Ch.18 - Chemistry of the Environment

Problem 21a

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Chapter 18, Problem 21a

The wavelength at which the O2 molecule most strongly absorbs light is approximately 145 nm. (a) In which region of the electromagnetic spectrum does this light fall?

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1. The electromagnetic spectrum is divided into several regions, each characterized by a specific range of wavelengths. These regions, in order of increasing wavelength (or decreasing frequency), are: Gamma rays, X-rays, Ultraviolet (UV), Visible light, Infrared (IR), Microwaves, and Radio waves.

2. The wavelength given in the problem, 145 nm (nanometers), falls into the Ultraviolet (UV) region of the spectrum. This is because the UV region encompasses wavelengths from about 10 nm to 400 nm.

3. Therefore, the light that the O2 molecule most strongly absorbs, which has a wavelength of approximately 145 nm, falls into the Ultraviolet (UV) region of the electromagnetic spectrum.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electromagnetic Spectrum

The electromagnetic spectrum encompasses all types of electromagnetic radiation, arranged by wavelength or frequency. It includes various regions such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Each region has distinct properties and applications, with ultraviolet light being particularly relevant for the absorption characteristics of molecules like O2.

Ultraviolet Light

Ultraviolet (UV) light is a type of electromagnetic radiation with wavelengths shorter than visible light, typically ranging from about 10 nm to 400 nm. It is divided into three categories: UVA, UVB, and UVC, with 145 nm falling within the UVC range. UV light is known for its ability to cause electronic transitions in molecules, which is why O2 absorbs strongly at this wavelength.

Molecular Absorption

Molecular absorption refers to the process by which molecules take in light energy, leading to electronic transitions. When a photon of light matches the energy difference between two electronic states of a molecule, it can be absorbed, resulting in excitation. For O2, the strong absorption at 145 nm indicates that this wavelength corresponds to a specific electronic transition, which is crucial for understanding its behavior in various chemical and physical processes.

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Related Practice

The dissociation energy of a carbon-bromine bond is typically about 276 kJ/mol. (a) What is the maximum wavelength of photons that can cause C-Br bond dissociation?

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Open Question

In CF3Cl, the C-Cl bond dissociation energy is 339 kJ/mol. In CCl4, the C-Cl bond dissociation energy is 293 kJ/mol. What is the range of wavelengths of photons that can cause C-Cl bond rupture in one molecule but not in the other?

Textbook Question

(b) Use the energy requirements of these two pro- cesses to explain why photodissociation of oxygen is more important than photoionization of oxygen at altitudes below about 90 km.

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Textbook Question

The ultraviolet spectrum can be divided into three regions based on wavelength: UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (100–280 nm). (b) In the absence of ozone, which of these three regions, if any, are absorbed by the atmo- sphere?

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Open Question

Do the reactions involved in ozone depletion involve changes in the oxidation state of the O atoms? Explain.

Textbook Question

Which of the following reactions in the stratosphere cause an increase in temperature there? (a) O(g) + O2(g) → O3+(g) (b) O3*(g) + M(g) → O3(g) + M*(g) (c) O2(g) + hv → 2 O(g) (d) O(g) + N2(g) → NO(g) + N(g) (e) All of the above

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