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Chemistry 360
Spring 2015
Dr. Jean M. Standard
March 27, 2015
Physical Chemistry Assignment 3: Chlorine Oxides and Ozone Depletion
(25 points)
This assignment is due on Wednesday, April 8, 2015.
The Destruction of Polar Stratospheric Ozone
It has been known for many years that chlorofluorocarbons (and even their replacements to some degree) contribute
to the destruction of the earth's protective ozone layer. In the stratosphere, ozone protects the earth from harmful
ultraviolet rays by absorbing ultraviolet photons and breaking apart to form O2 and O. Eventually, the products
recombine to form ozone.
Severe depletion of polar stratospheric ozone is observed in Antarctic winter, creating an ozone hole, as illustrated in
Figure 1. The ozone hole is most prominent in the Antarctic during the months from August through November, as
shown for the past several years in Figure 2.
Figure 1. Measured ozone amounts (Dobson units) around the south pole in October 2014 (from
http://www.theozonehole.com).
Figure 2. Area of ozone hole around the south pole during 2007-2014 (from
http://www.theozonehole.com).
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Many governmental and academic research group web sites provide overviews of ozone depletion. A few of these
sites are:
http://ozonewatch.gsfc.nasa.gov
http://www.ozonelayer.noaa.gov
http://www.epa.gov/ozone
http://www.nas.nasa.gov/About/Education/Ozone
http://www.atm.ch.cam.ac.uk/tour .
Some of these sites may be useful in learning more about the specific chemical reactions involved in polar
stratospheric ozone depletion.
1. The Role of ClO and (ClO)2 in Stratospheric Ozone Depletion (5 points)
In this part of the assignment, you will investigate some general characteristics of stratospheric ozone depletion and
the potential role that ClO and its dimer, (ClO)2, play in the destruction of the ozone layer.
a.) Using the on-line sources mentioned above, find and report the altitude range at which the stratosphere exists.
Cite your source.
b.) The image shown in Figure 1 gives ozone amounts measured in Dobson units. Define a Dobson unit.
c.) The lowest ozone amount observed in Figure 1 is about 130 DU. Calculate how many ozone molecules this
corresponds to in one square meter. (Assume ideal gas behavior.)
d.) In the polar regions, severe depletion of ozone is observed in part due to processes that take place in polar
stratospheric clouds (PSCs). What are the primary components of these clouds (there may be more than one
type)? Cite your source.
e.) The molecule chlorine oxide, ClO, plays a key role in at least two mechanisms that lead to the catalytic
decomposition of ozone in polar stratospheric regions. These two mechanisms are thought to account for about
60% of total ozone destruction in those regions. One mechanism involves the formation of the dimer, (ClO)2,
sometimes written as ClOOCl or Cl2O2. The other mechanism involves ClO and BrO. Using the sources listed
above, find and report the two key catalytic ozone destruction mechanisms in polar stratospheric regions. Give
both the individual steps in the mechanism and the overall balanced chemical reaction. Cite your source.
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2. Determination of tbe Thermochemistry of ClO and (ClO)2 (12 points)
The dimerization of ClO in the stratosphere during Antarctic winter is believed to play an important role in that
region's seasonal ozone depletion. Equilibrium constants for the reaction
(ClO) 2 ( g)
2 ClO ( g)
were determined at a variety of temperatures by Cox and Hayman in 1988 [1]. Their data is presented in the table
below.
€
€
T (K)
K eq
233
4.13×108
248
5.00×107
258
€
1.45×107
268
5.37×106
273
3.20×106
280
9.62×105
288
4.28×105
295
1.67×105
303
7.02×104
a.) From the data listed above, create a van't Hoff plot by graphing the natural log of the equilibrium constant,
ln K eq , on the y-axis with 1/T on the x-axis. Fit the data to a linear trend line and display the equation on the
graph. Turn in this plot with your assignment.
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b.) Polar stratospheric temperatures during the winter range from about 190 K to 210 K. Using the equation that
you obtained from your van't Hoff plot, calculate the equilibrium constant for the reaction at 190 K. Does the
equilibrium favor reactants or products at this temperature?
c.) From your van't Hoff plot from part (a), determine the standard molar enthalpy change of the reaction, ΔH R! ,
and the standard molar entropy change of the reaction, ΔSR! , for the dimerization of ClO. Report your results in
kJ/mol (for ΔH R! ) and J/molK (for ΔSR! ).
d.) Determine the standard molar Gibbs free energy change, ΔGR! , for the reaction at 190 K in kJ/mol.
e.) In part (c), you should have obtained a negative value for the entropy change. Explain qualitatively why you
would expect the entropy change to be negative (give a physical reason).
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3. Further Studies of the ClO / (ClO)2 Equilibrium (8 points)
Since the determination of the temperature dependence of the ClO/(ClO)2 equilibrium constant by Cox and Hayman
in 1988, many additional studies have been performed on the system because of its crucial role in ozone destruction.
The experimental studies are difficult to perform due a variety of factors, including the accurate determination of
ClO and (ClO)2 concentrations. A few recent articles related to the ClO/(ClO)2 equilibrium have been provided in
order to give a sampling of current research in the field.
a.) In 2007, von Hobe and coworkers [2] evaluated the data to date on the ClO/(ClO)2 equilibrium and made some
recommendations regarding the most consistent set of results to be used in atmospheric modeling. For the
equilibrium constant, results from the 2005 study by Plenge et al. [3] were recommended; the van't Hoff results
are shown in Table 3 of von Hobe. Note that the parameter B given in the table equals the slope of the van't
Hoff plot, −ΔH R! / R . Use the information from Table 3 to determine the standard molar enthalpy of reaction
from Plenge's results. Compare this quantitatively with your finding from part 2(c).
b.) Use the parameters A and B from Plenge's 2005 work given in Table 3 of von Hobe to determine the
equilibrium constant for ClO dimerization at 190 K. Note that the form of the van't Hoff equation used in von
Hobe for the equilibrium constant K is an exponential form,
K = A e B /T .
The equilibrium constant K determined from this equation involves concentrations expressed in units of
molecule/cm3 rather than mol/L. To convert
to K eq with concentrations in units of mol/L, multiply K by
€
N A /1000 ,
€
€
K eq =
NA
K.
1000
where N A is Avogadro's number. Compare quantitatively the calculated equilibrium constant at 190 K from
K
Plenge's results with your finding from
€ part 2(b). Is the agreement better or worse for eq compared with the
agreement for the ΔH R! values? Discuss.
€
€ further constraints on the A
c.) Recent work from the Jet Propulsion Laboratory by Santee and coworkers places
and B parameters in the van't Hoff expression for the ClO/(ClO)2 equilibrium [4]. What is the value of the A
parameter recommended by Santee and coworkers, JPL06 or JPL09? Based upon the recommended A value,
what is the new value of the B parameter that Santee and coworkers determined? Report both the A and B
values. Using these parameters, calculate the equilibrium constant at 190 K and compare it quantitatively to the
value determined from von Hobe's earlier recommendations in part 3(b). Note that the same units conversion as
in part 3(b) must be included.
d.) A high level computational study completed in 2008 by Matus et al. [5] shed new light on the structure of the
ClO dimer. These investigators reported structures and energies of three stable isomers of (ClO)2. What are the
structures of these three isomers? Draw a Lewis structure for each one.
e.) Which of the three isomers of (ClO)2 was previously thought to be the most stable? Which of the three isomers
is predicted by Matus et al. to be the most stable? What are the energy differences between the three isomers?
Why may this finding be important in terms of understanding polar stratospheric ozone depletion?
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References
1.
Cox, R. A.; Hayman, G. D. The stability and photochemistry of dimers of the ClO radical and implcations for
Antarctic ozone depletion. Nature 1988, 332, 796-800.
2.
von Hobe, M.; Salawitch, R. J.; Canty, T.; Keller-Rudek, H.; Moortgat, G. K.; Grooß, J.-U.; Müller, R.; Stroh,
F. Understanding the kinetics of the ClO dimer cycle. Atmos. Chem. Phys. 2007, 7, 3055-3069.
3.
Plenge, J.; Kühl, S.; Vogel, B.; Müller, R.; Stroh, F.; von Hobe, M.; Flesch, R.; Ruhl, E.; J. Phys. Chem. A
2005, 109, 6730-6734.
4.
Santee, M.; Sander, S. P.; Livesey, N. J.; Froidevaux, L. Constraining the chlorine monoxide (ClO)/chlorine
peroxide (ClOOCl) equilibrium constant from Aura Microwave Limb Sounder measurements of nighttime ClO.
Proc. Nat. Acad. Sci. 2010, 107, 6588-6593.
5.
Matus, M. H.; Nguyen, M. T.; Dixon, D. A.; Peterson, K. A.; Francisco, J. S. ClClO2 is the Most Stable Isomer
of Cl2O2. Accurate Coupled Cluster Energetics and Electronic Spectra of Cl2O2 Isomers. J. Phys. Chem. A Lett.
2008, 112, 9623-9627.