edited by GEORGE L. GILBERT Denison University (2anville. Ohio 43023
A Versatile Demonstration with Calcium Carbide SUSMITTEO BY Ariel A. Fender John Abbon College St. Anne de Bellevue, PQ. Canada Davld N. HarDD MCGIII unlversltG ' Montreal, PO, H3A 2KB Canada Joseph A. Schwarcz Vanier College Montreal. PO, H3X 2N9 Canada CHECKED BY
Erwln Boschmann IU-PU at lndlanapolis lndlanapolis, IN 46202 The generation of acetylene by the addition of calcium carbide to water and its subsequent comhustion have been reactions of great practical utility; such devices as "starter cannons" use this chemistry.' We wish to report a useful means whereby the necessity for the presence of oxygen in the comhustion is effectively demonstrated. More importantly perhaps, this technique provides a completely available and inexpensive methodology to carry out the experiment.
Fifty milliliters of water are placed in a 250-mL vacuum filter flask followed by -1 g of calcium carbide lumps. Powdered calcium carbide should not he used because the reaction proceeds too quickly. The flask is then lightly corkstoppered and immediately (2-3 s) a lighted match is placed next to the vacuum opening on the flask. Within a few seconds the acetylene in the flask explodes, rocketing out the stopper with a sharp report. The calcium carbide is allowed to react further (more can he added) and the flask is then lightly restoppered. After 10-15 s a match is placed next to the opening and with everyone expecting an even louder explosion, the acetylene now meekly ignites a t the opening with a soft squeak. The flame can he allowed to burn out, or extinguished by covering the opening with a small glass plate. The explanation for this seemingly surprising result is that initially the flask contains an explosive mixture of oxygen and acetylene leading to the expected effect; further evolution of acetylene, however, drives the remaining oxygen from the flask so that comhustion takesplace only where the cornhustible gas, acetylene, comes into contact with the oxidizing agent, oxygen gas. Initially, if the match is not quickly placed near the vacuum opening, the acetylene may have displaced the oxygen and no "explosion" will take place. This apparatus can he used to demonstrate the differ&es hetwee~n~controlled mmbustion and explosionand also clearly illustrares the principle of the6'carbidelamps" of the early mining days. This demonstration has been executed nearly 200 times
' Hague, G. R., Jr. J. Chem. Educ. 1983, 60, 355. 444
Journal of Chemical Education
with a perfect safety record. For added safety, the flask could he selectively taped (so as not to obscure the reaction) and placed in a glass or clear plastic container to reach just below the take-off arm. Further, safety shields can he placed in front and behind the demonstration for complete protection.
The Induction by lron(li) of the Oxidation of iodide by Dichromate SUMlrn BY Wllliam L. Jolly U n l ~ e r s i t y01 Calllordlt Berkeley, CA 94720
The ideal lecture demonstration is thought-provoking as well as pertinent to the topic under discussion. When, in a course in inorganic chemistry, I have almost finished discussine the mechanisms of inorganic reactions in aaueous soluti&s, I like to present this dekonstration. ~ l t h o u g hthe procedure and results are fairlv simole. . . one is able to deduce a rather surpriring mechanism. First consider the reductim potentials for dichromate and triiodide Cr(V1) + 3eC p Cr(II1)
Eo = 1.33 V
I t is obvious that dichromate (represented above by Cr(V1) for simplicity) is a stronger oxidizing agent than triiodide and that it is capable of oxidizing iodide to triiodide. The first step in the demonstration is actually to add an iodide solution to a weakly acidic dichromate solution. Nothing much happens. (Only after an hour or two is a gradual darkening of the solution perceptible, due to the formation of brown 13-.) The predicted reaction takes place very slowly, if a t all. The second step of the demonstration is to add a solution containing ferrous ion to the reaction mixture. Essentially instantaneous formation of brown IS- occurs upon adding the reducing agent Fe(II)! This result can serve as the basis for discussion with the class. Probably a student will suggest that the Fe(I1) acts as a catalyst for the oxidation of iodide by dichromate, that is, that the Fe(I1) is oxidized to Fe(II1) by dichromate, and that the Fe(III), in turn, oxidizes the iodide:
However, if the proposed catalytic mechanism is correct, then the reaction of Fe(II1) with I- must he at least as fast as the overall reaction, which was found to occur almost instantaneously. This point can he tested. The next step of the demonstration is to add an iodide solution to a solution of Fe(II1) having the same acidity as the dichromate solution. Nothing happens. Obviously the brown 13- color forms a t a rate much lower than the rate of
oxidation of iodide by dichromate in the presence of Fe(I1). Hence we conclude that the Fe(I1) ion does not simply a d as a catalyst for the dichromate-iodide reaction. Another explanation, and further discussion with the class, are required. It may take some prompting to get someone to suggest that perhaps the Fe(I1) reacts with the dichromate to form an intermediate species which is able to oxidize the iodide ion rapidly. Indeed, the reaction is believed to proceed by such a mechanism.' The Fe(I1) is said to "induce" the dichromate-iodide reaction. The following reactions are believed to be important. Cr(V1) + Fe(I1) followed by
-
CrW) + 31-
-
CrW) + 2Fe(II)
-
and
Cr(V) + Fe(II1)
Cr(II1) + I
