Ozone Photolysis in the Stratosphere
You've already seen that ozone in the stratosphere acts a filter to protect us from harmful UV light. How
does this happen?
Ozone and molecular oxygen are both stable molecules. From the Lewis structures we can see that the
electron count around the atoms in these molecules is 8, the same as the number for the nearest noble gas.
However, the oxygen atom is highly reactive (unstable) because it has only 6 electrons around this very
electronegative center. Because an oxygen atom is one of the products, it is easy to guess that the reaction
above is endothermic. It requires energy and this energy is supplied by electromagnetic radiation.
When UV or visible light reacts with a molecule to break a chemical bond, the process is called
photolysis. Photo = light and lysis = cutting or breaking
Other photolysis reactions in the stratosphere are important in forming
ozone and breaking it down.
Outline
•
Color and Absorption Spectroscopy
•
Bond Energy
•
Excited States and Photolysis
•
Homework
Color and Absorption Spectroscopy
Ozone molecules absorb ultraviolet light. This is radiation in a frequency too high (wavelength too short)
for us to detect with our eyes. We can detect and distinguish electromagnetic radiation between about 400
to 700 nm. Below is the picture representing the electromagnetic spectrum that you saw in the last lecture.
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Spectrometers can accurately distinguish and quantify radiation in the ultraviolet, visible, and infrared
regions of the spectrum.
You know that visible light is composed of a range of frequencies. The frequency of the radiation is
proportional to its energy and the wavelength of the radiation is inversely proportional to the energy. Red
is the lowest energy visible light and violet is the highest.
A solid object has color depending on the light it reflects. If it absorbs light in the red and yellow region of
the spectrum, it will have a blue color.
Here is an example. Chlorophyll, the
pigment that makes plants green, absorbs
light in the red end of the spectrum and light
in the blue end of the spectrum.
A green leaf is green to us because the
middle band of visible light is not absorbed
and is instead reflected into our eyes.
Our eyes have 3 types of specialized cells,
called cone cells. Each type of cone cell is
sensitive to a range of frequencies. Below
right is a graph of the wavelengths of light
absorbed by each of these cells.
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When a cone cell absorbs light in its range, it sends an electrical signal
to the brain. The intensity of the signals from each of these 3 types
of cells tells us the color of the light coming in. Each person
may have cone cells that are more or less sensitive so our
perception of color is not precise.
Instruments such as UV-visible spectrometers are precise and highly reproducible. They can also detect
and quantify electromagnetic radiation with frequencies higher and lower than the human eye can
perceive.
In a spectrometer, a beam of radiation is split into two. One beam passes through the sample and the other
goes straight to a detector.
The detector compares the sample and the reference beam to produce its signal.
Bond Energy
Ozone
In the photolysis of ozone, one of the oxygen-oxygen bonds in the molecule breaks. A specific quantity of
energy must be added to break the bond. This is the bond energy.
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We can determine the energy necessary to break the O-O bond in ozone with an experiment.
Remember that the energy in light is proportional to its frequency and inversely proportional to the
wavelength the light.
E = hc/λ
Lasers can emit light of a single wavelength, and certain types of lasers can be tuned so that they emit
light over a range of wavelengths. By changing the wavelength of laser light through a sample until the
radiation causes the O-O bond to break, we can experimentally determine the exact bond energy.
Absorption spectroscopy will tell us when absorption and bond-breaking happens.
When we find the longest wavelength (lowest energy) radiation that can break the bond, we can use it to
calculate the bond energy.
Tunable laser
For 1 molecule:
E = hc/λ
c = (3.0 x 108 m/s)(1.0 x 109 nm/m) = 3.0 x 1017 nm/s
h = 6.63 x 10-34 J s = 6.63 x 10-37 kJ s
For 1 mole, multiply the energy required to break the bond in 1 molecule by the number of molecules in a
mole:
E = (hc/λ molecule)(6.02 x 1023 molecules/mol)
E = (6.63 x 10-37 kJ s)(3.0 x 1017 nm/s)(6.02 x 1023 mol-1)/λ
E = 1.2 x 105 kJ nm mol-1/λ
The lowest energy light that can break the bond in ozone has a wavelength of 330 nm.
BE (ozone) = 1.2 x 105 kJ nm mol-1/330 nm = 364 kJ/mol
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Other O-O Bonds
We can obtain experimental data on bond energies of other molecules in the same way. Molecular oxygen,
O2, is photolyzed by light of 241 nm and has a bond energy of 498 kJ/mol. Hydrogen peroxide, HOOH,
has a very weak O-O bond and is photolyzed by light of 845 nm. Its bond energy is only about 142 kJ/
mol.
Why do we see such large difference in the strength of oxygen-oxygen bonds in these molecules. Let's
look at the Lewis structures.
The bond energy correlates with the bond order. Bond energies of some gas phase molecules are
listed below.
Excited States and Photolysis
The reactivity of a molecule depends on the number, arrangement, and energy of its valence electrons. In
this section we'll examine the changes to the energy of electrons when an atom or molecule absorbs light.
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Energy States
The electrons in a molecules have many possible energy
levels. When any molecule absorbs electromagnetic
radiation in the UV-visible range, the energy of the light
causes an electron to go from a low energy state to a higher
energy state. The molecule is in an excited state because of
this.
The red arrow represents an electron in a stable molecule.
There is a particular energy gap between the low energy
state typical for that electron and a higher energy state. The
molecule can only absorb light with a wavelength that
corresponds to the energy gap ΔE.
Energy is conserved because the energy of the ground state plus the energy of that wavelength of light
exactly equals the energy of the excited state.
At any temperature above absolute zero, the bonds in a molecules are bending and stretching. The exact
energy of the electron associated with the vibrating atoms will change a little as the atoms move.
So instead of one energy level, there can be a small
range of energy levels for the different vibrational
positions.
This is what gives absorbance bands, like the ones in
the chlorophyll spectrum, their shape. They are not
single sharp lines but have a width because the
absorbance is due to a combination of absorbances to
and from vibrational sub-levels of the electronic energy
levels.
(Note that I've used a red ball instead of an arrow to
represent the electron in this picture.)
Fluorescence and Phosphorescence
The excited state molecule has extra energy from absorbing a photon of electromagnetic radiation. It can
lose the energy in a number of ways.
Heat
Molecules collide with one another. The excited molecule can transfer its extra energy to one or more
molecules. This increases their kinetic energy. After transferring the energy, the electron falls back in its
original ground state energy.
Light
The molecule can also lose the excess energy as light. This is the reverse of the absorption step and the
excited state molecule returns to the ground state energy level.
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Light and heat
Usually when a photon of light is given off, the frequency of the emitted light is lower than that of light
that was absorbed. This is because some of the energy is dissipated as heat through vibrations of the
molecule.
When an excited state molecule returns to its ground state by losing a photon of light, we call the process
fluorescence or phosphorescence.
Photochemistry
Excited state molecules can also participate in chemical reactions. One of these reactions is photolysis,
where a bond is broken. Let's look again at ozone:
Ground state ozone absorbs radiation of 330 nm or below and becomes an excited state molecule.
The excess energy is equal to the O-O bond energy in the molecule and the excited state molecule can
break apart into products.
If there are collisions with molecules of nitrogen (for example) before the O-O bond breaks, the excess
energy is transferred to the nitrogen molecules instead. The N2 molecules move faster and have higher
kinetic energy. The average kinetic energy is proportional to temperature so the temperature of the
atmosphere in that region increases.
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