The Solubility of CaS04 To the Editor
Sawyer's finding [J. CHEM.EDUC.,60,416 (1983)j that his class determination of K,, for calcium sulfate is 10 times higher than the published value results from the presence of undissociated CaS04 molecules in the solution. The equilibrium CaSOAs) * CaSOdaq) Has K., 150 K., making i t a more important factor in the solution orocess than the iron oroduct.1 The dikolution of inorganic'solids almost always involves rom~licatinenluilibria which make s i m ~ l Kr,. calculations wildiy inacc&atk. I t is frightening that this oveEimplification oersists in freshman texts. deceiving students who eventuallv pass on the error to their kudentsetc.
-
'
blculated from the stability consttnt of CaSOdaq) interpolated between the two values (204 and 11) at p = 0.0 and 1.0 from Smith. Robert M.. and Martell. Arthur E.. "Critical Stabilih, Constants. Volume 4: Inorganic Complexes,'' Plenum Press. New ~ o r k 1976 , p. 79.
To the Editor:
Stephen J. Hawkes Oregon State University Cowalli~,OR 97331
My recent note in T ~ I JOURNAL S described an inexpensive laboratom. exoeriment on the solubilitv of CaSOa and noted . the difference hetween the K,, valut;;calrularedtrt,m s d u bilitv data and those found in haadhooks. Since then I have received several letters from interested individuals explaining the nature of this difference. In particular i t has been pointed out that in the case of CaS04 the presence of undissociated CaS04 molecules in solution contributes greatly to thts difference although other factors enter in. Particularlv, I w s h to refer to an earl~erreferencel which was passed on in one ot these letters titIed..Are Solubilities and Solubility Producls bliiti,d" and wh~vhtitkrs up the CaS04 problem in considerable detail. I wish to acknowledge helpful correspondence from Ralph Tykodi of Southeasterh Massachusetts University (North Dartmouth, Massachusetts), Stephen Hawkes of Oregon State University (Corvallis, Oregon), and William Guenther of the University of South (Sewanee, Tennessee). Meites, L., Pode, J., and Thomas, H., J. Cnm. EDUC.,43, 667 (1966). Alberl K. Sawyer University of New Hampshire Durham. NH 03824 The Oriains of Bondina- Conceots To the Editor
The article "A Needed Replacement for the Customary Description of Chemical Bonding" (May 1982, page 376) contains a factual error. The second paragraph of the article starts with the following two sentences: "A superior description of honding (1-1 I ) has been available for 14 years. Its supetiority has been demonstrated by the successful use of this description for the auantitative calculation of bond enereies in thousands of chemical hmds in most compounds ot common type." 'I'hr suwrior description ot hondinr-. riwn in the rrkrences in essentially that a hond between unlike atoms is intermediate between a pure covalent bond and a pure ionic hond. T o say that this superior description of honding has been available for 14 years is incorrect. I t has been available for two-thirds of a century. In his 1916 paper and 1923 hook G. 362
Journal of Chemical Education
N. Lewis said that the chemical bond consists of two electrons coupled together, lying between two atomic nuclei and held jointly in the outer shells of the atoms. He said that the shared-electron pair can lie nearer to one atom than to the other, giving partial ionic character to the bond, and that the bond could he anywhere in between the extreme polar (ionic) limit and the extreme nonpolar (covalent) limit. Lewis' ideas were extended and made more precise by the application of quantum mechanics, especially in papers that were published from 1931 on. In my paper "The Nature of the Chemlcal Bond. 111. The Transition from One Extreme Bond ' h o e to another" IJ. Amer. Chem. Soc.. 54. 983 (193211. I pointed out that according t o quantum mechanics a bond could be of any intermediate type and that increase in bond energy (extra stability) would result from partial ionic character of covalent bonds. Sanderson's reference to "the auantitative calculation of hond energies" accordingly refer; to a new idea that was introduced in chemistry in 1932. I t was applied in detail in the paper "The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativity of Atoms," [J. Amer. Chem. Soc. 54,3570 (1932)j. The electronegativity scale was introduced in this paper. It was accordingly 50 years ago, not 14 years ago, that a method for quantitative calculation of bond energies based on partial ionic character of bonds was devised. Llnus Pauling Linus Pauling institute of Science and Medicine 440 Page Mill Road Palo Alto, CA 94306 To the Editor:
1 regret this obvious misunderstanding. Prnfisuor Pauling is. of course, correct in his tracing of the history ot'the concept of polar covalence. In my very hkef article I took for granted that this history is very familiar to all chemists. The "superior description of &din.g," however, is not the general concept ot'polur honds originated many yexrs ago, although it is based upon that concept. Rather, I referred to the very specific quantitative theory brought to fruition about 14 years ago and continuallv imnroved since that time. This descri~tionis hased on (1) m i own demonstrably improved evaluation of t electroneeativitv electroneeativitv. (2) mv orieinal c o n c e ~of equalization, (3jmy methodhf calculating the partial charg& on combined atoms. (4) mv use of these charge values to determine the blending coefficients for weighting the relative contr~hutionsof covalence and ionicitv to the total hond energy, (5) my recognition of the true origin of the lone pair bond weakeninceffect and the evaluation of that effect, (6) my evaluation of multiplicity factors, and (7) the successful applicat~onof these to the accurate calculation of energies of more than 15,000 bonds among nearly all major group elements, in more than 1,000inorganic and organic molecules and nonmolecular solids. All chemists, and even beginning students, are thus provided with the potential for an improved understandins of the beautiful cause-and-effect relationshiu between the qualities of atoms and the nature of their compounds. They are also given unique insight regarding the cause of chemical reaction. Pauliug's pioneering work on electronegativity and m e chemical bond will never he forgotten or unappreciated, but until oerfection is achieved.. im~rovements will oresumablv . always be possible. Indeed, a major purpose of my article was to encouraee contributions bv others, in the hope that further improvem&ts might be suggested. ~~~~~~~~
~~~
~~~
R. T. Sanderson 4725 Piayer Drive
Fort Collins, CO 80525
