Nitrogen dioxide equilibrium

The brown nitrogen dioxide gas eondenses to a yellow liquid whieh freezes to eolourless erystals of dinitrogen tetroxide. Below 150°Cthe gas eonsists of moleeules of dinitrogen tetroxide and nitrogen dioxide in equilibrium and the proportion of dinitrogen tetroxide inereases as the temperature falls. Above 150°C nitrogen dioxide dissoeiates into nitrie oxide and oxygen. 

By using different membranes, it is possible to obtain potentiometric sensors for gases such as sulfur dioxide or nitrogen dioxide. Such sensors employ similar (acid-base) or other equilibrium processes. These devices, along with their equilibrium processes and internal electrodes, are summarized in Table 6-2. Membrane coverage... 

Nitrogen dioxide, N02 (oxidation number -t-4), is a choking, poisonous, brown gas that contributes to the color and odor of smog. The molecule has an odd number of electrons, and in the gas phase it exists in equilibrium with its colorless dimer N204. Only the dimer exists in the solid, and so the brown gas condenses to a colorless solid. When it dissolves in water, NOz disproportionates into nitric acid (oxidation number +5) and nitrogen oxide (oxidation number +2) ... 

The detailed chemistry describing the synthesis of ammonia Is complex, so we Introduce the principles of equilibrium using the chemistry of nitrogen dioxide. Molecules In a sample of nitrogen dioxide are always colliding with one another. As described in Chapter 15, a collision in the correct orientation can result In bond formation, producing an N2 O4 molecule 2 NO2 N2 O4... 

The tetroxide, which forms an equilibrium with nitrogen dioxide, is a very strong oxidant and an endothermic compound. To analyse all the dangerous reactions of... 

Nitrogen dioxide forms an equilibrium with dinitrogen tetroxide. When it is cold, the equilibrium favours the second compound. Depending on the thermal conditions, the dangerous reactions will involve one or the other of these two compounds. Their endothermic character makes them hardly stable. 

The reaction between arsenious oxide and concentrated nitric acid yields a mixture of nitric oxide and nitrogen dioxide. It also contains some nitrogen tetroxidc and perhaps trioxide, the amount in equilibrium depending upon the temperature of the gas. The compressed air forced in via flask A insures an excess of oxygen, and thus complete oxidation. Only a slow stream is necessary, two to three bubbles per second. 

The organic substrates in Chart 8 can be divided into two main categories in which (i) the oxidation of olefins, sulfides, and selenides involves oxygen atom transfer to yield epoxides, sulfoxides, and selenoxides, respectively, whereas (ii) the oxidation of hydroquinones and quinone dioximes formally involves loss of two electrons and two protons to yield quinones and dinitrosobenzenes, respectively. In order to provide a unifying mechanistic theme for the seemingly disparate transformations in Chart 8, we note that nitrogen dioxide exists in equilibrium with its dimeric forms, namely, the predominant N—N bonded dimer 02N—N02 and the minor N—O bonded isomer ONO—N02 (equation 88). 

Nitrogen dioxide is a stable radical with an unpaired electron and forms the dimer N204 in the equilibrium reaction ... 

This equilibrium reaction occurs with enthalpy A//=-58.2kJ mol-1 and entropy A S = — 177 J mol-1 K [167], Nitrogen dioxide decays at elevated temperatures in the gas phase by the reaction... 

Example Nitrate-sensing electrode is employed to cater for a cell which will be sensitive exclusively to nitrogen dioxide (N02). The equilibrium of such a reaction may be represented as follows ... 

Figure 7.7 shows how the law of chemical equilibrium applies to one chemical system. Chemists have studied this system extensively. It involves the reversible reaction between two gases dinitrogen tetroxide, which is colourless, and nitrogen dioxide, which is dark brown. 

These are not student tests. Your teacher may demonstrate this equilibrium if a suitable fume hood is available for the first test, and if sealed tubes containing a mixture of nitrogen dioxide, N02(g), and dinitrogen tetroxide, N204(g), are available for the second test. If either or both tests are not demonstrated, refer to the photographs that show the changes. 

Equation 2-4, [Oj] = kJNOjJ/JlTj [NO], should be tested in the real atmosphere, as well as in laboratory experiments. Simultaneous measurements of the concentrations of ozone, nitric oxide, and nitrogen dioxide and of the intensity of sunlight for a variety of conditions will provide a much-needed check on this dynamic equilibrium. 

The second example demonstrates another aspect of equilibrium by using the transformations between dinitrogen tetroxide and nitrogen dioxide. 

Nitrogen dioxide readily converts to other forms of nitrogen oxides. It coexists in equilibrium with its dimeric form, N2O4. The latter is more stable at ordinary temperatures. 

The effect (upon the overall rate of conversion of dichromate to chromate) of changing the chromate ion concentration was studied. The rate was inversely proportional to the square of the chromate concentration, as well as proportional to the dichromate concentration. Since oxygen and nitrogen dioxide had no effect on the rate, the nitryl ion, N(>2+ was postulated as intermediate. However, the equilibrium constant for the reaction could not be determined because too little N02+ was formed. 

The whole apparatus is baked as efficiently as possible without evaporating large quantities of the manometric fluid or test substance, and is finally sealed off. The apparatus is returned to the vertical position and wholly immersed in a thermostatic bath. The level of the manometer liquid should be exactly equai under these conditions. The apparatus is then arranged as shown in the sketch with bulb B in solid carbon dioxide or liquid nitrogen. When equilibrium is attained the difference of level in the manometer limbs is observed with a cathetometer. The thermostat temperature is readjusted and the process repeated. ... 

In the presence of nitrogen dioxide, nitric oxide reacts rapidly to establish the equilibrium... 

Ashmore and Burnett8 examined the thermal decomposition of nitrogen dioxide between 200 and 434°C. They found that log k 5 = 8.59 — 23.9/0. Using the values of 16.8 kcal/mole and 60 eu for the heat of formation and entropy of N03 (averaged from the results of Schott and Davidson371 and Ray and Ogg357), they found the equilibrium constant log J 5> 5 = 1.03 + 22.2/0, whence log k5 = 9.62 - 1.7/0. This leads to a value of 8.38 at room temperature. If thermodynamic values listed in JANAF tables are used, K5t 5 is larger at room temperature by a factor of 1.8 log k5 would be about 8.65. This paper is discussed further in Section IV-B-3. 

The very low value of Ashmore and Burnett is difficult to explain. It is easy to demonstrate that the discrepancy is not resolved by assuming the N03 intermediate in nitrogen dioxide decomposition is the pernitrite radical, in contradistinction to the symmetric nitrate radical. Their calculation of k5 depended on an experimentally obtained value for k 5 and an equilibrium constant K5- 5 calculated from thermodynamic properties for N03 measured by Schott and Davidson and Ray and Ogg. These results, obtained in a nitrogen pentoxide system, pertain to the nitrate radical, not the pernitrite radical. Guillory and Johnston176 reported an equilibrium constant based on estimated... 

In addition, there is an independent N02-catalyzed reaction, which only becomes important at elevated temperatures and when nitrogen dioxide is a major constituent. It operates by conversion of asyw-ONOO to jyw-N03 therefore, it interferes with the equilibrium ii.-n- Oxidation would then consist of reaction (11A) followed by reactions (42) and (5S)... 

Nitrogen dioxide molecules react with NO to form N203, in an equilibrium mixture. The equilibrium mixture of NO> and N204 also reacts with water... 

---