Approaching Equilibrium in the N2O4-NO2 System: A Common Mistake in Textbooks

Abstract

The paper deals with a widespread mistake in standard textbooks--an experiment with a shift of the equilibrium in gaseous mixtures of nitrogen oxides upon abrupt compression. Any explanation of this experiment should take into account two facts: noticeable heating upon compression and extremely fast chemical reactions in the system (relaxation time is of the order of a microsecond). The paper presents experimental data on kinetics in the system approaching equilibrium and the calculations of the equilibrium mixture just after compression. The paper will be suitable for an in-class lecture activity and will be useful for teachers and students in general chemistry and physical chemistry courses (gas laws, chemical thermodynamics and kinetics).

Photocatalytic Conversion of Nitric Oxide on Titanium Dioxide: Cryotrapping of Reaction Products for Online Monitoring by Mass Spectrometry

“…67 It is known that NO 2 dimerizes to dinitrogen tetroxide (N 2 O 4 ) at lower temperatures (i.e., <−10 °C), but at temperatures >140 °C, N 2 O 4 converts back to NO 2 gas. 68,69 To verify if NO 2 is detected intact (i.e., not partially lost due to degradation or dimerization) in the mass spectrometer after trapping (at ∼−196 °C, i.e., LN 2 temperature) and desorption (at >150 °C), we performed cryofocusing experiments utilizing pure NO 2 samples to monitor the effects of cryofocusing on NO 2 . Under identical experimental conditions (i.e., He purge period, cryofocusing duration, flash heating duration and temperature, and mass spectrometry parameters) to other photocatalytic experiments reported in the article, the standard NO 2 sample was detected in the mass spectrometer with no detectable NO 2 degradation products other than its fragment ions (e.g., ion at m/z 30 from fragmentation of NO 2 and potential residual NO (see Figure 5 Figure 4c) than in the absence of TiO 2 (Figure 4a).…”

Quenching of I(2P1/2) by NO2, N2O4, and N2O

“…As noted above, the N 2 O 4 dimer is present in equilibrium with NO 2 The partial pressure, P (N 2 O 4 ), depends on the total pressure P o = P (N 2 O 4 )+ P (NO 2 ) and the temperature T according to the expression where P (NO 2 ) is the partial pressure of NO 2 and K eq is the equilibrium constant. The latter is given by K eq = P (NO 2 ) 2 / P (N 2 O 4 ) = 760 exp(21.13−6880/ T )) (in units of Torr, ref ). Note that the fraction of N 2 O 4 in the mixture decreases as the total pressure is reduced and/or the temperature is increased.…”

Análisis de problemas de selectividad de equilibrio químico : errores y dificultades correspondientes a libros de texto, alumnos y profesores

“…Atkins (1992) y Ouahes y Dévallez (1998) muestran una foto en la que disminuye la intensidad del color de la mezcla por una disminución del volumen, y Feltre (1993) y Oró y otros (1997) realizan una predicción idéntica por un aumento de presión; Pérez y otros (2003) proponen este problema, pero no se pronuncian al respecto. En cambio, si se compara el color de la perturbación con la del equilibrio fi nal, sí que se puede decir correctamente que la concentración disminuye (Brown et al, 1997), aunque este caso es más bien un ejercicio didáctico y no una actividad práctica que demuestre el objetivo pretendido (Leenson, 2000). Existen simulaciones con ordenador en las que únicamente se estudian los cambios de concentración producidos (Van Putten, 1983).…”