Thermodynamics of chemical reactions

Thermod5mamics is a fundamental engineering science that has many applications to chemical reactor design. Here we give a summary of two important topics determination of heat capacities and heats of reaction for inclusion in energy balances, and determination of free energies of reaction to calculate equihbrium compositions and to aid in the determination of reverse reaction 

The design equations for a chemical reactor contain several parameters that are functions of temperature. Equation (7.17) applies to a nonisothermal batch reactor and is exemplary of the physical property variations that can be important even for ideal reactors. Note that the word ideal has three uses in this chapter. In connection with reactors, ideal refers to the quality of mixing in the vessel. Ideal batch reactors and CSTRs have perfect internal mixing. Ideal PFRs are perfectly mixed in the radial direction and have no mixing in the axial direction. These ideal reactors may be nonisothermal and may have physical properties that vary with temperature, pressure, and composition. 

Enthalpy. Enthalpy is calculated relative to a standard state that is normally chosen as T0 = 298.15 K = 25°C and /J0 = I bar pressure. The change in enthalpy with pressure can usually be ignored. For extreme changes in pressure, use 

The change in enthalpy with respect to temperature is not negligible. It can be calculated for a pure component using the specific heat correlations like those in Table 7.1  

This table provides data for calculating molar heat capacities at low pressures according to the empirical formula 

The science of chemical kinetics is concerned primarily with chemical changes and the energy and mass fluxes associated therewith. Thermodynamics, on the other hand, is concerned with equilibrium systems. .. systems that are undergoing no net change with time. This chapter will remind the student of the key thermodynamic principles with which he should be familiar. Emphasis is placed on calculations of equilibrium extents of reaction and enthalpy changes accompanying chemical reactions. 

The basic criterion for the establishment of chemical reaction equilibrium is that 

These equations are equivalent to a requirement that the Gibbs free energy change for each reaction (AG) be zero at equilibrium. 

The choice of the standard state is largely arbitrary and is based primarily on experimental convenience and reproducibility. The temperature of the standard state is the same as that of the system under investigation. In some cases, the standard state may represent a hypothetical condition that cannot be achieved experimentally, but that is susceptible to calculations giving reproducible results. Although different standard states may be chosen for various species, throughout any set of calculations it is important that the standard state of a component be kept the same so as to minimize possibilities for error. 

Certain choices of standard states have found such widespread use that they have achieved 

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Thermodynamics of Chemical Reactions 89 Heat of formation AHf(cal/gmol)... 

Thermodynamics of Chemical Reactions 95 Heats and Free Energies of Formation (continued)... 

What Do We Need to Know Already The concepts of chemical equilibrium are related to those of physical equilibrium (Sections 8.1-8.3). Because chemical equilibrium depends on the thermodynamics of chemical reactions, we need to know about the Gibbs free energy of reaction (Section 7.13) and standard enthalpies of formation (Section 6.18). Ghemical equilibrium calculations require a thorough knowledge of molar concentration (Section G), reaction stoichiometry (Section L), and the gas laws (Ghapter 4). 

In principle one can treat the thermodynamics of chemical reactions on a kinetic basis by recognizing that the equilibrium condition corresponds to the case where the rates of the forward and reverse reactions are identical. In this sense kinetics is the more fundamental science. Nonetheless, thermodynamics provides much vital information to the kineticist and to the reactor designer. In particular, the first step in determining the economic feasibility of producing a given material from a given reactant feed stock should be the determination of the product yield at equilibrium at the conditions of the reactor outlet. Since this composition represents the goal toward which the kinetic... 

The following texts contain adequate discussions of the thermodynamics of chemical reactions they can be recommended without implying judgment on others that are not mentioned. 

Thermochemistry has been defined in one of the most popular physical chemistry textbooks as the study of the heat produced or required by chemical reactions [1], The use of heat, instead of the more general word energy, immediately suggests a close association between thermochemistry and calorimetry—the oldest experimental technique for investigating the thermodynamics of chemical reactions. This view is, in fact, shared by many of our students and some of their teachers, together with the belief that thermochemistry, founded in the eighteenth century by Black, Lavoisier, and Laplace, has seen few major developments since the days of Berthelot and Thomsen, over 100 years ago [2],... 

Much of this chapter will be a review for those who have had courses in chemical kinetics. In this chapter we will also review some aspects of thermodynamics that are important in considering chemical reactors. For students who have not had courses in kinetics and in the thermodynamics of chemical reactions, this chapter will serve as an introduction to those topics. This chapter will also introduce the notation we will use throughout the book. 

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