Equilibrium Calculations for Gas-Phase and Heterogeneous Reactions

Equilibrium calculations, used throughout the science of chemistry in both basic and applied work, involve specific procednres. The present section presents these problem-solving techniques in the context of gas-phase and heterogeneons reactions, but they are applicable in all eqnilibrinm calcnlations. We illnstrate these calculation methods with several examples that fall into two broad classes evaluating the eqnilibrinm constant from reaction data, and calcnlating the amounts of products and reactants present at equilibrium when the eqnilibrinm constant is known. 

Evaluating Equilibrium Constants from Reaction Data 

In Section 14.3 we showed how to evaluate K from calorimetric data on the pnre reactants and products. Occasionally, these thermodynamic data may not be available for a specific reaction, or a quick estimate of the value of K may suffice. In these cases we can evaluate the equilibrium constant from measurements made directly on the reaction mixture. If we can measure the equilibrium partial pressures of all the reactants and products, we can calculate the equilibrium constant by writing the eqnilibrinm expression and substituting the experimental values (in atmospheres) into it. In many cases it is not practical to measnre directly the equilibrium partial pressure of each separate reactant and prodnct. Nonetheless, the equilibrium constant can usually be derived from other available data, although the determination is less direct. We illustrate the method in the following two examples. 

Phosgene, COCI2, is a chemical intermediate used in making polyurethanes for foams and surface coatings. 

Phosgene, COCI2, forms from CO and CI2 according to the equilibrium 

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Section 14.3 follows with the thermodynamic prescription for calculating the equilibrium constant from tabulated Gibbs free energy values for gas-phase, solution, and heterogeneous reactions, with specific examples for each. 

The last term arises from Eq. (15.32) applied to species C, Eq. (15.27) applied to B, and the fact that aA = fA for species A in the gas phase. Since K depends on the standard states, the value of K is not the same as that obtained when the standard state for each species is chosen as the ideal-gas state at 1 bar. However, all methods theoretically lead to the same equilibrium composition, provided Henry s law as applied to species C in solution is valid. In practice, a particular choice of standard states may simplify calculations or yield more accurate results, because it makes better use of the limited data normally available. The nature of the calculations required for heterogeneous reactions is illustrated in the following example. 

For batch, plug flow, and CSTR. Includes gas-phase isothermal, nonisothermal, and nonisobaric reactions, heterogeneous catalysis, and thermochemical database for calculation of equilibrium constants. Many subprograms for special situations (shock waves, flames, partially stirred reactors, etc.) are available. 

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