Unit IV: Reaction Equilibria

Problems involving reactions are affected by equilibrium limitations as well as their kinetics. Briefly, the Gibbs energy related to the “partitioning” of atoms between species must be minimized in a fashion analogous to the way that it was minimized for phase partitioning. For example, the formation of methanol from carbon monoxide and hydrogen is favorable based on the energy from the bond formation, but limited by the reduction in entropy as the carbon monoxide and hydrogen are squeezed into a single molecule. Thus, even though the heat of reaction may be favorable, a balance is struck between the loss in entropy and the favorable energy as the methanol is formed. At 100 bars and 510 K, this balance occurs at about 60% conversion of stoichiometric CO + H₂ to CH₃OH. This unit introduces the principles for determining the effect of temperature and pressure on equilibrium conversion. A few overly zealous practitioners of the past have erroneously assumed that conversion of reactants is determined solely by the rate of reaction, and have attempted to increase conversion beyond the equilibrium limit by increasing reactor size. For example, Le Châtelier (1888) wrote:

It is known that in the blast furnace the reduction of iron oxide is produced by carbon monoxide, according to the reaction: Fe₂O₃ + 3CO = 2Fe + 3CO₂, but the gas leaving the chimney contains a considerable proportion of carbon monoxide, which thus carries away an important quantity of unutilized heat. Because this incomplete reaction was thought to be due to an insufficiently prolonged contact between carbon monoxide and the iron ore, the dimensions of the furnaces have been increased. In England they have been made as high as thirty meters. But the proportion of carbon monoxide escaping has not diminished, thus demonstrating, by an experiment costing several hundred thousand francs, that the reduction of iron oxide by carbon monoxide is a limited reaction. Acquaintance with the laws of chemical equilibrium would have permitted the same conclusion to be reached more rapidly and far more economically.

Let us hope that this is one bit of history that you will not repeat.