Reactions with Gaseous Reactants and Products

In Chapter 3 we used balanced chemical equations to calculate amounts of reactants and/or products in chemical reactions—expressing those amounts in mass (usually grams). However, in the 

---

Figure 1.2. Potential energy diagram of a heterogeneous catalytic reaction, with gaseous reactants and products and a solid catalyst. Note that the uncatalyzed reaction has to overcome a substantial energy barrier, whereas the barriers in the catalytic route are much lower.

SECTION 11.4 Reactions with Gaseous Reactants and Products... 

Provided that gaseous reactants and products behave ideally and the specific volumes of liquid and solid reactants and products are negligible compared with the specific volumes of the gases, the internal energy of reaction may be calculated from Equation 9.1-5. (lliis quantity is required for energy balances on constant-volume batch reactors.)... 

Chemical reactions involving gaseous reactants or products can be influenced by high vacuum in the direction of greater molar volume, as in the case of the dissociation of certain metallic oxides into oxygen and free metals, and the reduction of such oxides by carbon with the formation of CO or C02. 

Given a balanced equation for a reversible chemical reaction that involves at least one gaseous substance, write its equilibrium constant, K/>, expression with the reactants and products described in terms of gas pressures. 

In Chapters 3 and 4, we encountered many reactions that involved gases as reactants (e.g., combustion with O2) or as products (e.g., a metal displacing H2 from acid). From the balanced equation, we used stoichiometrically equivalent molar ratios to calculate the amounts (moles) of reactants and products and converted these quantities into masses, numbers of molecules, or solution volumes (see Figure 3.10). Figure 5.11 shows how you can expand your problem-solving repertoire by using the ideal gas law to convert between gas variables (F, T, and V) and amounts (moles) of gaseous reactants and products. In effect, you combine a gas law problem with a stoichiometry problem it is more realistic to measure the volume, pressure, and temperature of a gas than its mass. 

Benzaldehyde hydrogenation reaction was carried out in a fixed-bed Pyrex reactor tube at atmospheric pressure with 200mg of samples and a total flow of 50ml/min. Before testing, the catalysts were in-situ reduced for 16 h at 350" C in a current of H2 (20ml/min). The gaseous reactant and products were analyzed on line by flame ionisation (FID) detector (Delsi ICG 121 Ml). 

Physical methods involve the measurement of a physical property of the system as a whole while the reaction proceeds. The measurements are usually made in the reaction vessel so that the necessity for sampling with the possibility of attendant errors is eliminated. With physical methods it is usually possible to obtain an essentially continuous record of the values of the property being measured. This can then be transformed into a continuous record of reactant and product concentrations. It is usually easier to accumulate much more data on a given reaction system with such methods than is possible with chemical methods. There are certain limitations on physical methods, however. There must be substantial differences in the contributions of the reactants and products to the value of the particular physical property used as a measure of the reaction progress. Thus one would not use pressure measurements to follow the course of a gaseous reaction that does not... 

We title this chapter the reactions of sohds and we deal mostly with gaseous and sohd reactants and products, but the same ideas and equations apply to gas-hquid and liquid-liquid systems. For example, in fiying of foods, the fluid is obviously a liquid that is transferring heat to the sohd and carrying off products. The same equations also apply to many gas-hquid and liquid-hquid systems. The drops and bubbles change in size as reactions proceed so the same equations we derive here for transformation of sohds wiU also apply to those situations. 

The computer-reconstructed catalyst is represented by a discrete volume phase function in the form of 3D matrix containing information about the phase in each volume element. Another 3D matrix defines the distribution of active catalytic sites. Macroporosity, sizes of supporting articles and the correlation function describing the macropore size distribution are evaluated from the SEM images of porous catalyst (Koci et al., 2006 Kosek et al., 2005). Spatially 3D reaction-diffusion system with low concentrations of reactants and products can be described by mass balances in the form of the following partial differential equations (Koci et al., 2006, 2007a). For gaseous components ... 

---