J. Chem. l%erdynamics
Note The enthalpy
M. E. ANTHoNEy
A. S. CARSON, P. G. [email protected]
, and M. mRFXL1
Departmentof Physical Chemistry, The University, LeedsLS2 9JT, U.K. (Received4 March 1976)
We report the enthalpy of formation of gaseous dimethyl oxalate based on the enthalpy of combustion of the solid and the enthalpy of sublimation derived from vapourpressure measurements. Dimethyl oxalate (from B.D.H. Ltd.) was purified by refluxing with methanol for 4 h. The solid which separated on cooling the mixture was recrystalhxed twice from methanol and sublimed in uacllo at 308 to 313 K. The melting temperature was 327.8 to 328.1 K and melting curves obtained with’s differential scanning calorimeter (Perkin Elmer, model DSC lb) indicated the purity to be 299.9 moles per cent. Dimethyl oxalate was burnt as pellets in melinex bags which prevented hydrolysis prior to combustion. The bags were filled and sealed inside a glove box through which dry nitrogen was passed. The combustion calorimeter has been described previously.(‘*‘) 1 cm3 of water was placed inside the combustion bomb which was filled with oxygen to 30 atm pressure at 292 K.t The sample was burnt in a platinum crucible. Ignition was at 298.15 K by means of a cotton thread and a platinum wire which was fused by an electric current. The vapour pressure of dimethyl oxalate was measured in the temperature range 289.21 to 306.09 K using a Bourdon tube gauge (Texas Instruments Ltd., model 144-01). Further details of the apparatus and the results of calibration experiments are in reference 2. The results of six combustion experiments are shown in table 1. The meaning of the symbols is given in reference 3 with the exception of AT and q,,,; AT is the temperature rise of the calorimeter corrected for the heat, exchanged with the surroundings and q,,,is the energy correction for the combustion of melinex. The method of applying the corrections and the subsidiary data were taken from reference 4. The correction for melinex was based on the value Au: = -22.837 kJ g-’ which was measured previously(5) for the same material as used in the present work. Using the values(@ AI$(COs, g) = -(94.Oql f0.031) kca&, mol-’ and AH”, kcal,,, mol-r we obtain for the enthalpy of formation (WA 1) = -(68.315+0.010) of solid dimethyl oxalate: AIf,“(C,H,O,, s, 298.15 K) = -(180.%6&0.20) kc&, mol-‘. [ t Throughoutthispaperc& = 4.184J; atm = 101.32UPa;Ton = (101.325/760) kPa.
TABLE 1. Combustion of dhnethyloxalate CIHe04 A4 = 118.089g mol-l p = 1.15g crnm3 E= (13.8341f- 0.0036)kJ K-’ @a&,,= 4.184J)
9&J 1.9179 0.0591
2.3839 1.9734 0.0531
1.6975 0.0476 2.1141 1.4190 0.0690
0.0363 o&97 0.0208
0.0065 0.0491 0.0373 0.0041 0.0500 0.0379 0.0053 0.0046 0.0024 0.0020
0.0492 0.0457 0.0484 0.0423
0.0373 0.0348 0.0350 0.0333
-A&]kJ g-’ 14.2027
14.1946 14.1899 14.1948
14.1995 14.1979 (L Mean: 14.1966
-AU: = (1676.46rt 0.63)kJ mol-‘; --AH: = (1675.22f 0.63)kJ mol-1 = (400.39f 0.15) kc& mol-l wheretheuncertaintyistwicethestandarddeviationof the meanandincludestheerror in the calibrationexperiments. DFor thisexperimente = (13.8542f 0.0032)kJ K-l.
There is excellent agreement between this result and the value -(180.86&0.07) kcaJr, mol-’ obtained recently by reaction calorimetry in which dimethyl oxalate was hydrolysed in both water and alkali. (‘) The present work indirectly confirms the more than 40 years old [email protected]
* ‘) - 341.0 kcal, mol-’ for the enthalpy of formation of oxalic acid dihydrate on which the hydrolysis result depends. The weighted mean of the two results is AHF(C4H604, s, 298.15 K) = -(180.85+0.07) kcal,, mol-‘. The vapour-pressure measurements are shown in table 2. A least-mean-squares treatment gives the relation [email protected]
/Torr) = (19.461_+0.180)- (5701.5+ 53.6)K/T from TABLE 2. Vapourpressure p of dimethyloxalateat temperatureT [Torr = (101.325/76O)kPa]
which AHsub = (11.33 +O. 11) kcal,, mol-I. Combining this value with the enthalpy of formation of the solid we obtain the standard enthalpy of formation of gaseous dimethyl oxalate: [email protected]
, g, 298.15 K) = -(169.52&0.13) kcal, mol-I. M.Y. wishes to thank The British Council for a Research Scholarship. REFERENCES 1. Carson,A. S.; Wilmshurst,B. R. J. Chem. Thermodynamics 1971,3, 251. 2. Anthoney,M. E.; Carson,A. S.; Laye, P. G. J. Chem. Sot. (Perkin ZZ) in press. 3. Carson,A. S.; Laye,P. G.; Morris, H. J. Chem. l%ermodjmamics 1975,7, 993. 4. Rossini,F. D. (Ed.).ExperimentalThermochemistry. Chaps.3 BE5.Interscience:NewYork 19% 5. Butler, R. S.; Carson,A. S.; Laye, P. G.; Steele,W. V. J. Chem. l%ermodynamics 1971, 3, 277. 6. CODATA key valuesfor thermodynamics. J. Chem. i%ermodynamics l!V2,4, 331. 7. Anthoney,M. E.; Finch, A.; Stephens, M. TAermochimica Acta 1975,12,427. 8. Becker,G.; Roth, W. A. Z. Ekctrochem. 1934,40,836. 9. Nat. Zhr. S&ad. Tech. Note 270-3,1968.