What is the reaction quotient?

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.