Question

Why is the photodissociation of \(\mathrm{N}_{2}\) in the atmosphere relatively unimportant compared with the photodissociation of \(\mathrm{O}_{2}\) ?

Short answer

The photodissociation of \(\mathrm{N}_2\) in the atmosphere is relatively unimportant compared to that of \(\mathrm{O}_2\) due to the higher bond dissociation energy of the nitrogen-nitrogen triple bond in \(\mathrm{N}_2\) and its lower absorption cross-section for sunlight. Despite the higher concentration of \(\mathrm{N}_2\) in the atmosphere, these factors make it much less likely to undergo photodissociation. In contrast, the photodissociation of \(\mathrm{O}_2\) plays a crucial role in atmospheric chemistry, particularly in the formation and destruction of ozone.

Step-by-step solution

Understanding photodissociation

Photodissociation, also known as photolysis, is a chemical reaction in which a molecule absorbs a photon of light and breaks apart into its individual atoms or smaller molecules. In the context of the atmosphere, this process plays a significant role in the formation and destruction of ozone as well as influencing other atmospheric chemical processes.

Factors influencing photodissociation rates

The rate of photodissociation depends on several factors including the energy of the light photons, the probability that a molecule will absorb the light (absorption cross-section), and the number of molecules available to dissociate (concentration).

Relative bond energies of N2 and O2

The energy required to break the bond in a \(\mathrm{N}_2\) molecule is higher than that in an \(\mathrm{O}_2\) molecule. The nitrogen-nitrogen triple bond in \(\mathrm{N}_2\) is very strong, with a bond dissociation energy of approximately \(941 \; \mathrm{kJ/mol}\). In contrast, the oxygen-oxygen double bond in \(\mathrm{O}_2\) has a lower bond dissociation energy of approximately \(498 \; \mathrm{kJ/mol}\).

Absorption cross-section for N2 and O2

The absorption cross-section of a molecule is a measure of its likelihood to absorb a photon of a specific wavelength. The photodissociation of \(\mathrm{N}_2\) is much less likely than that of \(\mathrm{O}_2\) because \(\mathrm{N}_2\) has a very low absorption cross-section for the wavelengths of light present in the Earth's atmosphere. This means that \(\mathrm{N}_2\) molecules have a lower probability of absorbing the incoming sunlight and undergoing photodissociation. \(\mathrm{O}_2\), on the other hand, has a relatively higher absorption cross-section for the ultraviolet (UV) light in the atmosphere, making its photodissociation more likely.

Concentration of N2 and O2 in the atmosphere

The concentration of a molecule in the atmosphere affects the rate of photodissociation. Although \(\mathrm{N}_2\) is more abundant in the atmosphere, making up approximately 78% of the air, while \(\mathrm{O}_2\) constitutes approximately 21%, the low absorption cross-section of \(\mathrm{N}_2\) makes its photodissociation much less likely, even though there are more \(\mathrm{N}_2\) molecules than \(\mathrm{O}_2\).

Conclusion

Taking into consideration the differences in bond energies, absorption cross-sections, and concentrations of \(\mathrm{N}_2\) and \(\mathrm{O}_2\) in the atmosphere, it becomes clear that the photodissociation of \(\mathrm{N}_2\) is relatively unimportant compared to that of \(\mathrm{O}_2\). The strong triple bond in \(\mathrm{N}_2\), coupled with its low absorption cross-section for sunlight, makes it significantly less likely to undergo photodissociation than \(\mathrm{O}_2\). The photodissociation of \(\mathrm{O}_2\) plays a crucial role in atmospheric chemistry, particularly in the formation and destruction of ozone.

Key concepts

Atmospheric Chemistry

To appreciate the complex interactions within our Earth's atmosphere, one must understand atmospheric chemistry. This branch of science focuses on chemical transformations in the air and the effects of both natural processes and human activities.

For instance, photodissociation, a pivotal atmospheric chemical reaction, involves the breakdown of molecules into simpler substances when they absorb light. The process has a profound impact on the composition and behavior of atmospheric gases, influencing phenomena like the formation of the protective ozone layer as well as playing a part in air pollution degradation.

The atmosphere is composed mainly of nitrogen (\textbf{N\(_2\)}), oxygen (\textbf{O\(_2\)}), and smaller amounts of other gases. While both \textbf{N\(_2\)} and \textbf{O\(_2\)} undergo photodissociation, \textbf{O\(_2\)} is more prone to this because of its lower bond dissociation energy and greater absorption cross-section, making it a key player in atmospheric reactions.

Bond Dissociation Energy

The bond dissociation energy represents the strength of the bond between two atoms in a molecule; it's the amount of energy required to break the bond and separate the atoms. In simple terms, bonds that require more energy to break are considered stronger and more stable, making them less likely to participate in reactions like photodissociation.

For instance, the nitrogen molecules in our atmosphere, comprised of two nitrogen atoms and connected by a triple bond, have a very high bond dissociation energy, approximately \textbf{941 kJ/mol}. This strong bond makes nitrogen fairly inert under normal atmospheric conditions. In contrast, the double bond between two oxygen atoms in an oxygen molecule has a lower bond dissociation energy of about \textbf{498 kJ/mol}, making oxygen more reactive and likely to participate in photodissociation and other atmospheric reactions.

This difference in bond strength can help us understand why \textbf{N\(_2\)}, though abundant, is less chemically active in the atmosphere compared to \textbf{O\(_2\)}.

Absorption Cross-Section

The absorption cross-section of a molecule is a critical factor in atmospheric chemistry. It denotes a molecule's capability to absorb light of a particular wavelength, which is vital for photodissociation to occur. A higher absorption cross-section means that a molecule is more likely to absorb photons and undergo chemical change.

\textbf{O\(_2\)} has a relatively higher absorption cross-section for ultraviolet (UV) radiation compared to \textbf{N\(_2\)}, which means that \textbf{O\(_2\)} will absorb UV light more readily, contributing to a higher rate of photodissociation. This is a crucial detail when picturing the Earth’s stratosphere, where \textbf{O\(_2\)}'s interaction with UV light results in the production of ozone (\textbf{O\(_3\)}), a molecule essential for shielding the surface from the sun's harmful rays.

In summary, the combination of a lower bond dissociation energy and a higher absorption cross-section makes oxygen molecules more susceptible to photodissociation. Consequently, this leads to their significant role in shaping the atmosphere’s chemical profile.