Equilibrium oxygen pressure compounds

Figure 7.5 Variation of equilibrium oxygen partial pressure (a) equilibrium between a metal, Ag, and its oxide, Ag20, generates a fixed partial pressure of oxygen irrespective of the amount of each compound present at a constant temperature (b) the partial pressure increases with temperature (c) a series of oxides will give a succession of constant partial pressures at a fixed temperature and (d) the Mn-O system. [Data from T. B. Reed, Free Energy of Formation of Binary Compounds An Atlas of Charts for High-Temperature Chemical Calculations, M.I.T. Press, Cambridge, MA, 1971.]...

Another peculiarity of the Earth s atmosphere is the relatively high nitrogen pressure. Considering the physicochemical conditions on our planet, this fact is contradictory to chemical equilibrium considerations. Thus under our conditions (temperature, oxygen pressure, pH in ocean waters etc.) the stable state of nitrogen would be in nitrate compounds dissolved in ocean waters (Sillen, 1966). 

With rising temperature, the free enthalpy approaches zero. This means that at a given temperature, equilibrium exists between metal, metal oxide, and oxygen (100 kPa pressure). All phases are stable next to one another under the prevailing conditions, the decomposition pressure of the metal oxide is equal to the oxygen pressure of the surrounding atmosphere. At lower oxygen pressure, the compound decomposes into its elements. 

If the pressure P and temperature T are fixed, then the total concentrations of point defects at thermodynamic equilibrium in a compound with n components can only be dependent upon the n — 1) independent chemical potentials fXi of the components (i.e. upon the (n — 1) component activities <2.). For example, the concentration of electronic defects in silicon that has been doped with aluminum (or phosphorus) is uniquely determined by the activity of the dopant element, since this is a binary system. In a completely analogous way, the concentrations of electron holes and cation vacancies in NiO are uniquely dependent upon the oxygen partial pressure as long as overall equilibrium can be assumed. 

These reduced compounds are found in living organisms, natural gas, oil and coal. Now clearly this situation is not one of equilibrium and forces us to look again at our general assumption that as oxygen partial pressure changed the elements in the... 

Eh-pH diagrams are sometimes used to predict or describe the major dissolved species and precipitates that should exist at equilibrium in aqueous solutions, including groundwaters, surface waters, laboratory solutions, and porewaters from soils, sediments, or rocks. However, as previously described, many natural aqueous systems are not at equilibrium and they often contain metastable species that are not predicted by Eh-pH diagrams. Metastable species refer to compounds, other substances, or ions that are present under redox, pH, pressure, temperature, or other conditions where chemical equilibrium indicates that they should be unstable and absent. Many metastable species (such as As(III) in oxygenated seawater) result from biological activity. 

In our environment, there are many substances that, like oxygen in our atmosphere, cannot further diffuse and/or react toward more stable configurations and may be considered to be in equilibrium with the environment. Neither chemical nor nuclear reactions can transform these components into even more stable compounds. From these components, we cannot extract any useful work, and therefore an exergy value of OkJ/mol has been assigned to them. This has been done for the usual constituents of air N2,02/ C02/ H20, DzO, Ar, He, Ne, Kr, and Xe at T0 = 298.15 K and P0 = 99.31 kPa, the average atmospheric pressure [1]. Their partial pressures P in air are given in Table 7.1. 

Kinetic results for the reduction of ethyl acetate and acetic acid on similarly prepared 5 wt% Cu/SiC>2 catalysts are shown in Figs. 12 and 13 (75). The experiments were performed at 570 K at various partial pressures of hydrogen and oxygenated compounds. Measurements were also made at temperatures from 500 to 580 K (Figs. 12 and 13). It was observed that ethyl acetate reacts to form only acetaldehyde and ethanol, in equilibrium with each other (75). Figure 12 shows the effects of reaction conditions on the formation of ethanol when ethyl acetate is converted on Cu/SiC>2. 

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