ROBERT C. PLUMB
chemical principles exemplified
Triboelectricity Illustrating the quantum mechanical tunnel effect
Information supplied by Professor William T . Scott, University of Nevada Virtually everyone living in the temperate low humidity climates of the world has experienced a nervetingling surprise as a result of the ability of electrons to tunnel through potential energy barriers! Next time you walk across a rug and get a jolt as you touch a light switch, think of what happened-you became electrostatically charged t,hrough the operation of the quantum mechanical tunnel effect. Triboelectricity (from the Greek "tribein" to rub) is the oldest-known manifestation of electricity. The electron energy levels in two nonconductors may be described as in the diagram below where the potential energy wells represent atomic nuclei. Place two different nonconductors in contact, and electron transfer will take place by electron tunneling.
The resulting electron deficiency in A will cause the energy levels in A to be lowered and the levels in B to he raised. If the two substances are metals, the electrons transfer between energy levels which reflect mobility over the entire pieces of matter, because the electrons are free t o diffuse throughout the entire conductor. Electron energy levels in insulators do not have this property. I n an insulator a local electron deficiency will lower the electron energy levels in a small area only. As the number of points of contact of substance A (an insulator with higher energy levels) with substance B (an insulator with lower energy levels) is increased, more and more localized energy levels in A will he lowered and progressively more positive charge 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 illustrations which we know will attract the students' interest. Your contributions and suggestions are invited. They may he sent to the author.
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will accumulate on substance A. Rubbing two insulators together makes t,his extensive contact. The idea that rubbing is necessary to get, t,he charge transfer is a common but fallacious notion; its purpose is to make many points of contact instead of the few formed when two molecularly rough surfaces are placed together. Your body, an electrical conduct,or, can he charged to quite a large electric potent,ial by the electrons tunneling from the ~ o oofl a rug to the sole of your shoe (both insulators) and thence leaking off to your body. Then when sou touch another conductor a shock results. The substances in the list below are arranged as a Triboelectric Series; each substance on the list will become more positive than one below it on contact under averacre conditions. As exnected from the molecular origin of trihoelectricity discussed above, as one goes down the list the work function, t,hat is the minimum energy required to remove an electron from the substlanceto free space, increases. Triboeledrie Series Asbestos Rabbit's fur Glass Mica. Wool Quartz dslaite Cat's fur Ca, Mg, Ph Silk Al, Zn, Cd, Felt, ~ u & nskin Cotton
Rock salt Woods, Iron Tinned Iron Cork, Ebony Amber Slate Resins Cu, NI, Co, Ag, Sn, As, Bi, Sb, Pd, C, Blass Para rubber Sulfur Pt. Au ~eiluloid India rubber
If electrons didn't tunnel, there would be no stat,ic electrification by contact of insulators. Cationic Starch and Water Pollution Illustrating principles of surface chemistry
Contribution by Alexander R. Malcolm, National Starch and Chemical Corporation The offensive water pollution of paper mills can be greatly alleviated by chemically modifying natural starch, one of the ingredients of paper, in such a way as to change its surface chemistry. Paper manufacturers add starch to the wood pulp and clay used in paper. Starch, a natural polymer, increases the strength of the paper by bonding together the fibers in the sheet. I n making paper a dispersion of the solid ingredients is filtered out onto a moving
0 +
endless belt screen of fine mesh. The solid ingredients, pulp fibers, filler clays, inorganic pigments, and starch are all anionic-that is, they bear a negative + charge with respect to the - + + solution. Thus they tend + + + to repel each other and stay dispersed-and as a result much of the finer pulp, the clay and the starch pass through the screen, not only contributing to water pollution but wasting raw material and costing money. A clever application of an elementary idea of surface chemistry provided a solution to this problem. Take naturally occurring starch +
+ + Q+
a
and treat it to make it cationic
a
Then the positively charged colloidal starch particles can act as a bridge between the negatively charged pulp, clay, and pigment particles, increasing retention by the metal screenon which the paper is filtered-improving the economics and reducing water pollution at the same time! This fun+ damental chemical concept + is the basis of a major seg+ ment of industrial starch chemistry.
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The "Critical Mass-Configuration" in Chemical Reactions Illustrating principles of chernicol reocfion kinetics
Idea provided by Uri Haber-Schaim, Education Development Center, Newton, Massachusetts A knowledge of the critical mass-configuration effect can help you realize one of the little pleasures of lifekeeping a cheery fire burning in the fireplace at home
or a t the family campsite on vacation. Suppose one builds fires from split logs in the two configurations shown below. Which continues to burn and which goes out?
This is but one of many chemical counterparts of the critical mass-configuration concept of nuclear physics, according to which there is a critical amount of fissionable material required to maintain a chain reaction for a particular configuration. In nuclear fission the perpetuation of a chain reaction requires that at least as many neutrons from the fission reaction be retained by the system, as reactants for further fission, as were consumed in the fission reaction from which they were produced. The mass of fissionable material required to maintain this balance and keep the chain reaction going depends on the shape and configuration of the components from which the critical mass is assembled. In every chemical reaction in which the products from one molecular transformation (energy, free radicals, carbenes, etc.) are required to initiate another molecular transformation event, the maintenance of a critical mass-configuration determines if a reaction will continue. The thermal and radiant energy from a wood fire needs to be used judiciously because some of the energy is necessary to provide the chemical activation energy for the combustion yet to come. Ignore the critical mass-configuration effect and the fire goes out; recognize and capitalize on theeffect and bask in the praise, "My, what anice fire!" The chemistry of combustion in a wood fire is enormously complicated, but an empiricist maintaining a fire soon discovers that two parallel logs bum better than a configuration which does not retain as much of the therinal energy released by combustion. A much simpler chemical example of the critical mass configuration is in the explosion of a stoichiometric hydrogen-oxygen mixture a t pressures below 200 torr. If too many of the H atoms and OH radicals necessary for continuation of the chain reaction are lost by collisions with the walls, an explosive propagation of the reaction will not take place. I t is found experimentally that the minimum pressure of hydrogen and oxygen at which an explosion will take place increases as the volume of the container decreases. This effect has been put to practical use in the Davy's mine safety detector for flammable gases in which a flame is enclosed by a fine mesh screen through which a flame will not propagate, because of losses of chain reaction propagating species on the surface of the screen.
Volume 48, Number 8, August 7977
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