Mixtures metastable equilibrium

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. 

Note 3 For a two-component mixture, a necessary and sufficient condition for stable or metastable equilibrium of a homogeneous single phase is... 

The earliest attempt to detect the presence of free radicals in this way was through the catalytic conversion of ortho-para hydrogen mixtures. At equilibrium at room temperature, ordinary hydrogen consists of a mixture of 75 per cent ortho-H2 (nuclear spins parallel) and 25 per cent para-H2 (nuclear spins antiparallel). At low temperatures (<90°K) equilibrium mixtures may be prepared which contain up to 100 per cent pure para-H2. The latter mixtures are metastable below 500 C and are slowly converted to the stable composition above that temperature. The thermal reaction has been well studied " and corresponds to a catalytic conversion by H atoms present at these temperatures. 

A pattern of this sort does not form directly in the primary Fischer-Tropsch reaction. It does, however, develop when a primary Fischer-Tropsch mixture remains in contact with the catalyst, for a day or so at 35(MOO °C (Fig. 3, bottom), or longer times at lower temperatures (Studier et al., 1968, 1972 Galwey, 1972). Under such conditions, a metastable equilibrium is approached, with methane and aromatic hydrocarbons forming at the expense of ethane and heavier alkanes (Dayhoff et al, 1964 Eck et al, 1966). The kinetics and mechanism of such aro-matization on the catalyst surface has been discussed by Galwey (1972). Of the 61 hydrocarbons in the meteorite, 42 (underlined) are also seen in the synthetic sample, though often not in the same amount. It remains to be seen whether the match can be made more quantitative by changes in the reheating conditions. 

The relaxation of certain properties of the system can often be described by simple phenomenological equations called relaxation equations. In chemical kinetics, for example, the constrained state may be a mixture of gases in metastable equilibrium—for example, hydrogen and oxygen. A spark is then introduced and the gas mixture reacts. The concentration of the reactants and products change with time until a new equilibrium state is achieved. The relaxation equations are the familiar phenomenological equations of chemical kinetics and the relaxation times are related to the chemical rate constants. 

The solubility of a substance B is the relative proportion of B (or a substance related chemically to B) in a mixture which is saturated with respect to solid B at a specified temperature and pressure. Saturated implies the existence of equilibrium with respect to the processes of dissolution amd precipitation the equilibrium may be stable or metastable. The solubility of a substance in metastable equilibrium is usually greater than that of the corresponding substance in stable equilibrium. (Strictly speaking, it is the activity of the substance in metastable equilibrium that is greater.) Care must be taken to distinguish true metastability from supersaturation, where equilibrium does not exist. 

Values of /H2 revealed by the relative concentrations of acetic and propanoic acid can be used to test explicitly whether the organic acids can be in metastable equilibrium with CO2, CH4, hydrocarbons, or other organic compounds. At present, little is known about the concentrations of hydrocarbons and other organic compounds in sedimentary basin brines. Little is also known about the mixing of hydrocarbons in petroleum and how to evaluate activities of individual components in such complex mixtures. Nevertheless, thermodynamic calculations allow the construction of a variety of plausibility arguments regarding the compositional extent of metastable... 

Fig. 12 indicates that metastable equilibrium with respect to reactions (3) and (28) does not appear to operate at the mineral-buffered values of log/H2 at 100 °C. It is possible that something else in the natural system might set the /H2 values that the acids are tracing. One likely candidate is the mixture of hydrocarbons in petroleum. The plausibility of this hypothesis can be tested with additional calculations as described below. 

Miscibility of polymer blends has been often defined as the capability of a mixture to form a single phase over certain ranges of temperature, pressure and composition. Whether or not a single phase exists depends on the chemical structure, molar mass distribution and molecular architecture of the components present. For a two-component mixture, a necessary and sufficient condition for stable or metastable equilibrium of a homogeneous, single-phase is... 

Melting point, 193, 203, 528 Meslin s theorem, 229 Metastable states, 181 Mixed liquids, 380 Mixture rule, 263 Mobile equilibrium, 304, 340 Model, thermodynamic, 240 Mol, 20, 135... 

Three different forms of beryllium hydroxide in the solid state have been described (10, 52, 53, 100). The amorphous form of Be(OH)2 is obtained as a gelatinous precipitate when alkali is added to a beryllium-containing solution at ambient temperatures. The gelatinous precipitate is slowly transformed into the metastable a form when the mixture is allowed to stand. The stable fi form is obtained after the mixture has aged for some months or by precipitation at 70°C. The value of log K for the equilibrium between a and (3 forms,... 

Although the formation of a large number of metastable materials that are far from equilibrium cannot be explained thermodynamically, thermodynamics predicts that they will with time transform to the stable phase or phase mixture, often via intermediate phases. More than one hundred years ago, Ostwald pointed out that... 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

Figure 16.5. Supersaturation behavior, (a) Schematic plot of the Gibbs energy of a solid solute and solvent mixture at a fixed temperature. The true equilibrium compositions are given by points b and e, the limits of metastability by the inflection points c and d. For a salt-water system, point d virtually coincides with the 100% salt point e, with water contents of the order of 10-6 mol fraction with common salts, (b) Effects of supersaturation and temperature on the linear growth rate of sucrose crystals [data of Smythe (1967) analyzed by Ohara and Reid, 1973],...

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