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 + hν → N + O (b) NO + hν → NO+ + e^- (c) NO + O3 → NO2 + O2 (d) 3NO2 + H2O → 2HNO3 + NO

Step-by-step solution

(a) Photodissociation of nitric oxide in the upper atmosphere

Photodissociation is a process where a molecule absorbs light and breaks into its constituent atoms. For nitric oxide (NO), the process can be represented as: NO + hν → N + O Where hν is the energy in the form of a photon. As the reaction is already balanced, no further steps are needed for this part.

(b) Photoionization of nitric oxide in the upper atmosphere

Photoionization is the process of ionizing a molecule through the absorption of light. For nitric oxide, this can be represented by the following equation: NO + hν → NO+ + e^- Here, NO absorbs energy from the photon and loses an electron, resulting in the charged ion NO+ and an ejected electron. The reaction is balanced in terms of atoms and charges.

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

The oxidation of nitric oxide by ozone (O3) in the stratosphere can be represented by the following chemical equation: NO + O3 → NO2 + O2 This reaction is also balanced in terms of atoms and no further steps are needed.

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

When nitrogen dioxide (NO2) dissolves in water (H2O), it forms nitric acid (HNO3) and nitric oxide (NO). The balanced chemical equation for this reaction is: 3NO2 + H2O → 2HNO3 + NO The balanced chemical equation shows that three moles of nitrogen dioxide react with one mole of water to produce two moles of nitric acid and one mole of nitric oxide.

Key concepts

Photodissociation in the Atmosphere

Photodissociation is a critical atmospheric process in which molecules absorb light (photons) and subsequently break down into simpler components. It commonly occurs in the upper layers of the atmosphere, where solar radiation is intense. For instance, nitric oxide (NO) is a molecule that can undergo photodissociation when exposed to solar radiation, as represented by the equation:
\[ NO + hu \rightarrow N + O \]
Here, the symbol \( hu \) denotes the energy of a photon absorbed by the nitric oxide molecule. The molecular bond in NO breaks, resulting in the formation of separate nitrogen (N) and oxygen (O) atoms. Notably, the balancing of atoms on both sides of the equation is crucial to accurately reflect the conservation of matter. Photodissociation plays a pivotal role in atmospheric chemistry, influencing the concentration and distribution of different compounds in the atmosphere.

Photoionization in the Upper Atmosphere

Photoionization is another photochemical reaction that occurs when a molecule absorbs energy from light and releases one or more electrons, becoming positively charged as a result. When nitric oxide experiences photoionization, the process is described by the following equation:
\[ NO + hu \rightarrow NO^+ + e^- \]
The photons (\(hu\)) provide sufficient energy that an electron (\(e^-\)) is ejected from the nitric oxide molecule, forming a positively charged nitric oxide ion (\(NO^+\)). This transformation is essential for understanding the behaviour of molecules in the presence of light, as well as their interactions with other atmospheric species. Moreover, balancing the charges in addition to the atoms is necessary to ensure the equation adheres to the laws of conservation of charge as well as mass.

Oxidation Reactions with Ozone

Oxidation reactions involve the transfer of electrons between chemical species, often resulting in changes in their oxidation states. In the stratosphere, nitric oxide undergoes oxidation through a reaction with ozone (O3), playing a significant role in the atmospheric cycles of nitrogenous compounds. The balanced reaction is given by:
\[ NO + O_3 \rightarrow NO_2 + O_2 \]
In this process, nitric oxide (NO) acts as a reducing agent and becomes nitrogen dioxide (NO2), while ozone is reduced to diatomic oxygen (O2). The conservation of atoms and charge is evident here, as the total number of oxygen atoms is the same on both sides of the equation. This type of reaction is essential for maintaining the delicate balance of reactive oxygen and nitrogen species in the atmosphere, which can have implications for both environmental and human health.

Dissolution of Nitrogen Dioxide in Water

The dissolution of gases in water is a common occurrence in environmental processes, one example being the dissolution of nitrogen dioxide (NO2) in water to form nitric acid (HNO3) and nitric oxide (NO). This represents an important chemical pathway for the removal of NO2 from the atmosphere and its incorporation into the hydrosphere. The reaction can be depicted as:
\[ 3NO_2 + H_2O \rightarrow 2HNO_3 + NO \]
Balancing this equation requires that the number of each type of atom and the total charge are equal on both sides, reflecting the conservation of mass. This reaction not only balances atoms but also demonstrates the principle of dissolution in water, where gases interact with water to produce new compounds, like the formation of nitric acid, which is a component of acid rain. Understanding such reactions helps explain the interconnectedness of atmospheric and hydrological cycles and their impact on environmental chemistry and ecosystem health.