ROBERT C. PLUMB
chemical principles exemplified
Worcaler P o l y t e h ~lnllilula ~ Worcesler. M a s s a c h ~ ~ t01609 h
Tire Inflation Thermodynamics lllustroting the first low o f thermodynomics, ond thermodynamics o f gores
Suggestion by John J . Connors, Cranston ( R . I.) West High School The backyard of your home can serve as a teaching laboratory where the children, and perhaps your spouse, can be enlightened on the subject of thermodynamics. Some interesting thermal effects are easily observable when youinflate a bicycle tire. Pump up an English style tire to 60 psi with a hand pump and feel the valve stem. It gets noticeably warm (AT = 22'F in our experiments). Is this due to "friction"? You can test the "friction" hypothesis, and other hypotheses such as that it is due to the JouleThompson effect, by another experiment. Observe the temperature rise of the valve stem when the tire is inflated a t a compressor at the neighborhood gas station. Usually the valve does not get hot when inflated from a compressor (AT < 1°F in our experiments), but it does get hot when inflated with a hand pump. Thus it can't be "friction" since the air flow is about the same in the two cases. Then what causes the effect? The work which you do with a hand pump in compressing the gas raises its internal energy. The process is sufficiently rapid as to be approximately adiabatic; that is, no thermal energy transfer takes place and q = 0. According to the first law of thermodynamics AE=y-ur
You are doing work on the gas, w is negative and hence AE is positive. From a molecular viewpoint this increase in internal energy is in the form of increased kinetic energy-i.e., the molecules are moving more rapidly and the temperature is higher. Thus t,he compressed gas from the hand pump, which u7e.expect from the first law to be hot, heats up the valve stem. Although the compressed gas in a storage tank at a service station was hot when delivered to the tank from the compressor, it has usually cooled to room temperature and when bled into the tire produces no change in the temperature of the valve stem. The exempla. are designed to show fundamental chemical principles in operation. They deal with phenomena in which students have intrinsic interest; they apply abstract ideas to easily visualized situations. All of us have our pet anecdotes and illust,rations which we know will attract the students' interest. Yonr contributions and suggestions are invited. They may be sent to the author.
A Footnote to the Champagne Recompression Exemplum of Henry's Law Contribution by Roger J . Hateley, Wi?atringham B o w School, Grimsby, Lincs., U.K. The March, 1971 Chemical Principles Exemplified column contained a story of how a group of dignitaries lost their dignity when, after imbibing champagne under high atmospheric pressure, the pressure was abruptly decreased. More specifics on the event follow. "I was most interested to read in the March edition of JCE your Champagne Recompression story. Although I cannot find written confirmation, I am certain that this incident occurred on Saturday, November 10, 1827, when the tunnel below the Thames in London (at Rotherhithe) was broken through. The tunnel was built by the father and son team of Marc and Isambard Brunel, and the celebration involved 50 VIP's (mainly shareholders in the company), with a band from the Coldstream Guards, and also 120 of the constrnction team. This tunnel was the first to be built under the Thames and also the first using the shield method, and was lined all through with brick. Originally intended as a road tunnel, financial snags caused construction delays and it was not opened until March 1843, and then to foot traffic. Now it is part of the London Underground system, but in perfect condition still. . . ." Entropy Makes Water Run Uphill-in
Trees
lllustroting entropy o f mixing and osmotic pressure
Contribution by Professor Philip E. Stevenson, Worcester Polytechnic Institute Place pure water in contact with cellular fluids (water with other substances dissolved in it), and the water will mix with the cellular fluids diluting them. The driving force for this mixing process is entropy, the tendency for a system to increase its state of disorder. In this case, the diluted fluids are more disordered than the originally separate fluids and water. This mixing will take place even if the contact is through a membrane which allows only the water but not the dissolved solutes of the cellular fluids to pass through. Such a membrane is called "semipermeable," and plant cells are enclosed by such membranes. For example, a carrot which has become flaccid (limp) through drying out can be restored to turgidity (crispness) by immersion in water. The Volume 48, Number 72, December 7977
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water passes through the membranes of the carrot in order to dilute its now highly concentrated cellular fluids. This process of water passing through a semi-permeable membrane in order to dilute the fluids contained within is called "osmosis." How strong is the force which causes osmosis? Consider a tree. The leaves (or needles) of trees are continuously losing water to the atmosphere (a process called transpiration). This results in increased solute concentrations in the fluids in the leaves, and generates an "osmotic pressure"
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which forces ground water into the roots of the tree and up through its trunk and branches to the leaves in order to dilute their fluids. The tallest trees known are the coastal redwoods of California (Sequoia sempervirens), one of which has been found to attain a height of 364 ft. A column of water that high would exert a pressure a t its base of about 12 atm, and in order to force water through a pipe from the ground to the top of such a tree, one would need a pump capable of generating that much pressure. Yet the tree accomplishes this feat simply through entropy of mixing!
