Reaction in a gas phase

If a reaction takes place in a gaseous mixture, the standard state of each reactant and product is the pure gas behaving ideally at the standard pressure p° (Sec. 9.3.3). In this case, each activity is given by aj(g) = fi/p° = Pi/p° where f i is a fiigacity coefficient 

On the right side of this equation, the first factor is the proper quotient of fugacity coefficients in the mixture at reaction equilibrium, the second factor is the proper quotient of partial pressures in this mixture, and the third factor is the power of p° needed to make K dimensionless. 

The proper quotient of equilibrium partial pressures is an equilibrium constant on a pressure basis, Kp  

Note that Kp is dimensionless only if v,- is equal to zero. 

The value of Kp can vary at constant temperature, so Kp is not a thermodynamic equilibrium constant. Eor instance, consider what happens when we take an ideal gas mixture at reaction equilibrium and compress it isothermally. As the gas pressure increases, the fugacity coefficient of each constituent changes from its low pressure value of 1 and the gas mixture becomes nonideal. In order for the mixture to remain in reaction equilibrium, and the product of factors on the right side of Eq. 11.8.12 to remain constant, there must be a change in the value of Kp. In other words, the reaction equilibrium shifts as we increase p at constant T, an effect that will be considered in more detail in Sec. 11.9. 

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Triethanolamine is produced from ethylene oxide and ammonia at 5 atm total pressure via three consecutive elementary chemical reactions in a gas-phase plug-flow tubular reactor (PFR) that is not insulated from the surroundings. Ethylene oxide must react with the products from the first and second reactions before triethanolamine is formed in the third elementary step. The reaction scheme is described below via equations (1-1) to (1-3). All reactions are elementary, irreversible, and occur in the gas phase. In the first reaction, ethylene oxide, which is a cyclic ether, and ammonia combine to form monoethanolamine ... 

Importance of ion-molecule reaction in a gas phase was recognized in this recoil atom reaction. A similar study on a bromine-methane mixture was performed, but no contribution of charge was found. 

The equilibrium conversion can be increased by employing one reactant in excess (or removing the water formed, or both). b. Inerts concentration. Sometimes, an inert material is present in the reactor. This might be a solvent in a liquid-phase reaction or an inert gas in a gas-phase reaction. Consider the reaction system... 

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

The method for calculating effective polarizabilitie.s wa.s developed primarily to obtain values that reflect the stabilizing effect of polarizability on introduction of a charge into a molecule. That this goal was reached was proven by a variety of correlations of data on chemical reactivity in the gas phase with effective polarizability values. We have intentionally chosen reactions in the gas phase as these show the predominant effect of polarizability, uncorrupted by solvent effects. 

In a recent experimental study of the femtosecond dynamics of a Diels-Alder reaction in the gas phase it has been suggested that both concerted and stepwise trajectories are present simultaneously It is interesting to read the heated debates between Houk and Dewar on the... 

Other conventions for treating equiUbrium exist and, in fact, a rigorous thermodynamic treatment differs in important ways. Eor reactions in the gas phase, partial pressures of components are related to molar concentrations, and an equilibrium constant i, expressed directiy in terms of pressures, is convenient. If the ideal gas law appHes, the partial pressure is related to the molar concentration by a factor of RT, the gas constant times temperature, raised to the power of the reaction coefficients. 

The hydrocarbon is carried in a stream of H2 or Ar, and P-SiC is formed or P-SiC is formed by reaction in the gas phase, under static conditions, of compounds such as SiO or CO formed in situ during the process. In this latter case the important reaction appears to be SiO + 3CO — SiC + 2CO2. This... 

Computer simulation techniques offer the ability to study the potential energy surfaces of chemical reactions to a high degree of quantitative accuracy [4]. Theoretical studies of chemical reactions in the gas phase are a major field and can provide detailed insights into a variety of processes of fundamental interest in atmospheric and combustion chemistry. In the past decade theoretical methods were extended to the study of reaction processes in mesoscopic systems such as enzymatic reactions in solution, albeit to a more approximate level than the most accurate gas-phase studies. 

A simple model for the formation and growth of an aerosol at ambient conditions involves the formation of a gas product by the appropriate chemical oxidation reactions in the gas phase. This product must have a... 

BrCl can be prepared by the reaction in the gas phase or in aqueous hydrochloric acid solution. In the laboratory, BrCl is prepared by oxidizing bromide salt in a solution containing hydrochloric acid. 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

Pertiaps the most obvious experiment is to compare the rate of a reaction in the presence of a solvent and in the absence of the solvent (i.e., in the gas phase). This has long been possible for reactions proceeding homolytically, in which little charge separation occurs in the transition state for such reactions the rates in the gas phase and in the solution phase are similar. Very recently it has become possible to examine polar reactions in the gas phase, and the outcome is greatly different, with the gas-phase reactivity being as much as 10 greater than the reactivity in polar solvents. This reduced reactivity in solvents is ascribed to inhibition by solvation in such reactions the role of the solvent clearly overwhelms the intrinsic reactivity of the reactants. Gas-phase kinetic studies are a powerful means for interpreting the reaction coordinate at a molecular level. 

For the reaction in the gas phase, Emmons proposes a liomolytic O—N fission and (possibly simultaneous) alkyl shift [Eq. (28)]. The... 

Polymerization reactions can occur in bulk (without solvent), in solution, in emulsion, in suspension, or in a gas-phase process. Interfacial polymerization is also used with reactive monomers, such as acid chlorides. 

The separation of n-alkanes from a kerosene or gas oil fraction by a molecular sieve can be performed in a liquid phase or in a gas phase process. In the gas phase processes there are no problems of cleaning the loaded molecular sieve from adherent branched and cyclic hydrocarbons. However, the high reaction temperature of the gas phase processes leads to the development of coke-contaminated sieves, which have to be regenerated from time to time by a careful burning off of the coke deposits. 

We are now ready to build a model of how chemical reactions take place at the molecular level. Specifically, our model must account for the temperature dependence of rate constants, as expressed by the Arrhenius equation it should also reveal the significance of the Arrhenius parameters A and Ea. Reactions in the gas phase are conceptually simpler than those in solution, and so we begin with them. 

Field and Lampe (23) established the occurrence of the hydride transfer reaction in the gas phase in 1958 by detecting secondary ions of mass one unit lower than the parent compound. Subsequently, Futrell (24, 25) attempted to account for most lower hydrocarbon products formed in the radiolysis of hexane and pentane by assuming that hydride transfer reactions play a dominant role in radiolysis. More recently, Ausloos and Lias (2) presented experimental evidence which indicated that some of the products in the radiolysis of propane are, in... 

For reactions in the gas phase, the free energy per mole of gas as a function of pressure is given... 

Despite their transient existences, it is possible to study transition states of certain reactions in the gas phase with a technique called laser femtochemistry Zewall, A.H. Bernstein, R.B. Chem. Eng. News, 1988, 66, No. 45 (Nov. 7), 24. For another method, see Ceilings, B.A. Polanyi, J.C. Smith, M.A. Stolow, A. Tarr, A.W. Phys. Rev. Lett., 1987, 59, 2551. See Smith, M.B. Organic Synthesis, McGraw-Hill NY, 1994, p. 601. 

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

Example 3.2 Consider the reaction 2A B. Derive an analytical expression for the fraction unreacted in a gas-phase, isothermal, piston flow reactor of length L. The pressure drop in the reactor is negligible. 

Homogeneous, liquid-phase reactions may also be important in trickle beds, and a strictly homogeneous term has been included in Equation (11.42) to note this fact. There is usually no reaction in the gas phase. Normally, the gas phase merely supplies or removes the gaseous reactants (e.g., H2 and H2S in hydrodesulfurization). ... 

CO6-OO6I. Acrylonitrile is an important starting material for the manufacture of plastics and synthetic mbber. The compound is made from propene in a gas-phase reaction at elevated temperature ... 

Where in this cycle is the essential influence of the catalyst Suppose we carry out the reaction in the gas phase without a catalyst. The reaction will proceed if we raise the temperature sufficiently for the O2 molecule to dissociate into two O atoms (radicals). Once these radicals are available, the reaction with CO to CO2 follows instantaneously. 

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