. Oxygen has 6 valence electrons and hydrogen has 1 valence electron. Oxygen will be the central atom. The total number of valence electrons is 7. 1. Place one single bond between oxygen (O) and hydrogen (H). We have used 2 electrons so far. 2. In a Lewis structure, there should be 8 electrons around an oxygen atom, except for peroxides. However, since hydroxyl radical has an unpaired electron, it will be an exception. Place 2 lone pairs on the oxygen atom (4 more electrons). 3. Put the unpaired electron on the oxygen atom. This completes the Lewis structure. The Lewis structure of the hydroxyl radical (OH) can be represented as: ``` O H : – . ```

(b) Reaction of Hydroxyl Radical With NO2

The reaction between OH and NO2 can be written as: \[ OH + NO_2 \longrightarrow HNO_3 \] When this reaction occurs, OH and NO2 react to form nitric acid (HNO3). This is significant for tropospheric pollution because nitric acid is a key contributor to acid rain, which can have harmful effects on ecosystems and aquatic life. As this reaction removes the hydroxyl radical and NO2 from the troposphere, it helps reduce the concentrations of these pollutants in the atmosphere.

(c) Reaction Series Involving Hydroxyl Radical, CO, O2, and NO

The given reaction series involves the two following reactions: 1. \( OH + CO + O_2 \longrightarrow CO_2 + OOH \) 2. \( OOH + NO \longrightarrow OH + NO_2 \) In the first reaction, OH reacts with carbon monoxide (CO) and oxygen (O2) to produce carbon dioxide (CO2) and a peroxy radical (OOH). This reaction helps reduce the concentration of carbon monoxide, which is a harmful air pollutant emitted by automobiles and industrial processes. In the second reaction, OOH reacts with nitrogen monoxide (NO) to regenerate the hydroxyl radical (OH) and produce nitrogen dioxide (NO2). These two reactions result in a net reduction of CO and NO, which both contribute to air pollution in the troposphere.

(d) Reaction Series Involving Hydroxyl Radical, Methane, O2, and NO

The given reaction series involves the following three reactions: 1. \( OH + CH_4 \longrightarrow H_2O + CH_3 \) 2. \( CH_3 + O_2 \longrightarrow OOCH_3 \) 3. \( OOCH_3 + NO \longrightarrow OCH_3 + NO_2 \) In the first reaction, the hydroxyl radical (OH) reacts with methane (CH4), a greenhouse gas, to form water (H2O) and a methyl radical (CH3). This helps to reduce the concentration of CH4 in the atmosphere. In the second reaction, the CH3 radical reacts with O2 to form a methoxyperoxy radical (OOCH3). In the third reaction, OOCH3 reacts with NO to regenerate NO2 and form a methoxy radical (OCH3). These subsequent reactions ultimately lead to the reduction of methane levels and removal of the methyl radical from the atmosphere. Overall, these reaction series contribute to the reduction of pollutants and greenhouse gases in the troposphere.

Key concepts

Lewis Structure

Understanding the Lewis structure of the hydroxyl radical ( \( \text{OH} \) ) is important because it highlights why this molecule is highly reactive. The hydroxyl radical has a unique arrangement of electrons, as it contains an unpaired electron. Here's how it is structured:

• Oxygen supplies 6 valence electrons, while hydrogen provides 1, resulting in a total of 7 electrons.
• A single bond is formed between the oxygen and hydrogen atoms, using 2 electrons.
• The oxygen atom holds 2 lone pairs of electrons, utilizing 4 more electrons.
• One electron remains unpaired on the oxygen atom, making it highly reactive.

This unpaired electron allows the hydroxyl radical to interact with various other molecules, especially in atmospheric chemistry, where it plays a fundamental role in the breakdown of air pollutants.

Troposphere Pollution

Pollutants in the troposphere, the lowest layer of Earth's atmosphere, greatly impact environmental and human health. The hydroxyl radical's reactivity plays a pivotal role in altering these pollutants. For instance, when the hydroxyl radical reacts with nitrogen dioxide ( \( NO_2 \) ), it forms nitric acid ( \( HNO_3 \) ):

\[ \text{OH} + \text{NO}_2 \longrightarrow \text{HNO}_3 \]

This reaction is significant as nitric acid contributes to acid rain, which affects soil, water bodies, and living organisms by altering their pH balance.

• Acid rain damages plants, aquatic life, and infrastructure.
• Reducing the presence of hydroxyl radicals and nitrogen oxides diminishes this problem.

In essence, the hydroxyl radical helps balance pollutants, decreasing their harmful concentrations in the troposphere.

Air Pollutants

Air pollutants like carbon monoxide (CO) and nitrogen monoxide ( \( NO \) ) are harmful to human health and the environment. The reactivity of the hydroxyl radical is beneficial in reducing these pollutants through a series of reactions:

• First reaction: \( \text{OH} + \text{CO} + \text{O}_2 \longrightarrow \text{CO}_2 + \text{OOH} \) reduces carbon monoxide, converting it into carbon dioxide.
• Second reaction: \( \text{OOH} + \text{NO} \longrightarrow \text{OH} + \text{NO}_2 \) changes nitrogen monoxide into nitrogen dioxide, continuing the cleansing cycle.

Together, these reactions play a role in minimizing the levels of CO and NO, which contribute to pollution and environmental harm. These transformations are part of a self-cleansing mechanism in the troposphere, leading to reduced air pollutant concentrations.

Greenhouse Gases Reactions

Greenhouse gases like methane ( \( CH_4 \) ) are significant contributors to climate change. The hydroxyl radical can initiate a sequence to reduce methane levels in the atmosphere, thus limiting its greenhouse effect:

• The first reaction: \( \text{OH} + \text{CH}_4 \longrightarrow \text{H}_2\text{O} + \text{CH}_3 \) breaks down methane, forming water and a methyl radical.
• The second reaction: \( \text{CH}_3 + \text{O}_2 \longrightarrow \text{OOCH}_3 \) creates a methoxyperoxy radical.
• The third reaction: \( \text{OOCH}_3 + \text{NO} \longrightarrow \text{OCH}_3 + \text{NO}_2 \) further transforms the radicals, with nitrogen dioxide formed as a byproduct.

These reactions help in mitigating methane, a potent greenhouse gas, reducing its concentration in the atmosphere. Consequently, the hydroxyl radical plays a valuable role in managing greenhouse gas emissions and addressing climate change challenges.