Problem 18 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

The photodissociation of \(\mathrm{N}_{2}\) is relatively unimportant compared to \(\mathrm{O}_{2}\) because \(\mathrm{N}_{2}\) has a stronger bond and weaker absorption spectrum than \(\mathrm{O}_{2}\). Additionally, the photodissociation of \(\mathrm{O}_{2}\) has significant atmospheric implications, such as the formation of the ozone layer, while the photodissociation of \(\mathrm{N}_{2}\) does not have such effects.

Step 1: Understanding the molecules

\(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\) are both diatomic molecules, with nitrogen and oxygen being the two most abundant elements in Earth's atmosphere. Nitrogen makes up about 78% of the atmosphere, while oxygen makes up about 21%.

Step 2: Analyzing Bond Strength

One factor that affects the photodissociation of molecules is their bond strength. \(\mathrm{N}_{2}\) has a triple bond, which is stronger than the double bond found in \(\mathrm{O}_{2}\). The bond dissociation energy of \(\mathrm{N}_{2}\) is about 941 kJ/mol, while that of \(\mathrm{O}_{2}\) is about 498 kJ/mol. This means that it requires more energy to break the bond in \(\mathrm{N}_{2}\) than in \(\mathrm{O}_{2}\).

Step 3: Comparing Absorption Spectra

Both \(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\) absorb ultraviolet (UV) radiation, causing them to photodissociate. However, their absorption spectra are different. \(\mathrm{O}_{2}\) strongly absorbs wavelengths below 240 nm, while \(\mathrm{N}_{2}\) has a much weaker absorption in this region. This means that \(\mathrm{O}_{2}\) will absorb more UV radiation and undergo photodissociation more readily than \(\mathrm{N}_{2}\).

Step 4: Atmospheric Implications

The photodissociation of \(\mathrm{O}_{2}\) in the atmosphere is important because it leads to the formation of ozone (O3). The ozone layer in Earth's stratosphere protects life on the surface from harmful ultraviolet radiation. In contrast, the photodissociation of \(\mathrm{N}_{2}\) has less significant atmospheric implications, as it does not contribute to the formation of any protective layer or participate in major life-sustaining processes like photosynthesis or respiration.

Step 5: Conclusion

The photodissociation of \(\mathrm{N}_{2}\) is relatively unimportant compared with the photodissociation of \(\mathrm{O}_{2}\) for a few reasons: 1. The bond strength in \(\mathrm{N}_{2}\) is stronger than in \(\mathrm{O}_{2}\), making it more difficult to break apart. 2. The absorption spectrum of \(\mathrm{N}_{2}\) is weaker than that of \(\mathrm{O}_{2}\) in the relevant wavelength region, meaning it absorbs less ultraviolet radiation. 3. The photodissociation of \(\mathrm{O}_{2}\) has significant implications for Earth's atmosphere, such as the formation of the ozone layer, while the photodissociation of \(\mathrm{N}_{2}\) does not have such effects.