How is the reaction quotient (Qc) related to the equilibrium constant (Keq) for a chemical reaction?

The reaction quotient can be used with the equilibrium
constant to determine if a chemical reaction is at equilibrium and in which direction it
is likely to move.

Take a chemical reaction: A + B
<--> C + D where the equilibrium constant is
Keq.

The ratio [C][D]/[A][B] which is determined by using
the concentrations at any moment of time is known as the reaction quotient
Qc.

Comparing Qc and Keq, three situtions can
arise:

• Qc > Keq : This is the case when
  the concentration of the products is too high and the reaction moves towards the left
  with the formation of the
  reactants.

• Qc < Keq: This is
  the case when the concentration of the reactants is too high and the reaction has to
  move in the forward direction with an increase in the concentration of the
  products.

• Qc = Keq : When this is
  the case, the reaction is at equilibrium, and for other conditions remaining the same,
  the ratio of the concentration of reactants and products will not
  change.