Textbook Question
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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Ch.18 - Chemistry of the Environment
Chapter 18, Problem 19a
(a) Distinguish between photodissociation and photoionization.
Verified step by step guidance1
Photodissociation is a process where a molecule absorbs a photon and breaks into two or more fragments. This typically involves the breaking of a chemical bond within the molecule.
Photoionization, on the other hand, occurs when a molecule absorbs a photon with enough energy to remove an electron, resulting in the formation of a positively charged ion.
In photodissociation, the energy of the absorbed photon is used to overcome the bond energy, leading to the separation of atoms or groups within the molecule.
In photoionization, the energy of the absorbed photon must be greater than the ionization energy of the molecule, which is the energy required to remove an electron from the molecule.
Both processes involve the absorption of photons, but photodissociation results in molecular fragmentation, while photoionization results in the formation of ions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Photodissociation
Photodissociation is a process in which a molecule absorbs light energy and breaks apart into two or more smaller fragments, typically atoms or smaller molecules. This occurs when the energy of the absorbed photon exceeds the bond dissociation energy of the molecule, leading to the breaking of chemical bonds. An example is the dissociation of ozone (O3) into oxygen molecules (O2) and oxygen atoms (O) upon exposure to ultraviolet light.
Photoionization
Photoionization is the process by which an atom or molecule absorbs a photon and subsequently loses one or more electrons, resulting in the formation of a positively charged ion. This process requires that the energy of the incoming photon be greater than the ionization energy of the atom or molecule. A common example is the ionization of sodium (Na) when it absorbs ultraviolet light, leading to the formation of Na+ ions.
Energy Considerations
Both photodissociation and photoionization are governed by energy considerations, specifically the relationship between the energy of the absorbed photon and the energy required to break bonds or remove electrons. In photodissociation, the photon energy must exceed the bond dissociation energy, while in photoionization, it must exceed the ionization energy. Understanding these energy thresholds is crucial for predicting the outcomes of light-matter interactions in chemical processes.
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Related Practice
Textbook Question
The dissociation energy of a carbon-bromine bond is typically about 276 kJ/mol. (b) Which kind of electromagnetic radiation—ultraviolet, visible, or infrared—does the wavelength you calculated in part (a) correspond to?
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 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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Textbook Question
The wavelength at which the O2 molecule most strongly absorbs light is approximately 145 nm. (b) Would a photon whose wavelength is 145 nm have enough energy to photodissociate O2 whose bond energy is 495 kJ/mol? Would it have enough energy to photoionize O2?