Gas phase equilibria

The strong dependence of the PES on molecular orientation also leads to strong coupling between rotational states, and hence rotational excitation/de-excitation in the scattering. This has been observed experimentally for H2 scattering from Cu surfaces. Recent work has shown that for H2 the changes m rotational state occur almost exclusively when the molecular bond is extended, that is, longer than the gas-phase equilibrium value [ ]. 

Cjo CO concentration in gas phase equilibrium with liquid phase, mmol COl-1... 

In porous media, liquid-gas phase equilibrium depends upon the nature of the adsorbate and adsorbent, gas pressure and temperature [24]. Overlapping attractive potentials of the pore walls readily overcome the translational energy of the adsorbate, leading to enhanced adsorption of gas molecules at low pressures. In addition, condensation of gas in very small pores may occur at a lower pressure than that normally required on a plane surface, as expressed by the Kelvin equation, which relates the radius of a curved surface to the equilibrium vapor pressure [25],... 

One of the earliest measurements of the gas-phase equilibrium acidity of propene involved measuring the rates of reaction of propene with hydroxide ion in both directions33. The resulting equilibrium constant gave A//acid = 391 1 kcalmol-1. In the case of ethylene, the acidity and independently measured electron affinity of vinyl radical were used to determine the bond dissociation energy, a quantity difficult to obtain accurately by other means8. 

Care the product of K and p could be written as K p (i.e. K x p), which could be mistaken for a gas-phase equilibrium constant... 

Nitric acid is the only oxyacid of nitrogen which is at all stable in the free condition. The kinetics of its decomposition are discussed below. Nitrous acid exists in the gas phase, although it cannot be isolated. Information on the kinetics of its decomposition is restricted to an estimate103 of the half-life of the forward reaction in the rapid gas-phase equilibrium... 

Besides the second law method, there is another way of extracting reaction enthalpies from gas-phase equilibrium constants. This alternative involves the determination of a single value of an equilibrium constant at a given temperature and the calculation of the reaction entropy at the same temperature. From equations 2.54 and 2.55, we obtain... 

A very fine example was provided by the extensive use of Professor Pitzer s electrolyte activity coefficient theory within several acid gas phase equilibrium models. 

The formulae given in Table 4.1 for the molecular partition functions enable us to write the partition function ratio qheavy/qiight or q2/qi where, by the usual convention, the subscript 2 refers to the heavy isotopomer and 1 refers to the light isotopomer if heavy and light are appropriate designations. Then, a ratio of such partition function ratios enables one to evaluate the isotope effect on a gas phase equilibrium constant, as pointed out above. Before continuing, it is appropriate to... 

No gas phase equilibrium exists between O2 and CO2 in the oxidant gas that could alter the composition or cathode potential. 

Mass-spectrometric investigation (83KGS1273) showed that the more substituted A-(3-oxoalkyl)thioureas 69 (R = R = R = Me R = H R = alkyl, Ar) exhibit a ring-chain equilibrium in the gas phase, but the less substituted derivatives 69 (R = R = H R = R = Me R = Me, Et, Bu) exist only as open-chain isomers. Clear evidence of the stabilizing effect of gem-dimethyl substitution on the cyclic form (Thorpe-Ingold effect) was presented for a gas-phase equilibrium. The mass-spectrometric data are rather similar to those obtained for this system in solution, which indicates that this equilibrium depends mainly on structural, rather than solvation, factors. 

The data from microwave spectroscopy have been interpreted with a dihedral angle H—O—O—H = 120.0° for the gas-phase equilibrium structure of H202. The nonplanarity of the peroxide gives rise to a stereogenic 0—0 axis . The computed total parity violating energy shift of —1.9 x 10 kJmoH between the two enantiomers, however, is too small in order to be measured with contemporary devices. ... 

The structures of protonated azoles in the gas phase (equilibrium ) can be determined by mass spectrometry in a chemical ionization experiment followed by collision-induced dissociation. The method has been used to study the protonation of benzimidazole (5), indazole (7), and 1-ethylimidazole (179) (all, as expected, on the pyridinelike nitrogen atoms) (80OMSI44) of 1-ethylpyrrole (probably at the -position) (80OMS144) and indole (at the -position) (85IJM49) (see Section IV.A). 

Owing to the strength of the B—F bond, die BF3 complexes are of widespread use as model compounds, for investigating Lewis acid-base interactions and the nature of the donor-acceptor bond. BF3 is frequently employed as a standard Lewis acid, for the quantitative characterization of the Lewis basicity of donor mojecules.62,63 The gas-phase equilibrium constants for some BF3 complexes are shown in Table 5. 

In 1995, Maciel and co-workers (118) synthesized the trityl cation in the supercages of zeolite HY by a clever application of Friedel-Crafts chemistry—13CC14 was reacted with an excess of benzene (Fig. 15). Maciel and co-workers carried out a number of spectroscopic and chemical manipulations that unambiguously demonstrated that the product was the trityl cation and that the cation was in the zeolite. Ab initio calculations at various levels of theory predict that the point group of isolated 16 is D3 rather than Dih. It is interesting to speculate about the extent to which the zeolite environment might force the degree of twist away from the gas-phase equilibrium value. 

A gas-phase equilibrium responds to compression—a reduction in volume—of the reaction vessel. According to Le Chatelier s principle, the composition will tend to change in a way that minimizes the resulting increase in pressure. For instance, in the formation of NH3 from N2 and H2, 4 mol of reactant molecules produces 2 mol of product molecules. The forward reaction therefore decreases the number of molecules in the container and hence the pressure. It follows that, when the mixture is compressed, the equilibrium composition will tend to shift in favor of product, because that minimizes the increase in pressure (Fig. 9.12). Expansion results in the opposite response, a tendency for products to decompose. Haber realized that to increase the yield of ammonia, he needed to carry out the synthesis with highly compressed gases. The actual industrial process uses pressures of 250 atm and more (Fig. 9.13). 

The determination of an Si—N bond dissociation energy has been carried out by Krasnoperov and coworkers114 who studied directly the gas-phase equilibrium ... 

Similar comparisons between the thermodynamic /J-silyl stabilization measured in the gas phase20,21 and the kinetic -silicon effect81,83 found in protonation experiments in solution are possible for the acetylenes 186 and 188 and for the alkene 190. The data for both solution study and gas phase equilibrium measurements are summarized in Table 5. 

Suppose we increase the volume of the above gas-phase equilibrium. The total pressure will decrease, and, according to Le Chatelier s principle, the system will... 

A sophisticated study of the calculation of gas-phase equilibrium constants Bohr F, Henon E (1998) J Phys Chem A 102 4857... 

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