CIATION ACHERS NEW ANGLES ON FAMILIAR DEMONSTRATIONS1 ELBERT C. WEAVER Phillips Academy, Andover, Massachusetts PREPARATION OF HYDROGEN
DIFFUSION OF HYDROGEN
A. The preparation of hydrogen, using the reaction of sodium with water, is familiar and its dangers are well known. The presence of sodium hydroxide must be assumed unless the demonstrator concentrates a large volume of solution and evaporates it to dryness, a timeconsuming process. The preparation of hydrogen by the reaction of metallic calcium on water proceeds a t a convenient rate, i t is free from danger, and furthermore, the students can see the calcium hydroxide. The solubility of this compound in water is so low that i t appears as a white precipitate. Procedure: A gas-collecting jar is filled with water. About a half-teaspoonful of calcium chips is placed in the jar. It is then immediately covered with a glass plate, inverted, and placed mouth downward below water in a pneumatic trough. The chips drop to the bottom of the trough, and the displaced hydrogen is collected in the inverted jar by displacement of water.
Just as' a small dog can run through a forest but the larger, slow-moving hunter must pick his path among the obstructions, so the agile hydrogen molecule diffuses thrdugh a porous porcelain cup. Procedure: Equip a porous cup with a one-hole stopper through which a long 6-mm. glass tube projects slightly. The lower end of the glass tube extends through a two-hole stopper that fits an Erlenmeyer flask. The other hole of the second stopper is supplied with a jet. 'The flask is partially filled with colored water. An inverted beaker encloses the porous cup. A jet of hydrogen from a generator is thrust between the beaker and the cup. The diffusionof hydrogen into the cup soon builds up enough pressure within the flask to squirt a stream of water out of the jet. A growing plant in a flowerpot stands ready t o receive the stream. Now the beaker is removed quickly, and the return of air into the system through the jet is noticed while the hydrogen diffuses outward through the walls of the porous cup.
Ca
+ 2HOH
-
+
~ a ( 0 h ) ~HZt
B. The reaction of metal and acid to produce hydrogen is trite, but the corresponding action of strong alkali on a metal is sometimes neglected until the study of the metal. Aluminum is selected as the metal, and the presence of the protective coating of oxide on its surface is demonstrated incidentally by 'the delay in starting the reaction. Once started, the exothermic reaction proceeds with vigor and steam is generated. Procedure: Two tall precipitating jars are placed erect in large (12-inch) evaporating dishes. The dishes serve to catch any overflow. Aluminum trimmings to a depth of two inches are placed in each jar. Enough liquid to cover the metal is added to each jar, hydrochloric acid to one and sodium hydroxide solution to the other. Try concentrated hydrochloric acid and saturated sodium hydroxide, each mixed with an equal volume of water. Retard the speed of the reaction if necessary by dilution. 2Al+ 2.41 2A1+ 2A1
6HC1
+ 6H+
+
2NaOH 20H-
--
+ 2AlC1,
+
+
2Al+++ 280 2NaAlOn 2H,O + 2A10~-
+ 3& t
+ 3H* t + 3H21
+ 3H2t
1 Presented at the 237th Meeting of the New England Asso. ciation of Chemistry Teachers, Colby Junior College, New London, New Hampshire, October 19, 1946.
HYDROLYSIS
The explanation of hydrolysis of aluminum sulfate solution on the basis of the formation of sulfuric acid and aluminum hydroxide has an apparent weakness due to the fact that the student does not see any aluminum hydroxide, although its solubility is very low (0.00001 g . . ) . A more satisfactory explanation is found in application of a modern theory of electrolytic solution^.^ Procedure: Dissolve 5 g, of aluminum sulfate in 500 ml. of distilled water. Remove a sample of the solution with a medicine dropper and squirt the liquid onto a sheet of blue litmus paper. PRECIPITATION OF ALUMINUM HYDROXIDE
The amphoteric nature of aluminum compounds may be discussed a t length, but no amount of verbal persuasion equals two minutes of demonstration. Especially impressive and incredible to the beginner is the formation of a hydroxide by use of a strong acid. Procedure: 100 ml. of aluminum sulfate solution is placedin a tall jar that has a dark card for a background. Sodium hydroxide solution is added with stimna until a WEATER,E. C., AND L. 5. FOSTER, School Sn'. and Mathe., 46, 547 (1946).
JANUARY, 1947
49
flocculent precipLtate appears. Continued addition of the hydroxide causes the precipitate to dissolve. Hydrochloric acid is now added cautiously until the hydroxide precipitate just reappears. More acid dissolves the precipitate. The precipitate can be recalled by means of sodium hydroxide solution, and so on, indefinitely. AIdSOJa AI(0H)s NaAIOz A1(OH)3
-- -
+ 6NsOH 3Na804 + 2AI(OHh 4 ++ NaOH NaAIOn + 2H20 Hz0 + HCI Al(OH), 1 + NaCl + 3HC1MC13 + 3 8 0
REDUCTION BY ALUMINUM
With the return of peace, Thermit and Thermitigniting mixture are once more available. This stimulating experiment should again appear on our lecture tables. For igniting Thennit, imbed the heads of several large wooden matches in igniting mixture. Avoid the use of amagnesiumfuse which, whenit burns, blinds the audience to the important reaction to follow. After the reaction, cool the molten material. Chip off the brittle aluminum oxide slag and brighten the hall of steel with a file. Procedure: Fill a sand crucible almost full of Thermit mixture. Place a teaspoonful of igniting mixture in the center of the top of the Thermit and allow the two to interpenetrate slightly. Imbed five match heads in the igniting mixture. Place the crucible on a dry-sandfilled pneumatic trough that in turn rests on a large sheet of asbestos paper. Ignite the mixture by means of the flame of the Bunsen burner. Immediately after the reaction, seize the crucible with tongs and pour the contents onto some nails that are imbedded in a depression in the sand in the pneumatic trough. When sufficiently cool, hold the erstwhile liquid mass under running water, break off the adhering slag, and file. 3FesOl
+ 8A1-
4ALOs
+ 9Fe
Other oxides that may he reduced by this method include: Fez03, Crz08, MnOz, Vz06, TiOn, NiO, Mooa, Bz03,and SiOz. 0
New Members
'Allan Cooney, Vermont Junior College, Barry, Vermont Elene Farello. Wilmington High School. Widmineton.
0
Official B v s i n ~
237th meeting, October 19, 1946. Colby Junior College, New London, New Hampshire. The following was given a t the 237th meeb ing of the N.E.A.C.T. Greetings, H. Leslie Sawyer, President of Colby Junior College; "Surface Activation," W. B. Thomas, Bates College; "Derivatives of Vitamin K and Dicourmarine," Douglas M. Bowen, Dartmouth College; Lecture Demonstrations, Otis E. Alley, Elbert C. Weaver, Phillips Andover Academy. A short business meeting followed the luncheon. 238th Meeting, December 7, 1946. Boston College, Chestnut Hill, Massachusetts. The following program was given a t the 238th meeting of the NEACT. Greetings, Rev. William L. Keleher, S.J., President of Boston College; "Some Impacts of Biology, Biochemistry, and Biophysics on the Prohblem of the Nature and Formation of Connective Tissue," Dr. Bernard S. Gould, Massachusetts Institute of Technology; "The Physics of Precipitation in the Atmosphere," Mr. Roland J. Boucher, Meteorologist of the United States Weather Bureau; "The Application of Science in the Detection of Crime as Used in the F. B. I. Laboratory," Mr. Jeremiah F. Buckley, S.A., Federal Bureau of Investigation; "Rocket Propellants," Dr. John K. Rouleau, Boston College. A short business meeting preceded the last paper. President Eldin Lynn has announced the appointment of the following committees. Ninth Summer Conference Committee: Paul F. Stockwell, Chairman, Dorothy Abel, Otis Alley, Lawrence H. Amundsen, Paul Bodell, Millard W. Bosworth, Harold Coburn, Helen Crawley, Gertrude Eastman, Rev. Bernard Fiekers, S.J., Ina Granara, George Hearn, Elizabeth Hollister, Harriet Hemenway, Walter Lynch, John Suydam, Carl Swinnerton, Elbert Weaver, and Saverio Zuffanti. Chemical Education Committee: Amasa F. Williston, Chairman, Dorothy Ahel, Harold Coburn, Marjory Fenerty, Ralph Keirstead, Evelyn Murdock, Andrew O'Comell, and William Stakeley.
Schedule of MeetinLouise Foering, Russell Sage College, Troy, New York 0 Robert Lyons, Marlborough High School, Marlborough, 239th meeting, February 15, 1947, Rumford Chemical Massachusetts Works, Rnmford, Rhode Island Paul Doinan. Research Fellow. Iowa State Collene. - , 240th meeting, March 22, 1947, Trinity College, HartArnes, fowi ford, Connecticut Carl Perkins, Pittsburgh High School, Pittsburgh, 241st meeting, May 10,1947, College of the Holy Cross, Massachusetts Worcester, Massachusetts
