Nitrogen dioxide, NO2, and dinitrogen tetroxide

The unpaired electron in NO2 appears to be more localized on the N atom than it is in NO and this may explain the ready dimerization. NO2 is also readily ionized either by loss of an electron (9.91 eV) to give the nitryl cation N02 (iso electronic with CO2) or by gain of an electron to give the nitrite ion NO2 (isoeleelronic with O3). These changes are accompanied by a dramatic diminution in bond angle and an increase in N-O distance as the number of valence electrons increases from 16 to 18 (top diagram). 

The structure of N2O4 in the gas phase is planar (D2h) with a remarkably long N -N bond, and these features persist in both the monoclinic crystalline form near the mp and the more stable low-temperaturc cubic form. Data for the monoclinic form are in the lower diagram together with those for the isoelectronie species B2F4 and 

In addition to the normal homolytic dissociation of N2O4 into 2NO2, the molecule sometimes reacts as if by heterolytic fission thus in media of high dielectric constant the compound often reacts as though dissociated according to the equilibrium N2O4, NO + NOj (see 

There is no noticeable tendency for pure N2O4 to dissociate into ions and the electrical conductivity of the liquid is extremely low (1.3 x 10 ohm cm at 0°). The physical properties of N2O4 are summarized in Table 11.11. 

N2O4 is best prepared by thermal decomposition of rigorously dried Pb(N03)2 in a steel reaction vessel, followed by condensation of the effluent gases and fractional distillation  

These changes are accompanied by a dramatic diminution in bond angle and an increase in N-0 distance as the number of valence electrons increases from 16 to 18 (top diagram). 

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The effect of temperature on the gas-phase equilibrium of nitrogen dioxide, NO2, and dinitrogen tetroxide, N2O4, can be seen because of the difference in color of NO2 and N2O4. The intense brown NO2 gas is the pollution that is responsible for the colored haze that you sometimes see on smoggy days. 

The study of physical equilibrium yields useful information, such as the equilibrium vapor pressure (see Section 11.8). However, chemists are particularly interested in chemical equilibrium processes, such as the reversible reaction involving nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) (Figure 14.1). The progress of the reaction... 

In the gas phase, nitrogen dioxide is actually a mixture of nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4). If the density of such a mixture is 2.3 g/L at 74°C and 1.3 atm, calculate the partial pressures of the gases and Kp for the dissociation ofN204. 

Tests and experience have shown that nitrogen dioxide NO2 and dinitrogen tetroxide N2O4, whether liquid or gaseous, have no action on aluminium from low temperatures (— 100 °C) up to relatively high temperatures and pressures (200 °C and 20 bar) [5]. 

Vapor Sttate. Here the situation is much simpler. Ionic species are no longer present, and in the absence of nitric oxide the choice lies merely between nitrogen dioxide and dinitrogen tetroxide. The proportions vary very much with temperature at 21.15° C., the tetroxide is 15.9% dissociated, whereas dissociation to NO2 is complete at 140°. It does not necessarily follow, however, that reaction at low temperatures involves dinitrogen tetroxide, as reaction with olefins clearly shows. 

A mixture of nitrogen dioxide and dinitrogen tetroxide are allowed to come to equilibrium at 30 °C, and their partial pressures are found to be 1.69 atm N2O4 and 0.60 atm NO2. 

Nitrogen dioxide, NO2, a red gas, and its dimer dinitrogen tetroxide, N2O4, a colorless, easily condensable gas, exist in equilibrium with one another ... 

Nitrogen dioxide (NO2) is also an odd-electron molecule and has a V-shaped structure. The brown, paramagnetic NO2 molecule readily dimerizes to form dinitrogen tetroxide,... 

Nitrogen dioxide rapidly forms an equiUbtium mixture with its dimer, dinitrogen tetroxide (AH g = —28.6 kJ/mol of NO2 consumed). The formation of tetroxide is favored by low temperature and high pressure. 

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