R. J. FRIESEN, editor
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University of Waterloo Waterloo. Ontario, Conoda
A Lab to Motivate Weak Students
Brass
GlenLoueridge Silver Heights Collegiate 350 Lodge Avenue, S t . James, Manitoba RM OM8
P. D. McCormick
Teachers need help motivating weak science students, motivating t h e m t o complete t h e course, a n d motivating t h e m t o t r y a science course. I'd like to help b y sharing m y ideas. One l a h which I have found to h e very good is described below. I c a n use i t with almost a n y type of class. If I have covered half reactions a n d t h e oxidation potential table, I c a n insist o n balanced half reactions or equivalent weights. B u t better still, I c a n use it with a n y class t o introduce electrolysis. Depending o n t h e ability of t h e class, I m a y a s k for observations alone.
T h i s experiment constitutes a rather spectacular illustration of a diffusion process i n solid copper. M r . A. T. Nathan, Central Technical School, Toronto, Ontario, first presented i t t o a group of teachers who were just heginning participation i n a Solid S t a t e Structure a n d Reactions Program sponsored by t h e American Society for Metals.
Thorold Secondary School Ormond Street North Thorold, Ontario
Object T o study a n electrolysis reaction. Introduction T h e electrolysis will he done i n a jelly. T h e n if t h e a p p a r a t u s is moved, t h e results of t h e electrolysis will not mix. Method 1) Put l.W g of compound into a 250-ml beaker. Add 20.0 ml of water and make a solution. Depending an your class: A) Give them the name potassium iodide, then insist on half reactions. B) Give them no name, and let them try to determine what was present. C) For some classes you may want to say it is a compound which liberates hydrogen ions and iodide ions. 2) Heat the solution until it boils, then add 0.2 g of agar. Continue to heat and stir until the solution boils again. 3) Obtain two pieces of copper about 10 em long, bend into a semi-circle and have one end extending up. Place into a Petri dish, as shown, then pour in the hot mixture. Do not let the two pieces of copper touch. 4) Saturate a sponge with cold water. Put the Petri dish on the sponge and let cool until a jelly forms. This will take 5-10min. 5) once the mixture h a s s e t completely into a jelly, attach the copper wires to a 6-V battery and subject the mixture to electmlysis. Let electrolysis reactions continue for about 5 min. 6) Then disconnect the terminals and pull the capper wires out of the jelly. Have a good look at everything. Put your fingers into the jelly and try to determine what happened. Smell the jelly and your fingers. 7) Put the copper and jelly into the garbage. Clean up and put away your apparatus. T h e s t u d e n t s c a n see t h e brown iodine formina, thev c a n smell it a n d some recognize i t . Some students kill ndt i r e bubhles forming o n t h e other wire, a n d , depending on their abilits. will recomtze t h e m as hubblesof hvdn,eetl. As a teacher, you will have t o decide o n w h a t t o a s k for i n t h e lab reports. This will depend o n t h e ability of t h e class. ~
102
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Journal of Chemical Education
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Experiment A small spoonful of zinc dust is placed in an evaporating dish, and covered with 3 M to 10 M sodium hydroxide solution. This is heated to a steaming temperature, whereupon a clean copper token is immersed in the hot zinc-plating bath. When the plating bath is active, numerous bubbles of hydrogen will escape from the mixture and heating may he temporarily discontinued. A silvery coating of zinc will develop on the copper. When this plating pmcessiscompleted, the token should be washed and dried. The zinc may be caused to diffuse into the copper by holding the token, with tongs, in the outer cone of a Bunsen flame. The token will suddenly turn yellow as a brassy coating is formed. It is heated for two or three more seconds and immediately quenched in water to prevent oxidation. The copper will appear to have been transformed into a brass token. Notes 1) A practice to be discouraged is the use of the copper tokens which are available, for the price of one cent, from the Canadian 0rU.S. Mint. 2) c u t the copper token in hall and examine it for depth of penetration of the brassy coating. 3) If the brassy token is left undisturbed for one or two days, the upper surface will revert to its original copper color while the lower surface remains brassy. Why? 4) The most frequent defect obtained by the student is oaidation of the brass by overheating. 5 ) Try reversing the brass-forming process by heating a piece of brass shim stock (0.005-in. or greater in thickness) in the flame of a Meker burner while the other end is in a beaker of cold water. You should be able to volstilize the zinc from the brass over the hottest part of the flame, and observe a return to a copper color. A bonus to this part of the experiment is obtained if you etch tbe brass with 30% HNOa-70% cone. H202 whereupon large crystals can he observed microscopically at the heated end of the brass strip. These become progressively smaller toward the colder end of the brass sample. 61 Brass is fun stuff to work with, and any old brass door knob should yield its crystalline secrets when etched either hv cantinuous usebr by the etchant described above. Editbr's Note: Chem 13 News is an informal publication of the Department of Chemistry of the University of Waterloo, addressed primarily to Ontario high school chemistry teachers. This column is the first of a bimonthly series of selections from Chem 13 News by R. J. Friesen, one of the editors. The other editors are J. L. Candida of Kitchener Waterloo Collegiate Institute and Vocational School and W. A. E. McBryde of the University of Waterloo.
