In Chemistry, Q stands for the reaction quotient.
The reaction quotient Q, plays a major role in a chemical reaction. It can be used to determine the value of the products and reactants at any point in a chemical reaction.
It is also used to determine the direction a chemical reaction is most likely going to follow, if the value of the products and reactants are given.
When Q is compared with K ( equilibrium constant), it is easy to tell the direction a chemical reaction will go.
If Q < K, the reaction will move forward.
If Q > K, the reaction will move in reverse.
If Q= K, the system is at equilibrium.