Though trite, "You get what you pay for" is also appropriate. My personal history may prove instructive. I teach a t a state college, the training ground for most future scientists and teachers. I have a PhD, six years of postdoctoral research, and 12 years of teaching, research, publications, and service as an assistant professor-all the "right" things for my profession. I recently was rewarded with the first of only two major promotions possible, that of associate professor rank. All the effort and sacrifice had finally "paid off', I was informed by an $80,00O/year administrator, as he awardedme a3.8% raise, whichmeans an extra $15 a week to my family. I am expectedto be grateful. What I am is insulted and demoralized. I have yet t o receive a salary equal to what a fresh PhD can expect to start with a local pharmaceutical firm. I have students who earn more waiting tables at local restaurants than I do. This situation, I find, is the norm. Far from being "alarmed" (6) a t the impending shortage of science and math teachers, I welcome it. I intend to do everything in my power to augment it by honestly and openly informing prospective teachers that society values the nurture of minds far less than the body. If they need to serve mankind, I'll suggest an MD program. People will pay dearly to unclog their nose, hut not to open what lies behind. And MD's have a better union, too. Terry L. Helser S t a t e University College Oneonta. NY 13820
Be Cautious of Hypothetlcal Treatments of Impossible Experiments To the Editor:
I-~should like to comment on examnle 2 of W. H. Seaton's paper on "Plosivity" [1989,66, ~ 1 3 7 j . Neelectine the certaintv that a mixture of nitric acid (1.04 mol),-acetic-anhydride (i.0 mol), and water (0.15 mol) will not exist as such for lone enoueh .. " to ~. ui ttinto a test avvaratus, all experience suggests that, though this mixture may not be "nlosive". the shravnel emanatine from auto-reaction will stili kill. here is evidence to suggest that it becomes detonable on standing ( I ) . Even if it be not ihork sensitive, I have seen runaway oxidations, which would have heen explosions at pilot-plant scale, with ahout a fifth of this proportion of nitric acid. Interestingly, if we assume that the water reacts, as it will, with anhdride. which means a lowered available ~ ~the ~-~~ - acetic potential energy, the mixture becomes more, not less, "plosive". On the other hand. if we assume acetvl nitrate formation, probably associatedwith increased detbnability of such mixtures on aeine. " -. the "~losivitv"declines. I would earnestly commend tdall your readers the study of texts covering experience of reality, such as L. Bretherick Handbook of Reactive Chemical Hazards, before they trust the lives of their students to hypothetical treatments of impossible experiments in that ideal universe inhabited by chemistry teachers but unknown to those who practice the art. ~
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Literature Clted
Literature Cited
I. Holden,C.Science 1989,244,1536-1537. 2. Judce, R. Chrm. Enp. Nexus 1989.67, 42. 3. On C o m p w 1989,9(1),2. 1. McKenns, B. OnCompus 1989.9111, 10-17. 5. Legowaki,J. J. J. Chrm. Educ 1989,66.535 6. Tifft. Time 1989.lSopt. 111,6b70.
I.BromT.A. Chrm. Brit. 1967.8, 504. Dingle L. E . e t al. Chrm. Rril. 1968.4. 136.
P. G. Urben 2 Upper Rosemary Hill Kenilworth.Warwickshire CV 8 2PA. England
S.
Aufbau Mnemonics To the Editor:
It would appear that a popular pastime of contributors to this Journal consists of devising new mnemonics for teaching electronic configurations, as well as debating the virtues and priority of various forms of these diagrams (1-5). I cannot resist participating in this sport by pointing out that, to date, nobody has cited the real originator of the idea, who was Sommerfeld (6). On a more serious note, however, the more concrete we make these mnemonics the more they are likely to be taken as a representation of the truth by students. Not only are there numerous cases of exceptions as many contributors have pointed out, but, more fundamentally, the assignment of electronic configurations to atoms represents an approximation that is stricrly inronsistent withquantum mechanics
(7). Perhaps we should also devise mnemonics to ingrain the latter fact into the minds of unsuspecting students. Literature Clted 3. 4. 6. 6. 7.
Strong. J . Cham. Edur. 1986.63. U. Adelhelm,M.:Hohn,E.G. J. Chem.Educ. 1988.65, 1114. ParsonsR. W. J. Chem.Educ. 1989.66.319. Sommerfeld.Mem. Roc. Monchesier Lit. Phil. Soe. 1925/26, 70,141 Scerri. E. R. J. Cham. Educ 1989.66.481.
Erlc R. Scerrl King's College, H.P.S. London University London SW3 6LX. U.K.
To the Editor;
Dubar and Calzia (1)conducted a series of experiments in which they subjected equilibrated mixtures of acetic anhydride. nitric acid. and water. within aluminum cylinders, to the impact imposed by a detonator that contained 5 g of mercurv fulminate. Thev vlotted their results on a ternary diagram to obtain a dome-shaped region that was based aeainst the acetic anhvdridelnitric acid side of the triangle. y h e area beneath this-dome defines all of the compositions that would he classified as plosive ifsuh.iected to this level of impact. In a second serie; of experiments, the impact was imposed by a detonator plus 10 g of "plastic" (believed to he about 10%more powerful than Tetryl). This series defined a second dome-shaped region that completely enclosed the first region. In a third series of experiments, the impact was imposed by a detonator plus 20 g of "plastic". This series defined a third dome-shaped region that completely enclosed the second region. No doubt, larger amounts of plastic would have enclosed even larger regions. However, the level of impact imposed by their third set of conditions can be regarded as very severe. As these results indicate, positive results can be accepted from almost any experimental test, but, when a negative result is obtained, there is always the possibility that more severe experimental conditions or the nresence of a (often unsus~ected)sensitizer will reverse the classification. 'one should always combine judgment with comvutational and experimental evidence to decide if a processbresents a signifiiant danger (2). Volume 68
Number 5
May 1991
445
Because Dubar and Calzia did not define the specific compositions or describe the specific results of any of their experiments, I did not use their data when I determined the plosive weight of nitric acid that was listed in my article. If I had included their data, then a plosive weight of 55.0 would have been required to classify properly all of the plosive compositions defined by their third set of conditions. I also did not discuss the f a d that nitric acid can react with some materials to form sensitizers, as indicated by Urben. All of the tested compositions in my data base contained nitric acid mixed with an oxidizable components. I should have specified that this was a requirement for application of the nitric acid plosive weight. In view of the comments of Urben and in the interest of greater caution. I now recommend the limitation of my method to compositions that do not contain nitric acid. I am grateful to Urben for his contribution. Literature Cited I. oubsr. J. Cslzia, J. J . Camp. Rend. Acad. Sci.,Pnrix.Ser. C,2261988,15,1114-111fi. 2. Coflee. R.D..fire Tpchnol. 1911.1,37-45.
Wllllam H. Seaton 1329 Belmean Drive Kingsport, TN 37664
"Entropy of Mlxlng" I s Just Confuslng To the Editor:
The article hy K. T. Brady [1989, 66, 3391 describes an apparatus that provides a wonderful demonstration of the laws of partial pressures and effusion, but to introduce "entropy of mixing" is totally unnecessary and serves only to perpetuate a concept that has led more to confusion than enlightenment [1987,64,676]. One need only imagine replacing air a t one atmosphere with a vacuum to realize that the same course of events would occur in the absence of any mixing. Furthermore, the entropy changeof the h\drogen,calculated from rhe drop in its oressure as it ef!'u$es into the porous cup, is identical to theso-called entropy of mixing presented in the article. Edwln F. Meyer OePaul University Chicago. IL 60614
Reaction of Phenolphthalein
Chem. Educ. 1989, 66, 7251. We have found this to be an excellent reaction system choice for the study of both batch and flow reactors in our chemical engineering laboratories for the past 15 years (see Andres, R.; Hile, L. Chem. Eng. Educ. 1976, 10, 18). I t may be of interest to note in our article a simple, inexpensive design for a flow-through spectrophotometer cell that allows not only study of flow systems but also easy temperature controlin batch systems by continuously circulating the reaction mixture from a temperature-controlled vessel through the cell. There are also some references to other literature studies of this reaction in our article. One of the earliest published observations of the this reaction is that of von Baeyer in 1880 [Annalen 1880,202, 361. One thing that was not mentioned in Nicholson's recent article is how the reversible nature of this fading reaction affects the analvsis of its kinetics. The analysis described in the article neglected the r w e r x reactton. Note that tllc final rwctionsolutionat euuilibrium isstill ninkand sothe fading reaction must be reversible. As a result it cannot be represented by a rate = k i [ P l ,since the reaction rate must go to zero a t equilibrium, which can only happen in this rate law if [PI, the concentration of the colored form of phenolphthalein, is zero. To account for the reverse reaction, we can propose the rate = ki[PI - kdPOH1, where [POHI is the concentration of the colorless carbinol form. For high hydroxide ion concentrations (greater than 0.3 N), the reverse reaction is negligible, and an irreversible reaction model is adequate. However, for a hydroxide ion concentration of about 0.1 N a n d an ionic strength of 0.3 N, the reverse rate constant kz is about 10% of the pseudo-frst-order forward rate constant k1' a t room temperature. I t can be shown that ignoring the reverse reaction (by taking k i a s the slope of the semilog plot of absorbance At vs. time) will result in a kl' value that is about 10% too low in this case. I n fact, the value for k i found from assuming the reaction irreversible is always too low. At lower hydroxide ion concentrations, the ratio kzlki increases, and the error becomes even greater. Also this ratio increases a s temperature increases since the reaction is exothermic. Barnes and LaMer [J.Am. Chem. Soc. 1942, 64, 23121 examined the apparently anomolous effect of ionic strength on the reverse rate constant. In order to determine both the forward and reverse rate constants, only one more piece of data must be collected: the absorbance a t equilibrium, A,,. Then a semilog plot of At -A,, vsersus time should yield a line with slope equal to kl' + kz. The ratio kilkz equals inff A,, where int =AaA, and is the extrapolated intercept a t theorigin of the semilog plot. To locate this intercept properly, it is necessary to time the experiment from the instant it is initiated when the phenolphthalein is injected.
To the Editor:
It is nlce to see yet another "rediscovery" of the color-fading rwction of phenolphthalein in alkaline solutim as an ideal one for studying reaction kinet~csINicholson. L J.
446
Journal of Chemical Education
Lloyd Hile Chemical Enaineerina Deoattment California State university. ~ o n gBeach Long Beach, CA 90840
