Joseph F. Castka Martin Van Buren High School I Demonstrations for High School

Queens, New York City, 11427 Chemistry

These demonstrations and the sequence in which their presentation is suggested may serve to initiate and main- tain student interest in such topics as (1) the development of acid-base theories and (2) bond strength. Sources of these demonstrations or variations of them are the texts by Alyea and Dutton' and the not so easily obtainable one by Fowles.2

Students readily frame an operational definition of acids on demonstrations or experiments dealing with taste, indicator reactions, replacement of hydrogen by ac- tive metals, and acid-base neutralization resulting in the formatton of water and a salt. Listing of common acid for- mulas results in student recoenition of hvdroeen as an " <~ ~ e ~ - - ~~~ - ~ - ~

acid constituent. In similar procedures they frame an op- erational definition of a hase and recognize that hases contain the OH group.

Conductivity demonstrations of ionization develop the Arrhenius conceptual definition (explanation) of acids and bases and their behavior. These may include the conduc- tivity of fused salts as well as that of aqueous solutions of acids, bases, and salts. The representative ionization (dis- sociation) equations are then written. Most high school chemistry courses include the concepts of the Bronsted- Lowry definitions and some include the Lewis definitions. Some of the demonstrations described below may he used for these purposes.

Chemical bonds and the role of energy in their rear- rangements are fundamental in the development of the basic concepts of high school chemistry. Some of the dem- onstrations descrihed below may he used in explaining the role of bond strengths and energies. . - Development of Acid-Base Theories

In Part I of a series of articles Jensen3 deals with the development of acid-base theories in the sequence: Ar- rhenius definitions-Solvent Systems definitions- Bmnsted-Lowry definitions-Lewis definitions. In dis- cussing the Solvent Systems definitions, he points out that such nonaqueous solvents as liquid NH3 and liquid SO2 undergo a small amount of dissociation into a solvent cation and anion. Therefore an Arrhenius acid is merely a species which, on dissociation in water, causes an increase in concentration of the solvent cation H30+ and an Ar- rhenius hase is a species which merely causes an increase in concentration of the solvent anion, OH-. The dissocia- tion (ionization of water) may he demonstrated by using a Ne or Ar glow lamp in the conductivity apparatus (Alyea- Dutton, p. 148). Other solvents such as absolute ethanol, glycerine, acetone, and glacial acetic acid also demon- strate this phenomenon. Indicator reactions in nonaque- ous solvent are described in Alyea-Dutton, p. 61. Jensen points out that the Solvent Systems definitions made

'Alyea, H. N., and Dutton, F. B., "Tested Demonstrations in Chemistry," 6th Ed., Chemical Education Publishing Co., Eas- ton, Pennsylvania, 1969, pp. 148,61,188, 103.

2Fawles, G., "Lecture Experiments in Chemistry," 4th Ed., G. Bell & Sons, LTD, 1957, pp. 272,276,277.

Vensen, W. B., Chemistry, 47(3), 11 (1974); 47(4), 13 (1974); 47(5), 14 (1974); Chemistry Reprint 123. "Lewis Acid-Base Theo- ry."

both the specific properties of the solvent and the phe- nomenon of ionic dissociation central to acid-base behav- ior and failed in the recognition and explanation of acid- base reactions in the gas phase, and in nonionizable sol- vents. Demonstrations of these reactions lead to the Bmnsted-Lowry definitions.

Gas Phase Reactions

1) Invert a bottle of dry hydrogen chloride over a bottle of dry ammonia and remove the cover plates.

2) Bring the moistened stoppers fmm bottles of concentrated hydrochloric acid and concentrated aqueous ammonia to within a distance of 2-3 in.

Both of these well known demonstrations or the equiva- lent Graham's Law diffusion (Alyea-Dutton, p. 188) result in the production of white fumes of ammonium chloride. They may be preceded by the neutralization of diluted so- lutions of the acid and base using methyl orange indica- tor.

Nonionizabie Solvents

Prepare benzene solutions of dry HCI and dry NH3. Samples of benzene and of each of the benzene solutions are tested for electrical conductivity to show nonioniza- tion of the solvent and of the respective solutions. Mix 20 ml of each of the solutions and stopper the bottle. The re- sultant mixture turns cloudy. Upon standing, the solid NH4CI settles out. The solvent is decanted and the pre- cipitate is permitted to air dry under the hood. Water is added to the residue and the aqueous solution is tested for electrical conductivity. ~l ternat ive tests include mixing some of the residue with powdered Ca(OH)z and detecting NH3 by odor or moist litmus. The test for chloride ion may also be used.

Jensen explains that the Arrhenius definitions are actu- ally a special case of the Bronsted-Lowry definitions. An Arrhenius acid is a Bronsted acid that has dissociated in water by transferring its proton to the solvent water. Defi- nition of an acid as a proton donor allows the transfer to occur in inert solvents and in the gas phase. The Arrhen- ius base, OH-, accepts a proton in neutralization and therefore is a base according to Bmnsted. However, the O H ion is not the only base that can exist in water. Any species that can accept a proton, whether in solution or in a gas phase reaction, is considered a base. The net result is that Bmnsted-Lowry generalized the definition of a hase.

Lewis took the next step-generalizing the definition of an acid. Jensen states and at great length explains and il- lustrates that Lewis definitions not only generalize the concepts of acids and hases but that the concepts are now stated in terms of the fundamental features of chemical change, i.e. electron rearrangement and the different types of bonds. Lewis definitions are: "A basic substance is one which has a lone pair of electrons which may he used to complete the stable group of another atom, and . . . an acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms." Lewis acids are electron acceptors, and Lewis hases, electron donors. The reaction product of

394 / Journal of ChemicalEducation

the two may be called an adduct. Briinsted acids are actu- ally adducts of the Lewis acid, H+. Jensen further ex- plains that interpretation of chemical reactions in terms of Lewis acid-base definitions can be used to include dis- placement reactions, ionic dissociation in a solvent, sol- volysis reactions, the formation of metal complexes, or- ganic reactions, and the complementary nature of redox acid-base reactions. In Part 111 of his series of articles Jen- sen deals with the generalization about Lewis acids and bases known as the principle of hard and soft acids and bases (HSAB).

Lewis Definitions

The gas phase reactions between NH3 and HC1 may he used to develop Lewis definitions. Many high school texts describe one method of salt formation in terms of the reaction between a metal oxide and a non-metal oxide. The reactions are related to acid-base reaction in terms of acidic and basic anhydrides. The following demonstration may he used to explain such reactions and lead to Lewis definitions.

A chunk (or several small chunks) of solid CaO from a freshly opened bottle is placed in a bottle to which a continuous supply of COz is furnished. The COz may be provided by the inclusion of small pieces of Dry Ice, from a laboratory bottle or from a gas generator. After approximately 10-min exposure, water is added to the Droduet. The demonstration is not to be confused hy the

fact that the unreacted CaO may show evidence of slaking. A portion of the product (CaC03) is tested for carbonate ion by the addition of acid and detection of the evolved COz by limewater. Details of the acid-base Lewis reaction are described in Master- tan and Slowinski.*

Bond Strength

Demonstration 20-10s in Alyea and Dutton (p. 103) de- scribes the production of 12 crystals by the reaction he- tween HI(g) and Cla(g). I t may be used as indirect evi- dence of the relative strengths of the H-C1 and H-I bonds.

The following series of demonstrations from Fowles may he used as more direct evidence of the relative bond strengths of H-CI, H-Br, and H-I. Dry HC1 may he prepared by the usual method (NaC1 + HzSOn). Dry HBr and HI may be prepared by heating the respective halides with phosphoric acid. A heated glass rod is plunged into test tubes containing the three hydrogen halides. The hot rod in HI results in the violet 12; Note: If the glass rod or an iron rod is heated red hot and plunged into HI, the lih- erated Hz may inflame. No evidence of halogen formation results from HCI and HBr. However, heating a loosely stoppered test tube of HBr strongly in the Bunsen flame does yield the brown color of Brz.

*Mastertan, W. L., and Slowinski, E. J., "Chemical Prinei- ples," 2nd Ed., W. B. Saunders Co., Philadelphia, 1969, p. 452.

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