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PICTURES o ACID-BASE REACTIONS'THOM S H. HAZLEHURST, JR.

Lehigh Univenity Bethlehem Pennsylvania

E of the obstacles to be surmounted in teachingthe Br6nsted theory of acids and bases is the veryone which caused the scientists themselves tohesitate to accept it. As far as Arrhenius knew, ions ofdifferent species had individual characteristics, bu t the

limits of variation from the average for all ions were notvery wide after all. Such things as ionic radii or

that the extent of solvation varies from ion to ion.Hence the insistence of Br(nnsted upon the fact thathydrogen ions are lw ys solvated seemed supexthous

and of no particular significance since all other ionsare solvated also.The basic difference between the hvdrozen ion and.other ions is primarily its size. Whereas all other ionshave diameters of the order of A (apart from anyshell of solvation), the diameter of the hydrogen ion isestimated to be lo- A This is smaller than even thenuclei of other atoms and le'ads to a tremendously highvalue of ionic potential = charge/radius) comparedto that of Lif. Hence the hydrogen ion, which is justa bare nucleus entirely devoid of electron shells, ac-tually buries itself in the electron shell of the atom towhich it happens to be attached.Some years ago it was pointed out 1) that mostmolecular models are inadequate in their representationof combined hydrogen because they used balls or blocksof the same size for both hydrogen and carbon, for in-stance. The same fault can be found with the black-board formulas

Presented before the Division of Chemical Education at thewere of the same Order of magnitude' ninety-seventh meeting of the A. C. S., Baltimore Md. April 4It was e&ly recognized that all ions are solvated and 1939.

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Here, too, the implication of comparable sizes of andH persists.To avoid this particular defect (at the risk of intro-ducing others) a method of representing molecularstructure has been tried out with considerable successin class work. This representation is based upon the

wave-mechanics picture of the atom as a nucleus sur-rounded by an atmosphere of negative electricitythe density of which is governed .by the value of b2,the square of the amplitude in the w%veequation ofSchrijdinger. Methods of representing this atmos-phere in single atoms have been worked out 2 ,3 ) . In

the present scheme involving polyatomic species only aqualitative representation is possible, but two featuresof the actual species are faithfully preserved: (1) theinternuclear distances, which are pretty well known forthese simple species, and (2) the arrangement of theatmosphere in layers or shells. On the other hand,since the representations are two-dimensional cross-sections, no structure with non-coplanar atoms can be

conveniently represented without distortion. Thebest that can be done is to preserve the essential ele-ments of symmetry, a pyramidal ammonia molecule orthe hydronium ion becoming triangular, and a tetra-

hedral ammonium ion or methane molecule becomingsquare.To prepare the pictures, the positions of the nucleiare first marked. The scale used in these diagrams was8 2.5 inches (a magnification of roughly one to

2H z0 OH- OHJ

half a billion). For the sake of convenience and in theabsence of definite evidence to the contrary i t is as-sumed that the- formation of .coiialent bonds of the

NH; HCI CI- NHli

type actually present in these species has no significanteffectupon any but the valenc~shel l,o tha t the othersare simple circles. The radii of these circles aretaken as the maxima in the electron atmosphere and

are determined by use of the screening constants ofPauliug and Sherman (4). Actually there are several

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maxima in one shell, bu t they are closely spaced, and combination of chlorine and hydrogen. These dia-nothing is to be gained by introducing such a complica- grams also show that the ammonia molecule and thetion. With artis t's pastel chalk in one of the more hydronium ion differ only negligibly in size and shape,intense shades such as dark green, dark blue, dark red, the single significant difference being the charge on theor brown it is fairly easy to show the varying density hydronium ion and the lack of charge on the ammoniaof the electron atmosphere by a varying intensity of molecule.The presence of electron shells in the cores of bothatoms bound by covalence causes a pronounced dif-ference in the shape of the diatomic molecule as isshown by hydrogen chloride and chlorine. In thelatter there is a decided neck between the atoms;in the former almost none. The representation of the

color. The nuclei are obvious in the diagram simplyas approximate centers of symmetry for the electronatmosphere in their immediate vicinity. If they arerepresented explicitly by a spot of color it must beclearly explained tha t they would actually be entirelyinvisible on the scale employed. The actual shape ofthe distorted valence shell being unknown except in thecase of the hydrogen molecule 5) , t has been arbitrarilyrepresented.The internuclear distances used are: O--H 1.0Cl-H 1.3 N-H 1.0 Cl-Cl 2.0 0=0 1.2all in h g s t r o m units. In the picture of the watermolecule the angle HOH is taken as l l O O The otherbond angles are obvious from symmetry.The object of these diagrams is to drive home to thestudent that the proton in any hydrogenous speaes isburied in the valence shell of another atom. Thus hy-drogen chloride is more properly considered as chlorideion distorted by the presence of a proton than as a

double bond in oxygen is probably fanciful and unreli-able, but may still be used with as much propriety asthe usual blackboard formulation, 0=0 which im-plies two parallel bonds.Such pictures are by no means restricted to acid-basereactions. The organic chemist frequently uses modelsto elucidate the properties of the molecules which he isinvestigating. The sets of balls and sticks commonlyused have only one size of ball for all atoms, a state ofaffairs which is tolerable for all the relevant atomsexcept hydrogen, and much confusion has resulted inthe past because of t he implicit assumption that ahydride is essentially the same as any other binarycompound. A picture of methane exhibited to theclass in organic chemistry would help overcome anysuch dificulty.

SUMMARYThe unique behavior of the proton is clarified byusing charts of electron density to represent mole-cules and ions containing hjdrogen. Representativecharts are sliown.

LITERATURE CITED1) H A Z L E H ~ S Tm NEVILLE,. CHEM. EDUC., 2 128 McGraw-Hill Book Co., Inc; New York City, 19301935). p. 565.2) Haz~~xonsrND KELLEY,bid. 12 309 1935). (4) PAULINCND SHERMAN,. K .; 81 1 1932).3) R u m AND UREY Atoms, r n ~ l e ~ ~ l e ,nd quanta, The (5) LONDON,. Physik, 46, 476 1928).