Nitrogen oxides aqueous equilibria

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

The reaction mechanism has been further investigated using cr-amino acids (96).8,62 In aqueous ethanol these amine derivatives exist as zwitterions in equilibrium with varying amounts of the uncharged molecules,6Ja which are oxidatively deaminated, via unstable imino acids (98), to stable detectable aldehydes (100). When ethanolic solutions of 2-(2-pyridyl)isatogen (90b) and 96a or 96b are refluxed under nitrogen,... 

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

Figure 1 Plot of log /Hz versus pH at 25°C and 1 bar total pressure showing fields of relative predominance of aqueous nitrogen species. Log /pj values represent numerical indices of the oxidation state of the system. Each line has a matching chemical reaction as listed in Table 3. Bold horizontal lines establish the stability limits for water. Dashed lines define the area of dissolved nitrogen in equilibrium with an atmosphere containing 78% N2.

Use values of Af/f ° and AfG° in Appendix H to evaluate the standard molar reaction enthalpy and the thermodynamic equilibrium constant at 298.15 K for the oxidation of nitrogen to form aqueous nitric acid ... 

The nitric oxide which is formed reacts with oxygen to form nitrogen dioxide. Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide. The nitrogen dioxide/dimer mixture is sent to a column, sometimes called an absorption tower. Water is added at the top of the column. The nitrogen dioxide is converted to nitric acid. Byproduct nitric oxide is oxidized to nitrogen dioxide by means of a stream of air passed into the absorption column. The aqueous nitric acid is removed continuously from the base of the column. Overall, the reaction can be written as ... 

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