Some chemical reactions go to completion. That means that when you add the reactants together they combine completely and all that you then have are one or more products. The reactants have all been used up.
Other chemical reactions form an equilibrium between the reactants and products. In these types of reactions there are three possible conditions:
1. the reaction can be moving forward from the reactants toward equilibrium.
2. the reaction can be at equilibrium
3. the reaction can be moving back from the products toward equilibriium.
In cases 1 & 3 above, the reaction quotient can be calculated. The reaction quotient (Qc) is the ratio of the concentration of the products divided by the concentration of the reactants.
For a general reaction of the form A + B --> C
Qc = [C]/[A][B]
If the reaction is at equilibrium, case 2 above, then the reaction quotient is equal to the equilibrium constant.