Reactions involving gases

Some types of reactions involving gases that have been studied in ILs are hydrogenations [16, 25-37 ], oxidations [38, 39], and hydroformylations [25, 40-45]. In addition, some dimerizations and alkylations may involve the dissolution of condensable gases (e.g., ethylene, propylene, isobutene) in the IL solvent [46-50]. 

90 Joan F. Brennecke, Jennifer L. Anthony, EdwardJ. Maginn 

Thermochemical measurements on reactions between gases and solids, other than combustions, are uncommon. Gross and his co-workers measured the enthalpy of reaction between alkali-metal fluorides MF (M = Li or Na) and BFg to form MBF4 using an isoperibol calorimeter operated at 110 The stream of argon flowing over the heated fluoride was replaced by BF for a period of 10 min. Duus and Mykytiuk used a flow method to determine the enthalpy of the reaction 

Reactions between gases in static systems require only simple reaction vessels in conventional calorimeters. The gases can be introduced into the vessel by simple Joule expansion, when account has to be taken of the energy of the expansion process, or they may be contained in a two-compartment glass reaction vessel and separated by a break-seal, a technique employed by Gunn for the chlorination of B2CI4 using a heat-flow calorimeter suitable for the study of processes of an hour s duration.  

Lacher and his co-workers have used a flow system operated at temperatures up to 250 °C to measure the enthalpies of the catalytic addition of hydrogen chloridff to olefins and the catalytic hydrogenation of alkyl halides. The calorimeter is maintained at constant temperature by 

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To proceed fiirther, to evaluate the standard free energy AG , we need infonnation (experimental or theoretical) about the particular reaction. One source of infonnation is the equilibrium constant for a chemical reaction involving gases. Previous sections have shown how the chemical potential for a species in a gaseous mixture or in a dilute solution (and the corresponding activities) can be defined and measured. Thus, if one can detennine (by some kind of analysis)... 

Chemical reactions involving gases carried out in closed containers resemble in many ways the H20(/)-H20(g) system. The reactions are reversible reactants are not completely consumed. Instead, an equilibrium mixture containing both products and reactants is obtained. At equilibrium, forward and reverse reactions take place at the same rate. As a result, the amounts of all species at equilibrium remain constant with time. 

In some cases, the catalyst is a solid substance on whose surface a reactant molecule can be held (adsorbed) in a position favorable for reaction until a molecule of another reactant reaches the same point on the solid. Metals such as iron, nickel, platinum and palladium seem to act in this way in reactions involving gases. There is evidence that in some cases of surface adsorption, bonds of reactant particles are weakened or actually broken, thus aiding reaction with another reactant particle. 

The data have been expressed in concentrations, although pressure units are more usual for a reaction involving gases. 

One word of warning when using the van t Hoff equation for reactions involving gases, the equilibrium constants must be K, not K(.. If we want a new value for Kc for a gas-phase reaction, we convert from K(. into K at the initial temperature (by using Eq. 12), use the van t Hoff equation to calculate the value of K at the new temperature, and then convert that K into the new Kc by using Eq. 12 at the new temperature. 

This calculation shows that reaction energies and reaction enthalpies are usually about the same, even when reactions Involve gases. For this reason, chemists often use A 5 reaction nd A reaction interchangeably. Because many everyday processes occur at constant pressure, thermodynamic tables usually give values for enthalpy changes. Nevertheless, bear In mind that these are different thermodynamic quantities. For processes with modest AE values and significant volume changes, A " and A H can differ substantially. 

Enthalpies of reactions involving gases vary with pressure (because of the PV term in the definition of H), so pressures must be specified. 

The values of E° for electrode reactions (or of E°, when sufficiently reliable activity data are not available) are listed in special tables some values of this type are shown in Table 3.1. When using such tables we must bear in mind that the val-nes of ii° for reactions involving gases have been calculated for partial pressures of 1 atm, which in SI units corresponds to 101, 325 Pa (about 0.1 MPa). Hence, in the Nemst eqnation we must use gas pressures in the now-obsolete unit atmospheres. 

In some reactions involving gases, the rate of reaction estimated by the simple collision theory in terms of the usually infened species is much lower than observed. Examples of these reactions are the oxidation of H2 and of hydrocarbons, and the formation of HC1 and of HBr. These are examples of chain reactions in which very reactive species (chain carriers) are initially produced, either thermally (i.e., by collision) or photochemically (by absorption of incident radiation), and regenerated by subsequent steps, so that reaction can occur in chain-fashion relatively rapidly. In extreme cases these become explosions, but not all chain reactions are so rapid as to be termed explosions. The chain... 

It is not true, however, that volumes must be conserved in reactions involving gases. The complete combustion of carbon monoxide is a case in point ... 

The only reactions that are significantly affected by pressure are reactions involving gases in which the stoichiometric coefficients of the gases add to different values on the two sides of the reaction. Pressure, therefore, would not affect the equilibrium of... 

At temperatures below the ignition point, the thermal decomposition of black powder provides an interesting insight into the processes which are thought to control the reaction rate during subsequent burning. In decomposition experiments it has been shown that the overall reaction proceeds in several steps. As the temperature is increased the steps become shorter and the reaction faster. Since these reactions involve gases, the effect of pressure is also important. 

When there are appreciable density changes, as for example in reactions involving gases or in some polymerisation reactions, these must be taken into account. In this case, at the reactor inlet, the volumetric flow rate Fi is... 

In the next sections we will consider two exothermic chain reactions involving gases, the oxidation of H2 and alkanes. Then we wiU consider combustion of liquid and solid fuels, a gas-liquid process. These processes all occur very rapidly and at very high temperatures. 

The term gasochromism has been applied to systems which change colour by redox reactions involving gases it is closely related to electrochromic behaviour. 

Many different types of reversible reactions exist in chemistry, and for each of these an equilibrium constant can be defined. The basic principles of this chapter apply to all equilibrium constants. The different types of equilibrium are generally denoted using an appropriate subscript. The equilibrium constant for general solution reactions is signified as or K, where the c indicates equilibrium concentrations are used in the law of mass action. When reactions involve gases, partial pressures are often used instead of concentrations, and the equilibrium constant is reported as (p indicates that the constant is based on partial pressures). and are used for equilibria associated with acids and bases, respectively. The equilibrium of water with the hydrogen and hydroxide ions is expressed as K. The equilibrium constant used with the solubility of ionic compounds is K p. Several of these different K expres-... 

Note that homogeneous electron transfer in the solution phase is negligible in the case of half-reactions involving gases, but that it may be very rapid for ionic redox systems. 

Equilibrium constants for reactions involving gases can be written either by using molar concentrations and writing Kc or by using partial pressures and writing K. The two forms of the equilibrium constant are related by Eq. 14. 

Here, K is Kp for reactions involving gases and Kc for reactions involving solutes in solution. For reactions involving both gases and solutes in solution, the equilibrium constant expression contains partial pressures of gases and molar concentrations of solutes.)... 

If powders are used for the sample material, the packing of the particles will have an effect on the rate of reactions involving gases. Tighter packing inhibits free diffusion of gaseous species in and out of the reaction zones. Hence, the decomposition... 

Many important chemical reactions take place in the atmosphere. All these reactions involve gases. There are two main types of gas chemistry in the atmosphere ... 

For an electrochemical reaction involving gases, the electrode potential is related to the pressure of the gases. In H2/02 fuel cells, the electrode potentials at both anode and cathode sides are given by the following equations ... 

Many chemical reactions involve gases. By assuming ideal behavior for these gases, we can carry out stoichiometric calculations if the pressure, volume, and temperature of the gases are known. 

However, when a reaction involving gases is studied at constant pressure, A E may not equal AH. The reaction... 

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