Chemistry 360
Spring 2017
Dr. Jean M. Standard
March 31, 2017
Physical Chemistry Assignment 3: Chlorine Oxides and Ozone Depletion
(25 points)
This assignment is due in class on Wednesday, April 12, 2017.
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. In recent years, an ozone hole also has been observed in the Arctic.
Figure 1. Measured ozone amounts (Dobson units) around the south pole in October 2016 (from
http://www.theozonehole.com).
Figure 2. Area of ozone hole around the south pole during 2014-2016 (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.theozonehole.com/sitemap.htm
http://www.esrl.noaa.gov/csd/assessments/ozone/2014/twentyquestions/
http://www.ozonelayer.noaa.gov
http://www.epa.gov/ozone-layer-protection .
Some of these sites may be useful in learning more about the specific chemical reactions involved in polar
stratospheric ozone depletion.
A. The Role of ClO and (ClO)2 in Stratospheric Ozone Depletion (6 points)
In this part of the assignment, you will investigate some general characteristics of stratospheric ozone depletion and
the potential role that chlorine oxide, ClO, and its dimer, (ClO)2, play in the destruction of the ozone layer.
1.
Using the on-line sources mentioned above, find and report the altitude range at which the stratosphere exists.
Cite your source.
2.
The images shown in Figures 1 and 2 give ozone amounts measured in Dobson units. Define a Dobson unit.
3.
The lowest ozone amount observed in Figure 1 is about 100 DU. Calculate how many ozone molecules this
corresponds to in one square meter. (Assume ideal gas behavior.)
4.
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.
5.
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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B. 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)
2ClO ( g)
were determined at a variety of temperatures by Cox and Hayman [1]. Their data is given in the table below.
€
€
T (K)
K eq
233
4.13×10
8
5.00×10
7
258
1.45×10
7
268
5.37×10
6
273
3.20×10
6
280
9.62×10
5
288
4.28×10
5
295
1.67×10
5
303
7.02×10
4
248
1.
€
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 and 1/T on the x-axis. Fit the data to a linear trendline and display the equation on the
graph. Turn in this plot with your assignment.
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2.
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 200 K. Does the
equilibrium favor reactants or products at this temperature?
3.
From your van't Hoff plot from part 1, 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/mol-K (for ΔSR! ).
4.
Determine the standard molar Gibbs free energy change, ΔGR! , for the reaction at 200 K in kJ/mol.
5.
In part 3, 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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C. Further Studies of the ClO / (ClO)2 Equilibrium and Ozone Depletion (7 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.
1.
In 2007, von Hobe and coworkers [2] evaluated the data on the ClO/(ClO)2 equilibrium that had been obtained
up to that point and made some recommendations regarding the most consistent set of results for use in
atmospheric modeling. For the equilibrium constant, results from the 2005 study by Plenge et al. [3] were
recommended; the van't Hoff results from this study 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 with your finding from
part B3.
2.
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 200 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 .
Also, the equilibrium constant determined from this equation involves concentrations expressed in units of
3
molecule/cm rather than mol/L. To convert
to K eq with concentrations in units of mol/L, multiply K by
€
N A /1000 (where N A is Avogadro's number),
€
€
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K eq =
NA
K.
1000
Compare the calculated equilibrium constant at 200 K from Plenge's results with your finding from part B2.
3.
€
Recent work from the Jet Propulsion Laboratory by Santee and coworkers places further constraints on the A
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 200 K and compare it quantitatively to the
value determined from von Hobe's earlier recommendations (from part C2). Note that the same units
conversion as in part C2 must be included.
4.
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 (see the
provided page from the article's supporting information for the geometries of these isomers). Draw a Lewis
structure for each of the three stable isomers of (ClO)2.
5.
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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6.
The Montreal Protocol of 1987 is the treaty that established a ban on chlorofluorocarbons (CFCs) and related
molecules as ozone-depleting substances. Recent simulations of chemical transport in the atmosphere by
Chipperfield and coworkers [6] have attempted to model how the polar ozone holes and worldwide
stratospheric ozone levels would have fared without the protocols in place.
For the simulations, assumptions about the amounts of ozone-depleting substances (ODSs) in the atmosphere
must be made. Some of the information required includes atmopsheric concentrations of chlorine. What was
the peak concentration value for atmospheric chlorine and in what year was this reached? What rate of growth
(in %) for production of ODSs did Chipperfield and coworkers employ in their simulations?
7.
From the simulations, by what % would the Antarctic ozone hole have grown by 2013? Loss of ozone in
northern mid-latitudes as a result of ODSs would have been more severe over the years without the Montreal
Protocol. Such global loss of ozone would lead to enhanced ultraviolet radiation at the surface and an expected
increase in skin cancers worldwide. From Fig. 5 in the paper by Chipperfield and coworkers, estimate the %
increase in surface ultraviolet index at Bloomington-Normal in 2011 if the Montreal Protocol had not been
enacted.
References
1.
Cox, R. A.; Hayman, G. D. The stability and photochemistry of dimers of the ClO radical and implications 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.; Rühl, E. Bond Strength of
Chlorine Peroxide. 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, 6588-693.
5.
Matus, M. H.; Nguyen, M. T.; Dixon, D. A.; Peterson, K. A.; Francisco, J. S. ClClO 2 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.
6.
Chipperfield, M. P.; Dhomse, S. S.; Feng, W.; McKenzie, R. L.; Velders, G. J. M.; Pyle, J. A. Quantifying the
ozone and ultraviolet benefits already achieved by the Montreal Protocol. Nature Commun. 2015, 1-8.