JUNE,
1941
SOME LECTURE EXPERIMENTS USING RUBBER BALLOONS EDWW T. MITCHELL Holy Cross College, Worcester, Massachusetts
I N DEMONSTRATING before a large class in general chemistry, many principles can be illustrated very clearly by using reactions which evolve gaseous products. Such reactions can be shown by attaching a rubber balloon to the reaction flask and noting the relative rates a t which the balloons are inflated. This idea was used by Waltonl to show the relative strengths of acids. Balloons have also been used very successfully by the author in demonstrating the following principles. (1) LAW OF MASS ACTION The common method of demonstrating this fundamental law is to employ either the iodine clock reaction or the arsenious sulfide clock reaction. A much simpler method of demonstrating this law, however, can be carried out as follows: Fit four 125-cc. Erlenmeyer flasks with one-hole rubber stoppers into which is inserted a short piece (two inches) of glass tubing, flared at,the outer end. Over each flared end is pushed the mouth of a balloon. The balloons can be fixed to the glass tube in a more permanent fashion by making them tight with a piece of string or a rubber band. To carry out the experiment, add to the four flasks 50 cc. of hydrochloric acid of the following concentrations: 6 M , 3 M , 1.8 M , and 1 M , respectively. Through the mouth of each balloon pour two grams of 20-mesh granular zinc. -Next insert into the mouth of each flask one of the stoppers, to which a balloon is attached. The balloons are then raised so that the zinc is allowed to fall into the acid solutions. The hydrogen generated fills the first balloon immediately, the second in approximately a minute and a half, the third in about four to five minutes, and the fourth only after a period of a half an hour. 1
WALTON, "Lecture demonstrations in general chemistry,"
J. CHEM.EDIJC.. 8,303 (1931).
(2) COMMON ION EFFECT This experiment shows the manner in which an excess of acetate ions decreases the acidity of the acetic acid. To each of two 125-cc. Erlenmeyerflasks fitted with stoppers, carrying balloons as described above, are added four grams of calcium carbonate. To one of the flasks 50 cc. of water are added and to the other 50 cc. of a saturated solution of sodium acetate. The solutions are shaken and allowed to settle. To each flask are added 15 cc. of 1.5 M acetic acid. Immediately insert the prepared stoppers in the mouths of the flasks. The repression of the hydrogenion is very strikingly shown by the Gery slow evolution of carbon dioxide, thus giving rise to a slow inflation of the balloon as compared with the rapid evolution of carbon dioxide in the other flask and the consequent rapid inflation of that balloon. . . If the experiment is carried ;&-as described very little frothing of the solutions occurs. (3) CATALYSIS To each of two 125-cc. Erlenmeyer flasks are added 50 cc. of 1 M sulfuric acid. For this experiment, as for the others, one-hole rubber stoppers should be available which have been fitted to balloons by means of glass tubes. Into each of the balloons are poured two grams of 20-mesh granular zinc. To one of the flasks containing the acid is added about a gGam of copper filings. The stoppers are put on the flasks and the balloons held up so that the zinc is allowed to fall into the acid solutions. The solution containing the copper filings evolves hydrogen much more rapidly than the other solution. When the zinc is all used up, the copper filings can be taken out and shown to the class. Platinum foil can also be used in dace of c o ~ ~ e r . ~ -
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Nole: The zinc must be in contact with the copper filings or platinum foil.
