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GEORGE L. GILBERT Denison University Oranville. Ohio 43023
pH Change Analogy
Reaction Stoichiometry and Theoretical Yield Submitted by:
Cheched by:
Martin G. Ondrns University of Wisconsin-Stout Menomie, Wisconsin David A. Franz Lycoming College Williamsport, Pennsylvania
Preparation A cardboard square 40 cm on the side is placed on a single pan balance with 0.1 g sensitivity, and a 10 cm asbestos square is placed in the center. The total mass is determined and written on the blackboard. From 5 to 10 g of ammonium dichromate are poured onto the asbestos and the new mass determined. Subtraction yields the weight of ammonium dichromate. Demonstration Initiate the reaction with a match or preferably a Bunsen burner. While the reaction is proceeding, the instructor may point out that the weight is decreasing (as evidenced by the falling balance pointer) because N2 and Hz0 are escaping as gases. When reaction is complete, reweigh and determine the weight of the solid product, Cr203, by subtracting the empty pan weight. With the aid of the reaction equation above,' calculate the theoretical yield. Observed yield will be in good agreement (97-9996 yield), usually a tenth of a gram or so low due to dispersal of the fluffy product, particularly in a drafty room. Remarks 1) Considerable class interest can be generated during a second trial. Using a different weight of ammonium dichromate, ask studenis to calculate & advance the yield and final weight a t which balance will come to rest. Results of the calculations can he posted, odds drawn, bets placed, and the balance preset to the desired weight. 2) Demonstration of H 2 0 as a reaction product may be made by inverting a preweighed 1000 ml heaker over the volcano after initiation. Condensation on beaker walls is immediate. Percent yield based on Cr203 + 4 Hz0 as the trapped products falls to 87-89%, hut most students notice the escape of steam from the pour lip of the heaker. Exothermicity of the reaction also becomes apparent. 3) For a good pun, it could he pointed out that this is normally called the volcano reaction, but, had we started with 252 g, we could refer to it as making a mountain out of a....
' Finholt, J. E., J. CHEM. EDUC., 47,533 (1970). 228 / Journal of Chemical Education
Submitted by: Cheched by:
Erwin Boschmann Indiana University-Purdue University Indianapolis, Indiana 46202 Paul T. Ruda Cleveland Hill Schools Cheehtowaga, New Yorh 14225
Preparation Obtain a 2 X 3-ft. piece of cardboard. Paint one side red and label "ACID" along the 3-ft. direction. Paint the other side blue and label " B A S E in a similar manner. Affix a 3-ft. metal rod along the 2-ft. center line and attach to the cardboard securely. Support the rod and cardboard in a double buret clamp mounted on a ring stand to allow rotation of the cardboard. Demonstration The rod is turned at a slow, even rate beginning with the hase (or acid) side in flat view. This represents the uniform addition of acid to hase (or hase to acid). The change of area visible represents the untitrated base and the rate of change is found to he greatest at the equivalence point. This analogy illustrates the point that the rate of change of D H increases as one approaches the endpoint. It is best to iotate the board several times while no
