edited by: RON DELORENZO Middle Georgia College Cochran. Georgia 31014
'biagona~Relationships-Descriptive or Theoretical? H. I. Feinstein' George Mason University Fairfax, VA 22030
I t is well known to inoreanic chemists that certain diazonally related pairs of elements (e.g., lithium and magnesium, bervllium and aluminum, and boron and silicon) possess a surprising number of similar properties.2 This behavior is generally explained by Fajans' Rules, namely that polarizability is a function of charge and of size of the charged species. The result is that the members of each of the above pairs are more similar to each other than would be expected simply from their positions in the periodic table in adjacent families. There are two areas where an understanding of this phenomenon is useful: as an aid to remembering periodic properties and as an aid to devising methods of analysis in certain cases. When I was a bench chemist I received a request for a direct determination of the free (elemental) beryllium in fairly pure beryllium metal. The major impurity was beryllium oxide. Other impurities were small and were determined spectrographically. As any bench chemist knows, analyses of highpurity metals are generally made by determining the impurities and then finding the purity from the difference. in this case a dGect determination was stipulated. I t was essential that an acceptable method be used and that 3 result br ubtained in a rea&ablt! time. A research projrct was out of the oueation. Thcrv were i u ~ too t many diiftwnt types oisamplej waiting for analvses. Reference u, the standard trcatiiri was of no help. Nor werr the chpmista of the ~-~~ ~ ~ ~ ~ burvllium -or~~ducers ~ ~ of~anv~assistancr. The thought then occurrei, based on the conceit of diagonal relationship, that perhaps a method for the determination of free aluminum could be found in the monographs published bv the aluminum nroducers. Eureka! A relatively simile method was described for the determination of metallic aluminum in aluminum skimmings and dross.3 It was readily adapted to beryllium with little change. The sample was reacted with sodium hydroxide solution in a closed system. The evolved hydrogen gas forced an equal volume of water into a beaker. The volume of the hydrogen was determined by weighing the water. This was compared with the volume of hydrogen obtained under the same conditions at the same time from astandard sample of aluminum (since no standard beryllium was available) and the free beryllium was calculated from this ratio. (If other elements and compounds (silicon, aluminum, zinc, etc.) which also evolve gases under the same test conditions are present in more than
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neelieiblv " small amounts. suitable corrections must he made.) There are other cases in which a method for one member of a pair of diagonally related elements can he applied to the other under appropriate conditions. One that comes immediately to mindis the spectrophotometric method for beryllium or aluminum using the ammonium salt of aurin tricarboxylic acid. Another is the determination of NazO in borates or silicates by titration with strong acid.
An Analogy for the Leveling Effect Roger S. Macomber University of Cincinnati Cincinnati. OH 45221 In most introductory chemistry courses the &scussion of relative strengths of acids inevitably leads to a description of the leveling effect of certain solvents. Our experience has been that most students have some difficulty appreciating how variations in the relative basicity of the solvent permit determination of the relative acidity of a series of acids which are equally dissociated in a leveling solvent. The strength of a Bronsted acid in solution is usually measured by the position of the dissociation equilibrium
ow ever,
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' Present address: 10411 Forest Ave., Fairfax,VA 22030.
Brown, G. I., "Introduction to Inorganic Chemistry," Longman, London, 1974, pp. 182-183; Puddephatt, R. J., "The Periodic Tableof the Elements," Clarendon Press, Oxford, 1972, pp. 54-55. "Analysis of Aluminium and its Alloys," The British Aluminium Co., London, 1941, pp. 120-121. 128
Journal of Chemical Education
HX
+ Solvent =Solvent-H+ + X-
However, the basicity of the solvent (or the acidity of its conjugate acid Solvent-Ht) clearly plays a central role in determinine the maenitude of the eouilibrium constant. The classic rxample u t this: eftcct is the relative acidirv o~IICIO.~, HiiOs, and HCI in wdtrr:md inaceticacid assulwnt. All three acids are cquoll~s t n q (essentially rully dissociawd~in water. Hmvever. in acetic i ~ : din less h3sicsdvent than n a t e r ~he acid strength (measured as the extent of dissociation) increases in the order HCl < HNOn < HClOa. Thus, althourh the three acids are not inherentlykqually stiong, they appear equally strong in water for it is sufficiently basic to dissociate each one fully. In this context, water is described as having a "leveline" effect on the acids' strengths. We have found that the following analogy helps bring hime the point. Imagine a father and his young . .son (representing the more and 11,~s basic sdvent6, rvjptxtively) are in the kitchen fixing lunch. In iront uithcm I:, n series or jars and buttkn, t w h fitted with a srrrw cap, including a half-fillrd jar of peanut hutter, a new butrle i i ketchup. and a large unopened jar 111 jelly. \Ve know from exnrrirncr that the iarher ruuld r e o d ~d~isoci;ite l~ --~ each cap from its container with little difficulty. ff his young son were to attemnt the same feat. he would find it easv to dissociate the peanur hurter jar, ior it had been opened prrviouslv. He would haw to itrueele with rhe ketchur, bottle. hut t h i cap would eventually yie% to his small hand. However, trv as he will. he would he unable to budge the top on the ielh jai. Thus, while the father sees all the containek as easy dissociate, the son experiences quite a range of difficulty opening them. Of course, asking the father to pull the cap off a soda pop bottle with his bare hands, like water trying to dissociate methane, would prove that even he has limits! ~~
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