The heat capacity of nitrogen tetroxide has been measured from 15°K to the boiling point, 294.25°K. The melting point is 261.90°K. The heats of fusion and vaporization were found to be 3502 and 9110 cal./mole, respectively. The vapor pressures of solid and liquid nitrogen tetroxide were measured with a mercury manometer by means of an arrangement in which carbon dioxide protected the mercury surface from reaction with the nitrogen tetroxide. The data have been represented by the equations: Liquid nitrogen tetroxide 261.90 to 294.9°K log10P(int. cm Hg) = — 1753.000/T + 8.00436 — 11.8078 × 10—4T + 2.0954 × 10—6T2. Solid nitrogen tetroxide 240.3 to 261.90°K, log10P(int. cm Hg) = — 2460.000/T + 9.58149 + 7.6170 × 10—3T — 1.51335 × 10—5T2. By applying the third law of thermodynamics to the calorimetric measurements, the entropy of the gas, which is dissociated to the extent of 16.1 percent into nitrogen dioxide at the boiling point, was found to be 80.62 cal./deg. per mole of gas as N2O4. From a consideration of the available data on the equilibria N2O4=2NO2=2NO+O2 in combination with spectroscopic data for the several substances, and the experimental entropy value given above, a number of quantities of thermodynamic interest have been evaluated. For N2O4(g), S0298.1=72.73 and for NO2(g), S0298.1=57.47 cal./deg. per mole. These values, which are the ones which should be used in ordinary thermodynamic calculations, do not include the nuclear spin entropy, R ln 3=2.183, for each nitrogen atom. The absolute entropies are N2O4(g), S0298.1=77.10; NO2(g), S0298.1=59.65 cal./deg. per mole. For the reactions: 2NO2=N2O4, ΔF0298.1=—1,110 cal., ΔH0298.1=—13,693 cal.; N2+2O2=N2O4, ΔF0298.1=23,440 cal., ΔH0298.1=2,239 cal.; NO+½O2=NO2, ΔF0298.1=—8,375 cal., ΔH0298.1=—13,562 cal.; ½N2+O2=NO2, ΔF0298.1=12,275 cal., ΔH0298.1=7,964 cal. The experimental entropy value obtained in this investigation, together with band spectrum data, has made possible a much better correlation of the various measurements on the above equilibria than has hitherto been possible. The good agreement demonstrates that the entropy value obtained from the experimental measurements and the third law of thermodynamics is the correct one. In the course of the calculations several points concerning the molecular structures of nitrogen dioxide and nitrogen tetroxide were investigated. Accepting the band spectrum evidence, that NO2 is a symmetrical nonlinear molecule, it was possible to show from the equilibrium measurements that the statistical weight of the normal electronic state is 2. One would also infer this from existing magnetic data which are consistent with the value to be expected from a 2Σ state. It is difficult to draw accurate conclusions with respect to the structure of the N2O4 molecule; however, the experimental entropy value seems to be more consistent with the symmetrical form O2N–NO2 which would have considerably less entropy than would be expected in the case of an unsymmetrical N2O4 molecule. The measurements exclude the possibility of free rotation of NO2 groups in symmetrical O2N–NO2. The calculations do not permit an estimate of the potential barrier concerned in the rotational oscillations of these groups. An extensive summary of the N2O4=2NO2=2NO+O2 equilibria data is given.

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