Study of a Reversible Gas Phase Reaction: An Integrated Physical Chemistry Project

The reversible gas phase reaction dinitrogen tetroxide to nitrogen dioxide is very suitable for an integrated physical chemistry project incorporating thermodynamics, statistical mechanics, spectroscopy, and kinetics. Furthermore, nitrogen oxides are molecules of current interest given their important role in atmospheric chemistry. Only relatively common equipment and chemicals are required for the experiment. It consists of the synthesis of NO2 and of the study of the UV–vis electronic absorption spectrum of the NO2/N2O4 gas mixture as a function of temperature. From the analysis of the spectra, the absorption spectra of the pure constituents are obtained as well as the temperature dependence of the equilibrium constant of the reaction. From this dependence, the enthalpy and the entropy of the reaction are obtained. The values are analyzed and compared with statistical mechanical calculations. The kinetics of the reaction is also discussed. The nitrogen dioxide molecule possesses an unpaired electron, essentially located in the nitrogen atom, and has for this reason some propensity to dimerize forming the colorless dinitrogen tetroxide, Introduction The nitrogen oxides are a family of compounds of industrial, biological, and environmental importance. Furthermore, some of their physicochemical properties are quite interesting. In Table 1, some properties of selected nitrogen oxides that are gases at room temperature are displayed [1]. (3) 2 2 2 NO N O → 4 Nitrogen dioxide, first characterized by Dulong [2], stands out as the only common nitrogen oxide that has a color. The fumes of concentrated nitric acid, for instance, are yellowbrown owing to NO2. Part of the yellow-brown tint of polluted air in urban areas is also due to NO2. The dimerization process occurs by an elementary reaction with negligible activation energy, as is typical of the recombination of simple radicals. The N–N dimer bond energy is, however, small (only 57 kJ mol), and the dimer is unstable at room temperature. The reverse reaction, a unimolecular dissociation, is for this reason also an important process at room temperature Nitrogen dioxide is present in the atmosphere in low but significant concentrations. Most of the tropospheric NO2 originates in combustion reactions where some N2 oxidation takes place. The primary oxidation product is NO, but this compound readily transforms quantitatively into NO2 at atmospheric temperatures [3, 4]. The formation of NO2 occurs mainly by the reactions [3, 4] (4) 2 4 2 N O 2 NO → It is, therefore, the study of the reversible reaction