The Naming of Evolving Theories: Retaining the Spirit of invention Gordon R. Freeman University of Alberta. Edmonton. Canada TBG X i 2 I t ia valuable to retain the soirit of invention and chanee in the names of scientific theories. In the growth of knowlkdge an old theory is replaced hy a new one when enough evidence accumulates that the old one is conceptually inadequate. An example from the field of chemical kinetirs is the change in unimolecular reactirrn theory from the mnlecular activation hv radiatinn mndel to the activation hy rollision mndel, ti0 years ago. A new model (theory) is usu.& very crude, and it p r c q s s e s thmugh a sequence of improvements. As a memnry device, a theory is usually designated by the name of its inventor. The name hrings to mind a whole concept, not just an equation. The names of suhsequent improvers are added to that of the inventnr, so the name of a theory is a hrief summary of its historv. If the list becomes loneer than three or fnur. names are u k l l y lopped off the front.which results in the of reference tn the most radical of the chanees in the swuence of ideas. Knnwledee ia not an absolute. It in a soiral. New idem should he kept in the perspective df what has hone before. T o retain a reference to the maior controversv that was resolved by a new theory, and there& to a major.step in the growth of knowledae. reference names in a lenethenina list should he lopped from the middle. Two illustrations are taken from ohvsical chemistrv the Milnrr-1)etrve-Hi~rkeltMI)H) t h e 0 6 of electrolyte nnl;tions and the 1.indemann-Hinphrlwml-Marcus(1.H.M) thenwof unimnlewlar rrartion rates. In I9l2 the Arrhrn~usthpory ( 1 ) ofineomoleledissnciatinn of electrolytes was still gene&ly accepted, even for strong electrol.ytes, hut it had a serious flaw. The value ofthe degree of diwriation of a strong electrolyte calculated from freezing point lowering agreed with that ohtained from conductivity measurement?, hut the variation with concentration did not confnrm to the law of mass action. S. K. Milner devised a theorv in which the ohenomena and their conrentration depend&~ were expiained in terms of complete dissnciation of the electroi.yte, with a slightly nonrandom spacial distrihutinn of the ion? (2,:J). The electrical f o r m between the ions increase the chance of finding oppositely charged (attracting) ions near to each other and decrease the chanre of finding like charged (repelling) ions near to each other. Milner used the Roltzmann principle and a slntinnaw stochastic (prnhahilistic) treatment to determine the potential energy due in the
nonrandom distribution c)f ions. the so-called ionic atmosphere. The potential e n e q y is a measure of the deviation of the behavior from ideality and is equal to the work which the electrical forces are capable of doing as the ions are moved to infinite distances apart (infinite dilution). The theory was successful in interpreting osmotic pressure (3) a s well a s freezing point lowering and condurtnnce. Unfortunately. Milner's mathematical technique was ohscure to chemists and the theory was ignored. During the next decade the pressure increased mabandon the Arrhenius m ~ d r in l application to strong electrolytes. In 1923 Dehye and Hiickel ( 4 . 5 ) streamlined the mathematies of Milner's model, with theaid of Poisson'sequation to relate the elertrostntir potential to the charge density a t any point. They obtained simple equations in the limit ofextreme dilution. The simple equations were popularized hy Nnvm (6)and continue in use today under the name Dehye-Hiirkel theory. However, the conreptual hreakthrough was made hy Milner ( 2 ) ;his treatment will hecome less opaque tochemistF. in the currently growing awarenmofthe utility ofstncha%icmndels (7). In my lectures the name Milner-1)rbye.Hiirkel. or MDH, is used to designate the ionic atmosphere theory and the associated equations. Milner is the one who made the advance from Arrhenius in the description of strong electrolyte solutions. The other example has to do with advances in u n i m o l d a r reaction rate theory. In the early 1920's there was a controversy over the two leading interpretations of unimolecular reaction rates in gases. The favored one in 1925 involved molecular activation hy ahsorption of radiation ( 8 ) .The alternative wasactivation hy collision. hut it was thought that collisions should give second order kinetin. In 1920 1,indemann ( 9 ) had demonstrated that the radiation theory predirtpd an enormous increaw in the rate of inversion of sucrose in acidic aqueous solution upnn exposure to sunlight. His experiments showed that the inversion rate was thr same in dark and in illuminated solutions (9).The radiation theory was therehy shown to he wrnng, hut an acreptahlr alternative was not proposed. In 1922 he pointed out that activation hy cnllision would give first order decomposition if a molecule did nnt decompose immediately upnn receiving the required amount of energy ( 1 0 ) . If the activntion and deactivation rates were rapid rompared to the deramposition rate. first order
Volume 62
Number 1 January 1965
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decomposition would occur from the steady state concentration of activated molecules. However, he predicted that deviation from first order should be observed a t low pressures where the deactivation rate by collision would not be sufficiently larger than the decomposition rate. In 1926 Hinshelwood and Thompson observed the predicted deviation from first order a t low pressure (11) and Hinshelwood demonstrated through classical statistical mechanics that it is necessary to use energy stored in internal degrees of freedom to account for the observed decomposition rate and its pressure dependence (12.13). The collision theory then became universallv accepted. Improvements of the model include the increase of specific d&omposition rate with increasing molecular energy content above a required minimum (Rice and Ramsperger (14); Kassel(15)), and a consideration of quantum effects (Kassel(l5); Marcus (16)). Experimental work on unimolecular reactions was complicated bv free radical reactions for 50 years or more, but the basic theoreticalmodel still in use was created by Lindemann and Hinshelwwd. The pedigree of this theory of gas phase unimolecular reactions rates should read LHRRKM, which is awkwardly long. In practice it is commonly shortened to RRKM and the Marcus formalism is used. We recently had a guest speaker from a famous US research institute who referred to RRKM in his talk, but he had never heard of Lindemann or Hinshelwood. The point of retaining L and H in the common name is to have a perpetual reminder that current models are never good enough, and that the last change of kind for the model was made in 1926. Hinshelwood got a Nobel Prize for that and other contributions to chemical kinetics. The recommended shortened pedigree of the theory is LHM. On a smaller scale, a recent article about stochastic treat-
58
Journal of Chemical Education
ments of quantum mechanics (17) noted that one of the dynamics equations in stochastic electrodynamics was originally written down by Kalitsin (18) but is usually named after Braffort (19) and Marshall (20). The reference name should be Kalitsin-Braffort-Marshall,or KBM. I mention this rather esoteric example because the stochastic approach to quantum mechanics will probably be more widely used in the future. Certain problems are not broached by wave mechanics. For example, while the electron and photon are taken for granted by most chemists, nobody has a clear idea about what they are. Perhaps such questions as "What does a de Broglie wavelength represent?'' and "How long and wide is a photon?" could he approached by a stochastic treatment of quantum mechanics. I t is worthwhile to begin now with appropriate memory device labels for the equations. Literature Cited ill (2)
ArrhenivsS.Z.Phva.Chem.. 1.631 (18871:Phil.Moc..5thSer..I.81
Mi1ner.S. ~ . , ~ h i l , ~ ~ a . , f i t h ~ ; r . , 2(1912). 3,551 (3) Milner, S. R.,Phil. Mag, 6th Ser.,25,742 (1913). (4) Debye, P. and Hockel, E.. Phys. 2..24.185. 305,334 (1923). (5) Debye.P.,Phys.Z.,25,97(1924). 1 J .~ Amsr ~ Cham.Soe..46.1080119241. ,.0 , Nover.A.A.. ~ ,~~, , ~ ~ (7) F ~G.R., J.~CHEM.EDUC., ~ i ~9 , k (1984). , (81 Lewis. G.N. and Smith. D. F..J Amer. Chom. Soe.. 47.1508 (1925).
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(121 Hinshelwood. C. N..Proe. Rov. Soc. (Londonl. 113A. 230 (19261. 1'1 H . n < n . l u d , C N ' Klnrllm of ,'h.m,cal C h a w in 1:niwu. Syslema." Oxlvrd I I r ~ ( ~ l i n1 '\l ~ ~ i . O x i n r 19?1. d. I 0 R l r ~0 ti a n d l l n m i p r d o H ( J A m r r ('horn i o r .49.161- 19?1, I;, ~ a , . r l I.3 . J Pn ( r i m .32.2.5.1ffii I92e i,l B ~ l Marcar. ~ , R. A.. J. Chem. Phvs..20.364 (19521 (17) de la Pens, L, and Cetta, ~:~:,~k~nd.'~hy;., 12,1017 (1982). (18) Kslifsin, N. S.,Zhur Ehsp. Teor Fiz. (USSR), 25,407 (1953). (19) Braffort. P. and Tazara. C., C. R. Acad. Sci. (Paria). 239.1779 (1954). (20) Marahall, T. W., Pror. Ray. Soc., AZ76,475 (1963).
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