Balasubramanian, R.

Sinha, B. P.

English

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IndiaDJournalof ChemistryVol. 15A,May 1977,pp. 413-415Kinetics of Oxidationof TICI) by CeCIV) CatalysedbyThiocyanateIon in AqueousPerchloricAcid MediumR. BALASUBRAMANIAN & B. P. SINHADepartmentof Chemistry,Universityof Jabalpur,JabalpurReceived26 June 1976; accepted8 September1976Kineticsof oxidationof TI(I) by Ce(IV)in aqueousperchloricacidmediumhavebeenin-vestigatedby followingthe depletionin concentrationof Ce(IV) with time. At a seriesofequivalentconcentrationof thetworeactants,thetotalorderof thereactionis two;bein~unityin eachreactant.Theorderwithrespecto thiocyanateion is alsounity. Therateis directlyproportionalto the squareroot of [H+].The reactionshowsa primarypositivesalt effect.Whiletheadditionof oneof thereactionproducts,viz.Ce(III) retardstherate,theadditionofTI(III) producesnoeffect.Arrheniusparametershavebeendeterminedanda reactionschemewhichexplainsthesalientfeaturesof thereactionhasbeenproposed.A SERIES of cationswerefoundto effectivelycatalyse,otherwiseimmeasurablyslow1,non-complementaryreaction betweenTl(I) andCe(I:V)and the oxidationkineticsin presenceofsuch cationswere investigatedby Sinha et al.2-6and also by other workers7-10.Of the variousexplanationsput forth to account for the slownatureof this reaction,onerelatesto the coulombicforceof repulsionwhichwouldbe existentbetweentwo positivelychargedcations particularlywhenoneof thesehappensto behighlypositivelychargEd.Obviously,the introductionof an anionshouldbeable to reducethis forceeitherthroughcomplexa-tion with anyof thecationsor by actingasa bridgeand in consequencecatalysethe reaction. It is inthis connectionthat the authorshavefound thatthiocyanateion, a well known ligandll,acts as acatalystandthe presentpaperdescribesthe oxida-tion kineticsof this reactionin thepresenceof thisanion. The catalysisof this anion in the abovereactionhasnot beeninvestigatedso fa~.MaterialsandMethodsCeric perchloratesolution was preparedandstandardizedas describedearlier5fromeerieammo-nium nitrate (E. Merck). Thallous perchloratesolution was prepared5from thallous carbonate(E. Merck) and thallous content in the solutiondeterminedas thallouschromatelZ. The solutionofammoniumthiocyanate (GR) was preparedbydissolvinga knownquantityof the samplein therequisitevolumeof cioublydistilledwater. Otherreagentsusedwereof AR gradeand all solutionswereinvariablypreparedin doublydistilledwater.The kinetic procedurewas the sameas describedearlier2(a).ResultsA numberof runsat equivalentconcentrationsofthallous perchlorateand eerieperchloratein therange 4,444X 1O-4N-5'715X 10-4N were made inperchloricacid mediumat constant[catalyst]andionic strength. It was found that the reactionfollowsan overallsecondorderkineticsas revealedby the linear plots of Ij(a-x) versustime. Thetotal orderof thereactionwasfurtherconfirmedbyvan't Hoff's differential method. In order toascertainthe orderin eerieion, a numberof runswith differentinitial [eerieperchlorate]were fol-lowedwhile keepingall othervariablesunchanged.The order in eerieion was determinedby plot-ting concentrationversustimecurvesanddetermin-ing the slopeat that point at which 10% of thereductantwas oxidized. Table 1 givesthe initialratefor four runswith differentinitial [Ce(IV)].Substitutionof the valuesof initial rate in van'tHoff's differentialformula13=log (-dcjdth-log (-dcjdt)z (1)n log C)-log Cz ...(wheren givestheorderin eerieion) confirmedthefirst orderkineticsin Ce(IV).It wouldbeseenthat therateconstants(Table1)decreasewith increasein the initial [Ce(IV)], al-thoughthereactionadheredto secondorderkinetics.It is interestingto notethat a reasonableconstancyis observedfor the productof kz?< [Ce(C104)4]forconcentrationsof Ce(IV)closeto theinitial [thallousion] (--5'262X 10-4N), but at higher [Ce(IV)] thevalue of the product decreasedmarkedly. Theeffectof increasinginitial [Ce(IV)] which leadstodecreasein the value of rate constantsmay beattributedto the formationof unreactivedimersofCe(IV) at higher[Ce(IV)].Sincethe total orderof the reactionis two andthe orderin Ce(IV) is unity, it may be naturallyexpectedthat the order in Tl(I) is also unity.However, for verification, the concentrationofthallousion wasvariedovera wide range(--5'262X 1O-4N-I0'00 X 1O-4N), keepingall other factors,including the ionic strengthunchanged.It wasobservedthat the reaction follows secondorder413INDIAN J. CHEM.,VOL. 15A,MAY 1977([ce(iI0')4] = 5·262X 10-4N; [TICI04] =5·262X 10-W;[H 10,J = 5·000X to-IN; temp.= 30°C; (.L = 1·00M)[NH.S NJ X 106N k2(litre g. equiv.-1 k2 X 106j[SCW]min-1)33·3033·0633·0133·19k2(litre gequiv-1min-1)2·505·007·50to·OOAmountaddedX 10'M30·0127·3324·2220·49k2(litre gequiv-1min-1)2·505,007·50to'OOAmountaddedX lO'M[HCI04] X ION k2(litre g equiv-1k2/[HCI04]tmin.-1)2'021·7048,533·05-46·364,9 69955'33 16,545 8877 817840 6299TABLE 5- VALUES OF SECONDORDERRATE CONSTANTSIN THE PRESENCEOF DIFFERENT CONCENTRATIONSOFTI(II1) AND Ce(II1)([Ce(CI04),]= 5·262X 10-4N; [TlCIO,] =5·262X 10-'N;[NH4SCN] = 1-000X lO-sN; [HCIO,] = 5·000X 10-IN;temp. = 30°C; (.L = l'OOM)TABLE 4 - VALUES OF SECONDORDER RATE CONSTANTSFOR DIFFERENT CONCENTRATIONSOF HYDROGENIONS([Ce(CI),),]=5·262X 10-W; [TICIOi] = 5'262X to-'N;[NH4SCN] = 1-000X lO-sN; temp. =30°C; (.L = l'OOM)21'4228'4932'7734'227-4616'8366,5706·233k2(litre g. [TICIO.] Xk2(litre gequiv.-1104Nequiv.-1min-1)min-1)32·777-50019·9126·85014- 43- VALUES OF' SECONDORDERRATE CONSTANTSAT DIFFERENT [CATALYST]6·005,5005·2625'0005·6·TAB E 1- INITIAL RATE AND THE VALUES OF RATECONS NTS AT DIFFERENT INITIAL [CERIC PERCHLORATE]{[TI O.J,= 5·262x l(t"'N; [NH,SCNJ =1·000X to-oN;[H 10J = 5'000X 10-IN temp.=30°; (.L =l'OOM}Init' I concentra- (-dc/dt) X 106 k2 (litre gtion f [Ce(CIO.),] (g. equiv. equiv.-1min-1)X 10'N litre-1 min-1)TABLE12- SECONDORDERRATE CONSTANTSFOR VARIOUSINITIAL [THALLOUSPERCHLORATE]([Ce(qO,),] = 5·262X 10-'N; [NH4SCN] = 1·000X 10-oN;[H ,04] =5·000X lO-lN; temp. = 30°C; (.L = 1·00M}kineti s. Again 'c-t' curves were drawn andemploing van't Hoff's differential formula theorder in thallousion was found to be unity. Aperus1 of Table 2 showsthat the rate decreaseswith i creasein theinitial [T1(I)J,analogousto thatobtai ed by varying initial [Ce(IV)]. This mayagain·be attributedto the formationof unreactivecompIx speciesof TI(I) with Ce(IV) or to thediffernt lessreactivethiocyanatecomplexof TI(I).As stated earlier, thiocyanateion effectivelycatal es the reaction. A linear proportionalitybetwen the rate and con<:entrationf thiocyanatewas0 servedasis evidentfromthedatain Table3.In all previous studies it has been generallyreportd that the rate of oxidation of Tl(I) byCe(IV dependsignificantlyon [H+]. An analogousobser.ationwas found in the presentcaseas well(Tabl· 4). The reactionratewas foundto dependdirecty on [H+Jlj2.The reactionwasfoundto showprimarypositivesalt e ect in the presenceof a numberof neutralelectrlytes. However,no quantitativeconclusioncould e drawnas theionic strengthregioninvesti-gatedwas beyondthe limit permittedby Debye-Huck 1 limiting law. The effectof addedCe(III)7·08·09·0110'01·02·03·021·9925·8929-4233·1936·7840·3443·733·1413·2363·2693·3193·3443·3823·364andTI(III) was alsoinvestigatedand it wasfoundthat while TI(III) has no influenceon the rate,Ce(III) causesretardation.For determiningthe activationparameters,thereactionwas studiedat differenttemperaturesandit was found that the data adhereto Arrheniusequation. The temperaturecoefficientis 1·78. Theheatof activationwasevaluatedfrom the slopeofArrhenius plots and was found AH* =10·48kcal mole-I. The entropyof activation,AS* wasevaluatedfwm Eyring's equation and the freeenergyof activation,AF* was calculatedfrom thethermodynamicrelationship (AF*=AH*- TAS*).These values are AS* =-25·22 e.u. and AF*=18·12kcal mole-I.DiscussionCe(IV) is known to exist in perchloric acidmediumin a varietyof formssuchasCe4+,CeOH3+and Ce(OH)~+etc.,alongsidetheformationof smallamountsof dimersor eventrimers. Variousequili-bria havealso beensuggestedl4-21and correspon-dingequilibriumconstantshavebeenevaluatedl7-l9.Sincehighly chargedCe(IV) has a strong affinityfor water22, it wouldbe morelogicalif it is writtenas Ce(H20)~t. The ion steps(2-5)aresuggestedtoaccountfor the observedkineticdata.KNH4SCN ~ NH~+SCN- ...(2)k,Ce(H~O)~++SCN- ~ (Ce(H20)n_ISCN)3++HaO...(3)k_, complex414IIIBALASUBRAMANIAN & SINHA: Ce(IV) OXIDATION OF Tl(1)•..(12)k,Complex+Tl+~ Ce3++Tl2++SCN- ...(4)(aq)k.Complex+TI2+~ Ce3++T13++SCN- ...(5)(aq)On the basisof abovestepsthe rate is givenas:[Ce(H0)4+]_ 2 n =k2[complex][TI+]+k3[complex]LTI2+]... (6)Assumingsteadystatefor Tl2+andthe complex[TI2+]=k2~TI+J •••(7)3[C I] - k1[Ce(H20)4,f] [SCN-] (8)ompex - k' .J...2k [T1+] ...-1 I 2wherek~1=k_1[H20].Substitutionof theseconcentrationtermsin Eq. (6)leadsto the rateequation:_ d[Ce(H20W] ~ 2k1k2[Ce(H20):+][TI+][SCN-] (9)dt - k~1+2k2[Tl+J '"when k~1>2k2[T1+], and at constant[SCN-] therate is givenas- d[Ce(H20W] ~ ko [Ce(H20)4,,+][Tl"f] .,.(10)2k k [SCN-]whereko = 1k' andis the observedsecond-1orderrate constant.The rate expressionis found to explain theprincipal kinetic features. However, it fails assuch to account for the decreasein rate withincreasein initial concentrationsof eitherCe(IV),TI(I) or Ce(III). In this connectionit would berelevantto add that at higher[Ce(IV)]the forma-tion of dimers or even trimers of Ce(IV) hasbeen reported18,19.Also the formationof mixeddimersof [Ce(IV)+Ce(III)] are now known9. Theformationof such species,in all probability,isexpectedto retard the rate becauseof the lessreactivity. The decreasein rate with increasinginitial [TI(I)] may be attributedto two reasons.(i) It is quite likely that on increasing[TI(I)], theformationof somecomplexsalt may take placesimilar to that at least found in sulphuricacidmedium24.Such an explanationhas earlierbeenput forth by Dorfmanand Gryder10and also bySinha and coworkerssa,5.While complexationinperchloricacid mediumare not generallyfavouredthe recentstudiesby Hala25do not precludesuchapossibilityaltogether.(ii) Thallousion may enterinto complexationwith thiocyanateion whichmayalsoaffecttherate.To accountfor rather complexdependenceofrate on [H+], an explanationsimilarto that givenby Duke andBorchers26appearsapplicable.If weconsiderthe equilibriumKCe(H20):+~ Ce(H20)n-lOH3++H+ •..(11)th K =HCeh-Ce(at~)][H+]en 4+e(aq.)]or4+ [(CehJ[H+][Ce(aq.)]= [H+]+K •••(13)SubstitutingEq. (13) in therateEq. (9),weget-d[(CehJ kW[(Ce)r] [TI+][H+]dt =(k~1+2k2[T1+])([H+]+K) •••(14)wherekW =2k1k2[SCN-].Under such circumstances,the fractionallineardependenceof rateon [H+]canat leastbe qualita-tively explained.AcknowledgementOne of us (R.B.S.) is grateful to the CSIR,New Delhi for the award of a junior researchfellowship.References1. (a) SCHAFFER,P. A., J. Am. chern.Soc.,55 (1933),2169.(b) SCHAFFER,P. A., J. phys. Chern.,40 (1936),1021.2. (a) KRISHNA, B. & SINHA, B. P., Z. phys. Chern.,212(1959),149; (b) 212 (1959),155; (c) 213 (1960),177.3. (a) SINHA,B. P., Z. phys.Chern.,233 (1966),161;(b)233(1966),412.4. SINHA, B. P. & MATHUR, H. P., Z. phys. Chern.,246(1971),342.5. SINHA,B. P. & MATHUR,H. P., J. inorg.nucl. Chern.,33(1971),1673.6. MATHUR, H. p, Ph.D. thesis, University of Jodhpur,Jodhpur.7. HIGGINSON,W. C. E., Disc. Faraday Soc., 29 (1960),135.8. HIGGINSON,W. C. E., ROSSEINSKY,D. R., STEAD,J. B.& SYKES, A. G., Disc. Faraday Soc.,29 (1960),54.9. ZELTMAN,E. W., ChBmAbstr., 68 (1968),72,723q.10.DORFMAN,M. C. & GRYDER, J. W., J. inorg. Chern.,1(1962),799.11. TREINDL, L., Chern.Abstr., 68 (1968),90205x.12. VOGEL,A. I., A textbookofquantitativeinorganicanalysis(Longmans,London), 1961,549.13. FROST,A. A. & PEARSON,R. G., KineticsandmBchanism(John Wiley, New York), 1961,17.14. SHERRILL,M. S., KING, C. B. & SPOONER,R. C., J. Am.chern.Soc.,65 (1943),170.15. EVANS, M. G. & URI. N., Nature, Lond., 166 (1950),602.16. DUKE, F. R. & PARACHEN,R. F., J. Am. chern.Soc.,78(1956),150.17. BAKER, F. B., NEWTON,T. W. & KHAN, M., J. phys.Chern.,64 (1960l,109.18. HEIDT, L. & SMITH, M., ]. Am. chern.Soc., 70 (1948),2476.19. HARDWICK,T. J. & ROBERTSON,E., Can. J. Chern.,29(1951),818.20. KING, E. L. & PANDOW,M. L., ]. Am. chern.Soc., 74(1952),1966.21. ARDON,M., J. chern.Soc., (1957).1811.22. COTTON,F. A. & WILKINSON, G., Advancedinorganicchemistry(Interscience,New York), 1967, 1068.23. BLAUSTEIN,B. D. & GRYDER,J. W., J. Am. chern.Soc.,79 (1957),540..24. MELLOR, J. W., A comprehensivetreatisein inorganicchemistry,Vol. V (Longmans,London), 1960,662.25. HALA, J., ]. inorg.nucl. Chern.,29 (1967),187.26. DUKE, F. R. & BoRCHERS,C. E., J. Am. chern.Soc.,75(1953),5186.415

Kinetics of Oxidation of T1(I) by Ce(IV) Catalysed by Thiocyanate Ion in Aqueous Perchloric Acid Medium

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Kinetics of Oxidation of T1(I) by Ce(IV) Catalysed by Thiocyanate Ion in Aqueous Perchloric Acid Medium

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