ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The chemical relaxation of hydrogen bonds in dimeric formic acid (methanoic acid) was studied in the gas phase by means of precise measurement of the speed of sound and the sound absorption as functions of the pressure and the temperature by exciting standing acoustic waves. The first radial acoustic resonance of a cylindrical cavity was excited by a modulated CO2 laser beam. Resonance profiles were measured and recorded by a computer-controlled system. Due to the high information content of this method, a consistent set of thermodynamic and kinetic parameters can be obtained. The equilibrium constants of the dimer/monomer and the cis-/trans-monomer equilibrium, the dissociation rate constant of (HCOOH)2 and the mean relaxation time of the vibrational states of the monomer–dimer mixture were determined by fitting a detailed theoretical model of the resonator to the measured values for the resonance frequency and the resonance broadening for total pressures in the range from 0.3 to 50 mbar and temperatures from 290 to 325 K. We obtained 159±2 J/(mol K) for the entropy and 61.8±0.5 kJ/mol for the enthalpy of dissociation at 300 K. We inferred a value for the enthalpy of isomerization of 10±5 kJ/mol. For the first time the pressure dependence of the dissociation rate constant was determined. It was found that the unimolecular decay is in the second-order regime at these low pressures as expected. The mean collision efficiency for the dissociation process relative to the dimer was obtained for HCOOH, He, and Ar to be 0.5±0.2, 0.05±0.02, and 0.08±0.02, respectively, independent of the temperature. We measured an average (pτ) value of 10±2 ns bar for the relaxation of the vibrational degrees of freedom. The activation energy of the dissociation of dimeric formic acid was determined to be 33±1 kJ/mol.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.466466
