Unit IX. Chemical Equilbria

    A. Chemical Equilibrium. The condition where the rates of opposing reactions are equal.  The changes in concentrations of

        reactants and products are zero.

        1. A process at equilibrium must consist of opposing processes.  As a reaction () begins, the concentration

            of A ([A]) is high and the concentration of B ([B]) is zero.  The rate of disappearance of A is high because A would

            have most particles containing the "critical Kinetic Energy".  As the reaction progresses, the concentration of B increases

            which increases the number of B particles that will possess their own "critical KE" for the reverse reaction. 

        2. The concentrations of the reactants and products at equilibrium are not necessarily equal, even though their rates of

            production are equal.  Their concentrations are dependent more on the thermodynamics associated with each process. 

            The forward reaction may be more favored if DG_f < DG_r, or activation energy becomes very large for the reverse reaction. 

            The more negative DG of the forward reaction is the larger the activation energy of the reverse reaction. This would be

            seen as favoring the production of the products and not reactants, so equilibrium tends to the right.

            a. Equilibrium Expression. A mathematical expression that relates the concentrations of reactants and products as a

                function of the equilibrium.  This was first proposed by Guldber and Waage according to their Law of Mass Action.

                    For the reaction:                 

                    The K_eq for the reverse reaction is the reciprocal for the forward reaction.  Keep in mind that equilibrium is

                    independent direction in which it was achieved. What is important through is making sure that each K_eq is written

                    to express the concentration of their specific products to reactants.

                    The concentration of a solid or a pure liquid is typically constant throughout a reaction.  This will be omitted from

                    equilibrium expressions.

            b. If the substances are gases, pressure is proportional to the number of moles of particles as is concentration for

                solutions. In an equilibrium expression, for gases, we can merely use gas pressures as the ratio of particles at

                equilibrium.               

               The equilibrium constant can tell us which substances are favored, reactants or products.

                1. If K_eq >> 1 then the products are favored and equilibrium lies to the right

                2. If K_eq << 1 then the reactants are favored and the equilibrium lies to the left.

            c. Solving for equilibrium constants or equilibrium concentrations.

                To solve for a Keq constant, the concentrations of each component needs to be known. It is easily approached by

                setting up a table in order to express concentrations and change in concentrations.

                For a reaction that attains equilibrium, you will have to approach it by finding the changes in concentrations ( D [ ] )

                from the concentrations at time zero ( [ ]_to ).  The difference will give you the concentrations at equilibrium ( [ ]_te)