WITHIN CHEMISTRY

1. Use of gases in stoichiometry (see Extensions).

2. Effect of pressure on changes of state; change of boiling point with change in atmospheric pressure.

3. Determination of molar masses of unknown gaseous substances by researchers.
 

TO THE CONTEMPORARY WORLD

1. Acid Rain. The gases NO₂ and SO₂ contribute to acid rain. These gases dissolve in water droplets or aerosol particles, causing the pH to decrease.  This moisture precipitates as acid rain or acid snow. NO₂ produces nitric acid (HNO₃ ) and nitrous acid (HNO₂ ); SO₂ produces sulfurous acid (H₂SO₃ ) and, after conversion to SO₃ , sulfuric acid (H₂SO₄ ) . Rain is naturally acidic (pH = 5.6) from the CO₂ gas that dissolves in water to form carbonic acid (H₂CO₃ ). Acid rain has pH values below 5.6.

2. Ozone Depletion by Chlorofluorocarbons (CFCs) . CFCs are halogenated hydrocarbons that find use as refrigeration fluids, degreasing solvents for electronic parts, fire extinguishers (CFCs are unreactive) and, at one time, aerosol propellants for hair sprays, deodorants, and food. Their use as propellants is now banned in the U.S. due to evidence that they help destroy the ozone layer. In the stratosphere where ozone is the most abundant, ultraviolet light breaks down CFCs forming a very reactive chlorine atom (Cl). The chlorine atom and an ozone molecule combine producing an oxygen molecule and another reactive atom. A chain reaction occurs consuming many more ozone molecules. The stratospheric ozone layer filters out harmful ultraviolet rays, preventing skin cancer.

3. Photochemical Smog . Originally smog meant a combination of smoke and fog. Today it generally refers to the polluted air found over cities as a result of industrial emissions and automotive exhaust. The major chemical substances involved are hydrocarbons (unburned fuel), nitrogen oxides (which form at the high temperatures and pressures of internal combustion engines) and ozone. Ozone production is a light activated reaction and an example of a process contributing to photochemical smog. NO ₂ decomposes in light to nitrous oxide and free oxygen. The latter combines with molecular oxygen to make ozone.

    NO₂ ---> NO + O

O + O₂ ---> O₃

The ozone then reacts with various hydrocarbons to produce aldehydes, peroxynitrates, acrolein, and other irritants. These induce health problems in some humans and produce a phytotoxic effect in plants. Even some carcinogens are found in smog. Air pollution problems are aggravated by local geographic conditions in cities such as Denver and Los Angeles.  Consequently, gaseous emissions are restricted in these places.

4. The Greenhouse Effect . Carbon dioxide is transparent to visible and ultraviolet light but absorbs infrared radiation strongly. As a consequence, the atmosphere of the earth is warmer than it would be without carbon dioxide. Incoming higher energy visible light is absorbed at the surface by pigmented objects and organisms. Some of the absorbed energy is emitted as lower energy infrared radiation that is then trapped in the atmosphere.  Water vapor also contributes (and, indeed, is the major contributor) to the effect. This warming process is referred to as the Greenhouse Effect.  The CO₂ composition of the atmosphere has increased with the combination of fossil fuel use and decreasing forested land acreage. There is much concern that this effect might, or already has begun to, lead to global warming that could have disastrous changes in the size of the polar ice caps and distribution of arable land. Scientists are carefully monitoring this situation, and many individuals and governments are acting to reduce CO₂ emissions.

5. Radon. A member of the noble gas family, radon-222 is a product of the uranium decay series and is radioactive. It is a direct result of the decay of radium-226.
                   ²³⁸U ---> ²²⁶Ra ---> ²²²Rn ---> ²⁰⁶Pb
Radon-222 is found in soil and rocks. When radon-222 decays it produces alpha particles and several short-lived radioisotopes—all of which are alpha emitters. Since radon is a gas, it can be present in the air one breathes. If it decays while in the lungs (half life of radon-222 is about four days) atoms of Po and Pb (both solid elements) and alpha particles are formed. Although the Po and Pb cause problems, alpha radiation damages lung tissue because of close contact with the tissue. This can result in a higher than normal risk of
lung cancer.

6. Commonly Encountered Gaseous Air Pollutants
 
 

Gaseous Pollutant	Comment
SO2	sulfur dioxide Related to acid rain
O3	ozone Pollutant in lower atmosphere, beneficial in stratosphere
CO	carbon monoxide From incomplete combustion of gasoline in automobile
NO2	nitrogen dioxide From combination of N2 and O2 in air, in automobiles; related to acid rain problem
CO2	carbon dioxide From burning oil, gasoline, coal, and natural gas
CFCs	chlorofluorocarbons Believed responsible for depletion of ozone layer
Rn-222	radon Alpha-emitting gas found in rocks and soil

Extensions

The stoichiometry of chemical reactions involving gases can be interpreted through the relationship between amount and volume. Because the volume of a gas is directly proportional to the amount of gas (in moles), volumes of gases can be used experimentally in place of amounts in stoichiometric problems, if temperature and pressure remain constant.

Example: For the reaction: 2H₂ (g) + O₂ (g) ---> 2H₂O(g)

2 volumes of H₂ (g) react with 1 volume of O₂ (g) to form 2 volumes of H₂O(g).  By measuring the volume of water vapor formed, one can determine the volume of H₂ that reacted.

To determine the number of moles or grams of H₂ involved, one would use the general gas law in the form n = PV/(RT).
 

MEDIA

1. CHEM Study film and video on gases, Gases and How They Combine. Available from Ward’s Natural Science Establishment, P.O. Box 92912, Rochester, NY; (800) 962-2660.

2. The Cable Weather Channel periodically runs films on environmental issues that may be taped for class.

3. Software published by JCE: Software, a publication of the Journal of Chemical Education, Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue. Madison, WI 53706-1396: (608) 262-5153 (voice) or (608) 262-0381 (FAX).  Ideal Gas, by Richard Hiatt, Vol. IV, No. 1, for the Apple II computer.

4. Software published by Project SERAPHIM, Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue. Madison, Wl 53706-1396: (608) 263-2837 (voice) or (608) 262-0381 (FAX).

    a. For the Apple II computer: AP 401, AP 402, AP 404
    c. For IBM PCs and PC-compatibles: PC 2601, PC 2602, PC 2603

5. Videodiscs published by JCE: Software, a publication of the Journal of Chemical Education, Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wl 53706-1396:

(608) 262-5153 (voice)
or
(608) 262-0381 (FAX).

“Experimenting with Gas Pressure I,”  “Experimenting with Gas Pressure II,”  “Effect of Temperature on Pressure,” and “Experimenting with Gas Pressure III,” four chapters on The World of Chemistry: Selected Demonstrations and Animations: Disc I (double sided, 60 min.), Special Issue 3.