Steve Marsden’s

One way in which the value of Kc can be used in a predictive way is by creating a new expression which can be used to tell which direction a reaction will take in order to establish equilibrium in a system. The expression is often called the reaction quotient and is given the symbol Q (or Qc, in these cases).

Algebraically the reaction quotient is the same as the equilibrium constant except for the fact that it is calculated using the initial concentrations of substances.