Entropy of the Monomeric Forms of Formic Acid and Acetic Acid

Abstract

A recent measurement of the entropy of gaseous formic acid at its equilibrium vapor pressure is combined with vapor density data to give 60.0±0.3 as the entropy of the monomer at 25° and one atmosphere. The corresponding dimer entropy is 83.1±0.3 e.u. Pauling's residual entropy of R ln 2 per dimer weight for random orientation of hydrogen bonds in the crystal has been included in the above figures in order to allow a reasonable entropy for the torsional motion of the hydroxyl group in the monomer. The same correction has been applied to a previously published result for the acetic acid monomer to yield 70.1±1.0 as a revised value. It is highly probable that only a single significant potential minimum of undetermined breadth and depth occurs in the rotational cycle of the hydroxyl group.