Question

1) Explain the difference between an equilibium position and an equilibrium constant.

2) Why are the concentrations of pure liquids and pure solids never used in equilibrium constant expressions?

Answer 1

Equilibrium position: a particular set of concentrations of reactant and product species. Equilibrium constant: a number that describes how far "forward" equilibrium lies for a particular system (as described by a particular equation).

Example: for the system A <--> B, the mass action expression is [B]/[A]. Let's say K = 200. This means, at equilibrium, [B] will be 200x greater than [A]. That's what the equilibrium constant tells you - equilibrium for this system lies to the right, and the K value is greater than 1.
There are infinitely many different equilibrium positions that satisfy K. We could have [B] = 20 M and [A] = 0.1 M. Or we could have [B] = 1 M and [A] = 0.005 M. Those are two different equilibrium positions that are both at equilibrium. In both cases, Q = K and the system is at equilibrium.

Some notes:

1) If we describe the exact same system with a different equation, B <--> A, then the mass action expression is [A]/[B]. We would find that K = 0.005 (that is, 1/200) and we would say that equilibrium lies to the left. Of course, the molecules don't behave any differently, but the equationwe use to describe the system is different, and that difference is reflected in a different K.

2) If we have a system with more than one product or reactant or with stoichiometric coefficients other than 1, then it's a bit more complicated than just saying something like "product concentrations are 200x greater than reactant concentrations". Instead, equilibrium may be achieved at many different equilibrium positions. You have to look at the mass action expression to interpret the meaning of K.

3) The equilibrium constant K is just one way of expressing how far "forward" a particular reaction will proceed. Gibbs free energy change (deltaG) is another way. And, for redox systems, cell potential is yet another way.

2)Pure solids and liquids are not included in theequilibrium constant expression. This is because they do not affect the reactant amount at equilibriumin the reaction, so they are disregarded and kept at 1. Remember that the activity, a, of any solid or liquid in a reaction is equal to 1.