Textbook Question
(a) Distinguish between photodissociation and photoionization.
Ch.18 - Chemistry of the Environment
Chapter 18, Problem 21b
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?
Verified step by step guidance1
First, understand that photodissociation occurs when a molecule absorbs a photon with enough energy to break a chemical bond. The energy of a photon can be calculated using the formula: E = \( \frac{hc}{\lambda} \), where E is the energy, h is Planck's constant (6.626 x 10^-34 J·s), c is the speed of light (3.00 x 10^8 m/s), and \( \lambda \) is the wavelength in meters.
Convert the given wavelength from nanometers to meters. Since 1 nm = 1 x 10^-9 m, 145 nm is equivalent to 145 x 10^-9 m.
Calculate the energy of a single photon with a wavelength of 145 nm using the formula E = \( \frac{hc}{\lambda} \). Substitute the values for h, c, and \( \lambda \) to find the energy in joules.
Convert the bond energy of O2 from kJ/mol to J/molecule. Since 1 mol contains Avogadro's number of molecules (6.022 x 10^23 molecules/mol), divide the bond energy by Avogadro's number to find the energy required to break one O2 molecule in joules.
Compare the energy of the photon calculated in step 3 with the energy required to break the O2 bond from step 4. If the photon's energy is greater than or equal to the bond energy, it can photodissociate O2. For photoionization, the photon must have even more energy to remove an electron completely, which typically requires more energy than bond dissociation.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Photon Energy Calculation
The energy of a photon can be calculated using the equation E = hc/λ, where E is energy, h is Planck's constant (6.626 x 10^-34 J·s), c is the speed of light (3.00 x 10^8 m/s), and λ is the wavelength in meters. For a wavelength of 145 nm, this calculation will determine if the photon has sufficient energy to break molecular bonds or ionize atoms.
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Bond Energy
Bond energy is the amount of energy required to break one mole of bonds in a substance. In this case, the bond energy of O2 is given as 495 kJ/mol, which indicates the energy needed to dissociate the O2 molecule into individual oxygen atoms. Understanding bond energy is crucial for determining whether the energy from the photon is adequate for photodissociation.
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Photoionization
Photoionization is the process by which an atom or molecule absorbs a photon and subsequently loses an electron, resulting in the formation of a positively charged ion. The energy required for photoionization is typically higher than that needed for photodissociation, making it essential to compare the photon's energy with the ionization energy of O2 to assess if it can cause ionization.
Related Practice
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 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). (a) Photons from which region have the highest energy and therefore are the most harmful to living tissue? (315–400 nm), UV-B (280–315 nm), and UV-C (100–280 nm).
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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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). (c) When appropriate concentrations of ozone are present in the stratosphere, is all of the UV light absorbed before reaching the Earth’s surface? If not, which region or regions are not filtered out?