Banait, Jagtar S.

Narula, Suraj P.

Paul, R. C.

English

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---1IndianJournalof ChemistryVol. l5A, June 1977,pp.497-499PhysicochemicalStudiesin Non-aqueousSolvents:Part XI - Thermo-chemicalStudiesof SomeI: I Electrolytesin HexamethylphosphotriamideR. c. PAUL, ]AGTAR S. BANAIT & SURA] P. NARULADepartmentof Chemistry,PanjabUniversity,Chandigarh160014Received2 APril 1976;revised5 October1976;accepted19January 1977Heatsof solutionof alkalimetal,silver,ammoniumandsubstitutedammoniumperchlo~rates,lithiumchlorideandpotassiumthiocyanatehavebeenmeasuredin the concentrationrange1,26-61,2x10-3molelitre-i in hexamethylphosphotriamide(HMPT) at 26·9°. Slopesofthelinearplotsofheatof solution(AHs) versussquare-rootof concentration(ylC) arecompar-ableto thatof Debye-Hiickellimitingslope(excepthoseofLiCI andLiCI04 whicharerespec-tivelysix andeighttimeshigher)indicatingcompletedissociationof theseelectrolytes.Stan-dardheatsof solutions(AH~).oftheelectrolytesin HMPT arehigherthanthosein formamideandN-methylformamideshowingtherebylack of hydrogenbondingin this solvent.A modi-fied Buckingham'smodelhas beenusedto computethesolvationenthalpiesof variousionsin HMPT. The resultsindicatethation solvationenthalpiesdecreasewiththeincreasein thesizeoftheionswhichis consistentwiththefindingsin thecaseof othersolvents.SaltLiClLiCiO.NaClO.KSCNKClO.RbClO.CsClO.AgClO.NH.Cl04Me4NCl04Et4NClO.CONDUCTANCE studiesl,2have revealedthatelectrolytes are completelydissociatedinhexamethylphosphotriamide(HMPT) at re-latively low concentrations.However, there isno report regardingthe thermochemicalstudiesofelectrolytesin this solvent. Alkali metal, silver,ammoniumandsubstitutedammoniumperchlorates,lithium chloride and potassiumthiocyanateareapprec.iablysolublein HMPT and their dissolutionis accompaniedwithheatchange. Wereportin thisp3.perthe heatsof solutionof theseelectrolytesinHMPT in orderto havesomeadditionalinformationregardingtheir behaviourat relativelyhighconcen-trationsandto evaluateionsolvationenthalpies.Materialsand MethodsHexamethylphosphotriamide(FlukaAG) waspuri-fiedasreportedearlier2. Theelectrolytesusedherewerepreparedand/orpurifiedasreportedearlier2.Heatsof solutionof theelectrolytesweremeasuredin an isothermalphase-changecalorimeterdesignedafter Daintonet at.3• Diphenylether (m.p.26·9°)was used as a dilatometricfluid. The detailsofexperimentalprocedureare the sameas reportedearlier4 (accuracy±0·2 kJ mole-J).ResultsandDiscussionHeats of solution of various electrolyteshavebeenmeasuredin the concentrationrange1,26-61,2X 10-3molelitre-]. Plots of heatsof solutiop(/)'Hs)versus square-rootof the concentration(Ve) ofthe electrolytes(exceptKCI04) are linear (Figs. 1and 2) and have been extrapolatedto infinitedilution to give standardheatsof solution (MIsO).The eXDerimentalslopesof theseplots agreewellwith the theoretical Debye-Hiickelslore exceptthoseof LiCI and LiCI04 whicharerespectivelysixand eight times higher than the theoreticalones.Generallythe higher valuesare attributedto theion-pair formationin the solution. However,con-ductancemeasurements2havealreadyshowntheseelectrolytesto be fully dissociatedin hexamethyl-phosphotriamide(HMPT) in the relativelylow con-centrationrange. In view of the high dielectricconstantand dipole momentvaluesof HMPT, thedeviationsaretoo smallto showsignificantion-pairformation of the electrolytes. HOWever,at themost,theseelectrolytesmay be consideredas onlyslightly associatedin this concentrationsrange(1'26-61'2 X 10-3 molelitre-I).Examinationof Table 1 revealsthat with theexceptionof ammoniumperchlorate,heatsofsolutionof the electrolyteshavingcommonaniondecreasewith increasein the size of the cation which isconsistentwith the retortedfindingsin mostof thesolvents5-8. However,no such generalizationcanbe given for electrolyteshaving commoncationsdueto the lack of sufficientdata.A comparisonof the thermochemicaldata invariousamidesolventsrevealsthatheatsof solutioncecreaseas: HexamethylphosrhotricLmic'e(HMPT)> Dimethylformamide(DMF) > N<Methylfor-mamide(NMF) > Formamide(F). The highervalues of heats of solutionin HMPT than thosegivenin DMF* indicatethatHMPT istheleasthydro-r,en-bondedsolvent. S;ectroscopic9andcryoscopic1odata alsosupportthis fact.TABLE 1- HEATS OF SOLUTION (AH;) , LATTICEENERGIES (U) AND HEATS OF SOLVATION(~H:ol) OF SOME ELECTROLYTES INHEXAMETHYLPHOSPHOTRIAMIDE AT 26'9°-~Ho -U -AH:01v(k] mole-i) (k] mole-i) (k] mole-I)67·02 842·22 909·2497'45 707,43 804·8856'34 643'81 700·1533'99 622-46 656·4528·09 584,78 612-8718·04 564,69 582·7313-90 538·32 552·2235,46 593'57 629·0348,68 569·71 618·3910·38 385·53 395·91-5,44 363'76 358'32*Thehydrogenbondingin DMF maybeexpectedthroughformylhydrogen.497INDIAN J. CHEM., VOL. 15A, JUNE 1977A2·0AKSCNB-19'0'-2('0'-230'-25'0'0'.20'b;04 0'-0'6 0'-0'8 0'·10' 0"12 0'<14 0',16 0'-18SQUARE RO'OT O'F CONCENTRAT ION (.;C )Fig. 1- Plots of I1Hs versus'lie for variouselectrolytesin HMPTO~OO· 0'-0'20·0-1-0'L -•-3'0' -5'0.; -7·<l...,zQ -9'0'I-::>-'a'" -II-a...0'\twJ: -13- ' Me4NCIC4C~Cla4RbCI04LiC..'..-NaCIO'40'·240"220"200"0'4 0'·0'6 0"0'6 0"10' 0"12 0"14 0"16 0':16SQUARE RaaT O'F CO'NCENTRATIO'N (JC )Fig. 2- Plots of I1Hs versusV C for variouselectrolytesin HMPT-190'0"0'0'0'0'2498PAUL et al.: STUDIES IN NON-AQUEOUSSOLVENTSThe combinedion-solvationenthalpies(M~oIv)for all the electrolyteshave beencalculatedfromthe standardheatsof solutionand lattice energies(U) usingEq. (1)Ml~oIv =Ml~+U ...(1)The latticeenergiesfor alkali metalsalts ammo-nium andsubstitutedammoniumperchlorateshavebeentakenfrom the literature4,8and that of silverperchloratehasbeencalculatedusingKapustinskii'sequationll..Ion-SolvationenthalPies-- Ion solvation enthal-pies (AH~oIv(±»)of variousionsin HMPT havebeenevaluated using Buckingham's modell2,13. Thismodelshowsthat ions of equalradii andoppositechargeshavedifferentsolvationenthalpiesand thedifference,whenfour-foldcoordinationaroundtheion is considered,is givenby Eq. 2 (ref. 14).UAH~oIv(M+)-~H~oIv(X-)J=-A(r.+rsoIvt)-3+B(r.+rsoIvtt4 ... (2)whereA =4Nz.eoPsoIvandB =(135v'2f32v'3)[J.PsoIvAll the symbols have their usual meanings.Relative solvation enthalpies (~H(rel)soIv(±») ofvariousionsin HMPT havebeencalculatedby as-suming~H(reIlsoIv(Na+)=0,0 kJ mole-I. Substitut-ing AH~oIv(M+land M~oIV(X-) in terms of relativeion-solvationenthalpies,Eq. (2) reducesto Eq. 3.![ ~H(rellsolv(M+)- ~H(rellsolv(X-)J=- ~H~olv(Na+l-A(r.+rHMPTt3+B(r.+rHMPTt4 ... (3)Differencein relative enthalpiesof hypotheticalion-pairsof equalradii and oppositechargeshavebeenevaluated14fromtheplots of cationandanionsolvationenthalpiesagainst(r.+rHMPTt3. Theradiusof the HMPT moleculel1asbeenfoundto be 3,6Aon constructinga Fischer modelof its molecule.The solvationenthalpyof sodiumion hasbeeneva-luatedfromEq. (3)by the methodof least-squares.Recently, Conway andSalomonI5,IG~.avepointedoutthation-dipoletermis notequalfor IOnSof equalradii andoppositechargesif the orientationsof thesolvent at the cation and anion are not mirrorimagesl7.SalomonI8has evaluatedion-solvationenthalpiesin water and propylenecarbon~tebymodifyingthe Halliwell and Nybug equationasshownbelow:![~H(rel)Solv(M+)-~H~ellsolv(X-lJ=-~H~alx(Na+)+C(r.+rsolvt )'2-D(r.+rsoIvt t3 .,. (4)The resultsso obtainedaresignificantlydifferentand the differencehas been attributed to thevariationof ion-dipoleinteractionenergiesbetweensolventandtheions,dueto thedifferentorientationof solvent molecules.The data obtainedin thecaseof HMPT asthesolventhavealsobeenaDalysedusingSalomon'sequation. However,the solvationenthalpiesof sodium ion obtainedby the twomethodsdifferonly by 3'76 kJ mole-Iwhichis notsignificant. This maysugv,estthat ion-dipoleinter-actionsare almostth.esamein HMPT.Knowingthesolvationenth.alpyof Na+,solvationenthalpiesof variousions (AH~olv(±»)canbe calcu-TABLE 2- RELATIVE ION-SOLVATION(~H(reaI)solv(±»)AND ION-SOLVATION(~H~oIv(±l) ENTHALPIESTO VARI USIONS INHEXAMETHYLPHO PHOTR AMIDEIon~H(real)soIv(±)~moIv(±)(kJ mole-I)Halliw ll andSalomonNyburg methodmethodLi+-106-41519'52523 28Na+ -0,0041 ·12'8K+ +87'3 5'833 9·60Rb + 1 -42295·7029 6Cs 4 '96 8670Ag7 12· 045 7NH~73 363H2Me4N+4 40 92Et4N+· 975'CI-8 ,82SC -30, 14,37CIO;j5 39latedusingEq. (5).M~oIv(±) =~H~e1)soIv(±(~H(soIv)Na+) •••(5)whereplusandminussignsareforcationsandanionsrespectively.It is clear from the Table 2 thatsolvationenthalpiesof ions decreasewith increasein thesizeof theion andfollowtheorder:Li+>Na+> Ag+> NHt > K-I' > Rb+> CST> Me4N+>Et4N+ and Cl- > SCN- > CI04".This orderis in accordwith the onereportedinthe caseof other solventsincluding water6,8,15,17and also with the solvationnumbersof the ionsobtainedin HMPT2.It maythus be concludedthat HMPT is an ex-cellentdissociatingsolventfor alkali metal,silver,ammoniumandsubstitutedammoniumperchlorates.References1. BRUNO,P., DELLA-MoNICA,M. & RIGHETTI, E., J. phys.Chem.,77 (1973),1258.2. PAUL, R. C., BANAIT, J. S. & NARULA, S. P., Z. phys.Chem.(N.F.), 94 (1975),199.3. DAINTON, F. S., DIAPER, J., IVIN, K. J. & SHEARD,D. R., Trans. Faraday Soc., 53 (1957),1269.4. PAUL, R. C., GILL, D. S. & NARULA, S. P., Indian J.Chem.,8 (1970),936.5. PAUL, R. C., JAUHAR, S. P" BANAIT, J. S. & NARULA,S. P., J. chem.Soc. (Faraday I), 73 (1977),833.6. SOMSEN,G., Reel.Trav. chim.Pays-Bas. Belg.,85 (1966),517; 526.7. SOMSEN,G. & CooPs, J., Reel. Trav. chim.. Pays-Bas.Belg., 84 (1965),985.8. GILL, D. S., SINLGA, J. P., PAUL, R. C. & NARULA,S. P., J. chem.Soc. (DaltonsTrans.),(1972), 522.9. HERAIL, F., C.r. hebd.Sianc Acad. Sci. Paris, Ser. C.,262 (1966),22.10. BANAIT, J. S., (unpublishedresults).11. KAPUSTINSKII,Q. Rev. chem.Soc., 10 (1956),283.12. BUCKINGHAM,A. D., Discuss.FaradaySoc.,24(1957),151.13. BOCKRIS,J. O'M. & REDDY, A. K. N., Modern electro-chemistry,Vol. I (Plenum Press, New York), 1970.14. HALLIWELL, H. F. & NYBURG, S. C., Trans. FaradaySoc., 58 (1963),1126.15. CONWAY,B. E. & SALOMON,M., Chemicalphysics ofionic solutions, edited by B. E. Conway & R. G.Barradas (Wiley, New York), 1966.16. CONWAY, B. E., Modern aspects of electrochemistry,editedby B. E. Conway and J. O'M. Bockris (Butter-worths, London), 1964.17. VERWAY, E. J. W., Reel. Trav. chim. Pays-Bas. Belg.,61 (1942),127.18. SALOMON,M., J. phys.Chem.,74 (1970),2519.499

Physicochemical Studies in Non-aqueous Solvents: Part XI - Thermochemical Studies of Some I:I Electrolytes in Hexamethylphosphotriamide

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Physicochemical Studies in Non-aqueous Solvents: Part XI - Thermochemical Studies of Some I:I Electrolytes in Hexamethylphosphotriamide

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