Problem 76 - Chemistry: The Central Science

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) Nitricoxide undergoes oxidation by ozone in the stratesphere. (d) Nitrogen dioxide dissolves in water to form nitric acid and nitric oxide.

Step-by-Step Solution

Answer

(a) Photodissociation of Nitric Oxide: \[ NO \xrightarrow{h\nu} N + O \] (b) Photoionization of Nitric Oxide: \[ NO + h\nu \rightarrow NO^+ + e^- \] (c) Oxidation of Nitric Oxide by Ozone in the Stratosphere: \[ NO + O_3 \rightarrow NO_2 + O_2 \] (d) Dissolution of Nitrogen Dioxide in Water to form Nitric Acid and Nitric Oxide: \[ 3NO_2 + H_2O \rightarrow 2HNO_3 + NO \]

Step 1: (a) Photodissociation of Nitric Oxide

When nitric oxide (NO) molecules undergo photodissociation in the upper atmosphere, they absorb energy from sunlight and break apart into individual nitrogen (N) and oxygen (O) atoms. The balanced chemical equation for this reaction is: \[ NO \xrightarrow{h\nu} N + O \]

Step 2: (b) Photoionization of Nitric Oxide

In the upper atmosphere, nitric oxide (NO) can also undergo photoionization, in which the molecule absorbs energy from sunlight and loses an electron, forming a positive ion. The balanced chemical equation for this reaction is: \[ NO + h\nu \rightarrow NO^+ + e^- \]

Step 3: (c) Oxidation of Nitric Oxide by Ozone in the Stratosphere

In the stratosphere, nitric oxide (NO) can react with ozone (O₃) to produce nitrogen dioxide (NO₂) and molecular oxygen (O₂). This process is known as oxidation. The balanced chemical equation for this reaction is: \[ NO + O_3 \rightarrow NO_2 + O_2 \]

Step 4: (d) Dissolution of Nitrogen Dioxide in Water to form Nitric Acid and Nitric Oxide

When nitrogen dioxide (NO₂) dissolves in water, it forms 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 process where a molecule is broken down into smaller components as a result of absorbing light, usually within the ultraviolet or visible spectrum. In the context of atmospheric chemistry, this phenomenon is critical in the formation and breakdown of various compounds. For instance, nitric oxide (NO) can undergo photodissociation when exposed to solar radiation, breaking down into nitrogen (N) and oxygen (O) atoms, as represented by the balanced chemical equation:

\[ NO \xrightarrow{hu} N + O \]

This reaction plays a pivotal role in the upper layers of the atmosphere where high-energy photons are abundant.

Photoionization

Photoionization occurs when a molecule absorbs a photon of sufficient energy to eject an electron, thus forming a positive ion. This is another crucial reaction in the upper atmosphere, particularly for species like nitric oxide (NO). When NO absorbs high-energy solar radiation, an electron is removed, creating a positive ion (NO+) along with a free electron (e-). The balanced chemical equation is as follows:

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

This process contributes to the ionization of the atmospheres, leading to the creation of the ionosphere, an important layer that affects radio wave transmission and has implications for atmospheric electricity.

Oxidation Reactions

Oxidation reactions describe a broad class of chemical processes involving the transfer of electrons, where one substance gets oxidized while another is reduced. In atmospheric chemistry, oxidation reactions are vital for converting pollutants into less harmful substances. An example is the reaction of nitric oxide (NO) with ozone (O₃) to form nitrogen dioxide (NO₂) along with diatomic oxygen (O₂), as shown in the equation:

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

This oxidation reaction helps regulate atmospheric composition and affects the concentration of ozone, which is a critical component of the stratosphere.

The Nitrogen Cycle

The nitrogen cycle is a series of natural processes by which nitrogen, essential for all living organisms, is converted between various chemical forms. This cycle plays a significant part in maintaining the balance of nitrogenous compounds in the environment. An example within this cycle is the dissolution of nitrogen dioxide (NO₂) in water to form nitric acid (HNO₃) and nitric oxide (NO), essential for the formation of soil nitrates. The balanced equation for this hydrolysis reaction is:

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

These nitrates are vital nutrients for plant growth, making them a cornerstone of agriculture and ecosystem functionality.

Atmospheric Chemistry

Atmospheric chemistry is the branch of science that explores the chemical composition of the Earth's atmosphere and the reactions that take place within it. It includes the study of naturally occurring gases, air pollutants, and their interactions, all of which have profound effects on climate, air quality, and the ozone layer. The chemical equations provided above are fundamental examples of atmospheric reactions that govern the behavior of crucial compounds such as nitric oxide, nitrogen dioxide, and ozone. Understanding these chemical transformations is essential for development of strategies to address issues such as climate change and ozone depletion.