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The standard enthalpy of formation of tetraphenylsilane and the PhM mean bond-dissociation energies of the Group IV elements
M-834 J. Chem. Thermodynamics 1978, 10,445-452
The standard enthalpy of formation of tetraphenylsilane and the Ph-lVl mean bonddissociation energies of the Group IV elements W. V. STEELE
Departmentof Chemistry, University of Stirling, Stirling FK9 4LA, Scotland, U.K. (ReceivedI September1977) The standardenthalpyof formation of tetraphenylsilane hasbeenmeasured by oxygen aneroid rotating-bombcalorimetry using vinyl&w-fluoride polymer to promote the combustionand form a well-definedfinal solutionof fluorosilicicacid in excesshydrofluoric acid. A standardenthalpyof formationAH@4Si, g) = (44.2zH.4) kcalthmol-1 wasdetermined and comparedwith previousvalues,and a reappraisal of the meanbonddissociation energies< D >(Ph-M), whereM = C, Si, Ge, Sn, and Pb, is made.The enthalpiesof combustionand formationof the vinylidene-fluoridepolymersampleused in the combustions have alsobeendetermined.
1. Introduction In recent years there has been considerable interest in the thermochemistry of the organometallic compounds of the Group IV elements. An appreciable number of reliable data exist on those of germanium, tin, and lead.” J) For organo-silicon compounds no such data exist. Most of the early-determined values have been derived from static-bomb calorimetry and have been shown to be inconsistent and unreliable, w the major cause for the inconsistency being incomplete combustion. In 1964 Good et aZ.@) devised the rotating-bomb procedure for organosilicon compounds. The silicon compound, mixed with C(,or,cC-triflurotoluene, was burned in oxygen in the bomb containing water and after combustion and rotation, a homogeneous solution of fluorosilicic acid in excess hydrofluoric acid was produced. The energy of combustion of silicon in oxygen with vinylidene-fluoride polymer as additive to give the same final solution as in the main experiment was also determined. Hence the enthalpy of formation of the silicon compound was calculated with respect to elemental silicon. Since then the only rotating-bomb calorimetry on silicon organometalhcs, using a similar method, is that of Pedley’s group at the University of Sussex.(4*5) In this paper the standard enthalpy of formation of tetraphenylsilane is reported. It has been obtained by the same method as that of Good et d.(j) but using sheet vinylidene-fluoride polymer as the fluorine additive. As a byproduct of this work the enthalpies of combustion and formation of the actual sample of vinylidene fluoride used has been determined. The result obtained has completed the determinations of 0021-9614/78/0501-0445 $01.00/O
0 (1978)AcademicPressInc. (London)Ltd.
446
W. V. STEELE
the standard enthalpies of formation of the tetraphenyls of the Group IV elements and hence a reappraisal of the mean bond dissociation energies (D)(Ph-M) within the group has been made. The fall in the mean-dissociation energy is remarkably smooth throughout the group.
2. Experimental MATERIALS The sample of tetraphenylsilane was obtained from P.C.R. Inc., U.S.A. It was recrystallized four times from diethyl ether and sublimed under reduced pressure. A curve of melting temperature against time obtained from a d.s.c. indicated a purity of better than 99.95 moles per cent and a fusion temperature of 510.7 K. The vinylidene fluoride polymer was supplied in the form of small pellets from Polysciences Inc., U.S.A. The material used for combustion calorimetry was formed into sheets by the standard melt-pressing method. @) The sample of triphenylmethane used in the combustion calorimetry was obtained from B.D.H. Ltd. and was purified by repeated sublimation followed by zone-refining (50 passes). A curve of melting temperature against time obtained from a d.s.c. indicated a purity of better than 99.98 moles per cent and a fusion temperature of 369.5 K. The benzoic acid was B. D. H. thermochemical standard. CALORIMETRIC METHOD The aneroid rotating-bomb calorimeter and the auxiliary equipment have been described previously. (‘) All samples were burned under 30 atm of oxygen in the presence of 0.010 dm3 of water.? The bomb was flushed with oxygen for 5 min before filling to the stated pressure. The oxygen was purified by passage over heated cupric oxide to remove any combustible material. Because of the success of the method of combustion of other compounds enclosed in polythene, (‘*s) it was decided to burn the sample of tetraphenylsilane enclosed in a bag of polyvinylidene fluoride. This method is a variation on that used by Good et ~1.c~)who used a mixture of silicon powder and the polymer in particle form enclosed in polyester film. During a series of trial combustions it was found that provided the ratio n(F)/n(Si) was at least 15 enough HF was produced in the combustion reaction to convert all the silicon to H2SiF6. It was also found that an outer bag of polythene ensured complete combustion with no solid products. Due to difficulty in analysis of aqueous mixtures of hydrofluoric and fluorosilicic acids the amount of reaction was determined from the mass of tetraphenylsilane used. Neutralization of the acidic mixture by standard sodium hydroxide indicated that both HF and H,SiF, were present in approximately the amounts to be expected from the stoichiometry. The presence of nitric acid was tested for by the u. v. method(7*g’ but none was found in the series of combustions. Mass spectroscopy of a sample of the hydrofluoric acid-free gas from a combustion run failed to show the presence of any gaseous combustion products other than COZ. tThroughout
this paper calth = 4.184 J; atm = 101.324 kPa.
ENTHALPY
OF FORMATION
447
OF TETRAPHENYLSILANE
Comparison experiments were used to minimize errors from inexact reduction to standard states caused by lack of relevant data. (3, lo) The samples consisted of benzoic acid and triphenylmethane pellets enclosed in a polythene bag. The amounts of these materials were so selected that the energy evolved and the CO, produced in the comparison experiment were nearly the same as in the corresponding combustion experiment. The bomb initially contained an aqueous solution of hydrofluoric and fluorosilicic acids which, upon dilution with the water produced by the combustion of the samples, gave a solution of nearly the same amount and concentration as the combustion experiment. The energy of combustion of the sample of vinylidene-fluoride polymer actually used in the teraphenylsilane series was determined in a third series of combustions. The polyvinylidene-fluoride bags were enclosed in polythene and burned under 30.0 atm of oxygen in the presence of 0.010 dm3 of water. In the comparison experiments the pellets of benzoic acid and triphenylmethane were enclosed in polythene and the bomb initially contained aqueous hydrofluoric acid. Analysis showed that polythene and vinylidene-fluoride polymer were C,,,,,H,,,, and &H,F, respectively. The standard specific energies of combustion of the polythene and benzoic acid were 46.350 and 26.435 kJ g-‘. The standard specific energy of combustion of the sample of triphenylmethane was determined in a separate series of combustions under the standard conditions used for C, H compounds. The value c) = -(40.618 + 0.008) kJ g-’ agrees well with that of -(40.618 4 WC19H16, 0.005) kJ g-’ obtained by Coops et al.(“) as corrected by Cox and Pilcher.(“) The experimental results are based on 1976 atomic weights.(r3) For reducing weighings in air to masses, converting the energy of the actual bomb process to that of the isothermal process, and reducing to standard states(14) the values in table 1 were
TABLE
1. Physical properties at 298.15 K; values in parentheses are estimates; r,, fusion temperature from purity determinations by d.s.c. (atm = 101.325 kPa)
Compound ____ polythene vinylidene-fluoride polymer benzoic acid triphenylmethane tetraphenylsilane
M/g mol - 1
p/g cm-3
13.343 64.035
0.900 1.755
122.123 244.335 336.508
1.320 1.014 1.190
(&-/+)l/J atm-* g-l c,/J K-l g-l .--__ -_.__ - 0.0293 1.94 --0.0079 1.38 -0.0117 -(0.0293) -(0.0293)
1.209 1.21 1.18
TdK 369.5 510.7
used for density p, specific heat capacity cp, and (&/i3p),. All values of density were measured in the laboratory. Values for the specific heat capacities were determined on the d.s.c. using sapphire as a standard. (15) The standard state for aqueous hydrofluoric acid was HF. IOH, and the enthalpies of dilution of aqueous hydrofluoric acid solutions were taken from reference 16.
448
W. V. STEELE
3.
Results
Results of a typical combustion and the corresponding comparison experiments for vinylidene fluoride and tetraphenylsilane are given in tables 2 and 3 resp&ve)y. It is impractical to list summaries for all experiments, but values of AE;/M, the specific energy of the idealized combustion reaction, for all experiments are given in table 4. The vinylidene-fluoride polymer combustion reaction is represented by the equation: (l/nXCH,CF,),,(s) + 20,(g) -I- 20H,O(l) = 2CO,(g) + 2 (HF.lOH,O(l)). The combustion reaction for tetraphenylsilane is represented by the equation : C,,H,,Si(c)
(1)
+300,(g)
+ 17.015HFe 1394.2H20 = 24CO,(g) $H~SiF,~11.015HF~1406.2H~0. (2) Derived values of the standard molar energy of combustion AE,“, the standard molar enthalpy of combustion AH:, and the standard molar enthalpy of formation AH: of polyvinylidene fluoride and teraphenylsilane are given in table 5. The values of AH; refer to the equations: w@-W + H,(g) +Wg) = WWH2CWnW, 24C(c, graphite) + 10Hz(g) +Si(c) = C,,H,,Si(c).
2W,
The uncertainties in table 5 are twice the uncertainties in all the materials kcal, mol -I; AZY’;(H,O, 1) = -68.315 AH,“(HF. f 0.2) kcal,, mol -1*(18) , SiO,(c, quartz)+17.015HF*1394.2H20
(3) (4) the standard deviation of the mean and include used. The values(“) AHi(C02, g) = -94.051 kcal,,mol-‘; AHi(Si02, c, quartz) = -(217.7 10HzO, 1) = -76.968 kcal,, mol-‘,(‘@ and = HsSiF6~11.015HF~1396.2HzO; AH,“=- 32.39 kcal,, mol- ‘,
(5)
TABLE 2. Vinyhdene-fluoride polymer: summary of typical combustion and comparison experiments. The symbols and abbreviations am those of references 3 and 7 Combustionexperiment Comparison experiment m(vinylidene fluoride polymer)/g m(wWend/g m(benxoic acid)/g m(triphenylmethane)/g n*(&O)mol
ni(HFJ/mol
AR/P -ARs(calor)/kJ -AR&ont)/kJ
hE6gn)lkJ
AfmJ -mAe~(polythene)/kJ -mA&(benzoic acid)/kJ -mAe~(triphenyhnethane)/kJ mAe~(vinylidenefluoride polymer)/kJ Ae@inylidene-fluoride polymer)/kJg-l
0.967838 0.084412 0.5534
-
1.06850 -14.4515 -0.1496 0.0014 0.0341 0.2045 -14.3611 - 14.8383
0.004575 0.441975 0.062938 0.5405 0.0302 1.06053 - 14.3437u -0.1428 0.0016 0.0328 0.2120s 11.68368 2.5564 -
“Valueusedto determinee(calor) for the corresponding combustionexperiment.
ENTHALPY
OF FORMATION
449
OF TETRAPHENYLSILANE
TABLE 3. Tetraphenylsilane (M = 366.508 g mol-l): summary of typical combustion and comparison experiments. The symbols and abbreviations are those of references 3 and 7
m(tetraphenylsilane)/g m(vinylidene-fluoride polymer)/g m(polythene)/g m(benzoic aicd)/g m(triphenyhnethane)/g n*(HaO)/mol ni(HF)/mol rtl(HzSiFp)/mol AR/D -ARe(calor)/kJ -AR.s(cont)/kJ AE(ign)/kJ AE,/kJ -mAe~(polythene)/kJ -ntAe&enzoic acid)/kJ -mAe&iphenylmethane)/kJ -mAe~(vinylidene-fluoride polymer)/kJ mAi(tetrapheylsilane)/kJ Ai(tetraphenylsilane)/kJ g- 1
“Value used to determine e(calor) for the corresponding
TABLE
Comparison experiment
Combustion experiment -__--.-~ 0.128926 0.269008 0.079908 --0.5534 -0.94402 -12.7660 -0.1287 0.0015 0.0183 3.7037 3.9923 -5.1789 -40.1696
0.079752 0.037534 0.198625 0.5460 0.00610 0.08038 0.93562 - 12.6524 a -0.1264 0.0020 0.0203 3.6965 0.9922 8.0678 -
combustion experiment.
4. Summary of experimental results: values of Aer at 298.15 K Ae,O/kJg - 1 vinylidene-fluoride polymer tetraphenylsilane reaction (1) reaction (2)
mean : standard deviation :
TABLE Compound vinylidene-fluoride tetraphenylsilane
- 14.8383 - 14.8398 -14.8416 - 14.8430 - 14.8424 - 14.8410 0.0009
5. Derived values for the condensed state at 298.15 K (calti, = 4.184 J) -AEL/kcal,,
polymer
“Refers to reaction (1). bRefers to reaction (2).
-40.1696 -40.1723 -40.1769 -40.1726 -40.1773 -40.1737 0.0015
mol - 1
227.14 f 0.11 a 3231.1 i 1.3b
-AH,‘/kcal,,
mol - 1
227.14 rt 0.11 = 3234.7 i 1.3 b
AH@&,
mol - 1
-114.9 f 0.2 44.2 i 1.4
450
W. V. STEELE
were used to derive the standard enthalpies of formation. The value AH: used in reaction (5) was obtained by extrapolation of the data in reference 19 and we assume an uncertainty of +0.20 kcal,, mol-’ in the derived quantity.
4. Discussion The sample-enclosure method used here has given complete combustion of the organosilicon compound under study and is more convenient than that used by Good et ~1.‘~’ Attempts to use the Good method resulted in incomplete combustions probably due to the incomplete mixing of the polymer and silicon compounds before combustion. However the high ratio n(F)/n(Si) of 15 means that more auxiliary compound is required in the combustions and hence the uncertainty in the overall result increases. Actually the precision for combustion of the total sample (tetraphenylsilane, vinylidene-fluoride polymer, and polythene) was normal; the higher overall uncertainty was the penalty for using a combustion reaction in which only 40 per cent of the evolved energy is produced by the substance of interest. The value obtained for the energy of combustion of the sample of vinylidenefluoride polymer actually used in the series of combustions, -(14.8410 f 0.0018) kJ g -I, can be compared with that of -(14.7670 + 0.0016) kJ g-r obtained by Good et c.zZ.(~’ The difference between the two values is well outside the uncertainty intervals but it can possibly be explained by the two samples being of different degrees of crystallinity. The sample of polyvinylidene flouride was opaque when obtained from the supplier but became transparent after the melt-pressing process used in the manufacture of the bags. A similar difference, - (0.063 f 0.008) kJ g- ‘, has been found by Splitstone and Johnson c2’) between samples of polythene with 96 and 72 per cent crystallinity respectively. The results give a warning to anyone using this method of sample enclosure to determine the energy of combustion of the actual sample of polymer in use and not to use a literature value in their calculations. In the literature there are three values for the standard enthalpy of formation of tetraphenylsilane which are listed in table 6. The first two measurements(21’22) were carried out in static-bomb calorimeters and incomplete combustion resulted. The third determination(23) involved combustion of the sample of tetraphenysilane in the presence of potassium nitrate as an oxidant in a rotating tantalum bomb calorimeter; the solid products with the exception of the finely dispersed hydrated amorphous silicon dioxide were dissolved in aqueous nitric acid. The decreasing standard enthalpies show an increase in the completeness of combustion within the series but the value obtained here, (44.2 ) 1.4) kcal,,, mol-‘, is lower again. The method used is complicated and little information is available on their analysis by Hajiev et LZ~.(~~) of the final bomb solutions. The presence of K2Si03, for example, in solution could affect their overall answer markedly and would not be easy to find in the presence of large amounts of other ions. Also, a method which gives a homogeneous final solution is preferable to one with a finely divided precipitate. The careful determination of the standard enthalpies of formation and sublimation of the other Group IV tetraphenyls in recent years means that a reappraisal of the mean bond-dissociation energies, (D)(Ph-M), within the group can be made. Table 7 lists the gas-phase standard enthalpies of formation of both the teraphenyls and the
ENTHALPY
OF FORMATION
OF TETRAPHENYLSILANE
451
Table 6. Previous values of the standard enthalpy of formation of tetraphenylsilane (calth = 4.184 J) Investigators
AH,(Cn4HzoSi, c)/kcal,, mol-’ ---~~- _----~.~ *in (21) 75.6 3: 1.0 Tel’noy and Rabinovich (W 68.0 f 3.0 Hajiev et al. GW 56.2 Ai.- 1.7
TABLE
7. Gas-phase standard enthalpies of formation and mean bond-dissociation the group IV tetraphenyls Ph,M a (calth = 4.184 J)
Compound
AHI” iZ& mol- ’
AH&s) ---___ mol-1
kcal,,
tetraphenylcarbon tetraphenylsilane tetraphenylgermanium tetraphenyltin tetraphenyllead
95.2 zt 1.2 o3*25) 79.8 f 1.4 d.(26) 104.7 & 3.4 (27*28) 136.5 31 1.4 (28*30)
energies for
< D>(Ph-M)/kcal,, mol- 1 * found recommended c
171.29 -c 0.11 Cl’) 107.0 I 2.0 (la’
161.1 i 3.6 G~ZXZ)
_
91.3 If 0.5 (2) 71.87 3~ 0.07 a) 46.50 & 0.20 a)
96.5 rt 1.2
96.5 xt 1.0
84.5 i 2.4 74.1, ztz 3.4 61.3 z!z 1.4 48.9 i 3.6
84.5 zt 1.0 72.5 31 1.0 60.5 i 1.0 48.5 i- 1.0
"Values derived using AH; (Ph *, g) = (77.7 * 2.5) kcal,, mol-’ from reference 24. bUncertainties are twice the standard deviation of the mean neglecting the uncertainty in AH,(Ph *,g) cSee text. dThis research.
/ soc !LLI--L-IA C‘
FIGURE
1. Mean Ph-M
Si
bond-dissociation
Ge
Sn
Pb
energies for the group IV elements.
452
W. V. STEELE
corresponding central atoms. Column 4 of this table gives (D)(Ph-M) and it can be seen (figure 1) that the progression within the group is remarkably linear. A similar linear progression is obtained, using the recommended standard enthalpies from reference 12, for D(Ph-X) where X denotes halogen except fluorine. We recommend the values given in column 5 of table 7 as “best” mean bond-dissociation energies within this series of compounds. They have been obtained from a least-squares analysis of the values in column 4 of table 7 using the reciprocal of the uncertainty as a weight. The uncertainty interval on the “best” values has been set at an arbitrary + 2.0 kcal,, mol-‘. I acknowledge grants from the Science Research Council for the purchase of the a.c. bridge potentiometer used in the resistance measurements and the d.s.c. used in sample determination. We thank Dr I. McEwan for preparation of the vinylidenefluoride polymer sheets. REFERENCES 1. Pilcher, G. In International Review of Science Physical Chemistry Series Two. Volume 10. Thermo. chemistry and Thermodynamics. Skinner, H. A.: editor. Butterworths, London. 1975. Chap. 22. Steele, W. V. Ann. Rep. Prog. Chem. Sect. A 1974, 103. Good, W. D.; Lacina, J. L.; De Prater, B. L.; McCullough, J. P. J. Phys. Chem. 1964, 68, 579. :: Iseard, B. S.; Pedley, J. B.; Treverton, J. A. J. Chem. Sot. A 1971, 3095. 5. Pedley, J. B. Personal communication cited in reference 2. 6. Sorenson, W. R.; Campbell, T. W. Preparative Methods of Polymer Chemistry 2nd edition. Interscience: New York. 1968, Chap. 2. 7. Heath, G. A.; Hefter, G. F.; Steele, W. V. J. Chem. Thermodynamics (M-822). a. Steele, W. V. J. Chem. Thermodynamics 1977, 9, 311. 9. Buck, R. P.; Singhadeja, S.; Rodgers, L. B. Anal. Chem. 1954, 26, 1240. 10. Good, W. D.; Scott, D. W. In Experimental Thermochemistry Vol. 2. Skinner, H. A.: editor. Interscience: New York. 1962. Chap. 4. 11. (a) Coops, J.; Mulder, D.; Dienske, J. W.; Smittenberg, J. Rev. Truv. Chim. 1946, 65, 128. (b) Coops, J.; Van Nes, K.; Kentie, A.; Dienske, J. W. Rev. Truv. Chim. 1947, 66, 113. 12. Cox, J. D., Pilcher, G. In Thermochemistry of Organic and Organometallic Compounds. Academic Press: London. 1970. Chap. 5. 13. I.U.P.A.C. Commission on Atomic Weights 1975. Pure Appl. Chem. 1976, 47, 75. 14. Good, W. D.; Scott, D. W. In Experimental Thermochemistry Vol. 2. Skinner, H. A. : editor. Interscience: New York. 1962. Chap. 2. 15. Barrall, E. M.; Johnson, J. F. Tech. Methods Polym. Evsl. 1970, 2, 1. 16. Johnson, G. K.; Smith, P. N.; Hubbard, W. N. J. Chem. Thermodynamics 1973, 5, 793. 17. CODATA Key values for thermodynamics, J. Chem. Thermodynamics 1971,3,1. CODATA Bulletin 1971, 5, 1. 18. CODATA Key values for thermodynamics. J. Chem. Thermodynamics 1976, 8, 603. CODATA Bulletin 1976, 17, 1. 19. Kilday, M. V.; Prosen, E. J. J. Res. Nut. Bur. Stand. Sect. A 1973, 77A, 205. 20. Splitstone, P. L.; Johnson, W. H. J. Res. Nut. Bur. Stand. Sect. A 1974, 78A, 611. Birr, K. H. Z. anorg. allgem. Chem. 1962, 315, 175. i:: Tel’noy, V. M.; Rabinovich, I. B. Russ. J. Phys. Chem. 1966, 40, 842. 23. Hajiev, S. N.; Nurallaev, H. G.; Martynovskaya, L. N.; Mosin, A. M. IV International Conference on Chemical Thermodynamics, Montpelier, France 1975, l/41. 24. Chamberlain, G. A.; Whittle, E. Trans. Faraaiay Sot. 1971, 67, 2077. Kana’an, A. S. J. Chem. Thermodynamics 1972, 4. 893. 32: Calle, L.; Kana’an, A. S. J. Chem. Thermodynamics 1974, 6, 935. Adams, G. ; Carson, A. S.; Laye, P. G. Trans. Faraday Sot. 1969, 65, 113. iii Kana’an, A. S. J. Chem. Thermodynamics 1974, 6, 191. 29: Adams, G.; Carson, A. S.; Laye, P. G. J. Chem. Thermodynamics 1969, 1, 393. 30. Keiser, D.; Kana’an, A. S. J. Phys. Chem. 1969, 73, 4264. 31. Carson, A. S.; Laye, P. G.; Spencer, J. A. ; Steele, W. V. J. Chem. Thermodynamics 1972,4,783. 32. Kana’an, A. S.; Morrison, T. I. J. Chem. Thermodynamics 1977, 9, 423.
The standard enthalpy of formation of tetraphenylsilane and the Ph-lVl mean bonddissociation energies of the Group IV elements W. V. STEELE
Departmentof Chemistry, University of Stirling, Stirling FK9 4LA, Scotland, U.K. (ReceivedI September1977) The standardenthalpyof formation of tetraphenylsilane hasbeenmeasured by oxygen aneroid rotating-bombcalorimetry using vinyl&w-fluoride polymer to promote the combustionand form a well-definedfinal solutionof fluorosilicicacid in excesshydrofluoric acid. A standardenthalpyof formationAH@4Si, g) = (44.2zH.4) kcalthmol-1 wasdetermined and comparedwith previousvalues,and a reappraisal of the meanbonddissociation energies< D >(Ph-M), whereM = C, Si, Ge, Sn, and Pb, is made.The enthalpiesof combustionand formationof the vinylidene-fluoridepolymersampleused in the combustions have alsobeendetermined.
1. Introduction In recent years there has been considerable interest in the thermochemistry of the organometallic compounds of the Group IV elements. An appreciable number of reliable data exist on those of germanium, tin, and lead.” J) For organo-silicon compounds no such data exist. Most of the early-determined values have been derived from static-bomb calorimetry and have been shown to be inconsistent and unreliable, w the major cause for the inconsistency being incomplete combustion. In 1964 Good et aZ.@) devised the rotating-bomb procedure for organosilicon compounds. The silicon compound, mixed with C(,or,cC-triflurotoluene, was burned in oxygen in the bomb containing water and after combustion and rotation, a homogeneous solution of fluorosilicic acid in excess hydrofluoric acid was produced. The energy of combustion of silicon in oxygen with vinylidene-fluoride polymer as additive to give the same final solution as in the main experiment was also determined. Hence the enthalpy of formation of the silicon compound was calculated with respect to elemental silicon. Since then the only rotating-bomb calorimetry on silicon organometalhcs, using a similar method, is that of Pedley’s group at the University of Sussex.(4*5) In this paper the standard enthalpy of formation of tetraphenylsilane is reported. It has been obtained by the same method as that of Good et d.(j) but using sheet vinylidene-fluoride polymer as the fluorine additive. As a byproduct of this work the enthalpies of combustion and formation of the actual sample of vinylidene fluoride used has been determined. The result obtained has completed the determinations of 0021-9614/78/0501-0445 $01.00/O
0 (1978)AcademicPressInc. (London)Ltd.
446
W. V. STEELE
the standard enthalpies of formation of the tetraphenyls of the Group IV elements and hence a reappraisal of the mean bond dissociation energies (D)(Ph-M) within the group has been made. The fall in the mean-dissociation energy is remarkably smooth throughout the group.
2. Experimental MATERIALS The sample of tetraphenylsilane was obtained from P.C.R. Inc., U.S.A. It was recrystallized four times from diethyl ether and sublimed under reduced pressure. A curve of melting temperature against time obtained from a d.s.c. indicated a purity of better than 99.95 moles per cent and a fusion temperature of 510.7 K. The vinylidene fluoride polymer was supplied in the form of small pellets from Polysciences Inc., U.S.A. The material used for combustion calorimetry was formed into sheets by the standard melt-pressing method. @) The sample of triphenylmethane used in the combustion calorimetry was obtained from B.D.H. Ltd. and was purified by repeated sublimation followed by zone-refining (50 passes). A curve of melting temperature against time obtained from a d.s.c. indicated a purity of better than 99.98 moles per cent and a fusion temperature of 369.5 K. The benzoic acid was B. D. H. thermochemical standard. CALORIMETRIC METHOD The aneroid rotating-bomb calorimeter and the auxiliary equipment have been described previously. (‘) All samples were burned under 30 atm of oxygen in the presence of 0.010 dm3 of water.? The bomb was flushed with oxygen for 5 min before filling to the stated pressure. The oxygen was purified by passage over heated cupric oxide to remove any combustible material. Because of the success of the method of combustion of other compounds enclosed in polythene, (‘*s) it was decided to burn the sample of tetraphenylsilane enclosed in a bag of polyvinylidene fluoride. This method is a variation on that used by Good et ~1.c~)who used a mixture of silicon powder and the polymer in particle form enclosed in polyester film. During a series of trial combustions it was found that provided the ratio n(F)/n(Si) was at least 15 enough HF was produced in the combustion reaction to convert all the silicon to H2SiF6. It was also found that an outer bag of polythene ensured complete combustion with no solid products. Due to difficulty in analysis of aqueous mixtures of hydrofluoric and fluorosilicic acids the amount of reaction was determined from the mass of tetraphenylsilane used. Neutralization of the acidic mixture by standard sodium hydroxide indicated that both HF and H,SiF, were present in approximately the amounts to be expected from the stoichiometry. The presence of nitric acid was tested for by the u. v. method(7*g’ but none was found in the series of combustions. Mass spectroscopy of a sample of the hydrofluoric acid-free gas from a combustion run failed to show the presence of any gaseous combustion products other than COZ. tThroughout
this paper calth = 4.184 J; atm = 101.324 kPa.
ENTHALPY
OF FORMATION
447
OF TETRAPHENYLSILANE
Comparison experiments were used to minimize errors from inexact reduction to standard states caused by lack of relevant data. (3, lo) The samples consisted of benzoic acid and triphenylmethane pellets enclosed in a polythene bag. The amounts of these materials were so selected that the energy evolved and the CO, produced in the comparison experiment were nearly the same as in the corresponding combustion experiment. The bomb initially contained an aqueous solution of hydrofluoric and fluorosilicic acids which, upon dilution with the water produced by the combustion of the samples, gave a solution of nearly the same amount and concentration as the combustion experiment. The energy of combustion of the sample of vinylidene-fluoride polymer actually used in the teraphenylsilane series was determined in a third series of combustions. The polyvinylidene-fluoride bags were enclosed in polythene and burned under 30.0 atm of oxygen in the presence of 0.010 dm3 of water. In the comparison experiments the pellets of benzoic acid and triphenylmethane were enclosed in polythene and the bomb initially contained aqueous hydrofluoric acid. Analysis showed that polythene and vinylidene-fluoride polymer were C,,,,,H,,,, and &H,F, respectively. The standard specific energies of combustion of the polythene and benzoic acid were 46.350 and 26.435 kJ g-‘. The standard specific energy of combustion of the sample of triphenylmethane was determined in a separate series of combustions under the standard conditions used for C, H compounds. The value c) = -(40.618 + 0.008) kJ g-’ agrees well with that of -(40.618 4 WC19H16, 0.005) kJ g-’ obtained by Coops et al.(“) as corrected by Cox and Pilcher.(“) The experimental results are based on 1976 atomic weights.(r3) For reducing weighings in air to masses, converting the energy of the actual bomb process to that of the isothermal process, and reducing to standard states(14) the values in table 1 were
TABLE
1. Physical properties at 298.15 K; values in parentheses are estimates; r,, fusion temperature from purity determinations by d.s.c. (atm = 101.325 kPa)
Compound ____ polythene vinylidene-fluoride polymer benzoic acid triphenylmethane tetraphenylsilane
M/g mol - 1
p/g cm-3
13.343 64.035
0.900 1.755
122.123 244.335 336.508
1.320 1.014 1.190
(&-/+)l/J atm-* g-l c,/J K-l g-l .--__ -_.__ - 0.0293 1.94 --0.0079 1.38 -0.0117 -(0.0293) -(0.0293)
1.209 1.21 1.18
TdK 369.5 510.7
used for density p, specific heat capacity cp, and (&/i3p),. All values of density were measured in the laboratory. Values for the specific heat capacities were determined on the d.s.c. using sapphire as a standard. (15) The standard state for aqueous hydrofluoric acid was HF. IOH, and the enthalpies of dilution of aqueous hydrofluoric acid solutions were taken from reference 16.
448
W. V. STEELE
3.
Results
Results of a typical combustion and the corresponding comparison experiments for vinylidene fluoride and tetraphenylsilane are given in tables 2 and 3 resp&ve)y. It is impractical to list summaries for all experiments, but values of AE;/M, the specific energy of the idealized combustion reaction, for all experiments are given in table 4. The vinylidene-fluoride polymer combustion reaction is represented by the equation: (l/nXCH,CF,),,(s) + 20,(g) -I- 20H,O(l) = 2CO,(g) + 2 (HF.lOH,O(l)). The combustion reaction for tetraphenylsilane is represented by the equation : C,,H,,Si(c)
(1)
+300,(g)
+ 17.015HFe 1394.2H20 = 24CO,(g) $H~SiF,~11.015HF~1406.2H~0. (2) Derived values of the standard molar energy of combustion AE,“, the standard molar enthalpy of combustion AH:, and the standard molar enthalpy of formation AH: of polyvinylidene fluoride and teraphenylsilane are given in table 5. The values of AH; refer to the equations: w@-W + H,(g) +Wg) = WWH2CWnW, 24C(c, graphite) + 10Hz(g) +Si(c) = C,,H,,Si(c).
2W,
The uncertainties in table 5 are twice the uncertainties in all the materials kcal, mol -I; AZY’;(H,O, 1) = -68.315 AH,“(HF. f 0.2) kcal,, mol -1*(18) , SiO,(c, quartz)+17.015HF*1394.2H20
(3) (4) the standard deviation of the mean and include used. The values(“) AHi(C02, g) = -94.051 kcal,,mol-‘; AHi(Si02, c, quartz) = -(217.7 10HzO, 1) = -76.968 kcal,, mol-‘,(‘@ and = HsSiF6~11.015HF~1396.2HzO; AH,“=- 32.39 kcal,, mol- ‘,
(5)
TABLE 2. Vinyhdene-fluoride polymer: summary of typical combustion and comparison experiments. The symbols and abbreviations am those of references 3 and 7 Combustionexperiment Comparison experiment m(vinylidene fluoride polymer)/g m(wWend/g m(benxoic acid)/g m(triphenylmethane)/g n*(&O)mol
ni(HFJ/mol
AR/P -ARs(calor)/kJ -AR&ont)/kJ
hE6gn)lkJ
AfmJ -mAe~(polythene)/kJ -mA&(benzoic acid)/kJ -mAe~(triphenyhnethane)/kJ mAe~(vinylidenefluoride polymer)/kJ Ae@inylidene-fluoride polymer)/kJg-l
0.967838 0.084412 0.5534
-
1.06850 -14.4515 -0.1496 0.0014 0.0341 0.2045 -14.3611 - 14.8383
0.004575 0.441975 0.062938 0.5405 0.0302 1.06053 - 14.3437u -0.1428 0.0016 0.0328 0.2120s 11.68368 2.5564 -
“Valueusedto determinee(calor) for the corresponding combustionexperiment.
ENTHALPY
OF FORMATION
449
OF TETRAPHENYLSILANE
TABLE 3. Tetraphenylsilane (M = 366.508 g mol-l): summary of typical combustion and comparison experiments. The symbols and abbreviations are those of references 3 and 7
m(tetraphenylsilane)/g m(vinylidene-fluoride polymer)/g m(polythene)/g m(benzoic aicd)/g m(triphenyhnethane)/g n*(HaO)/mol ni(HF)/mol rtl(HzSiFp)/mol AR/D -ARe(calor)/kJ -AR.s(cont)/kJ AE(ign)/kJ AE,/kJ -mAe~(polythene)/kJ -ntAe&enzoic acid)/kJ -mAe&iphenylmethane)/kJ -mAe~(vinylidene-fluoride polymer)/kJ mAi(tetrapheylsilane)/kJ Ai(tetraphenylsilane)/kJ g- 1
“Value used to determine e(calor) for the corresponding
TABLE
Comparison experiment
Combustion experiment -__--.-~ 0.128926 0.269008 0.079908 --0.5534 -0.94402 -12.7660 -0.1287 0.0015 0.0183 3.7037 3.9923 -5.1789 -40.1696
0.079752 0.037534 0.198625 0.5460 0.00610 0.08038 0.93562 - 12.6524 a -0.1264 0.0020 0.0203 3.6965 0.9922 8.0678 -
combustion experiment.
4. Summary of experimental results: values of Aer at 298.15 K Ae,O/kJg - 1 vinylidene-fluoride polymer tetraphenylsilane reaction (1) reaction (2)
mean : standard deviation :
TABLE Compound vinylidene-fluoride tetraphenylsilane
- 14.8383 - 14.8398 -14.8416 - 14.8430 - 14.8424 - 14.8410 0.0009
5. Derived values for the condensed state at 298.15 K (calti, = 4.184 J) -AEL/kcal,,
polymer
“Refers to reaction (1). bRefers to reaction (2).
-40.1696 -40.1723 -40.1769 -40.1726 -40.1773 -40.1737 0.0015
mol - 1
227.14 f 0.11 a 3231.1 i 1.3b
-AH,‘/kcal,,
mol - 1
227.14 rt 0.11 = 3234.7 i 1.3 b
AH@&,
mol - 1
-114.9 f 0.2 44.2 i 1.4
450
W. V. STEELE
were used to derive the standard enthalpies of formation. The value AH: used in reaction (5) was obtained by extrapolation of the data in reference 19 and we assume an uncertainty of +0.20 kcal,, mol-’ in the derived quantity.
4. Discussion The sample-enclosure method used here has given complete combustion of the organosilicon compound under study and is more convenient than that used by Good et ~1.‘~’ Attempts to use the Good method resulted in incomplete combustions probably due to the incomplete mixing of the polymer and silicon compounds before combustion. However the high ratio n(F)/n(Si) of 15 means that more auxiliary compound is required in the combustions and hence the uncertainty in the overall result increases. Actually the precision for combustion of the total sample (tetraphenylsilane, vinylidene-fluoride polymer, and polythene) was normal; the higher overall uncertainty was the penalty for using a combustion reaction in which only 40 per cent of the evolved energy is produced by the substance of interest. The value obtained for the energy of combustion of the sample of vinylidenefluoride polymer actually used in the series of combustions, -(14.8410 f 0.0018) kJ g -I, can be compared with that of -(14.7670 + 0.0016) kJ g-r obtained by Good et c.zZ.(~’ The difference between the two values is well outside the uncertainty intervals but it can possibly be explained by the two samples being of different degrees of crystallinity. The sample of polyvinylidene flouride was opaque when obtained from the supplier but became transparent after the melt-pressing process used in the manufacture of the bags. A similar difference, - (0.063 f 0.008) kJ g- ‘, has been found by Splitstone and Johnson c2’) between samples of polythene with 96 and 72 per cent crystallinity respectively. The results give a warning to anyone using this method of sample enclosure to determine the energy of combustion of the actual sample of polymer in use and not to use a literature value in their calculations. In the literature there are three values for the standard enthalpy of formation of tetraphenylsilane which are listed in table 6. The first two measurements(21’22) were carried out in static-bomb calorimeters and incomplete combustion resulted. The third determination(23) involved combustion of the sample of tetraphenysilane in the presence of potassium nitrate as an oxidant in a rotating tantalum bomb calorimeter; the solid products with the exception of the finely dispersed hydrated amorphous silicon dioxide were dissolved in aqueous nitric acid. The decreasing standard enthalpies show an increase in the completeness of combustion within the series but the value obtained here, (44.2 ) 1.4) kcal,,, mol-‘, is lower again. The method used is complicated and little information is available on their analysis by Hajiev et LZ~.(~~) of the final bomb solutions. The presence of K2Si03, for example, in solution could affect their overall answer markedly and would not be easy to find in the presence of large amounts of other ions. Also, a method which gives a homogeneous final solution is preferable to one with a finely divided precipitate. The careful determination of the standard enthalpies of formation and sublimation of the other Group IV tetraphenyls in recent years means that a reappraisal of the mean bond-dissociation energies, (D)(Ph-M), within the group can be made. Table 7 lists the gas-phase standard enthalpies of formation of both the teraphenyls and the
ENTHALPY
OF FORMATION
OF TETRAPHENYLSILANE
451
Table 6. Previous values of the standard enthalpy of formation of tetraphenylsilane (calth = 4.184 J) Investigators
AH,(Cn4HzoSi, c)/kcal,, mol-’ ---~~- _----~.~ *in (21) 75.6 3: 1.0 Tel’noy and Rabinovich (W 68.0 f 3.0 Hajiev et al. GW 56.2 Ai.- 1.7
TABLE
7. Gas-phase standard enthalpies of formation and mean bond-dissociation the group IV tetraphenyls Ph,M a (calth = 4.184 J)
Compound
AHI” iZ& mol- ’
AH&s) ---___ mol-1
kcal,,
tetraphenylcarbon tetraphenylsilane tetraphenylgermanium tetraphenyltin tetraphenyllead
95.2 zt 1.2 o3*25) 79.8 f 1.4 d.(26) 104.7 & 3.4 (27*28) 136.5 31 1.4 (28*30)
energies for
< D>(Ph-M)/kcal,, mol- 1 * found recommended c
171.29 -c 0.11 Cl’) 107.0 I 2.0 (la’
161.1 i 3.6 G~ZXZ)
_
91.3 If 0.5 (2) 71.87 3~ 0.07 a) 46.50 & 0.20 a)
96.5 rt 1.2
96.5 xt 1.0
84.5 i 2.4 74.1, ztz 3.4 61.3 z!z 1.4 48.9 i 3.6
84.5 zt 1.0 72.5 31 1.0 60.5 i 1.0 48.5 i- 1.0
"Values derived using AH; (Ph *, g) = (77.7 * 2.5) kcal,, mol-’ from reference 24. bUncertainties are twice the standard deviation of the mean neglecting the uncertainty in AH,(Ph *,g) cSee text. dThis research.
/ soc !LLI--L-IA C‘
FIGURE
1. Mean Ph-M
Si
bond-dissociation
Ge
Sn
Pb
energies for the group IV elements.
452
W. V. STEELE
corresponding central atoms. Column 4 of this table gives (D)(Ph-M) and it can be seen (figure 1) that the progression within the group is remarkably linear. A similar linear progression is obtained, using the recommended standard enthalpies from reference 12, for D(Ph-X) where X denotes halogen except fluorine. We recommend the values given in column 5 of table 7 as “best” mean bond-dissociation energies within this series of compounds. They have been obtained from a least-squares analysis of the values in column 4 of table 7 using the reciprocal of the uncertainty as a weight. The uncertainty interval on the “best” values has been set at an arbitrary + 2.0 kcal,, mol-‘. I acknowledge grants from the Science Research Council for the purchase of the a.c. bridge potentiometer used in the resistance measurements and the d.s.c. used in sample determination. We thank Dr I. McEwan for preparation of the vinylidenefluoride polymer sheets. REFERENCES 1. Pilcher, G. In International Review of Science Physical Chemistry Series Two. Volume 10. Thermo. chemistry and Thermodynamics. Skinner, H. A.: editor. Butterworths, London. 1975. Chap. 22. Steele, W. V. Ann. Rep. Prog. Chem. Sect. A 1974, 103. Good, W. D.; Lacina, J. L.; De Prater, B. L.; McCullough, J. P. J. Phys. Chem. 1964, 68, 579. :: Iseard, B. S.; Pedley, J. B.; Treverton, J. A. J. Chem. Sot. A 1971, 3095. 5. Pedley, J. B. Personal communication cited in reference 2. 6. Sorenson, W. R.; Campbell, T. W. Preparative Methods of Polymer Chemistry 2nd edition. Interscience: New York. 1968, Chap. 2. 7. Heath, G. A.; Hefter, G. F.; Steele, W. V. J. Chem. Thermodynamics (M-822). a. Steele, W. V. J. Chem. Thermodynamics 1977, 9, 311. 9. Buck, R. P.; Singhadeja, S.; Rodgers, L. B. Anal. Chem. 1954, 26, 1240. 10. Good, W. D.; Scott, D. W. In Experimental Thermochemistry Vol. 2. Skinner, H. A.: editor. Interscience: New York. 1962. Chap. 4. 11. (a) Coops, J.; Mulder, D.; Dienske, J. W.; Smittenberg, J. Rev. Truv. Chim. 1946, 65, 128. (b) Coops, J.; Van Nes, K.; Kentie, A.; Dienske, J. W. Rev. Truv. Chim. 1947, 66, 113. 12. Cox, J. D., Pilcher, G. In Thermochemistry of Organic and Organometallic Compounds. Academic Press: London. 1970. Chap. 5. 13. I.U.P.A.C. Commission on Atomic Weights 1975. Pure Appl. Chem. 1976, 47, 75. 14. Good, W. D.; Scott, D. W. In Experimental Thermochemistry Vol. 2. Skinner, H. A. : editor. Interscience: New York. 1962. Chap. 2. 15. Barrall, E. M.; Johnson, J. F. Tech. Methods Polym. Evsl. 1970, 2, 1. 16. Johnson, G. K.; Smith, P. N.; Hubbard, W. N. J. Chem. Thermodynamics 1973, 5, 793. 17. CODATA Key values for thermodynamics, J. Chem. Thermodynamics 1971,3,1. CODATA Bulletin 1971, 5, 1. 18. CODATA Key values for thermodynamics. J. Chem. Thermodynamics 1976, 8, 603. CODATA Bulletin 1976, 17, 1. 19. Kilday, M. V.; Prosen, E. J. J. Res. Nut. Bur. Stand. Sect. A 1973, 77A, 205. 20. Splitstone, P. L.; Johnson, W. H. J. Res. Nut. Bur. Stand. Sect. A 1974, 78A, 611. Birr, K. H. Z. anorg. allgem. Chem. 1962, 315, 175. i:: Tel’noy, V. M.; Rabinovich, I. B. Russ. J. Phys. Chem. 1966, 40, 842. 23. Hajiev, S. N.; Nurallaev, H. G.; Martynovskaya, L. N.; Mosin, A. M. IV International Conference on Chemical Thermodynamics, Montpelier, France 1975, l/41. 24. Chamberlain, G. A.; Whittle, E. Trans. Faraaiay Sot. 1971, 67, 2077. Kana’an, A. S. J. Chem. Thermodynamics 1972, 4. 893. 32: Calle, L.; Kana’an, A. S. J. Chem. Thermodynamics 1974, 6, 935. Adams, G. ; Carson, A. S.; Laye, P. G. Trans. Faraday Sot. 1969, 65, 113. iii Kana’an, A. S. J. Chem. Thermodynamics 1974, 6, 191. 29: Adams, G.; Carson, A. S.; Laye, P. G. J. Chem. Thermodynamics 1969, 1, 393. 30. Keiser, D.; Kana’an, A. S. J. Phys. Chem. 1969, 73, 4264. 31. Carson, A. S.; Laye, P. G.; Spencer, J. A. ; Steele, W. V. J. Chem. Thermodynamics 1972,4,783. 32. Kana’an, A. S.; Morrison, T. I. J. Chem. Thermodynamics 1977, 9, 423.