Reaction Modeling with Limited Data

Frequently chemical engineers must model chemical reactions without information on the Gibbs energy of formation. Although it can be estimated, often a better method is to conduct experiments to find equilibrium conditions and calculate K_a directly from compositions. Experiments can be performed over a temperature range, resulting in an empirical representation of Eqn. 17.28. In fact, methods like this are used to experimentally determine Gibbs energies of formation. The data can be fitted to determine constants I and J. If the data are scattered, it is better to use the simplified van’t Hoff Eqn. 17.29 in the form lnK_a = a + b/T, where a and b are fitted to the experimental data, and b is an indirect measurement of –ΔH_R^o/R. Thus, equilibrium data for new compounds or for liquid or solid reactions (discussed next) are often presented in the literature in the form of an equation for K_a instead of providing Gibbs energies of formation.