4 THERMOCHEMISTRY by H. A. Skinner (Department of Chemistry, The University, Manchester, M13 9PL) THE 23rd Annual Calorimetry Conference (U.S.A.), the 4th All-Union Calorimetry Conference (U.S.S.R.), the 3rd Experimental Thermodynamics Conference (U.K.), and the 4th Calorimetry Conference of Japan all took place during 1968. Each of these National Calorimetry Conferences is by now well-established, and their success augers well for the 1st International Conference on Calorimetry and Thermodynamics, planned to take place at Warsaw, Poland in September 1969. 1968 also saw the 1st International CODATA Conference, held at Frankfurt-am-Main, at which the special problems facing the compilers of standard reference tables of thermodynamic, spectral, and nuclear data were thoroughly examined.Of significant new books and publications, mention should be made of ‘Experimental Thermudynamics’, Vol. I, edited by J. P. McCullough and D. M. Scott, and the announcement from Academic Press of the Journal of Chemical Thermodynamics, to appear in January, 1969. The present status of thermal and thermochemical data for lanthanide metal chalcogenides, borides, pnictides, and halides has been assessed by Westruml, Darby2 has reviewed the thermodynamic properties of transition-metal alloys, and Hubbard, O’Hare, and Feder3 have reviewed recent progress in experimental inorganic thermochemistry. The thermo- dynamic properties of the platinum-group metals and their compounds were reviewed by Goldberg and H e ~ l e r . ~ Continuing development in the design and operation of reaction calori- meters is exemplified by the twin-flow and the batch-type microcalorimeters described by Wadso5 and by Monk and Wadso.6 These are conduction calorimeters using thermopiles made from semi-conductors, and both are available from the L.K.B.Instrument Co. An isothermal titration calorimeter in which the heat of titration is neutralised by a constant cooling Peltier device is described by Christensen, Johnston, and I ~ a t t . ~ Peltier cooling is also employed in the microcalorimeter described by Evans, McCourtney, and Carney.8 Franks and Watsong have reported on a twin differential calorimetric 1 E. F. Westrum jun. in ‘Advances in Chemistry Series No. 71,’ American Chemical Society, Washington D.C., 1967, p. 25. 2 J. B. Darby jun., Znd.and Eng. Chem., 1968, 60, 28. 3 W. N. Hubbard, P. A. G. O’Hare, and H. M. Feder, Ann. Rev. Phys. Chem., 1968, 19, 1 1 1 . * R. N. Goldberg and L. G. Hepler, Chem. Rev., 1968, 68, 229. I. Wadso, Acta Chem. Scnnd., 1968, 22, 927. P. Monk and I. Wadso, Acta Chem. Scand., 1968, 22, 1842. 7 J. J. Christensen, H. D. Johnston, and R. M. Izatt, Reo. Sci. Znstr., 1968, 39, 1356. W. J. Evans, E. J. McCourtney, and W. B. Carney, Analyt. Chern., 1968, 40,262. F. Franks and B. Watson, J . Sci. Znstr., [J. Phys. (E)] 1968, 1, 940. 4950 H. A . Skinner system for the measurement of heats of solution of non-electrolytes at high dilutions (mole fraction, 0-05-2.0 x I. Key Compounds.-Careful measurements by Greenberg and Hubbard'" of the energy of combustion in a bomb calorimeter of graphite in fluorine led to AHf"(CF4, g) = - 223.04 & 0.18 kcal./mole, and have largely settled any remaining doubts on this important thermochemical datum. In conjunction with other seemingly reliable data,ll the sharp value for AHfO(CF4, g) 'fixes' the value for the enthalpy of formation of aqueous hydrofluoric acid at AHf0(HF,20H20) = - 76.75 0.10 kcal./mole.A direct measurement of the latter from the heat of combustion of hydrogen in a fluorine flame calor- imeter has given a value in close agreement.12 New measurements of the enthalpy of hydrolysis of crystalline phosphorus pentachloride by Birley and Skinner,l3 in conjunction with the enthalpy of synthesis of PCI,, gave AHfo(H3P04,100Hz0) = - 309.80 & 0 3 5 kcal./mole, in agreement with the value derived from recent rneas~rementsl~ of the heat of hydrolysis of P4010.The heat of oxidation of phosphorous to phosphoric acid has been measured by solution calorimetry,l5 yielding AHf0(H3P03,c) = - 229.1 kcal./mole. Sousa-Alonso, Chadwick, and Irvingl6 have re-measured the heat of oxidation of iron(rr) ions with dilute hydrogen peroxide. Earlier measurements of this quantity179 18 varied over several kcal./mole. The new measurements, using dilute solutions, are perhaps more relevant to the determination of AHf"(Fe2+, aq) [with respect to AHf"(Fe3+ aq)], and correspond to AH = 10.57 & 0.4 kcal./mole for the process: Fe2+(aq) + H+(aq)+ Fe3+(aq) + &Hz(g) This value is confirmed (AH = 10.6 & 0.9 kcal./mole) by independent rneasurement~l~ of the enthalpy of oxidation of iron(1r) ions by aqueous bromine.The enthalpies of formation of the simpler alkyl radicals (e.g. Me., Et., MezHC., Me&.) are of key importance for the thermochemical evaluation of bond dissociation energies of carbon bonds in organic molecules. Although acceptable values for many of these have been available for several years, they have carried uncertainty limits of at least &l kcal./mole. Moreover, neither the kinetic nor the electron-impact techniques of measurement of dissociation energies in polyatomic molecules have seemed capable of higher 10 E. Greenberg and W. N. Hubbard, J . Phys. Chem., 1968,72,222. l1 J. D. Cox, H. A. Gundry, and A. J. Head, Trans. Faraday SOC., 1965,61, 1594; see l2 R. C . King and G. T. Armstrong, J .Res. Nat. Bur. Stand., 1968,72, A , 113. l3 G . I. Birley and H. A. Skinner, Trans. Faraday SOC., 1968, 64, 3232. l4 R. J. Irving and H. McKerrell, Trans. Faruduy SOC., 1967, 63, 2582. l5 A. Finch, P. J. Gardner, K. S. Hussain, and K. K. Sen Gupta, Chem. Comm., 1968, l6 A. Sousa-Alonso, I. Chadwick, and R. J. Irving, J . Chem. SOC. (A), 1968,2364. l7 D. K. Bewley, Trans. Faraday SOC., 1960, 56, 1629. 18 M. F. Koehler and J. P. Coughlin, J . Phys. Chem., 1959,63,605. l9 J. W. Larson, P. Cerutti, H. K. Garber, and L. G. Hepler, J . Phys. Chem., 1968, also H. A. Skinner, Ann. Reports ( A ) , 1967, 64, 4. 872. 72, 2902.TItermochemistry 51 accuracy than this, so that sharp values have remained elusive. A significant advance is now reported by Chupka20 in the evaluation of threshold appear- ance potentials of parent and fragment ions produced by photo-ionization of molecules and radicals.Improved apparatus21, 22 of much higher sensitivity and photon energy resolution has increased the accuracy of measurement by a factor of 5 or more, and enabled the effect of thermal rotational energy at room temperature in the threshold region to be taken into account. From studies of the photo-ionization of methane, Chupka20 reports Doo(Me-H) = 4.477 & 0.005 ev 103.24 -+ 0.1 kcal./mole; the noteworthy feature here is the reduction (by about a factor of 10) of the uncertainty previously attached to this quantity. The new value corresponds to D(Me-H) = 104.8 kcal./mole at 298"~, and toAHt-O(Me, g) == 34.8 kcal./mole. Chupka and Lifshitz23 have studied the photo-ionization of methyl radicals by the same technique, and its threshold yields Do0(CHz-H) = 107.9 & 1.0 kcal./mole: this corresponds to D(CH2-H) = 109.0 & 1.0 kcal./mole at 298"~ and to AHf"(CH2, g) = 91.7 & 1 kcal./mole.The main source of errar in the threshold from the Me radical arises from uncertainty in the amount of thermal energy in the radicals at the instant of photoionization, since the radicals may have lost thermal energy on leaving the pyrolysis tube. Never- theless, the estimated uncertainty of & 1 kcal./mole represents a five-fold improvement relative to existing uncertainty in AHfO(CH2, g) obtained by other methods. Zmbov, Uy, and Margrave24 have studied the thermal decomposition of tetrafluorethylene over the temperature range 1 127-1244"~, and measured the temperature dependence of the equilibrium constant for the process CzF4(g) + 2CFz(g).The gas C2F4 (produced by pyrolysis of Teflon) was introduced into a heated graphite Knudsen cell, and the effusing gases were ionized by electron impact and analysed by mass spectroscopy. From these measurements, the enthalpy of disssociation was determined, AHr(298 OK) = 76.3 & 3 kcal./mole; in conjunction with AHfO(C2F4, g) = - 157.4 & 1 k~al./mole,~~ this leads to AHf"(CF2, g) = - 403 & 2 kcal./mole, in good agreement with the values obtained by Modica and La GraffZ6 from shock- tube studies on C2F4 and CHF3. Electron impact studies27 on CCl2 radicals (produced by pyrolysis of C13CSiC13) gave 9.76 ev for the vertical ionization potential; in conjunction with measured appearance potentials for the dissociative ionization of CHC13 and CzC14, a value AHf"(CC12, g) = 56-5 & 5 kcal./mole was derived.The new values (i.e. AH,' for CH2, CFZ, and CC12) lead to the following thermo- 2o W. A. Chupka, J . Chem. Phys., 1968,48,2337. 31 J. Berkowitz and W. A. Chupka, J. Chem. Phys., 1966,45, 1287. 22 W. A. Chupka and J. Berkowitz, J . Chem. Phys., 1967, 47, 2921. 23 W. A. Chupka and C. Lifshitz, J . Chem. Phys., 1968, 48, 1109. 24 K. F. Zmbov, 0. M. Uy, and J. L. Margreave, J . Amer. Chem. SOC., 1968,90, 5090. 25 J. R. Lacher and H. A. Skinner, J . Chem. SOC. (A), 1968, 1034. 26 A. P. Modica and J. E. La Graff, J . Chem. Phys., 1965,43, 3383; 1966,44, 3375. 27 J.S. Shapiro and F. P. Lossing, J . Phys. Chem., 1968, 72, 1552.52 H. A . Skinner chemical bond dissociation energies of C= C double bonds (in kcal./mole): D(CH2=CH2)= 170.9 & 2, D(CH2=CF2)= 133.7 & 3, D(CH2=CCl2) = 147.9 5, D(CF2=CF2) = 76.3 & 3, D(CC12=CC12) = 116.4 7, D(CF2=CCl2) = 96.2 & 6. Kinetic studies of thermal bromination of CF3H (361-431 "c) and C2F,H ( 3 2 3 4 5 8 " ~ ) by Amphlett and WhittleZ8 have established the Arr- henius parameters for the broinination reactions, and provide the bond dissociation energy values, D(CF3-H) = 106.7 0.5 and D(C,F,-H) = 102.6 & 1.2 kcal./mole. The value for D(CF3-H) is in good agreement with earlier measurements from independent kinetic studies291 30, and there seems no reason to doubt that it is of the right order of magnitude.In con- junction with the seemingly reliable value for AHf0(CF3H, g) = - 166.2 & 1 k~al./mole,~~, 31 the value AHf"(CF3, g) = - 1 1 1.6 & 1-5 kcal./mole is derived. This in turn with AHfo(C2F~, g) = - 321 -& 1 kcal./mole25 gives D(CF3-CF3) = 97.8 & 3 kcal./niole, in good agreement with the direct determination of this quantity by Tschuikow-Roux32 using a shock tube. However, this background of internal consistency is severely jolted should the conclusions of Modica and Sillers33 prove valid; from measurements of the rate of formation of CF2 radicals from the decomposition of CF,I, C,F,, and CF4 in an excess of argon behind shock waves over a temperature range 170&3000"~, these authors arrived at the value AHfO(CF3, g) = - 104 kcal./mole.The analysis of the experimental results is involved, and requires the enthalpies of formation of the reacting species as input data. The values used (from the JANAF tables) can be questioned. 2. Combustion Calorimetry.--Satisfactory values are now available for the standard enthalpies of formation of the oxides of most of the metallic ele- ments. Many of these were determined from high-precision measurements of their heats of combustion in oxygen by bomb combustion techniques. The thermochemistry group at Los Alamos have contributed notably to this programme over the past decade, and new measurements from this source include the heats of combustion of neptunium34 (to form NpOa), hafnium35 (to form HfOz), cerous oxide36 (Ce203-+ CeOz), and thorium carbide37 (ThC1.91).Solution calorimetric methods were used for obtaining the standard heats of formation of selected non-stoicheiometric oxides of 28 J. C. Amphlett and E. Whittle, Trans. Faraday SOC., 1968, 64, 2130. 29 J. W. Coomber and E. Whittle, Trans. Faraduy SOC., 1966, 62, 2183. 30 C. A. Goy, A. Lord, and H. 0. Pritchard, J . Phys. Chem., 1967, 71, 1086. 31 H. A. Skinner, Ann. Reports (A), 1967, 64, 4. 32 E. Tschuikow-Roux, J . Chem. Phys., 1965,43, 2251. 33 A. P. Modica and S. J. Sillers, J . Chem. Phys., 1968,48, 3283. S4 E. J. Huber jun., and C. E. Holley jun., J . Chem. and Eng. Data, 1968,13, 545. 35 E. J. Huber jun., and C. E. Holley jun., J . Chem. and Eng. Data, 1968,13, 252. 36 F. B. Baker and C. E. Holley jun., J . Chem. and Eng.Data, 1968,13,405. 37 E. J. Huber jun., C. E. Holley jun., and N. H. Krikorian, J . Chem. and Eng. Data, 38 G . C. Fitzgibbon and C. E. Holley jun., J . Chem. and Eng. Data, 1968, 13, 63. 1968, 13, 253.Thermoclzemistry 53 (TbO1.51, Tb01.709, Tb01.817, and Tb01.975), and of hexagonal Nd203.39 The heat of combustion of copper in a bomb calorimeter to form a mixture of CuO and Cu20 was reported by Mah, Pankratz, Weller, and King40, and used in combination with dissociation pressure measurements on CuO to yield more definitive values than hitherto for AHf"(Cu0) and AHf"(Cu20). Gal'chenko, Gedakyan, and Timofeev4I have used a nickel bomb calori- meter fitted with an internal micro-heater to measure the heats of combustion of Zr, Hf, and Ta metals in chlorine to form ZrC14, HfC14, and TaC15. The heat of combustion of B4C in fluorine has been reported,42 and the heat of formation of chlorine trifluoride measured by King and Armstrone3 using fluorine flame calorimetry [AHfo(CIF3, g) = - 38.6 kcal./mole].The heat of combustion of chlorobenzene has been measured by Kolesov, Tomareva, Skuratov, and A l e k h i ~ ~ ~ ~ using a platinum-lined rotating bomb calorimeter, with As203 solution as reductant to remove chlorine from the combustion gases. The value AHf"(CsH5C1, 1) = -2.82 & 0-25 kcal./niole was derived. Measurements by Robb and Zimmer45 of the solubility and heat of solution of COS in aqueous solutions containing As20.5, As203, and HCl provide improved information necessary for the standard state correc- tions in rotating bomb calorimetric studies with arsenic(rI1) oxide as reductant.Kolesov, Shtekher, and M a r t y n ~ v ~ ~ have continued their measurements on the heats of combustion of gaseous fluorinated hydrocarbons and find AHc" = -291.5 & 2 kcal./mole for 1,l-difluoroethane, which corresponds to AHfO(CF2HCH3, g) = -118.4 & 2 kcal./mole; this agrees well with the value25 derived from measurements of the heat of hydr~genation~~ of CH2 = CF2. Bomb combustion studies on trib~tylboron~~ and trihexylb~ron~~ have yielded AHfo[B(Bu")3, 11 = -84.3 & 0.5 and AHf"[B(n-hexyl)s, I] = - 116.8 0-7 kcal./mole. Combustion methods have also been used to obtain the heats of formation of a number of cyclopentanyl titanium compounds.50 39 G . C. Fitzgibbon, D. Pavone, and C.E. Holley jun., J . Chem. and Eng. Data, 1968, 4O A. D. Mah, L. B. Pankratz, W. W. Weller, and E. G. King, U.S. Bur. Mines Report 41 G. L. Gal'chenko, D. A. Gedakyan, and B. I. Timofeev, Russ. J . Inorg. Chem., 42 E. S. Domalski and G. T. Armstrong, J. Res. Nut. Bur. Stand., 1968,12, A , 133. -33 R. C. King and G. T. Armstrong, Nat. Bur. Stand. Report No. 9905, 1968. 44 V. P. Kolesov, E. M. Tomareva, S. M. Skuratov, and S. P. Alekhin, Zhur. fiz. 45 R. A. Robb and M. F. Zimmer, J. Chem. and Eng. Data, 1968,13,200. 46 V. P. Kolesov, S. M. Shtekher, and A. M. Martynov,Zhur.fiz. Khim., 1968,42,1847. 47 J. R. Lacher and P. B. Howard, to be published. 4* G. L. Gal'chenko and N. S. Zaugol'nikova, Zhur. $2. Khim., 1967,41, 1018. 49 G. L. Gal'chenko and N. S. Zaugol'nikova, Zhur.$z.Khim., 1967, 41, 2181. 50 V. I. Tel'noi, 1. B. Rabinovitch, V. D. Tikhonov, V. N. Latyaeva, L. I. Vishinskaya, 13, 547. No. 7026, 1967. 1968, 13, 159. Khim., 1967, 41, 1528. and G . A. Razuvaev, Doklady Akad. Nauk S.S.S.R., 1967, 174, 1374.54 11. A . Skinner The heats of combustion of resorcin01,~~ trans-stilbene and 22,44,66-hexa- nitr~stilbene,~~ several azido-s-triaz~les,~~ selected nitroso-naphthols,54 hydroxylammonium per~hlorate~~ and other explosive compounds56 are reported (the use of diethyl oxalate and diethyl phthalate as desensitisers to control the combustion is of interest in dealing with the latter). 3. Reaction Calorimetry.-Gross, Hayman, and Joel5' have measured the enthalpies of the reactions MF(c) + BF3(g)+ MBF4(c), (M = Li or Na), by passing BF3 gas over the solid alkali fluorides in a calorimeter maintained at 110"c.The enthalpy of formation of KBF4(c) was determined by comparing solution heats in HF(aq) of the mixtures (NaBF4 + KF) and (KBF4 + NaF). Greene, Gross, and Hayman5* reported the enthalpy of formation of LisAIFs(c) from the reaction 3LiF(c) + AlF3(c), measured at room temperature in a calorimeter fitted with an electrically heated reaction zone. A 'hot zone' calorimeter was also used by Vorob'ev, Monaenkova, and Skuratov59 to measure AHf"(BaH2, c) directly from the reaction of the metal with hydrogen gas at 2 atm. pressure. Solution calorimetric methods were used by CubicciottPO. 61 to measure AHf"(BiBr3, c) = - 66.0 and AHr"(Bi13, c) = - 36.0 kcal./mole. From equili- brium studies on Q Bi(1iq) + Q BiXs(g)+ BiX(g), (X = Br or I), and measure- ments of AHsub (BiXs), the dissociation energies D(Bi-Br) = 64 and D(Bi-I) = 52 kcal./mole were obtained; the average bond dissociation energies in BiBrs(g) (5 = 56) and BiIs(g)(E = 43 kcal./mole) are less than in the respective diatomic molecules.Froin measurements of the enthalpies of solution of the metals and their chlorides in hydrochloric acid solutions, Stuve62 has obtained Akl~f"(HoCl3, c) and AHf"(TbCI3, c); Gvelesiani and YashvW3 measured solution heats of the metals and the oxides in hydro- chloric acid to obtain AHf"(La203, c) andAHrO(Smz03, c). Solution calorimetry was also used by Irving and McKerrelP4 to obtain the enthalpies of formation of the crystalline salts Na,P,O,,, NazHzPz07, Na4Pz07, (NaP03)3, and (NaP0314.j1 P. D. Desai, R. C. Wilhoit, and B. J. Zwo!inski, J . Chem. and Eng. Data, 1968, 13, 334. 52 S. Marantz and G. T. Armstrong, J . Chem. and Eng. Data, 1968, 13, 1 1 8. 53 G. C. Denault, P. C. Marx, and H. H. Takimoto, J . Chem. and Eng. Data, 1968,13, 514. S4 J. V. Hamilton and T. F. Fagley, J . Chem. and Eng. Data, 1968, 13, 523. 55 M. F. Zimrner, E. E. Baroody, G . A. Carpenter, and R. A. Robb, J . Chem. and Eng. 56 E. E. Baroody, G. A. Carpenter, R. A. Robb, and M. F. Zimrner, J . Chem. atid Eng. 5' P. Gross, C. Hayrnan, and H. A. Joel, Trans. Faraday SOC., 1968,64, 317. 58 P. D. Greene, P. Gross, and C. Hayman, Trans. Faraday SOC., 1968,64, 633. 59 A. F. Vorob'ev, A. S. Monaenkova, and S. M. Skuratov, Doklady Akad.Nauk 6o D. Cubicciotti, Inorg. Chem., 1968, 7 , 208. D. Cubicciotti, Znorg. Chem., 1968, 7 , 211. 62 J. M. Stuve, U.S. Bur. Mines Report No. 7046, 1967. 63 G. G. Gvelesiani and T. S. Yashvili, Zhur. Neorg. Khinz., 1967, 12, 3233. 64 R. J. Irving and H. McKerrell, Trans. Faraday SOC., 1968, 64, 875, 879. Data, 1968, 13, 212. Data, 1968, 13, 215. S.S.S.R., 1968, 179, 1129.Thermocheniistry 55 Hill and Irving65 have continued their studies on the acetylacetonates of transition metals and have found values for AHf" of Fe(acac)s and Mn(acac)a, using reaction calorimetry (reactions with HCl and with FeCl2-HCl respec- tively). For the average M-0 bond dissociation energies in M(acac)3 molecules, the authors quote Mn-0 = 44, Fe-0 = 47, Cr-0 = 56, and Al-0 = 64 kcal./mole respectively.Clark and Price,66 from measurements of the enthalpy of reaction of indium trimethyl with Br2-CHC13 solution, obtained AHf"(InMe3, c) = 29.5 kcal./mole. FromAHsub = 1 1*6,AHf"(In, g) = 58.2 and AHp"(Me, g) = 34-8 kcal./mole, the average bond dissociation energy, D(In-Me) = 40.5 kcal./mole is derived. From previous kinetic studies, the authors found D(Me2In-Me) + D(In-Me) = 87.9 kcal./mole, which, in conjunction with 5 = 40.5, yields D(Me1n-Me) = 33.6 kcal./mole. Enthalpies of hydrolysis in aqueous HCl of nine trimethylsilane derivatives have been reported by Baldwin, Lappert, Pedley, and Tre~erton,~~ and Hill and Wadso68 have measured the enthalpy of hydrolysis of triacetylammonia. An improved value for AHp"(SOBr2,l) is available from enthalpy of hydrolysis measurement^,^^ and for AH," (sulphamic acid, c) from measurementsT0 of the enthalpy of reaction with sodium nitrite in acidic solution.The heat of explosion of CF2(0F)2 has been measured,71 and AHf"(ClF5,l) was obtained from rnea~urements~~ of its enthalpies of reaction with H2, and with NH3. Izatt, Eatough, Snow, and Chri~tensen~~ have measured the enthalpies of formation of complexes between Ag+ and Cu2+ with pyridine (AH for Ag pyzf = - 11.2, and for Cu py42+ = -21.5 kcal./mole), and the enthalpy of forma- tion of Co(CN)s3- has also been measured.74 Carson, Laye, and Smith75 used a diphenyl ether fusion calorimeter in measurements of the enthalpies of formation of the complexes of the divalent ions of Mg, Ca, Sr, and Ba with diethylene triaminepenta-acetic acid, and of the complexes formed by the trivalent lanthanide group of metals. The enthalpies of formation of pyridine complexes with aluminium halides are reported by Wilson and W0rrall.7~ A sensitive differential calorimeter was used by Brunetti, Lim, and Nancollas77 to measure the heats of complexing of Cu2+ with diglycine and triglycine.65 J. 0. Hill and R. J. Irving, J . Chem. SOC. ( A ) , 1968, 1052, 31 16. W. D. Clark and S. J. W. Price, Canad. J . Chem., 1968,46, 1633. 67 J. C. Baldwin, M. F. Lappert, J. B. Pedley, and J. A. Treverton, J . Chem. Soc. (A), 8* J. 0. Hill and I. Wadso, Acta Chem. Scand., 1968, 22, 1590. :g A. Finch, P. J. Gardner, and K. Radcliffe, J . Chem. and Eng. Data, 1968, 13, 176. 60 G. A. Nash, H. A. Skinner, T.A. Zordan, and L. G. Hepler, J. Chern. and Eng. 71 G. D. Foss and D. A. Pitt, J. Phys. Chem., 1968, 72, 3512. 72 W. R. Bisbee, J. V. Hamilton, J. M. Gerhauser, and R. Rushworth, J. Chem. and 73 R. M. Izatt, D. Eatough, R. L. Snow, and J. J. Christensen, J. Phys. Chem., 1968, 74 R . M. Izatt, G. D. Watt, C. H. Bartholomew, and J. J. Chriitensen, Inorg. Chem., 75 A. S. Carson, P. G. Laye, and P. N. Smith, J . Chem. SOC. (A), 1968, 141,527, 1384. 76 J. W. Wilson and I. J. Worrall, J . Chem. SOC. ( A ) , 1968, 316, 2389. 77 A. P. Brunetti, M. C. Lim, and G. H. Nancollas, J . Amer. Chem. Soc., 1968, 90, 1967, 1980. Data, 1968, 13, 271. Eng. Data, 1968, 13, 382. 72, 1208. 1968,7, 2236. 5 120.56 H. A. Skiitner The available enthalpies of formation of fluoro- and of fluoro-halogenated- hydrocarbons have been re-evaluated by Lacher and Skinner25 using improved values for ANm"(CF4, g) and AHf"(HF, as).The revised data correlate well with the bond additivity and bond interaction scheme of Allen78 in the case of the halogen-substituted methanes and fluorocarbons, but in halogen-sub- stituted ethanes steric repulsion energies of the order of 1 kcal./mole between gauche C-X bonds across the central C-C bond are indicated (X = F or Cl). 4. Equilibrium Studies.-The combination of the mass spectroscope and the Knudsen cell continues to prove fruitful in studies of gaseous equilibria at high temperatures. The vapour species over liquid SnF2, over liquid SnF2 + Sn, over CaFz(s) + Sn(l), and over liquid and solid PbFz have been studied mass spectroscopically by Zmbov, Hastie, and Margrave.79 The presence of dimers and trimers as well as monomers of SnFz was noted.There are no dimers or polymers in PbFz vapour, but the disproportionation 2PbFz(g) -+ PbF4(g) + Pb(g) takes place. The equilibrium constant of the process Ca(g) + SnF(g)-+ CaF(g) + Sn(g) was measured over the range 1304- 1401"~ from analysis of the vapours over CaF2 + Sn, and the derived en- thalpy of reaction, with Do(CaF) = 127-5 -+ 2-5 kcal./mole,*O gives D"(SnF) = 114 5 3 kcal./mole. Values were obtained for Do(Pb-F) = 85 & 2 kcal./mole, and for the enthalpy of dissociation, SnzFe(g)-+ 2SnFz(g), AHo = 39 5 2 kcal./mole. The vapours from NaSnFa and KSnF3 over the range 725-895 OK have been similarly investigated,*l and the species SnF2, NaSnF3, NazSnFe, and NaSnzFs have been identified.Ficalora, Hastie, and Margraves2 have analysed the species present when Al(g) is equilibrated with Sz(g), Sea(g), and Tez(g). A similar study, replacing A1 by In, is reported by Colin and D r o ~ a r t . ~ ~ AHf" Values for the gaseous species MX, M2X, and M2X2 were derived (M = A1 or In; X = S, Se, or Te). The vapour species from Gad%, Ga2Se3, and GazTe3 have been investi- gated.** Dimer molecules were detectedg5 in the vapours over KOH ; AH(dinwriza- tion) = -43 kcal./mole dimer. The vapours over Cs-02 contained Cs20 and Cs202, and the enthalpies of formation of both were determined.s6 The vapours from BeF2 at 9 4 7 " ~ contained some dimer molecules ; AH(dimerisa- tion) = -34.5 kcal./mole dimer.87 Small amounts (ca.1 %) of dimer species 78 T. L. Allen, J . Cfzem. Phys., 1959, 31, 1039. 79 K. Zmbov, J. W. Hastie, and J. L. Margrave, Trans. Faraday SOC., 1968, 64, 861. J. W. Hastie and J. L. Margrave, J . Chem. arid Eng. Data, 1968, 13, 428. J. W. Hastie, K. F. Zmbov, and J. L. Margrave, J . Inorg. Nuclear Chem., 1968,30, 729. 82 P. J. Ficalora, J. W. Hastie, and J. L. Margrave, J . Phys. Chem., 1968, 72, 1660. 83 R. Colin and J. Drowart, Trans. Faraday SOC., 1968, 64, 2611. s4 0. M. Uy, D. W. Muenow, P. J. Ficalora, and J. L. Margrave, Trans. Faraday SOC., 85 A. V. Gusarov and L. N. Gorokhov, Z h u r . 5 ~ . Khim., 1968, 42, 860. 86 A. V. Gusarov, L. N. Gorokhov, and A. G . Efimova, Teplofiz. Vys. Temp., 1967, 87 V. I. Belousov, L. N. Siderov, S. A. Kamarov, and P.A. Akishin, Zhur. fiz. Khim., 1968, 64,2998. 5, 584. 1967,41, 2691.Thermochemistr); 57 were detected88 in the vapours over LaCls(c) and EuCk(c), but none were found over EuCh(1). Mass spectral and microbalance studies of the sublima- tion of CrO3 at 150-250"c have shown the presence of polymers in the vapoursg, and provided values for the enthalpies of the reactions, n[CrO3,(g)] + (CrO&(g), for n = 3, 4, and 5. Hildenbrandgo has studied the high temperature gaseous equilibria in the Mg-Cu-F system by mass spectroscopy, and measured the enthalpy of the reaction MgF(g) + Cu(g)+ Mg(g) + CuF(g), leading to D(CuF) = 103 & 2.2 kcal./mole, so that the second dissociation energy in CuFz(g) is sub- stantially higher than the first [D(FCu-F) = 79 kcal./mole], and in accord with expectation from the valence-state excitation model for s-p-bonding in CuF2.Similar studiesg1 involving the reactions M(g) + MFz(g)+ 2MF(g), (M = Mg, Sr, or Ba), have established that D(FMg-F) > D(MgF), but the reverse order applies in the case of BaFz where D(BaF) > D(FBa-F). There is qualitative accord with the valence-state s-p-bonding model for the lighter Group I1 metal fluorides, but for the heavier metals the electrostatic model (in which MF2 is treated as polarisable ions in contact) is much sup- erior. The high temperature reactions AuMn(g) + Au(g)+ Mn(g) + Auz(g), and AgMn(g) + Ag(g)+ Mn(g) + Agz(g) have been studied, giving AHo" = -9-6 0.692 and -1043 & 4 kcal./moleg3 respectively, and leading to Do"(AuMn) = 43.4 & 3 and Do"(AgMn) = 26-8 & 5 kcal./mole.Molecular beam sampling and mass spectral detection were used by Kohl and Carlsong4 to analyse the equilibria in vapours over liquid solutions of Bi-Sb mixtures, in the range 225-625 "c. Dissociation energies and enthalpies of formation of the gaseous species BiSb3, BizSbz, BisSb, and BiSb were obtained and are additive in relation to the molecules Biz, Sbz, Bi4, and Sb4 [e.g. D(BiSb) = iD(Biz) + +D(Sbz); AHfO(Bi3Sb) = gAHf"(Bi4) + $AHfo(Sb4)]. The temperature and concentration dependence of an absorption band in iodine vapour95 in the region 265 mp has been interpreted in terms of an equilibrium 212 $14; in this event, the equilibrium constant and its temp- erature dependence over the range 150-420"~ require AHo (605"~) = 2-9 & 0.4 kcal./mole, and indicate 1.4 mole "/, T4 in the vapour at 240"c and 2.5 atm.pressure. 5. Bond Dissociation Energies.-Budinikas, Edwards, and WahlbeckS6 have developed a novel double-oven effusion cell, suspended as a single unit, enabling the simultaneous measurement of Knudsen effusion- and torsion 88 J. W. Hastie, P. Ficalora, and J. L. Margrave, J . Less Common Metals, 1968, 14,83. 89 J. D. McDonald and J. L. Margrave, J . Inorg. Nuclear Chem., 1968, 30, 665. 90 D. L. Hildenbrand, J. Chem. Phys., 1968,48, 2457. 91 D. L. Hildenbrand, J . Chem. Phys., 1968, 48, 3657. g2 S. Smoes and J. Drowart, Chem. Comm., 1968, 534. 93 A. Kant, J. Chem. Phys., 1968,48, 523. 94 F. J. Kohl and K. D. Carlson, J . Amer. Chem. SOC., 1968, 90, 4814. 95 A. A.Passchier and N. W. Gregory, J . Phys. Chern., 1968,72, 2697. 96 P. Budinikas, R. K. Edwards, and P. G. Wahlbeck, J . Chem. Phys., 1968, 48, 2859, 2867,2870.58 H. A. Skinner effusion-data. The combined measurements on the dissociation processes SZ-, 2S, Se2+ 2Se, and Te2- 2Te have provided the values 101.7 f. 2-9,75-7 If: 2.5, and 61.3 & 1.1 kcal./mole respectively for DO0(S2), Doo(Sez), and Doo (Te2). The latter is confirmed by photo-ionisation threshold meas~rements,9~ DO0(Te2) = 60.85 & 0.2 kcal./mole. Photo-ionisation yield curves for molecule and fragment ions have been measured by Dibeler and List0n~8~~9 from H2S, S02, BF3, and B2F4, and have led to the dissociation energy values Doo(HS-H) = 89.3 and Doo(S-H) = 83.2 kcal./mole; Do0(FzB-F) = 169.3, Doo(FB-F) = 117.6 and Doo(B-F) = 173.6 kcal./mole; and Doo(F2B-BF2) = 103 kcal./mole.The second bond dissociation energy in BF3, i.e. D(FB-F), is thus decidedly less than the first or third; it is noteworthy that the B-B dissociation energy in B2F4 is larger than the C-C dissociation energy in C2F4. The dissociation energies in TlF, TlCl, and TlBr were measured from photo-ionisation thresholds for the formation of TI+ by Berkowitz and Walter;100 the existence of small barriers in the potential energy curves of 3 ~ 0 and 3 ~ 1 states of TlF is confirmed by the new results. Setser and StedmanlOl have investigated a series of bond-rupture reactions in which metastable excited argon atoms act as rupture agent, Ar* + RCN -> Ar + R + CW. The Ar* carries 270 kcal./mole available energy for transfer.The emission spectra of the products were analysed to evaluate Emax of the highest populated vibrational level of CN", and thus set upper limits to the bond dissociation energies D(R-CN). The values so obtained, uiz: D(H-CN) < 124, D(NC-CN) < 133, D(Me-CN) < 116, D(HzN-CN) < 124, and D(F3C-CN) < 119 kcal./mole, establish that AHf"(CN, g) < 103 kcal./mole, in 'agreement with shock-tube studies1OZ which gave 100 & 4 kcal./mole. A method of assessment of differences between different C-C bond dissociation energies in chemically activated alkyl radicals is described by Larson, Tardy, and Rabinovitch.103 The scheme involves the steps (1) and (2) : (1) H + olefin-+ R* (vibrationally activated) (o1efin)l + Ri 7 L ole fin)^ + R2 Measurement of the yields of the olefins provides the relative rates of decom- position of R* by the alternative routes.Studies have been made in this way g7 J. Berkowitz and W. A. Chupka; referred to in ref. 96. 98 V. H. Dibeler and S . K. Liston, J . Chem. Phys., 1968, 49, 482. V. H. Dibeler and S. K. Liston, Inorg. Chem., 1968, 7 , 1742. loo J. Berkowitz and T. A. Walter, J. Chem. Phys., 1968, 49, 1184. lol D. W. Setser and D. H. Stedman, J . Chem. Phys., 1968,49, 467. lo2 W. Tsang, S. H. Bauer, and M. Cowperthwaite, J. Chern. Phys., 1962, 36, 1768. lo3 C. W. Larson, D. C. Tardy, and B. S. Rabinovitch, J . Chern. Phys., 1968, 49, 299.Thermochemistry 59 on the room-temperature decomposition of the octyl-4, heptyl-3, and 3- methyl-hexyl-2 radicals, and have indicated that Doo(Me-H) > Doo(Et-H) by 5.irLkcal./mole, and DoO(Et-H) > DoO(Pr-H) by 0.8 kcal/mole.A similar method was used by Cadman, Phillips, and Trotman-Dickenson,lO4 in studies of the decomposition of chemically activated CF3CH2Me. The active species, prepared by method (3) or (4); From Me. + *CH2CF3+ MeCH2CF3* From MeCHz * + * CF3+ MeCH2CF3** thereafter may deactivate by collision, or eliminate HF to form CF2=CHMe. The propene :propane yield ratio gives the relative rates of decomposition. The results indicate higher activation in CF3CH2Me** than in CF3CH2Me*, to the extent that D(CF3-CH2Me) > D(CF3CHz-Me) by ca. 4.5 kcal./mole. Tachikawa and Rowlandlo5 have studied recoil tritium reactions with hydrocarbons in a series of moderators (C-Cd?s, Ar, Nz), and have measured the HT:RT yield ratios in the reactions ( 5 ) and ( 6 ) : (3 1 (4) T* + RH+HT + R ( 5 ) T*+RH+RT+H (6) Wide variations in the yield ratio were noted, and a correlation emerged between the bond dissociation energies of the R-H bonds and the HT yields.The correlation proved unsatisfactory with MeCD :CD2, the HT yield being less than expected; the suggestion was made that the relaxation of CHzCD:CD2 to the allylic configuration is too slow to gain the benefit of n-electron delocalisation during the rupture process.lo6 HT Yields from recoil tritium abstraction with H-N < bonds also appear to correlate with N-H bond dissociation energies.lo7 The photolysis of alkyl halides in the presence of HSiC13 initiates a chain reaction in which the rate-determining step is RX + -SiC13+ R - + SiC13X.The relative rates of abstraction RIX + SiC13+, and R2X + SiC13-+, have been measured for mixtures of several alkyl clilorides and bromides by Kerr, Smith, Trotman-Dickenson, and Young,loS and the Arrhenius parameters determined. The activation energy differences, E(MeC1) - E(RCI), and E(MeC1) - E(RBr), increase as R ascends the series Et, Bun, Bui, Pri, Bus, But, and parallel the decreasing bond dissociation energies D(R-CI), D(R-Br), for similar changes in R. The pyrolysis kinetics of MesSi-SiMes have been investigated in a static system over the range 523-555" at pressures 0-2-0.8 mm., by Davidson and lo* P. Cadman, D. C. Phillips, and A. F. Trotman-Dickenson, Chem. Comm., 1968 lo5 E.Tachikawa and F. S. Rowland, J. Amer. Chem. SOC., 1968,90, 4767. lo6 E. Tachikawa, Y-N Tang, and F. S. Rowland, J. Amer. Chem. SOC., 1968,90, 3584. lo7 T. Tominaga and F. S. Rowland, J. Phys. Chem., 1968,72, 1399. lo8 J. A. Kerr, B. J. A. Smith, A. F. Trotman-Dickenson, and J. C. Young, J. Chem. 796. SOC. (A), 1968, 510. C60 H. A. Skinner Stephenson.log First-order kinetics were obeyed, and the activation energy (67.3 & 2.2 kcal./mole) is identified by the authors with the Si-Si bond dissociation energy. Band, Davidson, and Lambert ,11* from electron-impact studies and measured appearance potentials of fragment ions, have evaluated bond dissociation energies D(Me3Si-X) in a series of trimethylsilyl deriva- tives, including D(Me3Si-CI) = 88 3: 2 and D(Me3SiBr) = 78.5 & 2 kcal./mole.Combined with AHf"(Me3SiC1, g) = - 84.6 kcal./m0Ie,~7 the value D(Me3Si-Cl) = 88 yields AHfo(Me3Si-, g) = -25.5 kcal./mole: combined with A.Hf"(MesSiBr, g) = -70.1 k~al./mole,~~ the latter yields D(Me3Si-Br) = 71.3 kcal./mole. There is thus a greater inconsistency be- tween the thermochemical and electron-impact results than is indicated by the attached limits of error. The vertical ionisation potentials of Me3Sn, MezSn, and MeSn radicals, formed by electron impact on hexamethylditin, have been measuredlll by a mass-spectrometric method. In conjunction with appearance-potential measurements112 of Me3Snf from several trimethyltin derivatives, bond dissociation energies, D(MeaSn-R), are obtained for R = Me, Et, Pr, and Me&. The Sn-C bond in SnMee was determined at D(Me3Sn-Me) = 60 kcal./mole ; the average Sn-Me bond strength, from thermochemical measurements, is somewhat lower (E = 54.0 kcal./mole).The kinetics of thermal decomposition of CF30F in the presence of SO3 have been followed by pressure over the range 200-230"~. The kinetic analysis yields D(CF30-F) = 43.5 5 0.5 kcal./mole. 6. Miscellaneous. Kebarle114 has described gas-phase measurements of the relative concentrations of clustered ionic species of the type A+Sn (A+ = Hf, NH4+, S = HzO, NH3, MeOH), using mass spectral detection of A+&, sampled from an ion source containing a known pressure of the 'solvent' species, S. The enthalpies of the gas-phase reactions NH4+(NH3)n + NH3 -+ NH4+(NH3)n+l have been derived,1l5 and are -27 (n = 0), - 17 (n = l), - 16.5 (n = 2), - 14.5 (n = 3), and -7.5 kcal./mole (n = 4).The sharp fall at n = 4 is noteworthy, and contrasts with the steady decrease in- A H for the successive addition of H2O molecules to HC. Measurements of the attach- ment of HzO molecules to F-, C1-, Br-, and I- by Kebarle, Arshadi, and ScarboroughllG indicate that the negative ions solvate more readily and more strongly than do alkali-metal positive ions. The standard absolute free energy, enthalpy, entropy, and heat capacity for hydration of the proton have been evaluated by de Ligny, Alfenaar, and log I. M. T. Davidson and I. L. Stephenson, J . Chern. SOC. (A), 1968, 282. 111 F. W. Lampe and A. Niehaus, J . Chem. Phys., 1968,49, 2949. 112 A. L. Yergey and F. W. Lampe, J . Amer.Chem. SOC., 1965, 87, 4204. 114 P. Kebarle, in 'Advances in Chemistry Series No. 72,' American Chemical Society, 115 S. K. Searles and P. Kebarle, J . Phys. Chem., 1968, 72, 743. 110 P. Kebarle, M. Arshadi, and J. Scarborough, J . Chetn. Phys., 1968, 49, 817. S . J. Band, I. M. T. Davidson, and C. A. Lambert, J . Chem. Sor. ( A ) , 1968, 2068. J. Czarnowski, E. Castellano, and H. J. Schumacher, Chem. Comm., 1968, 1255. Washington D.C., 1968, p. 24.Thennocheniistry 61 Vander Veen,l17 based on a cyclic process involving ferrocene and the ferro- cinium ion : H+(aq) + Ferro(aq) - Ferro+(aq)-+ iHz(g) t .1 H+(g) + Ferro(g) - Ferro+(g) --f H+(g) + e-(g) The assumption was made that AGO (hydration) of the ferrocinium ion differs from AHo (hydration) of neutral ferrocene only by the electrostatic contribu- tion (calculable from the Born model), and was justified on the grounds that the ion is large and that the ion and neutral molecule are virtually equal in size.The absolute values, S"(H+)(aq) = -4.8 & 1-2 cal.deg.-1 mole-1 and AH" = -263 5 3 kcal./mole were derived. The interaction of alkyl groups with water is an important factor in relation to micelle formation, hydrophobic bonding in aqueous protein solutions, and adsorption processes at oil-water interfaces. The large negative entropy of hydration of alkyl-chain segments in water is acknowledged, but consistent results have not been obtained from measurements of the enthalpy of hydra- tion per -CH2- group of an alkyl chain. Aveyard and Mitchell118 have considerably clarified the situation from an analysis of their calorimetric measurements on the heats of solution of a series of alcohols and n-carboxylic acids to high dilution in water at 25"c.It is concluded thatAHsoln per -CHz-- (liquid) group is ca. 350 cal./mole for alkyl chains up to c8 in length. The thermal effects of intramicellar dissolution (solubilisation) of hydro- carbons in aqueous sodium oleate solutions have been estimated from refrac- tometric measurements of the intramicellar solubility over a range of concen- trations and temperature.llg For the process octane (I)+ octane-sodium oleate micelle, the value AH = 2 - 4 kcal./mole was derived, on the basis, however, of a model that may be oversimplified. Measurements of the standard partial molar heat capacity of NaBPh4 in aqueous solution from 0 to 90"c have been made by Subramanian and, Ahluwalia.120 The curves ofACpO and zp2O us. temperature showzd two breaks at the minimum near 50", and the maximum near 70"c. The minimum is ascribed to hydrophobic interactions, and the maximum to a sharp reduction in the structure-making ability of the solvent above 70"c. The denaturation of P-lactoglobulin by urea at pH 2-5-3.5 at several temperatures between 5 and 55"c has been studied by Pace and Tanford.l2I The isothermal denaturation, brought about by exposure to urea, was fol- lowed by the change in optical rotation measured at 365 mp. The monomeric native protein was used in all these studies. The equilibrium constant, defined 117 C. L. de Ligny, M. Alfenaar, and N. G. Vander Veen, Rec. Trav. chim., 1968,87, 585. 11* R. Aveyard and R. W. Mitchell, Trans. Faraday SOC., 1968, 64, 1757. lL9 Z . N. Markina, E. V. Rybakova, A. V. Chinnikova, and P. A. Rebinder, Doklady 120 S. Subramanian and J. C. Ahluwalia, J . Phys. Chem., 1968, 72, 2525. Phys. Chem., 1968, 179, 246. N. C. Pace and C. Tanford, Biochemistry, 1968, 198.62 H. A . Skinner by (fraction of unfolded molecules)/(fraction of native molecules), was found to be strongly dependent on the urea concentration, and to be strongly temperature dependent. From the temperature dependence, AH(denaturation) changes from -40 kcal./mole at 15"c, to zero at 35"c, and to 40 kcal./mole at 52"c. This sharp change in A H indicates a large change in heat capacity associated with the unfolding. The behaviour of P-lactoglobulin resembles that of ribonuclease, chymotrypsinogen, and myoglobin and may be charac- teristic for giobular proteins in general.