chemical principles ROBERT C. PLUMB Worce3ter Polytechn8c lnrtnlula Worcs.m, Mosmchu~etts01 609
Champagne Recompression Illustrating Henry's low o f portial pressure o f a gas dissolved in o liquid
Contribution by T . C. Loose, North Yarmouth Academy Henry's law states that the partial pressure of a dilute component of a two-component system is directly proportional to its mole fraction. The proportionality constant is known as the Henry's law constant. For example, for COzdissolved in water Pcol
=
Xcoz.Kcoa
and KOo2for liquid water has a value of 1.25 X lo6, Pcazis in tom, and Xoozhas dimensions of mole fraction. The time and place of the following incident are not known, but the comedy of the events overrides any question of authenticity. During the construction of a tunnel under a river, a party of politicians went down to celebrate the meeting of the two shafts. They drank champagne and were disappointed that it was flat and lifeless. Because it was under depth pressure, the carbon dioxide bubbles remained in solution. When the town fathers arrived at the surface, the wine popped in their stomachs, distended their vests, and all hut frothed from their ears. One dignitary had to be rushed back into the depths to undergo champagne recompression.
Chinook Winds-The
Fohn Phenomenon
Illustrating the thermodynamics o f gas expansion and phase transition
Information provided by Mishi Olci, University of Tokyo, and Georg M . Schwab, University of Munich Sugpsted by Philip E. Stevenson, Worcester Polytechnic Institute To experience a chinook wind (or as known in Japan and the German speaking countries, the Fohn phenomenon) is an eerie sensation. Hot, dry, high speed wind blowing as though from Hades-melting away drifts of snow, shaking houses on their foundations-even, The exemple, are designed to show fundamental chemical pprinciples in operation. They d e d with phenomena in which students have intrinsic interest; they apply abstract idea to easily visualized situations. All of us have our pet anecdotes and illustrations which we know will attract the studenis' interest. Your contributions and suggestions are invited. Thty may be sent to the author.
154 / Jovrnol o f Chemical Education
according to some more superstitious folk, causing men to beat their wives, dogs to go mad, and people to hecome insane. The temperature in such winds is known to jump as much as 50DFabove the prevailing temperatures, and t,he winds to be as great as 80 mph. I t is no wonder that they are frightening. There may even be some basis for the belief that they trigger irrational behavior because -1 Hz (inaudible) compressional waves have been detected during them. I n Germany, surgeons do not operate when these strange winds are blowing. The abnormally high temperature and low humidity of the chinook winds are readily understood by considering the geological conditions prerequisite to their appearance and thermodynamic principles. These effects only appear on the downwind side of mountain ranges when there are large atmospheric pressure differences forcing air masses over the mountains; e.g., the low pressure of a typhoon in the Sea of Japan draws air from the Pacific over the mountain range of Japan, producing the Fohn phenomenon on the west side of Japan. In the United States and Canada, chinook winds appear on the eastern slopes of the Rockies, especially around Boulder, Colorado, and in the plains of Alberta adjoining the continental divide a t the boundary with British Columbia. In Southern Germany the effect is produced when air masses from the Mediterranean move over the European Alps. As an air mass is siphoned over a high mountain, it undergoes a substantial expansion and recompression, the pressure being only 0.60 atm at an altitude of 13,000 ft. To the extent that the expansion is adiabatic, the work that the expanding air mass does in pushing back the surrounding air is reflected in a decrease in its internal energy and to the extent that the air is an ideal gas, this change
in internal energy is entirely a decrease in molecular kinetic energy and hence temperature 3 AE = AKE = n- RAT 2
= -w
When the air mass descends, it undergoes a recornpression, w is negative, and the temperature increases. The temperature of a dry air mass after expansion from sea level pressure to that at 13,000 ft may be calculated, assuming ideality and using the experimental value for the molar heat capacity of air T, = R lag P c, lag --1 T, PI C , of air is 6.96 cal/mole-degree and for PI and T I of 1.0 atm and 283°K and P, of 0.60 atm, T , is 245°K Thus dry air would drop 38°C in temperature and return to the initial temperature after recompression.
If instead
df
dry air, the air mass is saturated with
HzO,a strikingly different effect results. At 50°F or 10°C the vapor pressure of water is 9 torr or about 0.010 atm. If the temperature drops to even 0°C where the vapor pressure of water is 4.5 tom, one half of the moisture will condense out as liquid and each mole of air will have added to it the thermal energy of vaporization of 0.005 moles of water. This energy, about 45 calories, is sufficient to raise the temperature of the air by 7°C or 13°F. If all of the water of 50°F saturated air is removed in this way, the temperature after recompression will be 26°F higher than initially and if the air is initially a t 70°F where the vapor pressure is 18.6 tom, the temperature after recompression will be 52'F higher, or about 120°F; truly giving the impression to the susceptible that supernatural powers are at work!
Volume 48, Number
3, March 1971
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