Dec., 1959
NOTES
T H E HEAT O F FORMATION OF FORMIC ACID BY G. C. SINKE Thermal Laboratory, The Dow Chemical Company, Midland, Michigan ICeceiued April $4, 1969
The heat of formation of formic acid derived by Waring' from equilibrium studies is not in agreement with the heat of combustion (determined prior to 1900). The disagreement is unusual in that the equiIibrium data indicate the heat of combustion to be too high, while most of the older heat of combustion values tend to be too low. A new determination of the heat of combustion is in agreement with the equilibrium yalues. Experimental Commercial formic acid was purified by fractional crystilllieation until freezing curve analysis indicated a purity of better than 99.8 mole %. Analysis of the combustion gases indicated the sample had a carbon content of 99.9 f 0.1% of theoretical. The calorimetric system and technique was the same as that used previously2 except that the sample was enclosed in a small bag of Mylar film. The film had a heat of combustion 1.8 cal. g.+ and analyzed 62.12% of A U R / M = 5461.7 carbon and 4.18% hydrogen. The remainder was assumed to be oxygen. Washburn corrections were calculated by the ~ are given in Table I. The method of P r o ~ e n . Results sample mass is based on the mass of carbon dioxide produced, after correction for the carbon dioxide calculated from the weighed Mylar bag. Results based on sample weight were slightly lower which would be expected if the impurity was water.
TABLE I PTotal
6224.36 6313.54 6168.03 6008.06
PMylar
QHNOa
905.22 0.16 950.12 .OO 907.30 .32 .16 947.82
Sample mass, g.
-AUR/N,
PWaahburn
14.40 14.43 13.81 13.04
3.99148 4.02558 3.05442 3.79585
1329.0 1328.8 1326.8 1329 A
cal. g. -1
Av. 1328.6 Stand. dev. 0.6
ResuIts and Discussion Employing a molecular weight of 46.027 and calculating to constant pressure gives a H o c 2 9 , (1) = -60.86 f 0.06 kcal. mole-I. From heats of formation of liquid water and gaseous carbon dioxide2 the heat of formation is calculated as AHOf 298 (1) = -101.52 f 0.06 kcal. mole-l. The entropy of liquid formic acid a t 298°K. is given by Stout and Fisher4 and of carbon, hydrogen and oxygen by Stull and Sinke,6 from which the free energy of liquid formic acid is -86.38 0.06 kcal. mole-'. Waring derived -86.45 and -86.39 kcal. mole-l from two different dissociation equilibria. The excellent agreement is evidence against the existence of residual entropy (1) W.Waring, Chem. Revs., 81, 171 (1952). (2) G. C. Sinke, D. L. Hildenbrand, R. A . McDonald, W. R. Kramer and D. R. Stull, THIBJOURNAL, 62, 1461 (1958). (3) E . J. Prosen, Chapter 6 in "Experimental Thermochemistry," edited by F. D . Rossini, Interscience Publishers, New York, N. Y., 1956. (4) J. W. Stout and L. H. Fisher, J . Chem. Phys., 9, 163 (1941). (5) D. R. Stull and G. C. Binke, "Thermodynamic Properties of the Elements, No. 18 of the Advances in Cliemistry Series, Edited by the Staff of Industrial and Engineering Chemistry," American Chemical Society, Washington, D. c.
2063
in formic acid and supports recent spectroscopic and X-ray studies.6 The most likely explanation for the high previous combustion data is that the formic acid contained some acetic acid, a common impurity which has a considerably higher heat of combustion. (G) R. C. Millikan and I
