THE EQUILIBRIUM CONSTANT (Kc)

1. The equilibrium constant does not have fixed units.


2. Concentrations of certain substances remain constant... so they are not included in the equilibrium equation. Solids have a constant concentration and a fixed density... so they are omitted. The concentration of water is also taken as a constant in aqueous solutions.
   
3. Coefficients in this equation are represented by raised powers.
   
   4NH3 + 5O2 ↔ 4NO + 6H2O
   
   Kc = ([NO]^4)([H2O]^6)/([NH3]^4)([O2]^5)
   



4. Because forward and reverse constants never change at a given temperature, their ratio will always give the same value at equilibrium. This is known as the equilibrium constant.

Kf[A][B] = Kr[C][D]

Therefore:

Kf/Kr = [C][D]/[A][B]

OR

Kc = [C][D]/[A][B]

**What can Kc values tell us?

1. If Kc is very large, the reaction goes to completion.
2. If Kc is less than 1, it will not go to completion and may be an equilibrium reaction.
3. If Kc is very small, the reaction may not occur at all.
   
   Go to http://www.geocities.com/junebug_sophia/equilConst.htm for an online Equilibrium Constant quiz!