Chemical Kinetics PDF
Preview Chemical Kinetics
THE GEORGE FISHER BAKER NON-RESIDENT LECTURESHIP IN CHEMISTRY AT CORNELL UNIVERSITY CHEMICAL KINETICS BY FARRINGTON DANIELS THE GEORGE FISHER BAKER NON-RESIDENT LECTURESHIP IN CHEMISTRY AT CORNELL UNIVERSITY Chemical Kinetics BY FARRINGTON Professorof Chemistry in the University of Wisconsin ITHACA NEW YORK CORNELL UNIVERSITY PRESS LONDON: HUMPHREY MILFORD OXFORD UNIVERSITY PRESS 1938 COPYRIGHT 1938 BY CORNELL UNIVERSITY PRINTFD IN THE UNITEDSTATES OFAMERICA Qtt)t<Eiill*giatrfJreaa C.EORC.E BANTA PXTBMSHING COMPANY, MFNASHA, WISCONSIN PREFACE THIS book is based on lectures delivered at Cornell University under the George Fisher Baker Non-Resident Lectureship in Chemistry from February to June 1935. So rapidly is our under- standingof Chemical Kineticschanging that ithasbeen necessary to include additional material up to the time of the completion of the manuscript in 1937. Iappreciatetheopportunity accorded me by theGeorgeFisher Baker Fund. This lectureship, established a decade ago, has en-, abled CornelltodomuchforChemistryinAmerica. Ithasbrought the stimulus of foreign chemists to us and it is now leading in a much needed visitation of professors among our own universities. Wisconsin University and Cornell University have much in com- mon. I am glad to have had the privilege of telling each about the other. Particularly I wish to thank Professor J. Papish and all the members of thechemistry department for theircordial hospitality during our pleasant visit in Ithaca. FAKRINGTON DANIELS Madison, Wisconsin, October, 1936. CONTENTS III.. GINETNREORDAULCTPIROINNCIPLE.S....... .... 81 Orderofreaction . . . 8 STCphheeecmiifAicrcarlrheeeanqcmutiisolniebqrruaiatutemi..o.n.. . . 111584 The kinetictheoryofgases . . . ... . 24 III. THTTCCEhohhOameeRipInlcrEoaiSlrdcleiaiOaastctFietiodionUonNrhnIeshyaMypcpoOtotLithoEhenCesssUiisLsAR REACTIONS.... . . . ..... 4333217147 Freeradicals 49 IV. GAS PHASE REACTIONS 56 Experimentaltechnique ... 57 Thedecompositionofnitrogen pentoxide 60 Thedecompositionofethylbromide 72 Literature 86 V. CHEMICAL KINETICS OF REACTIONSINSOLUTION. ... 90 SCCooollmlvpiaastirioionnsofrnebqueetnwceyen reactionsin gasand insolution... . . ,. . 999136 Thedecompositionofnitrogen pentoxide 100 Thedecomposition ofchloroacetateion 107 Ionicreactions ... 110 VI. PHTOhTeOCdHeEcMoImpSoTsRiYtion ofammonium amalgam . 111174 Photochemical principles 118 Absorptionofradiation 120 Photochemical kinetics 124 Experimentaltechnique 127 QTThhueeanppthhuoottmoollyyyisseiilssdooffnaictertoogneen oxides: N2O6, N2O4 and NOa 111334577 Thephotobrqmination ofcinnamicacid 157 TThheepfhoortmoaltyisoinsooffvbiettaa-mcianryDophyllenenitrosite 116679 Photosynthesisbychlorophyll 172 Literature 176 VII. ELECTRICALACTIVATION 181 Electricaldischargesingases 182 Chemicalreactionsin electrical discharges 185 Chemicaleffectsofalpha particles 188 Chemical effectsofcathoderays 191 ChemicaleffectsofX-rays 195 Chemicaleffectsoflowvoltageelectrons 198 VIII. INFRARED SPECTROSCOPY 201 Experimentaltechnique 202 CHEMICAL KINETICS viii Ethyl bromide ... 204 Aceticacid. . . . . . .214 Theoreticalconsiderations 216 IX. THEORETICAL CALCULATION OF ACTIVATION ENERGIES.. ... 220 . Energyand interatomicdistance 222 Thesemi-empirical method. Thebromination ofethylene.. . 227 Chemicalapplications 234 The statistical mechanical method .... 239 X. ISOTOPICTRACERS . 244 Isotopiccarbon. . . 245 Literature 257 INDEX 263 CHAPTER I INTRODUCTION ACCORDING to theannouncementof the Departmentof Chemistry of Cornell University, I am "to present the most recent advance- mentsandtheresultsofmyowninvestigationsin thefield." I shall trytodo thisinchemicalkinetics, and, indescribingmyresearches and thoseofothers, I shall try to illustrate current theories and to giveglimpsesofpresentfrontiers. Itismyhopethatinterestin the subject will not beconfined to physical chemists alone, for impor- tant applications are waiting in many fields. With this in mind I shall endeavor to make the discussions rather elementary. In these lectures the author has a special privilege for he need not "address himself only to experts in the field," and he is not handicapped by the limitations of space which are necessary in technical journals. Moreover, he is not only permitted but ex- pected to talk chiefly about his own work. Chemical kinetics deals with the speed and mechanisms of chemical reactions. It is a recent development in chemistry and a difficult one one in which it is easy to raise more questions than answers. Itismoredifficultthan the predictionofchemical equilib- ria and the applications of thermodynamics, for these are con- cerned merely with the initial and final states and not with time nor with intermediate steps. At the close of these lectures you will realize that in chemical kinetics we have averycomplex field, in which we have not asyet established many exact laws nor simple generalizations, but you willrealizealsothatitisafieldfullofopportunityforpioneerwork. For this reason it is an intriguing field of research. The prediction ofreaction rates is a matter of practical impor- tance as well as theoretical interest. This is particularly true in organic chemistry where reactions are slow and many possible reactions are competing for the available material. That reaction CHEMICAL KINETICS 2 which goes fastest will produce the greatest yield. An ability to predict reaction rates enables an investigator to suppress unde- sired reactions just as a gardener eliminates weeds. At the present time the organic chemist does this empirically, but in the future we expect that chemical kinetics will provide a more certain ap- proach. Looking at chemical kinetics from an historical viewpoint we find that the mass law proposed in 1867 by Guldberg and Waage was the first fundamental contribution to theory. According to this law, now thoroughly established by experiment, the speed of a chemical reaction is proportional to the active masses of the reacting substances, and, as a first approximation, concentra- tions may be substituted for the active masses. One of the earliest experimental researches in this field was that of Harcourt and Essen in 1880 on the reaction between oxalic acid and potassium permanganate. The several factors involved in this complex reac- tion were varied, one at a time, and the speed of the reaction was measured experimentally. In the beginnings ofclassical physicalchemistry, starting with the publication of the ZeitschriftfiirPhysikalische Chemie in 1887, wefind the problemofchemicalkineticsbeingattacked in earnest. Ostwaldfound that thespeedofinversion ofcanesugar (catalyzed byacids) couldberepresented byasimplemathematicalequation, the so-called "compound interest law." Nernst and others meas- ured accurately the rates of several reactions and expressed them mathematically as first order or second order reactions. Arrhenius made a very important contribution to our knowledge of the influence of temperature on chemical reactions. His empirical equation forms the foundation of much of the theory of chemical kinetics which will be discussed in the following chapter. Following the period of early exploration and experimental measurement came three decades in which reaction rates in solu- tion were studied and classified. A tremendous amount of semi- quantitative work of an empirical nature has been accumulated, especiallyinthefieldoforganicchemistry. Manyusefulgeneraliza- tions have been drawn from these experimental investigations but the progress ofchemical kinetics suffered for lack of stimulat- ing hypotheses. Theradiationhypothesisin 1918,wrongthoughitwas, ushered in a period ofstimulating theoretical approaches which have done INTRODUCTION 3 much to advance our understanding of the subject. In fact the attack along theoretical lines has now progressed so rapidly that confusion exists and the weakest link in the chain of development now seems to lie in the determination of exact experimental facts regarding significant, simple reactions. It is one of my tasks to evaluate critically with you the many hypotheses, designed tocorrelatefactsandpredictbehavior,which are now springing up all over the field of chemical kinetics. The answer to theriddleofchemical action is notsimpleand thesitua- tion is rapidly becoming more complicated rather than less com- plicated, but there is no reason to be pessimistic. The progress of chemistry,oranyscience,maybepicturedasacooperativeprogram between the fact finders and the generalizers and it will be unfor- tunate for the next generation of students if the fact finders get toofaraheadofthegeneralizers.Therearealreadyenough isolated factstoremember. Butitwillbejustasunfortunateifthetheorists get too far ahead. In attacking the problem of the mechanism and speed of chemicalreaction I havefollowed severaldifferentlinesofresearch each of which will be described in one of the following chapters.1 The study of reactions in gases constitutes the simplest ap- proach from a theoretical standpoint, and it is here that much helpcan beobtained from thekinetic theoryofgasesasdeveloped by mathematical physics. Many of the interesting gases are cor- rosivein naturesothatithasbeennecessarytoavoidexperimental pitfalls such as stopcock grease, rubber tubing and mercury sur- faces. Elimination of these standard laboratory tools has forced us to adopt an all-glass technique in nearly all branches of the work. The decompositions of nitrogen pentoxide and of ethyl bromide constitute the principal work which I will describe in the chemical kinetics of gas reactions. Reactions in solution are more difficult from a theoretical, but easier from an experimental standpoint. From a practical stand- point they are vastly more important than gas reactions because the large majority of reactions in chemistry and biology occur in solutions. The field, however, is so enormous that we shall not go into it extensively and we shall exclude from consideration the 1 I wish toexpressat this time my appreciation for thesubstantial support which I havereceived formy research program from the Wisconsin Alumni Re- search Foundationandfrom CharlesS. Slichter, Dean EmeritusoftheGraduate SchooloftheUniversityofWisconsin.
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