1.* (1997 F 13)

A. Although chlorofluorocarbons (CFC’s) are several times heavier than air, thousands of measurements have been made from balloons, aircraft, and satellites that demonstrate that the CFC’s are present in the stratosphere. How do the CFC’s get up there?

Wind, collective mixing.

B. What are the three major ways that molecules are removed from the stratosphere?

Oxidation, photodestruction, rainout

C. What is the fate, i.e., destruction mechanism, of CFC’s in the stratosphere?

Ultimately dissociated by UV light

D. Why are Cl or Br atoms so effective in removing ozone molecules in the stratosphere?

They catalyze the conversion of O3 to O2. 105 O3 are converted for each Cl or Br atom:

O3 + Cl à O2 + ClO

ClO + O3 à O2 + O2 + Cl

ClO + O à O2+ Cl

2O3 à 3O2

E. The oceans are salty and four-fifths of the surface of the Earth is covered by oceans. Consequently, dried salt spray is a continual source of Cl in the troposphere. Why is this source of Cl atoms unimportant for the stratosphere?

Salt molecules dissolve in water and are removed as rain or snow before being transported to the stratosphere.

F. Why is an ozone hole observed over Antarctica at certain times of the year?

Sunlight releases stored Cl on ice crystals in the form of ClONO2 (ClONO2 + hn à ClO + NO2), and we are off to the catalytic conversion of O3 to O2. This occurs once per year in the antarctic spring.

G. The NO3 radical absorbs in the visible; the OH radical absorbs in the ultraviolet. These two molecules are responsible for oxidation in the stratosphere but NO3 is more important in the night time wereas OH is more important in the daytime. Offer a possible explanation for this behavior.

  1. Sunlight destroys NO3 but not OH.
  2. NO3 is made from O3 + NO2. OH is made from O + RH. With sunlight, O3 à O2 + O, so [O] increases while [O3] decreases. Hence OH concentration is higher in sunlight, while NO3 concentration is higher at night.

H. Suppose that a trace molecule, such as ozone, is present in the Earth’s atmosphere at the part per billion level by weight. Calculate the total weight of this trace molecule in the atmosphere to one significant figure, given that the pressure at sea level is 15 pounds per square inch and the circumference of the Earth is 25,000 miles. Carefully show your work.


2.* (1996 F 11B) Write a balanced equation for the decomposition of ozone. What is the sign of DG, DH, and DS?

This reaction forms 3 molecules of gas from 2 molecules, so S is positive.

It's not immediately obvious, but this is exothermic (H is negative.)

This guarantees that DG=DH-TDS is negative.

3.* (1996 F 16) At an altitude of 30 km and a latitude of 00 the atmospheric temperature is 233 K. The concentration of ozone and oxygen atoms are chlorine atoms are 2 x 1012 molecules/cm3, 1 x 108 atoms/cm3, and 1 x 105 atoms/cm3, respectively. Under these conditions the following reactions occur, each of which obey the Arrhenius rate equation: k=Aexp(-Ea/RT):




O3 + Cl à O2 + ClO



ClO + O3 à O2 + O2 + Cl



ClO + O à O2+ Cl



Where the units are cm3molecule-1s-1for A and K for Ea/R.

A. Write down a differential equation for the disappearance of ozone from the first reaction.

These are all elementary reactions, so back reactions may be ignored.

. There is a negative sign because O3 is being lost.

B. Find the rate for ozone dissapearance from reaction 1.

From A,

We also know that


C. Write down a differential equation for the disappearance of ozone from the second reaction.


D. Find the rate for ozone disappearance from the second reaction.

E. Use the information you have to estimate the steady-state concentration of ClO.

From reaction (2),

From reaction (3),

The total rate of change in [ClO] is the sum of these:

We are to estimate the steady-state concentration, so:

F. What is the role of Cl in the second and third reactions

Cl is a catalyst.

G. How does the rate of destruction of ozone from the first reaction compare to the rate of destruction from the second and third reactions?

Rate from (1) is

Rate from (2) and (3) is

Thus the rate of (2) and (3) is 8.4 times faster than the rate of (1).

This is why CFC's are bad.


4.* (1995 F 1C) Which of the following materials are implicated in the production of smog found over urban population centers?

  1. porphyrins.

No. These aren't even volatile.

b. alkenes.

Yes, alkenes react with O3 to form epoxides which are components of smog.

c. O2.

Yes and No. O2 doesn't directly contribute to smog, but one could argue that O3 comes from O2.

  1. O3.

Yes. (see b)

e. (O2)2.

No. This doesn't even exist.

f. H2O.


g. sunlight in the IR.


h. sunlight in the visible.


i. electrical transmission lines:

No. These do produce O3 but not in significant amounts.

j. aerosols:

YES. Aerosols are tiny droplets of liquid suspended in the air. They have very large surface area to volume ratios and supply surface for smog forming reactions to take place.

k. chlorofluorocarbons (freons)

No. CFC's are a problem for the environment but only the O3 layer which is separate from smog-causing O3

l. LACK OF atmospheric circulation.


5.* (1994 F 8) Write chemical reactions leading to destruction of the ozone layer in the upper atmosphere by the introduction of chlorofluorocarbons (freons), explaining carefully the role of sunlight.

Sunlight is necessary in the inital step of homolytic cleavage of the C-Cl bond to form Cl.

B. Write chemical reactions leading to the formation of the ozone layer in the upper atmosphere, explaining carefully the role of sunlight.

Sunlight causes the dissociation of molecular oxygen to oxygen atoms.

In the presence of a heterogeneous catalyst, oxygen atoms can add to O2 to form ozone.

C. Explain briefly what causes the appearance of an "ozone hole" only certain times of the year above Antarctica.

The reactions:

are normally slow. During the Antarctic night, wind currents cause the atmosphere above Antarctica to be isolated from global atmospheric circulation. The temperature falls to ~-90C and polar stratospheric clouds form. The ice crystals that form these clouds act as heterogeneous catalysts for the reactions above. The total surface area upon which catalysis can occur is immense and Cl2 and HOCl are produced rapidly.

When the Antarctic spring arrives, the sunlight causes rapid dissociation of Cl2, HOCl, and CFC’s to form Cl in the stratosphere. The resulting Cl radicals rapidly attack ozone molecules to form O2. The rapid decrease in [O3] causes a "visible" hole in the ozone layer in the early Antarctic spring (September).

D. True or False:

Ozone is paramagnetic.


Ozone is polar.


Ozone is chiral.


Ozone is denser than air.


Ozone attracts organic molecules containing double bonds.


Ozone is the most thermodymaically stabe form of oxygen.


The three O atoms in ozone are magnetically equivalent.


Ozone has an infrared spectrum.


Sunshine destroys ozone.


Ozone is also called singlet oxygen.