The Entropy of Formic Acid. The Heat Capacity from 15 to 300°K. Heats of Fusion and Vaporization

Abstract

The heat capacity of solid and liquid formic acid has been measured from 15 to 300°K. The melting point is 281.40°K (0°C=273.10°K). The heat of fusion is 3031 cal. mole-1 and the heat of vaporization at 298.10°K is 4754 cal. mole-1. (1 mole=46.0260 grams.) The vapor pressure at 298.10°K is 4.31 cm of mercury. From the third law of thermodynamics and the calorimetric data the entropy has been calculated. To the value obtained from the calorimetric measurements an entropy of R/2 ln 2, due to the random orientation of hydrogen bonds in the solid which has been predicted by Pauling, has been added. This gives 31.51 cal. deg.-1 mole-1 for the entropy of the liquid at 298.10°K and 47.46 for the entropy of 46.0260 grams of gas, P=4.31 cm, in equilibrium with the liquid at 298.10°K. These are the values that should be used in thermodynamic calculations.