Question

Write balanced chemical equations for each of the following reactions: (a) The nitric oxide molecule undergoes photodissociation in the upper atmosphere. (b) The nitric oxide molecule undergoes photoionization in the upper atmosphere. (c) Nitric oxide undergoes oxidation by ozone in the stratosphere. (d) Nitrogen dioxide dissolves in water to form nitric acid and nitric oxide.

Short answer

(a) \( NO \xrightarrow{h\nu} N + O \) (b) \( NO + h\nu \rightarrow NO^+ + e^- \) (c) \( NO + O_3 \rightarrow NO_2 + O_2 \) (d) \( 3NO_2 + H_2O \rightarrow 2HNO_3 + NO \)

Step-by-step solution

(a) Photodissociation of nitric oxide in the upper atmosphere

In this reaction, nitric oxide (NO) undergoes photodissociation. Photodissociation is the process by which a molecule absorbs a photon of light and breaks apart into smaller fragments. For NO, it will dissociate into nitrogen (N) and oxygen (O) atoms. The balanced chemical equation for this reaction is: \[ NO \xrightarrow{h\nu} N + O \]

(b) Photoionization of nitric oxide in the upper atmosphere

In this reaction, nitric oxide (NO) undergoes photoionization. Photoionization is the process by which a molecule absorbs a photon of light and becomes ionized by losing an electron. For NO, it will form a positively charged nitric oxide ion (NO+) and an electron (e-). The balanced chemical equation for this reaction is: \[ NO + h\nu \rightarrow NO^+ + e^- \]

(c) Oxidation of nitric oxide by ozone in the stratosphere

In this reaction, nitric oxide (NO) undergoes oxidation by ozone (O3) in the stratosphere. This reaction produces nitrogen dioxide (NO2) and oxygen (O2) molecules as products. The balanced chemical equation for this reaction is: \[ NO + O_3 \rightarrow NO_2 + O_2 \]

(d) Dissolving nitrogen dioxide in water to form nitric acid and nitric oxide

In this reaction, nitrogen dioxide (NO2) dissolves in water (H2O) to form nitric acid (HNO3) and nitric oxide (NO). The balanced chemical equation for this reaction is: \[ 3NO_2 + H_2O \rightarrow 2HNO_3 + NO \] These are the balanced chemical equations for the given reactions involving nitric oxide and related molecules.

Key concepts

Photodissociation

Photodissociation is a type of chemical reaction where a molecule absorbs a photon of light, often from the sun, and consequently breaks apart. This is particularly significant in atmospheric chemistry. In the upper atmosphere, various molecules can undergo photodissociation.
For nitric oxide (NO), this process involves the absorption of a photon, which provides enough energy to break the chemical bond between nitrogen (N) and oxygen (O), resulting in separate atoms. It can be represented by the equation:

• \[ NO \xrightarrow{hu} N + O \]

The photodissociation of NO plays a role in the ozone layer's chemistry and can influence environmental conditions. It's a natural process occurring due to the abundance of solar radiation in the atmosphere. Understanding photodissociation helps us comprehend how sunlight contributes to breaking down pollutants.

Photoionization

Photoionization involves the absorption of photons by a molecule, which results in the ejection of an electron. This leads to the formation of a charged ion. In the case of nitric oxide (NO), when it absorbs a photon, it loses an electron and transforms into a positively charged ion (NO⁺), along with an electron (e^-).
The equation for this reaction is:

• \[ NO + hu \rightarrow NO^+ + e^- \]

Photoionization is crucial in understanding the chemical processes occurring in the upper atmosphere, especially in regions exposed to intense solar radiation. It is also important in fields like astronomy, where it helps explain the ionization of gases around stars.

Oxidation Reactions

Oxidation reactions are chemical processes where a substance loses electrons, often involving oxygen. In the stratosphere, nitric oxide (NO) undergoes oxidation when it reacts with ozone (O₃). This interaction results in nitrogen dioxide (NO₂) and molecular oxygen (O₂).
This reaction can be written as:

• \[ NO + O_3 \rightarrow NO_2 + O_2 \]

Such reactions are significant in understanding how pollutants are transformed in the atmosphere. This particular oxidation reaction contributes to the regulation of ozone levels and thus plays a part in protecting the Earth from harmful ultraviolet rays.

Acid Formation

Acid formation in the atmosphere often involves the dissolution of gases in water. When nitrogen dioxide (NO₂) dissolves in water, it can lead to the formation of nitric acid (HNO₃) and nitric oxide (NO).
This process is depicted by the equation:

• \[ 3NO_2 + H_2O \rightarrow 2HNO_3 + NO \]

The formation of acids such as nitric acid in the environment is of particular interest due to their roles in acid rain. Acid rain can lead to environmental damage by lowering the pH of soils and water bodies. Understanding these reactions is key in environmental chemistry for developing strategies to mitigate pollution and its effects.