0. E. Weigang, Jr. Texas Lutheran College Seguin, Texas
A Model for Demonstrating Dynamic Equilibria
O n e of the most important, and yet difficnlt, concepts to visualize is the dynamic nature of all chemical and many physical eqnilibria. The model described below is dist,inctive in comparison to demonstrations report,ed previouslyl-3 in it,s aut,omat,ic, cont.inuous, and rapid operation. A steady condition can be perturbed in various ways while the model continues to operate so that effects and/or new eqnilihria can be observed. The fundamental unit of the model, as shown in Figures 1 and 2, is a columnar reservoir emptied through an orifice a t the base. Analogons to a chemical reaction, the volume of water delivered in a period (rate of chemical r e a h o n ) depends directly on the depth of water in t,he colnmn (amonnt of reactant in a given volume, i.e., concent,ration) and t,he diameter of thc orifice (react,ion rat,e constant). I t can easily bc demonstrat,ed that as "reactant concentration" decreases, a "rate of chemical reaction" decreases.
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to the original reservoir by a similar process (reacts in t,he back direction). Left t,o themselves, the crossed flowing reservoirs attain a dynamic st,ate of equilibrium in which thc depth of water in cach reservoir remains unchanged. At, t,his point one can ronvincingly see that a t equilibrium the two rates of flow must he equal, and t,hat flow through a smaller orifice can be compensat,cd by a greater depth, while a larger orifice requires less depth. Thc reservoirs consist of 14-in. lengths of 35-mm o.d. Pyrex tubing, held by Fisher Cast,aloy clamps srrcwed directly into the backboard. Short pieces of varying small-diarnekr glass tubing in #7 one-hole ruhher stoppers provide a selection of orifice sizes which arc casily exchanged. The two pumps are of a centrifugal type which is used in small portable cvaporat.ive coolers. Glass int,ake tubes are cemented t o the pumps t o allow for t,hcir operation in the horizontal posit,ion. It should
Figure I . The model for demonstrating dynamic equilibria a f t e r the levels hove attained on equilibrium. The pointer indicates the larger orifice of rmoller orifice hor b e e n used for the right ride. gloss tubing;
Figure 2. The actual workings of the model, two centrifugal pumpr driven b y on electric motor. Equilibrium has b e e n attained ogoin, with a differen, set of orificer. however.
The extension t o a reversible chemical equilibrium rcquirrs that the issuing water (reacting material) be quickly transferred to another reservoir (hence becomrs thc product.), which in turn may be delivered
be emphasized that the proper operation and behavior of thr model requires that the pumps operat,e a t a level well below thcir capacity with essentially no head of wat,rr. Thr pumps arc connected to a surplus hp ac motor ~vit,hshort lrngt,hs of rubber tubing for Hexihlr conplings.
Prcae~rt~ccl l~eforrthe Division of Chemical Education at the 115th Sont,hweat ltogianal Meeting of t,he ACS, Oklahoma City, Deremher, IOFO. ' CARMOLW, Mr. R . , J. C m a . Eoac., 37, 312 (1960). KA~FFMAX. G. B., J. CHEM. I ~ u c , 36, 150 (1950). SORTIM, C. H., J . CHEM.XDI:C.,25, 18!1 (1948).
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Journal of Chemical Fducafion
Demonstrations
The model is capable of vividly representing t,he properties of dynamic equilibria in a n understandable manner. It shows a smooth asympt,ot,icapproach t,o a stat,c of rquilihrium and readily s h o w t,hat an approach
from either the left or right gives the same final concentrations. (Rubber hands conveniently mark the levels.) An "excess of reagent" can he shown to displace an equilibrium in accord with the Le Chatelier principle. With change of orifices, the role of temperature or catalyst on equilibrium can be shown. For the latter, a second set of orifices, each larger than the first but
producing the same state of equilibrium, is used to represent a positively catalyzed reaction. A siphon from one of the reservoirs to an adjacznt heaker can illustrate the effect of an irreversible side reaction. Indeed, variations and extensions via additional units to visualize secular equilibrium, the bottleneck principle, etc., can go as far as the ingenuity and initiative of the demonstrator allow.
Volume 39, Number 3, Morch
1962
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