Category: 1. Reaction Equilibria

We have previously introduced the energy balance in Section 3.6 and also discussed adiabatic reactors. In this section we consider that there may be a there is a maximum possible value of ξ (outlet conversion) due to chemical equilibrium. Equilibrium may affect both adiabatic and nonadiabatic reactors, but we cover adiabatic reactors, and the extension to nonadiabatic…

Frequently, one may encounter a reaction that is not favored by Ka, and manipulation of temperature or pressure or feed composition provides only limited benefit for the desired conversion. In these cases, it may be possible to couple the reaction to another, more favorable, reaction to drive the overall production forward. Biological systems use coupling extensively. The…

When the equilibrium state in a reacting system depends on two or more simultaneous chemical reactions, the equilibrium composition can be found by a direct extension of the methods developed for single reactions. Each reaction will have its own reaction coordinate in which the compositions can be expressed. Some of the products of one reaction…

Plots of equilibrium constants provide a rapid method to visualize the gross trends and orders of magnitude. Fig. 17.2 illustrates how several reactions can be illustrated in a single graph. The equilibrium constants are calculated with the full temperature dependence. Note that the plots are nearly linear as would be approximated by the short-cut van’t Hoff. Exothermic…

Recall Eqn. 17.25, which we refer to as the general van’t Hoff equation: We can make rapid estimates of the equilibrium constant when we make the approximation that ΔHTo is independent of temperature. That is, suppose ΔCP = Δa = Δb = Δc = Δd = 0, which means the sensible heat effects for the reactants and products are the same. This is most…

Always remember that  depends on the standard state, which changes with temperature. Comparing Examples 17.2 and 17.3,  at 298 K (Ka = 1E-14), but decreases to  at 900 K (Ka = 0.242). In order to calculate , it may seem that we need to know ΔGfo for each compound at all temperatures. Fortunately this is not necessary because the  can be determined from the Gibbs energy…

In our preliminary examples, we have assumed rather idealized cases where none of the products are present in the inlet. However, in some cases, products may be present and then the reaction direction may not be as we anticipate. We can look at the reaction thermodynamics in a slightly different way to determine the direction…

At a given temperature, equilibrium values of the reaction coordinate are affected by pressure, inerts, and feed ratios. The principle that changing the quantities affects equilibrium conversions is known as Le Châtelier’s principle in honor of Henry Louis Le Châtelier who first characterized the phenomenon. An understanding of Le Châtelier’s principle is important for operating industrial reactions.…

The first term on the right side of Eqns. 17.12 and 17.16, , is called the standard state Gibbs energy of reaction at the temperature of the reaction, which we will denote . The standard state Gibbs energy of reaction is analogous to the standard state heat of reaction introduced in Section 3.6. The standard state Gibbs energy for reaction can be…

We now focus on the second summation of Eqn. 17.12. The ratio appearing in the logarithm is known as the activity, (cf. Eqns. 11.23 for a liquid, but now in a general sense):  activity The numerator  represents a mixture property that changes with composition. We have developed methods to calculate  in Eqns. 10.61 (ideal gases), 10.68 (ideal solutions), 11.14 (real solution using…