PICTURES of ACID-BASE REACTIONS' THOMAS H. HAZLEHURST, JR. Lehigh Univenity, Bethlehem, Pennsylvania
0
NE of the obstacles to be surmounted in teaching the Br6nsted theory of acids and bases is the very one which caused the scientists themselves to hesitate to accept it. As far as Arrhenius knew, ions of different species had individual characteristics, but the
that the extent of solvation varies from ion to ion. Hence the insistence of Br(nnsted upon the fact that hydrogen ions are always solvated seemed supexthous
limits of variation from the average for all ions were not very wide after all. Such things as ionic radii or and of no particular significance since all other ions are solvated also. The basic difference between the hvdrozen ion and other ions is primarily its size. Whereas all other ions have diameters of the order of 1 A. (apart from any shell of solvation), the diameter of the hydrogen ion is estimated to be lo-' A. This is smaller than even the nuclei of other atoms and le'ads to a tremendously high value of "ionic potential" (= charge/radius) compared to that of Lif. Hence the hydrogen ion, which is just a bare nucleus entirely devoid of electron shells, actually buries itself in the electron shell of the atom to which it happens to be attached. Some years ago it was pointed out (1) that most molecular models are inadequate in their representation of combined hydrogen because they used balls or blocks of the same size for both hydrogen and carbon, for instance. The same fault can be found with the blackboard formulas
.
u
-
were of the same Order of magnitude' It was e&ly recognized that all ions are solvated and
'
Presented before the Division of Chemical Education at the ninety-seventh meeting of the A. C. S., Baltimore, Md., April 4, 1939.
conveniently represented without distortion. The best that can be done is to preserve the essential elements of symmetry, a pyramidal ammonia molecule or the hydronium ion becoming triangular, and a tetraHere, too, the implication of comparable sizes of C and
H persists. To avoid this particular defect (at the risk of introducing others) a method of representing molecular structure has been tried out with considerable success in class work. This representation is based upon the
hedral ammonium ion or methane molecule becoming square. To prepare the pictures, the positions of the nuclei are first marked. The scale used in these diagrams was 1 8.= 2.5 inches (a magnification of roughly one to
2Hz0 = OH-
+ OHJ'
half a billion). For the sake of convenience and in the absence of definite evidence to the contrary i t is assumed that the- formation of .coiialent bonds of the
wave-mechanics picture of the atom as a nucleus surrounded by an "atmosphere" of negative electricity the density of which is governed .by the value of !b2, the square of the "amplitude" in the w%veequation of Schrijdinger. Methods of representing this atmosphere in single atoms have been worked out ( 2 , 3 ) . In
NH;
+ HCI
=
CI-
+ NHli
type actually present in these species has no significant effectupon any but the valenc~shell,so that the others are simple circles. The "radii" of these circles are taken as the maxima in the electron atmosphere and
the present scheme involving polyatomic species only a qualitative representation is possible, but two features of the actual species are faithfully preserved: (1) the internuclear distances, which are pretty well known for these simple species, and (2) the arrangement of the "atmosphere" in layers or shells. On the other hand, since the representations are two-dimensional "crosssections," no structure with non-coplanar atoms can be
are determined by use of the screening constants of Pauliug and Sherman (4). Actually there are several
maxima in one shell, but they are closely spaced, and nothing is to be gained by introducing such a complication. With artist's pastel chalk in one of the more intense shades such as dark green, dark blue, dark red, or brown it is fairly easy to show the varying density of the electron atmosphere by a varying intensity of
combination of chlorine and hydrogen. These diagrams also show that the ammonia molecule and the hydronium ion differ only negligibly in size and shape, the single significant difference being the charge on the hydronium ion and the lack of charge on the ammonia molecule. The presence of electron shells in the cores of both atoms bound by covalence causes a pronounced difI ference in the shape of the diatomic molecule as is shown by hydrogen chloride and chlorine. In the latter there is a decided "neck" between the atoms; in the former almost none. The representation of the double bond in oxygen is probably fanciful and unreliable, but may still be used with as much propriety as the usual blackboard formulation, 0=0, which imcolor. The nuclei are obvious in the diagram simply plies two parallel bonds. Such pictures are by no means restricted to acid-base as approximate centers of symmetry for the electron atmosphere in their immediate vicinity. If they are reactions. The organic chemist frequently uses models represented explicitly by a spot of color it must be to elucidate the properties of the molecules which he is clearly explained that they would actually be entirely investigating. The sets of balls and sticks commonly invisible on the scale employed. The actual shape of used have only one size of ball for all atoms, a state of the distorted valence shell being unknown except in the affairs which is tolerable for all the relevant atoms case of the hydrogen molecule (5),it has been arbitrarily except hydrogen, and much confusion has resulted in the past because of the implicit assumption that a represented. The internuclear distances used are: O--H = 1.0, hydride is essentially the same as any other binary Cl-H = 1.3, N-H = 1.0, Cl-Cl = 2.0, 0=0 = 1.2, compound. A "picture" of methane exhibited to the all in h g s t r o m units. In the picture of the water class in organic chemistry would help overcome any molecule the angle HOH is taken as l l O O . The other such dificulty. bond angles are obvious from symmetry. SUMMARY The object of these diagrams is to drive home to the The unique behavior of the proton is clarified by student that the proton in any hydrogenous speaes is buried in the valence shell of another atom. Thus hy- using charts of "electron density" to represent moledrogen chloride is more properly considered as chloride cules and ions containing hjdrogen. Representative ion distorted by the presence of a proton than as a charts are sliown. LITERATURE CITED
(1)
H A Z L E H ~aSmT NEVILLE, J. CHEM. EDUC.,12, 128 (1935).
(2) (3)
Haz~~xonsr AND KELLEY, ibid., 12, 309 (1935). R u m AND UREY,"Atoms, r n ~ l e ~ ~and l e , quanta," The
McGraw-Hill Book Co., Inc;. New York City, 1930, p. 565. (4) PAULINC AND SHERMAN, Z. K&.; 81, 1 (1932). (5) LONDON, Z. Physik, 46, 476 (1928).
