ROBERT C. PLUMB Worcerfer Polylerhni~Institute Worcester. Mormchurettr 01 61
chemistry should appeal to the students. After all-is there any phenomenon, process, or event which takes place in our universe which is not either immediately or in principle traceable to elementary molecular processes-the domain of knowledge where chemistry t,eachers have expertise? A large number of chemistry teachers have recently become convinced that we ourselves have failed to communicate to our students the importance of chemistry, the universal influence which chemistry has on our everyday lives. To a considerable extent this failure results from our own limited spheres of knowledge; we need to specialize and, as individuals, have difficulty in supplying our students with a broad perspective on the relevance of chemistry. The editorial staff of the JOURNAL OF CHEMICAL EDUCATION will address itself to this problem in this new column. Each month we expect to publish a few brief essays, vignettes, about phenomena which are of intrinsic interest to the general student, written from the viewpoint of the scientist in a mauner to illustrate basic chemical principles. Thc precise but awkward Euglish term for teaching examples of this type is L'Exemplifications of Chemical Principles." The Latin exemplum, plural exempla, meaning anecdotes or short narratives used to point a moral or sustain an argument, conveys the concept clearly so t,hat. the items in this series will be called exempla. The exempla are designed to appeal to student interest by dealing with phenomena of which the student is aware or which are of intrinsic interest to him; thus exempla have obvious benefits in motivation. At the same time, exempla increase comprehension by apply-
ing abstract ideas to easily visualized real life situations. Exempla can he used to liven up lectures, and they can be rephrased to make interesting exam questions and homework problems. All of us have our pet anecdotes and illustrations which we know will attract the students' iut,erest. Your contributions and suggestions are invitcd. Sea-Lab Experiment lllustrofing principles of the kinetic theory o f gases
The ocean floor is one of our unexplored geographical frontiers. TO extend exploration of the ocean, scientists and engineers are developing hardware and procedures to permit people to live for days or weeks in chambers on the ocean floor. In these chambers oxygen and helium are used as an atmosphere. A strange physiological effect is noted when a person lives in an atmosphere in which nitrogen has been replaced by helium. If the temperature in the chamber is a normal, comfortable 70°F such as is used in buildings on the earth's surface, the aquanauts feel decidedly chilly, Why? The answer lies in the kinetic theory of gases. Principally. the effect is caused because molecules of He move more rapidly than molecules of Nz a t the same temperature (about 2.6 times as fast on the average) and hence impact with the human skin about 2.6 times more frequently. Since the body is about 27°F warmer than the atmosphere, it is losing heat to the atmosphere continually aud the molecules carry away this energy as kinetic energy. He molecules are much more efficient than Nz molecules in carrying this heat away. Volume 47, Number 3, March 1970
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175
The Snowmaking Machines
Sunglasses that Respond to Brightness
lllustroting principles of thermodynamics in gos expansions and phase changes
lllustroting principles of chemical equilibrium
When you are skiing on artificial snow, you are benefiting from the cleverness of man in applying some elementary principles of thermodynamics. How does a snowmaking machine work? Spraying cold water into cold air will not make much snow. You can prove for yourself from the enthalpy change on fusion of ice (1436 cal/mole) and the heat capacity of air (7.0 cal/mole deg) that the product will be wet, warmed-up air and the snow produced will be soon washed away. The trick in operation of a snowmaking machine is that amixture of compressed air and water is sprayed out. The compressed air undergoes an adiabatic expansion, pushing back the surrounding atmosphere a t the expense of the internal energy of the gas. The expanding gas then is colder than the water droplets, so that the water is converted to ice particles, and you have the pleasure of skiing even though nature has been parsimonious. The description by thermodynamic equations follows from the first law
Contribution by Dr. S . D . Stookey, Director of Fundamental Chemical Research, Corning Glass Works A new kind of glass has become commercially available which darkens in sunlight and becomes clear again in the dark, by means of a simple chemical equilibrium which shifts with the intensity of the light. This glass provides one of the very few visible examples of a truly reversible chemical reaction. The "photochromic" glass contains minute silver chloride crystals which are transparent to light and when ultraviolet light photons strike it the energy is absorbed and produces metallic silver particles and chlorine atoms, according to the reaction AgCl
+ h"
Ago
+ C1°
The reaction products, AgO and C1° are trapped in the glass structure so they can't escape; thus they can react with each other, producing AgCl and giving off the energy again and the reaction is reversible. The equilibrium concentration of Ago is determined by the "concentration" of photons, hv (that is the number of photons available in a unit volume of the glass a t a given time) and the equilibrium is shifted to the right if the light level is high, and to the left if the light level is low. One of a multitude of possible uses is shown in the figure.
AE=4-w
q = 0 to the extent that heat transfer between the cloud of expanding gas and the surroundings can he neglected. The work done on the surroundings, w, is the pressure of the surroundings times the volume of atmosphere displaced w
=
PAV
To do this work, the internal energy of the expanding gas must drop by a corresponding amount AE = - P A V
The internal energy is the sum of the kinetic and intermolecular potential energies of the gas molecules and, to the extent that the gas is ideal, the intermolecular potential energy effects may be neglected. The kinetic energy is related to the temperature so that AE
=
3 RAT = -PAP 2
and the cold gas can then freeze the water.
Fisherman wearing runglosses that respond tobrightnerr under incandescent light (left1 and wnlight Iright).
High-Temperature Superconductivity High-temperature superconductivity-which means electricity flowing without resistance near the temperature of liquid nitrogen (77'K, or -320PF) or above-would undoubtedly have dramatic effects on technology. But the highest transition temperature-the point where superconductivity begins for a material-achieved so far is 21PK, or slightly above the boiling point of liquid hydrogen. This was accomplished last year by a group led by Dr. Bernd T. Matthias of the University of California, San Diego, and Bell Telephone Laboratories.
. . .Taken from "Physics in 1969" published by the AmericanI~nstituteof Physics. 176
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Journal of Chemical Education
