Difference Between Equilibrium Constant and Reaction Quotient

Equilibrium Constant vs Reaction Quotient

Some reactions are reversible, and some reactions are irreversible. In a reaction, reactants are converting to products. And in some reactions, the reactants can be generated again from the products. This type of reactions is called reversible. In irreversible reactions, once the reactants are converted to products, they cannot be regenerate again from the products. In a reversible reaction when reactants are going to products it is called the forward reaction and when products are going to reactants, it is called the backward reaction. When the rate of forward and backward reactions is equal, then the reaction is said to be at equilibrium. So over a period of time the amount of reactants and products are not changing. Reversible reactions always tend to come to equilibrium and maintain that equilibrium. When the system is at equilibrium, the amount of products and the reactants have not to be necessarily equal. There can be a higher amount of reactants than the products or vice versa. The only requirement in an equilibrium equation is to maintain a constant amount from both over time.

What is Equilibrium Constant?

For a reaction, in equilibrium, an equilibrium constant can be defined; where it is equal to the ratio between concentration/activity of products and concentration/activity of reactions.

K= [product]n/[reactant]m ; n and m are the stoichiometric coefficients of the product and reactant.

Equilibrium constant may have a unit or may not. If n is equal to m, all the concentration units cancel out giving no units to K. If n is different from m, according to the sum, K will be remained with a unit. Equilibrium constant is a constant at constant temperature. Equilibrium constant has the same value at constant temperature regardless of the amount of reactants or products in the medium. Catalysts or the pressure changes do not affect it. There can be several types of equilibrium constants. For example, when a weak acid is dissolved in water it establishes equilibrium the equilibrium constant is also known as the acid dissociation constant. Physical state of the products or reactants also affects the equilibrium constant, because solid state species are not included in the equation.

What is Reaction Quotient?

Reaction quotient is the ratio between the products and the reactants in a reaction, at a particular point of time. It is also denoted as Q. It is calculated in the same way as the equilibrium constant, but it is not a must to use it in reactions, which have reached equilibrium state. If the calculated reaction quotient is higher the value of the equilibrium constant, then the reaction will proceed in the reverse direction. In contrast, when the reaction quotient is lower than the equilibrium constant, the reactants will proceed forward. Therefore, reaction quotient is important in determining the direction of a reaction.


What is the difference between Equilibrium Constant and Reaction Quotient?

• Reaction quotient can be calculated for a reaction at any moment, not just for the reactions in equilibrium like equilibrium constants.

• If the equilibrium constant is equal to the reaction quotient, then the reaction is at equilibrium. By comparing the K and Q value, we can determine where the reaction is directed.