edited bv P. B I R ~
JAMES
Arizona State University Tempe. AZ 85287
Experimental Work with Tin(ll) Chloride in a High School Manuela Martin Sanchez E.U. "Pablo Montesino", Santisima Trinidad 37, 28010 Madrid, Spain The experiment described in this paper has been used by the author as a final ~ r o i e cat t the end of the first vear of a high school chemistry course. Students typically-work in groups of two to four as they seek to apply the concepts of hydrolysis, Le Cha~elier'sprinciple, and electrolysis to the sssrem. They seek answers to questions such as: "What rkactions were involved; why is an-aqueous solution of SnCI2 acidic; how can elemental tin be recovered from the system?" The Experiment Students are given about 1g of pure tin(I1) chloride (he sure the sample is free of oxide contamination. which will not dissolve even upon the addition of HCI), a small dropping bottle of 2 M HCI, a pH test paper strip, and a piece of tin/ lead solder wire about 5 cm long. They are instructed to brina the followina items from home: a 4.5-V pocket battery. two pieces of copper wire about 10 cm long, an old newspaper to put on the tahle, a small hottleof water, and the bottom of a small transparent plastic bottle about 6 cm in diameter and 3 cm tall. The latter will be used as the reaction vessel and electrolvtic cell. Once all Laterials have been readied, the students are instructed to place the SnC19 into the reactionvessel andadd water until it is about half fuil. They swirl the container until all the SnCIz has been dissolved. The resulting solution is . the pH shows turbid due to the pressure of s n ( 0 H ) ~Testing the solution to he acidic. Upon addition of a few drops of 2 M HCI, the solution clears. Students are warned not to add too much HCI as excess HC1 will result in the formation of Hz during the elrrtrdysis step itmead of tin. Once the turbidity has been disspelled, theelectrolysiscell is set up by connecting a piece of copper wire to each of the poles of the battery. A piece of tin wire is then connected to the copper wire that is connected to the positive pole. The copper wire a t the negative pole is left free. Both wires are then immersed into the solution in the cell. Students are cautioned to take care to keep the copper and tin wires from touching during this process. Shortly, the students will begin to see brilliant leaves of tin appearing on the copper wire as the tin becomes thinner. At the comuletion of the activity the entire setup, except for the batteiy, is disposed of. st;dents are then asked to explain the reactions they ohserved. They are further asked to trace the etymological meaning of hydrolysis, electrolysis, cathode, and anode. The Chemistry
As the tin(I1) chloride dissolved in water, the tin(I1) iron hydrolyzed to produce the acid turbid solution according to the equation
898
Journal of Chemical Education
Upon the subsequent addition of HCI, the turbidity disappeared as the equilibrium shifted to the left in accordance with Le Chatelier's principle. The brilliant leaves of tin that appeared at the cathode during the electrolysis can he accounted for by the reduction half reaction, The oxidation half reaction can he written to explain the reduction of size of the tin wire. Conclusion Students typically enjoy this experiment very much and have good success interpreting their observations and exolainin~the chemistw of process thev have observed. The experiment can he made quantitative if the electrodes are weighed before and after thev electrolvze. I t is difficult to coliect and weigh the tin leaves that r e s h t from the electrolysis.
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James P. Birk received his BA degree trom St. John's University (Minnesota) in 1963 and received his PhD from Iowa State University in 1967. Following a postdoctoral year at lhe University of Chicago. he joined the faculty of the University of Pennsylvania, where he held the Rhodes-Thompson Chair of Chemistry in 1972-1973. in 1973, he ioined the facuiw of Arizona
State Un "err ty Our ng lhe 1987 1988acaoemocyear, ne receweda SERAPhlM te o w n p an0 was a V.smg Prolessor a1 Easlern M cn gan University. Birk ha5 a long-standing interest in chemical education, with several undergraduate lab manuals and experimental modules currently in publication. He has twice received the Award for Distinction in Undergraduate Teaching given by ASU's Depaltment of Chemistry He Currentiv teaches an underaraduate course in methods of teachina h gh schoo Chemslry an0 5 Coord nator of ASL's hlgn Schooi O w reach program Granls that he ootalnea f.nded me thee-year Program to Enhance the Tra n ng of h q n Scnooi Chem rtry Birk is a Fellow of the American institute of Chemists and is a member of Phi Kappa Phi, American Chemical Society. Sigma Xi, Asso~iationfor Computers in Science and Math Teaching. Society for College Science Teachers, and Arizona Association for the Gifled and Talented. He was on the Bmrdaf Directors for Odyssey School farthe Gifled and Talented from 1985 to 1987 and currently serves on the Advisory Board for ASV's Center tar Academic Precocity.
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