Introduction; Chemical Equilibrium

Chemical reactions take place because of chemical affinity. The reaction rate r in general is

(6-1)

where n _A is moles of reactant A, V is volume, and t is time. If the volume is constant

(6-2)

where C _A is the concentration of reactant A in moles per volume.

Chemical reactions proceed at a rate r, reaching a condition of equilibrium at which the rate approaches zero asymptotically on a plot of rate versus time (Figure 6-1).

Figure 6-1: Equilibrium occurs when the rate of reaction approaches zero. (Reproduced with permission from reference 2. Copyright 1997 American Chemical Society.)

On a plot of conversion versus time, equilibrium occurs at the final percentage conversion attained. It indicates the likelihood that a reaction will produce an appreciable amount of product (Figure 6-2).

Figure 6-2: As a reaction approaches equilibrium, the conversion rate reaches the equilibrium value. (Reproduced with permission from reference 2. Copyright 1997 American Chemical Society.)

Hence, chemical equilibrium calculations are important when evaluating chemical reactions. A chemical equilibrium constant K _a is defined by the relationship

(6-3)

where ? G is the change in Gibbs free energy between products and reactants at a standard state. K _a can be defined by its relation to activities. If a reaction can be written as

(6-4)

then the equilibrium constant K _a in terms of activities is

(6-5)

where activity is represented by a.

The chemical...