Nitric acid thermodynamic properties

Table 2. Thermodynamic Properties of Nitric Acid and Its Hydrates...

The thermodynamic properties of nitric acid and its hydrates are given in Table 2 (Refs 32 34)... 

It is convenient to use phase diagrams [46] to represent the thermodynamic properties that determine the stability and equilibrium composition of water-containing aerosols. The properties of interest are the temperature, the vapour pressure and composition of the various components in the condensed phases. This is particularly important with respect to the composition and stability of the various hydrates formed at low temperature in the nitric acid-water [47] and sulfuric acid-water binary systems [48], and the ternary systems HjSO/HNOj/HjO and HjSO/HCl/HjO [49],... 

All thermodynamic data for air, the reaction gas mixture, and feed ammonia are taken from Ref.TDI (Section 3). Heat capacities for the various gas mixtures are calculated from the correlations in Ref.TD2. (Table E.1, p.538). Steam tables in Ref.TD3 are also used. Nitric acid properties are taken from Ref.TD4 (p.D-126 and D-77). Reaction equilibrium data are obtained from Refs. PT1 and PT2. 

Under -> open-circuit conditions a possible passivation depends seriously on the environment, i.e., the pH of the solution and the potential of the redox system which is present within the electrolyte and its kinetics. For electrochemical studies redox systems are replaced by a -> potentiostat. Thus one may study the passivating properties of the metal independently of the thermodynamic or kinetic properties of the redox system. However, if a metal is passivated in a solution at open-circuit conditions the cathodic current density of the redox system has to exceed the maximum anodic dissolution current density of the metal to shift the electrode potential into the passive range (Fig. 1 of the next entry (- passivation potential)). In the case of iron, concentrated nitric acid will passivate the metal surface whereas diluted nitric acid does not passivate. However, diluted nitric acid may sustain passivity if the metal has been passivated before by other means. Thus redox systems may induce or only maintain passivity depending on their electrode potential and the kinetics of their reduction. In consequence, it depends on the characteristics of metal disso-... 

The bare proton has an exceedingly small diameter compared with other cations, and hence has a high polarising ability, and readily forms a bond with an atom possessing a lone pair of electrons. In aqueous solution the proton exists as the H30+ ion. The existence of the H30+ ion in the gas phase has been shown by mass spectrometry [4], and its existence in crystalline nitric acid has been shown by NMR [5], Its existence in aqueous acid solution may be inferred from a comparison of the thermodynamic properties of HC1 and LiCl [6]. The heat of hydration of HC1 is 136 kcal mole"1 greater than that of LiCl, showing that a strong chemical bond is formed between the proton and the solvent, whereas the molar heat capacity, molar volume and activity coefficients are similar,... 

The sequence of reactions involved in the overall reduction of nitric acid is complex, but direct measurements confirm that the acid has a high oxidation/reduction potential, -940 mV (SHE), a high exchange current density, and a high limiting diffusion current density (Ref 38). The cathodic polarization curves for dilute and concentrated nitric acid in Fig. 5.42 show these thermodynamic and kinetic properties. Their position relative to the anodic curves indicate that all four metals should be passivated by concentrated nitric acid, and this is observed. In fact, iron appears almost inert in concentrated nitric acid with a corrosion rate of about 25 pm/year (1 mpy) (Ref 8). Slight dilution causes a violent iron reaction with corrosion rates >25 x 1()6 pm/year (106 mpy). Nickel also corrodes rapidly in the dilute acid. In contrast, both chromium and titanium are easily passivated in dilute nitric acid and corrode with low corrosion rates. 

Lustrous white, hard, ferromagnetic metal face-centered Cubic crystals, mp 1555". bp (calc) 2837" (3110 K) D. R. Stull. G. C. Sinke, Thermodynamic Properties of the Elements, Advances in Chemistry Series 18 (A.C.S.. Washington, 1956). d 8-90. Heat capacity (25 ) 6.23 cal/g -atom/°C. Mohs hardness 3-8. Latent heat of fusion 73 cal/g. Electrical resistivity (20 ) 6.844 uohms-cm. E taq) Ni/Ni2 0.250 V. Stable in air at ordinary temp burns in oxygen, forming NiO not affected by water decomposes steam at a red heat. Slowly attacked by dil hydrochloric or sulfuric acid readily attacked by nitric acid. Not attacked by fused alkali hydroxides. 

Several chemical compounds (water, ammonia, nitric acid, organics, etc.) can exist in both the gas and aerosol phases in the atmosphere. Understanding the partitioning of these species between the vapor and particulate phases requires an analysis of the thermodynamic properties of aerosols. Since the most important solvent for constituents of atmospheric particles and drops is water, we will pay particular attention to the thermodynamic properties of aqueous solutions. 

As soon as I remmed to Mainz, I contacted Dr. Frank Arnold of the Max Planck Institute for Nuclear Physics in Heidelberg to explain my idea about NO removal from the gas phase to him. After about a week he had shown that under stratospheric conditions, solid nitric acid trihydrate (NAT) particles could be formed at temperatures below about 200 K, that is, at temperature about 10 K higher than that needed for water ice particle formation. The paper about our findings was published in Nature at the end of 1986 [69]. Independently, the idea had also been developed by Brian Toon, Rich Turco, and co-workers [70]. Subsequent laboratory investigations, notably by David Hanson and Konrad Mauersberger [71], then of the University of Minnesota, provided accurate information on the thermodynamic properties of NAT. Next it was also shown that the NAT particles could provide efficient surfaces to catalyze the production of ClOx by reactions (23) and (24) [72, 73]. Finally, Molina and Molina [74] proposed a powerful catalytic reaction cycle involving ClO-dimer formation [Eqs. (21), (37), and (38)], which... 

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