26 Bull. Hist. Chem. 6 (1990)
1 A S o s o o e emia C ea e a Ma c 1 5 Ostwald, Arrhenius and van't Hoff (1). Though Muir himself ow oi Co ege i a y A c i es did not succeed in establishing a British school of physical 11 A S o s o o a ke C ea e a o em e 1 chemistry and did not make any significant experimental ow oi Co ege i a y A e i es contributions to the new field, he did play a role in disseminat- 1 e e e ce 5 3 ing its early results through his review of Ostwald's work on 13 [ o s o ] A b l r , C nt n n C n Expl n the measurement of affinity coefficients (2), the writing of a t n f C rt n r U d n Ch tr ..., ai c i So monograph on thermochemistry (3), the editing of an influen- Ca e o ia Y 1 35 is wo k was a e isio a e a geme o tial dictionary of chemistry (4) and, of course, through his b l r , C nt n n n Expl n t n f C rt n Ch l r ..., textbook. u is e a o ymous y a u swick Mai e i 1 a a i u e Muir, who was later to write an important history of o a ke C ea e a o s o s am e use mos o e e ms i chemistry (5), also had an unusual appreciation of the history C ea e a s e isi g a ou 15% e a e ew e ms o aise e of his suoject and in his textbook attempted to use the new um e o e ies om C ea e a s 159 o a o a o 9 As i views on chemical equilibrium and kinetics to unravel some C ea e a e as wo ages we e e o e o a is o "a cie " long-standing paradoxes of chemical affinity that had been c emica ames wi ei co es o i g "sys ema ic" ames e e known since the end of the 18th century. Among these were the we e ames i C ea e a e sus 5 i o s o (C ea e a s problems of predisposing affinity, contact actions, and the so- b l r was i s u is e i e ea o is 1 1 El nt r called status nascens or nascent state. The first of these topics r t n M n r l nd G l . a s o e e sio o o y has long since disappeared from the textbooks, whereas the i ems co ai e ma y e ies i e ica o qui e simi a o e 1 second, under the rubric of heterogeneous catalysis, has sur- e i io e ow oi Co ege a c i es co ai wo u a e a ia s vived. In many ways, however, it is the third topic that is the o C ea e a s b l r . O e is a ma usc i co y w ic was most fascinating, as not only the explanation of the nascent a a e y a ecu so o e 1 1 r t . e o e is a i e state, but the very question of whether it really exists, are still e i io wi ou i e age w ic e a e e 1 e i io I as i e unresolved problems. A history of the various attempts to ewe e ies a a ew e i i io s a a e ee e ise i e 1 explain this phenomenon provides one with an interesting e i io cross section of I9th and 20th century chemical theory and, 1 A S o s o o a ke C ea e a 7 o em e 1 though an explicit treatment of this subject has been missing ow oi Co ege i a y A c i es from textbook literature since the 1940's, it is of interest to note 15 A S o s o o A S acka 1 May 1 7 ow oi that the term is still to be encountered, albeit in passing and Co ege i a y A c i es without explanation, in more recent textbooks (6). 1 e au o wou ike o a k Wes eya U i e si y o i s A knowledge that freshly prepared gases, when generated e i o ai i g a co y o o s o s o ai in situ within a reaction system, are frequently more reactive than when added already prepared from an external source seems to date from the late 18th century. This enhanced, but William D. Williams is Professor of Chemistry at Harding short-lived, reactivity appeared to be associated with the gases University, Searcy, AR 72143. He collects and studies early only at the moment of their "chemical birth", so to speak, and American chemistry texts. the resulting metaphor became enshrined within the chemist's lexicon in the phrase "nascent state", though the chemical poet who first coined the term is not known with certainty. The first explicit use that I have been able to locate occurs WHATEVER HAPPENED TO THE in the 1790 edition of Joseph Priestley's work, Experiments NASCENT STATE? and Observations on Different of Kinds Airs (7). Having incor- rectly postulated that both fixed air (carbon dioxide) and William B. Jensen, University of Cincinnati nitrous acid (nitric acid) were compounds of inflammable air (hydrogen) and dephlogisticated air (oxygen), Priestley at- In I884 the British chemist, M. Pattison Muir, puhlished a tempted to rationalize the different products as a function of textoook on theoretical chemistry entitled A Treatise on the differences in the reaction conditions, arguing that: Principles of Chemistry in which he attempted to summarize many of the recent results "on the subjects of dissociation, w e ei e i amma e o e ogis ica e ai is e ac e om chemical change and equilibrium, and the relations between my su s a ce i co ac wi e o e ki o ai so a o e o em chemical action and the distribution of the energy of the is ma e o u i e wi e o e i w a may e ca e i s n nt t t , changing system" - in short, most of the topics which would, e esu wi e f x d ai ; u a i o o em e com e e y within the next ten years, come to become identified with the o me e o e e u io e esu wi e n tr aci new and rising field of physical chemistry and the work of u is C em (199 7
A year earlier, William Higgins, in his book, A Compara- tive View of the Phlogistic and Antiphlogistic Theories, also invoked the unusual reactivity of hydrogen "at the very instant of its liberation" to rationalize the necessity of water for the oxidation of iron (8). Though he did not describe the hydrogen as nascent, he did use the term when paraphrasing his original statement in 1814 (9). In the examples cited by both Priestley and Higgins, the nascent gases were generated chemically. In 1807 Humphry Davy reported that a similar enhanced reactivity was also present in the case of electrochemically generated gases (10). Upon electrolyzing water, he observed that some nitric acid was invariably formed at the anode and a trace of ammonia at the cathode. These he attributed to the "combination of nascent oxygen and hydrogen respectively with the nitrogen of the common air dissolved in water", since he knew that these reactions did not occur at room temperature when the fully formed gases were allowed to interact. In modern terms, the observed secondary electrode reactions can be represented as:
anode: 50* + N + H O —> 2H(NO 3 ) (1)
cathode: 6H* + N —> 2NH3 (2) ose ies ey (1733-1 was o e o e i s o use e e m where we have used the starred symbol to represent the nascent asce s a e i is c emica w i i gs state of the element in question in order to avoid the explicit representation of any hypothesis concerning its molecularity to generate arsine in the famous Marsh test for arsenic (13): or structure. An examination of 19th century chemical literature shows 9H* + Asp, —> AsH 3 + 3H 0 (7) that the nascent state soon became the accepted rationale for a number of otherwise puzzling chemical reactions. Among in the reduction of nitrobenzene to aniline: these were the varying products observed when metals lying above hydrogen on the activity series reacted with either 6H* + C H5NO2 —> C5H5N 2 + 21-1 0 (8) sulfuric (11, 77) or nitric acid (I2). At low acid concentrations dihydrogen gas is the major product in both cases, whereas at in the Clemmensen reduction of carbonyls, and in the Jones higher acid concentrations sulfur dioxide, nitrogen oxides and, reduction column. In each case the hydrogen is generated in in some cases, even ammonia are obtained instead. To explain situ by means of zinc and hydrochloric acid and the reactions this concentration dependency it was assumed that nascent are not observed under the same conditions if externally hydrogen was initially generated in both systems. At low acid generated dihydrogen gas is used instead. concentrations this reverted to normal dihydrogen gas, whereas In all of these examples the actual generation of dihydrogen as at higher acid concentrations it was entirely consumed in can be observed as a competing reaction. However, the nascent reducing the respective acids and dihydrogen gas was no state concept was soon extended to systems in which the gas longer observed among the reaction products: corresponding to the nascent intermediate is never observed under the conditions of the experiment and which apparently involve a single overall net reaction rather than two competing 2H* + H2(SO4) ---> SO + 2H 0 (3 reactions. For example, nascent oxygen was used to explain
H* + H(NO 3 ) --> NO + H O (4) the production of dichlorine oxide in the reaction (14):
3H* + H(NO 3 ) —> NO + 2H 0 (5) 2Cl + HgO Cl 0 + HgCl (9
5H* + H(NO3 ) NH3 + 311 0 (6) via the mechanism:
Other classic examples include the use of nascent hydrogen Hg0 --> Hg + 0* (1 u is C em 6 (199
Ma s s a a a us o es i g polyatomic molecules rather than isolated atoms. Contrary to o e ese ce o a se ic e the usual accounts found in most histories of chemistry, the u k ow is ace i e u e supporting evidence for this proposition was based not just on a o g wi y oc o ic aci Gay-Lussac's law of combining volumes, but on considera- a a iece o a se ic- ee i e tions related to the energetics of chemical reactions as well. e asce y oge eac s Rephrasing the original arguments of Laurent and Gerhardt wi a y a se ic i e sam e in terms of modern bonding terminology, the implied assump- (usua y i e o m o e o i e o ge e a e a si e e tion was that chemical bonds could be broken in the course of y oge -a si e mi u e issu- a spontaneous reaction only if they were replaced by stronger i g om e e o e u e is bonds. Given this premise, and the assumption that the ele- i e a si e ecom oses i ments were monoatomic, it was difficult to understand why e ame a ea es a mi o some compounds spontaneously dissociated into their ele- o e eme a a se ic o a coo ments and, conversely, why it was not possible to synthesize su ace e a o e e ame certain compounds directly from the elements themselves, though they could be made indirectly by means of displace- ment reactions. However, once one accepted the existence of 0* + CI --> Cl 0 elemental polyatomic molecules, these paradoxes vanished, since in both cases the breaking or making of the bonds in the Hg + Cl --> HgCl (12) compounds was competing with the making or breaking of bonds within the elemental molecules, or as Gerhardt put it, where again it was not possible to produce the highly endoener- with the synthesis or decomposition of a "hydride of hydro- getic dichlorine oxide by direct reaction between molecular gen", a "chloride of chlorine ", etc (17). dioxygen and dichlorine, though in step I1 it is presumed to Applying this concept to the nascent state, Laurent wrote occur with nascent oxygen. (18): Likewise, nascent oxygen was frequently invoked to ex- plain the bleaching (or oxidizing) ability of eau de Javelle or A i a y associa io o a oms mig a ow us a so o accou o a hypochlorite solutions (I5), via the initial step: ce ai e e o e a i i y ossesse y su s a ces i e asce s a e I wo ee mo ecu es o omi e a o y oge a MCIO ---> MCI + 0* (13) a e oug oge e e a i i y o o a o o may su ice o e e e com i a io o a wi a u i a supposition apparently supported by the fact that their water e su s a ces ese a e a ese wo w ic a e o a i i y solutions eventually decompose, on standing, to give the corresponding metal chloride and dioxygen gas. Many of the oxidation reactions of nitric acid with the less active metals and with the nonmetals were also rationalized in the same fashion (16):
2H(NO3 ) 2NO + H O + 30* (14) and the ability of aqua regia, in contrast to either dichlorine or hydrogen chloride gas, to chlorinate gold was likewise attrib- uted to the initial production of nascent chlorine (I6):
3HCI + H(NO 3 ) —> NOCI + 2H 0 + 2C1* (15)
As will be seen, the use of the nascent state in these cases became the basis of much of the criticism later directed at the concept. The first, and by far the most historically significant, attempt to rationalize the nascent state was made by the French chemist, Auguste Laurent, in 1846. Laurent and his collabora- tor, Charles Gerhardt, were early proponents of the concept Augus e au e (1 7-1 53 was e i s o o ose a that the elements in their standard states were composed of asce gases co ai e ee a oms u is C em 6 (199 9
o o e come wi e a e o com i e ea i y is is w a wi occu atom theory of the nascent state and in many ways the opposing w e y oge is i e asce s a e a is w e e e i is e o e experimental evidence was more convincing than the support- om y oc o ic aci y e ac io o a me a we s a a e e ing evidence. Much of this was the result of the efforts of the equa io Italian electrochemist, Donato Tommasi (30-33). He pointed out that, if nascent hydrogen was really atomic in nature, its C + M = C M + H properties should be independent of the metal - acid system used to generate it. But in actual fact, as his own experiments a e e wi e a e e cy o a eco s uc io o a i a y mo ecu e showed, its reducing ability was highly dependent on the ei e y com i a io wi omi e o wi a o e a om o y o- nature of the metal. Thus nascent hydrogen generated by ge means of zinc and hydrochloric acid was able to reduce chlo- rates to chlorides, whereas that generated either electrolyti- Similar arguments were also put forward by the English cally or by means of sodium amalgam produced no reaction. chemist, Benjamin Brodie, in the early 1850's (19-21). Tommasi further noted that all of the reductions observed for The value of this interpretation of the nascent state as a nascent hydrogen at room temperature could also be observed minor, but useful, piece of evidence in favor of elemental for fully formed dihydrogen gas at higher temperatures. He polyatomic molecules was fully appreciated by the I9th cen- therefore postulated that nascent hydrogen was nothing more tury chemist, and it is in this context, rather than as a topic in than thermally hot dihydrogen that had not yet had time to chemical kinetics, that the subject found a place in most thermally equilibrate with the surrounding solvent molecules chemistry textbooks after I860. Adolphe Wurtz devoted three and that its reducing abilities should directly parallel the pages to the subject in his classic historical study of the enthalpy of the reaction used to generate it. Thus he wrote in development of the atomic theory in 1881 (17) and Pattison I879 (31): Muir was quite explicit about its pedagogical value in his I884 text (1): A i is gas i e asce s a e ossesses g ea e a i i y a i e a u a s a e i is so ey ue o e ac that the hyärogen the A s u y o e eac io s i w ic asce su s a ces ay im o a nt t fr b n t n f nd t b p n d b a s a ea s o me o kee e o e e s u e a a im o a th h l nt t f th h t pr d d d r n th tt n fr f th is i c io e wee e a om a e mo ecu e w ic is so i a i h dr n. C n ntl , n nt h dr n n th n l th n c emica co si e a io s a a so o aw a e io i a ma ke way rd n r h dr n i [ i e e ] th r nd t n r, p n o e com e i y o a c emica c a ges n r ll , n d ff r nt ph l nd t n . o my mi e e es- sio asce y oge is sy o ymous wi y oge + ca o ies I
Though several 19th century chemists (I1, - 3 felt that ac a e eac io s o uce y asce y oge ca e o ai e their experimental investigations of the nascent state were not qui e as we wi o i a y y oge a a ig em e a u e; a e inconsistent with the free-atom interpretation, there was in fact i e e ces o se e e wee e y oge esu i g om i e e very little direct experimental evidence for the theory until the c emica eac io s a e sim y ue o e ac a ese eac io s o second and third decades of the 20th century. This came from o e e o e same qua i y o ca o ies studies by Irving Langmuir (24), Karl Bonhoeffer (25-26), Hugh Taylor (27) and others of the gas-phase chemical reac- Aside from a correction of some of his thermodynamic data tions of atomic hydrogen that had been generated either by by Julius Thomsen (34), Tommasi's theory was generally well thermal dissociation on a heated wire or by means of high received and was often quoted by the authors of advanced voltage electrical discharges. But even here the proof was textbooks and chemical dictionaries after I880 (1,4, 35). Muir really indirect and was based on the similarity between the expressed a somewhat more sophisticated version of it in his reactions observed in the gaseous phase for atomic hydrogen 1884 text, writing (1): and those observed in the liquid phase for chemically or electrochemically generated nascent gases. Attempts to bubble I a eac io w e ei [a] gi e com ou is o uce e e mus e gaseous atomic hydrogen into solutions gave negative results a mome o ime w e is com ou ca o y e sai o e is due to rapid recombination into dihydrogen gas, whereas o e ia y w e e a oms w ic co s i u e i s mo ecu es a e o attempts to detect the existence of a soluble, diffusible, nascent se e ow i o s a e co igu a io s; a is mome e com ou species in the liquid-phase chemical and electrochemical sys- may e sai o e is i e asce s a e I e a omic i a io s a tems were inconclusive at best ( 29). Indeed, the results i e ac io s a e a owe o u w a mig e ca e ei o ma seemed to indicate that in these systems the nascent activity cou se e com ou mo ecu es a e ce ai y o uce u i ese was confined to the surface of either the chemically reacting i e ac io s a e i e e e wi a ew se o mo ecu es may e metal or the active electrode. o me w ic mo ecu es ea a mo e o ess sim e ge e ic e a io Not all 19th century chemists were enamoured of the free- o ose o uce i e o ma ocess o c emica c a ge 3 u is C em (199
This view of the nascent state has recently been revived a a ogous o o o e a a ey a e i is s a e a e mome o among chemists working in the field of molecular reaction e e i g i o com i a io o w e ey a e i e n nt t t . dynamics (36), where it is applied to the vibrationally and/or electronically excited species which are the initial products Osann's results were criticized hy J. Löwenthal in I858, observed in molecular cross-beam studies of gas-phase reac- who suggested that the enhanced reducing properties of the gas tions. Indeed, in 1976, Simon Bauer of Cornell University were due to the presence of sulfur dioxide from the Nordhausen wrote a short note in which he argued that (37): acid used in its preparation (42). This criticism was disproved by Osann (43), but more serious problems were uncovered by I is ow ime o es o e e e m " asce " o egi imacy Muc Gustav Magnus (44), who was unable to duplicate many of e i e ce as accumu a e o e e ime a a eo e ica w ic Osann's results. He concluded that Osann's materials were emo s a es a i ma y eac io s e a i io o asce o uc s contaminated with iron and that this was actually responsible amo g ei c a ac e is ic o a io a i a io a a e ec o ic s a es for the observed reductions. Though Hermann Fehling sum- i e s su s a ia y om a e ec e we e ese ge e a e i marized Osann's work in his 1864 Handbuch (45), he was s a is ica equi i ium nt co as s wi t t r l x d, i e a critical and, by I895, Albert Ladenburg felt that Osann's sys em o w ie a n l em e a u e a co es o i g e mo y- hypothesis had been "aus der Welt geschafft" (35). amic u c io s ca e e i e Actually Ladenburg's funeral oration proved to be prema- ture as, in a series of papers published between I920 and I922, Though Bauer cited several examples of how the reactivity of Gerald Wendt and Robert Landauer of the University of these nascent products differed from that of the corresponding Chicago revived a form of Osann's hypothesis ( - 7 y state-relaxed products, it is open to question, as in the earlier means of a-radiation, electrical discharge under reduced pres- case of the gas-phase atomic hydrogen experiments, whether sures, and high potential coronas at atmospheric pressure, they this explanation can be extended to the classical liquid-phase generated an activated form of hydrogen which showed a chemical and electrochemical systems which gave rise to the reactivity different from both normal dihydrogen gas and concept of the nascent state in the first place. atomic hydrogen. This they equated with the H 3 molecule Yet a third theory of the nascent state was proposed by the postulated by Joseph J. Thomson in 1913 (now known to be the German chemist, Gottfried Wilhelm Osann, in a series of more ion). Because of both its increased reactivity relative to than a dozen papers published between 1852 and 1864 (38). By dihydrogen and the analogy between its formula and that for electrolysis of a dilute solution of freshly distilled Nordhäusen 0 3 , they proposed that the new species be called hyzone. sulfuric acid, Osann obtained a form of activated hydrogen As with the work on atomic hydrogen, these results could which was capable of reducing solutions of silver salts to not be extended to the traditional liquid-phase chemical and metallic silver and mixtures of potassium hexacyanoferrate(III) electrochemical methods of generating nascent hydrogen ( 7 and iron(III) chloride to Berlin blue. Platinum and porous though A. C. Grubb, in a short note published in 1923, claimed carbon cathodes were capable of storing the active hydrogen to have prepared traces of the species by dropping acid on and could be removed from the electrolysis apparatus and pieces of suspended metal so as to avoid emersion of the metal placed in other solutions, where they produced reactions not in liquid ( normally observed for dihydrogen gas under similar condi- The phenomenon of allotropism was a difficult problem for tions. the 19th century chemist. Rationales based on variations in
Heavily influenced by Christian Schönbein's discovery of either the degree of molecular complexity (i.e. polymeriza- ozone, Osann decided he had discovered a similar allotrope of hydrogen and since, like ozone, it was much more chemically reactive than the normal form of the element, he gave it the name of ozon wasserstoff (39). In later work he reported that the atomic weight of his new gas was 0.66 or about two thirds that of normal hydrogen (40). Similar ideas were expressed by T. L Phipson in I858 (41):
W a is ca e e " asce s a e" is, I i k o i g mo e o ess a e a o o ic s a e o o ies e e i g i o com i a io i seems i co es i e a v r t x n nt r nt r l v p nd , Osa s a a a us o e t i th t t f z n . W e we e ec u e o e esu s a ea y e a a io o o o -wasse - o ai e wi y oge c o i e omi e su u a os o us s o ia e e ec o ysis o we a e i c i e o e ie e a a sim e o ies s ou e a e i e a wa e so u io o es y same ma e ; a is o say a a o ies may a e a a o o ic s a e is i e o äuse aci Bull. Hist. Chem. 6 (1990) 31 tion) or variations in molecular structure (i. e., isomerism) had were the reduction of chlorate to chloride, nitrate to nitrite and to await the work of Brodie in the 1850's on the concept of ammonia, hexacyanoferrate(III) to hexacyanoferrate(II), ni- elemental polyatomic molecules (49). Indeed, it was because trobenzene to aniline, sulfurous acid to sulfur, and diarsenic of this very problem that Berzelius had introduced the distinc- trioxide to arsine (57). tions between isomerism and polymerism, on the one hand, As a result of their experiments, they became convinced and allotropism, on the other (50), and he himself favored the that the zinc portion of the couple reduced the water, that the idea that allotropes corresponded to different electrical states resulting hydrogen became occluded in the copper, and that of an element's atoms. this occluded hydrogen was responsible for the observed This concept was widely used to rationalize allotropism in reductions (56). In support of this model they compared the the first half of the 19th century and was applied in great detail reducing abilities of the couple with those of hydrogen oc- by Schöbein in his attempts to explain the behavior of ozone cluded in platinum, palladium, copper and porous carbon, (thought to be negative) and its relationships to normal oxygen concluding that (57): (thought to be neutral) and the antozones (i.e., peroxides, thought to be positive) (5I). Not surprisingly, it also made its e i c ease owe o e y oge is ue o i s co e se appearance in discussions of the nascent state via the allotropic co i io [u o occ usio i e me a ] w i e e o se e i e - theory. Thus Phipson contended that the transient allotropic e ces e wee e ac io o e i e e com i a io s esu om modification of an element which appeared at the moment of a ia io s i is es ec a e a s a so om e mo e o ess i m its formation was equivalent to having rendered the element o w ic e me a as u o e gas "infinitely more electropositive or electronegative" than its e a o e c emica c a ges a e e ec e mo e o ess e ec y y normal state (52). asee y oge u is y oge i e e y case is se ee i Like many of the other rationales of the nascent state, this co ac wi o i e y c ose o imi y o a me a w ic i i ue o so-called "polar" theory experienced a brief revival in the early e owe k ow o e ossesse y suc so i s e y o a y 20th century, this time within the context of early electronic co e ses a i es some o is gas I may e e o e e co cei e theories of the chemical bond. This modernized version was a e ac i i y o e y oge u e ese ci cums a ces is u e apparently suggested by Harry Shipley Fry of the University of co seque ce o i s i ima e associa io wi e me a s o i o e Cincinnati, who was an early proponent of a polar theory of wo s o i s ei g i e occ u e co i io organic chemistry based on the electron transfer model of chemical bonding first proposed by J. J. Thomson (53). This hypothesis is perhaps the most satisfactory when it Consistent with the fact that this theory viewed all bonding as comes to explaining the known facts about chemically and ionic, diatomic molecules were written as H*H - , Cl+Cl - , etc. electrochemically generated nascent hydrogen in the liquid This formulation meant that in the generation of dihydrogen phase. It explains the dependency of the reducing power of the gas by reduction of aqueous F' solutions, the reduction of the hydrogen on the method of its preparation (variations in the
H' ion had to proceed all the way to H - . This then combined ability of the metals to adsorb or occlude hydrogen), the with another H+ ion from the solution to generate the dihydro- absence of a soluble, diffusible, active intermediate in the gen gas. Likewise, generation of dichlorine via oxidation of solutions (it is really a surface reaction at the metal or elec- aqueous chloride solutions required the generation of Cl+ , trode), and the problems with reproducibility and sensitivity to which then combined with Cl- to generate the final product. contamination. (both highly characteristic of surface reac- Thus Fry equated the nascent state with the generation of tions). transient free ions having unusual and highly reactive oxida- Of course, Gladstone and Tribe were necessarily vague tion states. This view was also supported by A. Pinkus (54), but about the exact nature of the chemical species corresponding fell out of favor with the eclipse of the polar bonding model by to the occluded hydrogen, though at one point they did draw an the electron-pair covalent model of G. N. Lewis. analogy with the reducing ability of copper hydride. Indeed, Even these permutations fail to exhaust the speculations of this question is still a subject of debate among chemists (59). the I9th century chemist relative to the nature of the nascent In 1927 Joseph Mellor drew attention to the close relation- state. Yet a fifth model - the so-called occlusion model - was ship between this subject and the problem of the hydrogen developed by John Gladstone and Alfred Tribe in the late overpotential in electrochemistry (60). In general, metals with 1870's as a result of their work on the copper-zinc couple (55- low overpotentials tend to strongly occlude hydrogen, and to 58). Made by placing a strip of zinc foil in a copper sulfate act as effective catalysts for the recombination of hydrogen solution for a few minutes in order to plate out a small quantity atoms into dihydrogen gas, the hydrogenation of alkenes, etc. of copper on the zinc, Gladstone and Tribe found that in water (61). solutions the resulting couple was able to perform many of the A close examination of these five theories - the free-atom, reductions observed for nascent hydrogen, whether generated the nonequilibrium, the allotropic, the polar, and the occlusion electrochemically or via metal-acid reactions. Among these - shows that, in spite of their apparent diversity, they actually 3 u is C em (199 have some fundamental points in common. In all five cases, o e y oge -a om [i e e asce i e me ia e o e acce e one is postulating that the generation of a gas, X, either e ee-a om mo e ] wi e e ic su a e imi is es e o o - chemically, via the reaction of A and B, or electrochemically, io o y oge w ic oes wo k as a e uci g age y i c easi g proceeds through a short-lived intermediate, I, which, upon e amou o ee aci we i c ease e a e a w ic e y oge is further reaction with more B, gives the final product: e o e wi ou o a equa eg ee i c easi g e mo ecu a mo e- me o e e ic su a e a e ce a i c ease amou o A + B -4 [I] (16) y oge esca es y kee i g e amou o aci co s a a ea i g e iqui we i c ease e c a ces o co ac e wee e B + [I] —> X (I7) e ic su a e a y oge -a om a acco i g y o ai a i - c ease e uc io y i u i g e e ic su a e so u io e c a ces If this intermediate is generated in the presence of another o co ac e wee e y oge [a om] a e ic su a e a e o chemical species, D, capable of reacting with it to form an cou se imi is e a e ce mo e o e y oge esca es i e ee alternative product Z s a e
D + [I] Z (18) Given that the active intermediate model requires an altera- tion in both the kinetics and thermodynamics of the reactions, then one has an example of nascent reactivity. These common the modem chemist cannot resist asking which of these factors features might be characterized as belonging to a generalized is the most important in explaining the enhanced reactivity of "active intermediate" model of the nascent state. the nascent state. There were in fact proponents of both the Since the intermediate, I, is presumably more endoener- thermodynamic and kinetic points of view during the I9th getic than the product gas, X, this means that reaction I8 will century, though both necessarily rejected the active intermedi- be thermodynamically more favorable than the corresponding ate models, which imply the simultaneous alteration of both. reaction between X and D: In a series of papers and books published in the late 1860's and I870's, the French thermochemist, Pierre Eugène Mar- D + X Z (19) celin Berthelot, argued that the nascent state did not exist and that the phenomena usually attributed to its operation could be and will necessarily entail a change in its kinetics as well. The explained in terms of thermochemistry alone (I4, 63). All of only disagreement among the five theories is over the precise his examples involved the use of the nascent state to explain the nature of the intermediate. indirect synthesis, by means of displacement reactions, of This model further implies that nascent activity can be endoenergetic compounds that could not be directly synthe- manipulated by controlling the relative rates of reactions I6- sized from the elements themselves. A case in point is the 18. Increasing the rate of reaction I7 will increase the rate of synthesis of dichlorine oxide outlined earlier in equations 9-I2. gas evolution and decrease the amount of product Z, whereas The difference between the direct synthesis and indirect syn- decreasing its rate and/or increasing the rate of reaction 18 will thesis, Berthelot argued, had nothing to do with nascent inter- increase amount of product Z. This fact was explicitly recog- mediates but was due to the fact that in the latter case the nized by several investigators in the 19th century. production of the unstable endothermic product (Cl 0) was Thus, Gladstone and Tribe tested this hypothesis in their accompanied by the simultaneous production of a highly study of the reduction of nitric acid during electrolysis, postu- exothermic by-product (HgCl ) and this made the overall lating that the failure to observe the production of dihydrogen process exothermic and hence thermodynamically favorable. at the cathode in the case of concentrated acid solutions was In 1918 Alexander Smith pushed Berthelot's observations due to the fact that reaction I8 consumed all of the intermediate on the imaginary role of the nascent state in indirect versus (occluded hydrogen in their theory) in the production of NO direct syntheses a step further, sarcastically remarking that and NO , leaving none for the generation of dihydrogen gas via (74): reaction I7 (58). By increasing the rate of reaction I6 for a given acid concentration, via an increase in the rate of elec- Si ce y oge a c o i e o o u i e i e co w e su u ic trolysis, they were able to provide sufficient intermediate for aci a commo sa gi e y oge c o i e o e co sis e we both reactions 17 and I8 to be observed simultaneously. mus su ose a asce y oge a asce c o i e we e Likewise, Thomas E. Thorpe, in his 1882 study of the o me a com i e I o e wo s e e y u io o wo e eme s reduction of ferric salts with nascent hydrogen generated by o e a i ec u io mus e e ai e y asce ac io a oug various acid-metal systems, wrote (23, 62): i ou e ecom osi o is ogica ecessi y is u i o ma y o e - ooke A y co i io w ic i c eases e a i i y o e e o u io o e
y oge wi ou o a equa eg ee i c easi g e c a ce o co ac Nevertheless, not everyone was satisfied by Berthelot's ap- u is C em (199 33 approach. Adolphe Wurtz, for example, characterized Ber- famous hrb h (65). He pointed out that, when initially thelot's arguments as "useless', largely because they were not formed, small submicroscopic bubbles m ust be under enormous mechanistic in nature and therefore failed to provide a "natu- pressure due to the high surface tension of the surrounding ral" explanation (17). More telling were the comments of water. At molecular dimensions, a newly formed bubble with Tommasi in 1880 (64): a diameter of 10 cm would contain gas under a pressure of approximately 15,000 atmospheres, and this increased pres- M. e e o o o g ago e e e a e was e i s o gi e a sure, via the law of mass action, might well account for the a io a eo y o e asce s a e u i you o e ese wo enhanced reactivity of so-called nascent hydrogen. Today we o umes you wi o e a e o i a si g e se e ce w ic makes would also emphasize the importance of the increased surface o e e e sus ec w y asce y oge is mo e ac i e a o i a y area between the smaller bubbles and the surrounding liquid. y oge This latter aspect was further pursued by the Greek chemist, Constantine Zenghelis, in a series of papers published between In other words, though Berthelot had given a proper thermody- 1920 and 1921 (66-68). He created superfine gas bubbles by namic rationale forcing the of the indirect gases through CU ES S OWI G synthesis of en- fine filter paper E UCI G OWE S O I E E ME A S dothermic com- or through di- pounds, these alysis mem- cases had repre- branes. When sented question- this was done in able extensions solutions of of the nascent various reac- state concept in tants, he ob- the first place, tained many of and he had com- the reactions pletely ignored usually attrib- the competitive uted to the nas- gas-generating cent state. In the reactions which case of dihydro- had originally gen, these in- given rise to the cluded the re- concept. Y OGE E AI E duction of chlo- Indeed, the rate to chloride, inappropriate- E o e s cu es o y oge e o u io e sus e uc io o i o (III usi g nitrate to nitrite, ness of Ber- asce y oge ge e a e y a ious aci -me a sys ems and mercury(II) thelot's rationale to mercury(I). in the case of these competitive systems becomes apparent if In the case of mixtures of dihydrogen and dinitrogen, he one evaluates the thermodynamics of most of the classic reduc- obtained ammonia, and in the case of carbon dioxide, a variety tion reactions involving nascent hydrogen, for one quickly dis- of reduction products, including formaldehyde. Zenghelis covers that the corresponding reductions with normal dihydro- concluded that (68): gen gas are all thermodynamically feasible at room tempera- ture. This is true, for example, of the reduction of chlorate to We e ie e a o e asis o e ece i g esu s we mus e ec e chloride, nitrate to ammonia, diarsenic trioxide to arsine, ge e a y ecei e y o esis w ic a i u es e co si e a e ac i - nitrobenzene to aniline, iron(III) to iron(II), permanganate to i y o ce ai gases i e asce s a e o e e cess e e gy o ee manganese(II), etc. Thus our failure to observe these reactions a oms O e co a y we e ie e we a e emo s a e a is must be kinetic rather than thermodynamic in nature, and a ac i i y is ue o e e eme su i isio o e ac i e masses i favorable change in the reaction kinetics is both necessary and co ac sufficient to produce an observed reaction at room tempera- ture. Though a change in mechanism may also alter the Using this theory to account for the dependency of the activity reaction thermodynamics, this is not a requirement. of the nascent hydrogen on the nature of the metal-acid system The concept of a purely kinetic rationale of the nascent state used to generate it, would require that different metals nucleate brings us finally to our last model. This was proposed by different size bubbles, a factor that should, in turn, depend as Wilhelm Ostwald in 1902, almost as a passing thought, in his much on the physical roughness of the surface as on the 3 u is C em (199 c emica a u e o e e a i se C ai e e i e o ec e o e use o e e m "s a e" i Some a ici a io o e ki e ic a io a e was a ea y co u c io wi a a sie s ecies w ic y i s e y e i i- ese i e occ usio eo y e e o mous ec ease i io cou o e iso a e a assig e e i i e o e ies o u e accom a yi g e occ usio o y oge i a a ium (73 Mui o e o e a a em e o weig o e os a ee o e y omas G a am as ea y as 1 ( 9-71 a co s o e co ce i is 1 e (1 I ee e ca cu a e a e e si y o e y oge we om a a ue o 95 g/m i e gas ase o a a ue o 733 e e m " asce ac io " as o a y ee a o ce e u a g/m i e me a C emis s a o g s ecu a e a y o- a m u o e og ess o c emis y y c assi g u e a commo ge was a a a og o e a ka i me a s a u e is e eme ame ma y e ome a a mig o e wise a e ee os i e as co e sa io G a am e a i a i ac ecome a me a mass o ac wi w ic e scie ce as o ea e e essio as I o w ic e o ose e ame h dr n . e a so e a i k o e goo se ice; u i so a as i s as e e o e e e o e iew o occ u e y oge i a a ium was a i i es iga io - o i is a ways easie o say o a y u usua eac io s was a me a ic a oy o a i um a y oge ium " ese a e o asce ac io " a o e ami e ca e u y i o ei e i ea a e i c ease co ce a io o occ u e cou se a co i io s e use o e e essio as I i k ee y oge e sus gaseous y oge was a y es o si e o u a o a e o e es i e es s o c emica scie ce i s i c ease ac i i y was a ea y i e a i e s a eme om G a s o e a i e quo e ea ie a a simi a i ea was I ee e am es o i s use as a c emica sca egoa a e o sugges e y E wa Wi m i 1 73 (7 is is o cou se a o i (7 -79 a u e y ki e ic e ec o y i o e assumes a e occ u e u i was Smi i e 191 e i io o is o u a y oge is s i ia omic a oes o c emica y i e ac wi Intr d t n t In r n Ch tr , w o came c oses o ou e me a a ice - i e oi s o w ic e 19 ce u y c emis ow co c usio w e e w o e (7 was u e s a a y ague a i g come o e e o ou is o ica su ey w a ca e e m asce y oge is use i i e e se ses i a e y e sai a ou e a e o e e m asce s a e? es i e i s co usi g way (1 I may mea asce i e a y a is ew y o a a e e aissa ce amo g c emis s i e ie o c emica o i e a e ( I is a so use o mea i e e - om-o i a y o i eac io y amics i is u ike y a e e m wi e u o e ac a a o o ic o m o y oge (3 I is o e use o mea o e i o uc o y e ook Wi e emise o e ee-a om a icu a a o o e ame y a omic y oge ( I is use o mea mo e i o o ge as a ue as a mi o iece o e i e ce o y oge ac i a e y co ac wi a me a (5 i a y i s ac i i y is e o ya omic a u e o e eme a mo ecu es a oug o e e ai e as ei g ue o e a ge amou o ee e e gy co ai e mig a gue a is o ica y i e ese s o e o e i s sus- i i c us aci us e uci g age as com a e wi e ee e e gy ai e a em s o ea wi eac io mec a isms i s s a us as co ai e i ee y oge us e uci g age e wo asce a we - e i e co ce i c emica ki e ics is e e mo e is o cou se a mis ome e ce i g i co ec io wi [usage] 1 e uous Ou su ey oi s o e ac a asce ac i i y is ima - i a y i c osi g we mig ie y o e a osi i e e - i y a ki e ic e ome o a a i asica y su sumes a y am e o e use o e asce s a e co ce I 1 e c a ge i eac io co i io s a wi a o a y a e e Ge ma c emis Mo i au e success u y use i o u - ki e ics o a o e wise e mo y amica y a owe eac io a e e s uc u e o y oge e o i e (75-7 e ious o e ecise a u e o is a e a io may a y om o e sys em is wo k c emis s a iewe is com ou as o yge a e o e e us e occ usio o gas a so io mo e seems wa e I co as au e i sis e a i "was o a o i a io es sui e o esc i e asce ac i i y i e case o gases o uc o wa e u a e a com ou o a u amage ge e a e e ec oc emica y o ia e e oge eous eac io s i mo ecu e o o yge wi wo a e y oge a oms" is e e iqui ase; e o equi i ium mo e seems es sui e o e y com a i g e o uc s o ai e y assi g i y- o e i a io a y e ci e o uc s o me i mo ecu a oge gas o e asce o yge ge e a e a e a o e ia e c oss- eam s u ies a e gas u e mo e o ac i a io o e ec o ysis o wa e wi ose o ai e y assi g io yge a so e gases i o ous ca o I s o e e is o si g e gas o e asce y oge ge e a e a e ca o e Assumi g e a a io o asce ac i i y a e ce o we - e i e e ee-a om i e e a io o e asce s a e au e ea- asce s a e E e i o e acce s e ge e a i e ac i e so e a i e a i io a iew o y oge e o i e was i e me ia e mo e e ac emai s a simi a ac i e gases co ec e i s ou e o uce a e a o e ia e eac io ca e s o e i a i um a o ous ca o o o uce y mea s o su e i e u es a a e i e a mea i g o e + — zO ( e m asce o o ge a ies u e ese co i io s I is o i e es o com a e ese co c usio s wi ose o w e eas i is i e e a io was co ec i s ou e o uce ea ie w i e s I a memoi w i e i I 7 e i Sai e- a th ca o e ia e eac io u is C em (199 35
+ 1-1 ( ( 1 19 C o ie "O e Co i io o Ce ai E eme s a e Mome o C emica C a ge" h l. r n . . S ., 1850, 759- I ac o y wa e was o uce a e a o e w e eas a sma 13 amou o y oge e o i e was o me a e ca o e us C o ie "O e Co i io o Ce ai E eme s a e o e i yi g au e s y o esis a o i i g a so e i g Mome o C emica C a ge" . Ch . S ., 1852, 4, 19 emi e o ow i co ec assum io s (i e e ee-a om 1 C o ie "O e o ma io o y oge a I s i e e a io o e asce s a e ca some imes ea o omo ogues" r . . In t., 1853, 1 3 5-3 co ec esu s A a e "E u e su es ac io s c imiques a ai e e a c a eu em u e à a i e" C pt. nd., 1866, 3 3 9-37 References and Notes 3 E o e "O e e a iou o i c Mag esium a I o as e uci g Age s wi Aci u a e So u io s o e ic Sa s"
1 M M Mui A r t n th r n pl f Ch tr , . Ch . S ., 1882, 4 , 1 - 9 Cam i ge U i e si y ess Cam i ge 1 -1 I a gmui "A C emica y Ac i e Mo i ica io o
M M Mui "C emica A i i y" h l. M ., 1879, 8, y oge " . A . Ch . S ., 1912, 4, 131 - 13 5 1 1- 3 Mui a so a s a e a o Os wa s hrb h W. 5 K o oe e " e a e o ak i e Wasse s o "
Os wa S l t n , o gma s G ee o o 1 91 . h . Ch ., 1924, , 199 - 19 9 3 M M Mui a M Wi so El nt f h r l E o m a K o oe e "O e ie Gas eak io es Ch tr , Macmi a o o 1 5 ak i e Wasse s o " h . Ch ., 1926, , 3 5-399 M M Mui a Mo ey W tt t n r f 7 ay o " e C emica eac io s o y oge A oms" .
Ch tr , o s 1- o gma G ee o o 1 9 A . Ch . S ., 1926, 48, - 5 M M P. Muir, A t r f Ch l h r nd , S D. o mes "E e ime s wi asce Gases" . Ch . Wi ey ew Yo k Y 19 9 Ed ., 1934, , 3 G ee woo a A Ea s aw Ch tr f th El 29. J. ee y a E igge s " e asce S t t ", . Ch . nt , e gamo O o 19 5 Ed ., 1942, , 3 7 ies ey Exp r nt nd Ob rv t n n ff r nt 3 ommasi "O e o -E is e ce o asee y oge " K nd f A r, o 1 ea so i mi g am 179 171 is quo e Ch . . 1879, 40, 171 Ea ie summa ies o a ious I a ia may a so e ese i e i s e i io o 177 u I a e ee u a e a e s a e e o e i r ht , 1878, , 3 5 a Ib d., 1879, 2, o e ami e a co y 17 1 W iggi s A C p r t v f th hl t nd 31. D. ommasi "O e asce S a e o o ies" Ch . ,
Ant phl t h r , Mu ay o o 17 9 13 1879, 40, 5 - 9 W iggi s Exp r nt nd Ob rv t n n th At 3 ommasi "Su y ogè e aissa " ll. S . Ch .,
h r , o gma s o o 1 1 53 1882, 8, 1 - 15 1 a y "O Some C emica Age cies o E ec ici y" h l. 33 ommasi w o w o e a eas ee ooks o e ec oc emis-
r n . . S ., 1807, , 1 -5 e i e i E a y e C ll t d y ( r té thé r t pr t d l tr h , a is 1 9 r té W r f S r phr v . o 5 Smi a E e o o 1 d p l él tr , a is 1 9 a M n l pr t d lv n (quo e o 11 pl t , a is 1 91 a o e c emica a ook ( r l r ph
11 S ike i g " e Ac io o Su u ic Aci o Co e " . h , a is 1 97 a so e gage i se e a mi o o emics
Ch . S ., 1878, , 11 - 139 wi w i e s w o a o e ooke is eo y o e asce s a e o 1 eg au C r élé nt r d h , o 1 Masso e o emie wi i so Ch , 1879, 40, 1 5 a is 1 51 1 -1 3 57 a Ib d., 1880, 4 , 1-3 17 _ o e o emic wi a c o
13 A aque r n pl fCh tr , e s aw o o 1 see ll. S . Ch ., 1896,'17, 9 1 -9 3 a f. h . Ch ., 1896, , 17 555-55 1 E M e e o E d é h n h , o 34. J. omse "Ue e ie a o o isc e us ä e es i ai e es Co s es o s e C auss es a is 1 79 -33 Wasse s o s" r ht , 1879, 2, 3 15 A Smi Intr d t n t In r n Ch tr , e Ce u y 35 A a e u g e nd ört rb h d r Ch , o 3 Co ew Yo k Y 191 ewe es au 1 95 - 7 1 e e e ce 15 53 -537 3 e i e a e s ei M l l r t n 17 Quo e i A Wu h At h r , A e o ew n , O o U i e si y ess O o 197 Yo k Y 1 1 7- 1 37 S aue " e aissa ce o Co ce wi e asce
1 A au e " ec e c es s i es com i aiso s a o e" Ann. St t ", . Ch . Ed ., 1976, , 37 -373 Ch . h ., 1846, 8, - 9 (quo e o 97- 9 See a so A 3 G Osa "Ue e ei e Mo i ica io es Wasse s o s" . au e Ch l M th d, t t n, Cl f t n nd n l r . Ch ., 1853, 8, 3 5-391 A is i g o Osa s u a e s ca t r , Ca e is Socie y o o 1 55 9-7 e ou i G l n ndb h d r An r n h Ch , W r 3 u is C em (199
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