In a chemical reaction with certain initial concentrations of reactants and products, it is useful to know if the reaction will shift to the right (increasing the concentrations of the products) or if it will shift to the left (increasing the concentrations of the reactants). For this purpose we define the reaction quotient for reactions of the type:

where A, B, C, and D are four chemical species involved in this reaction and k, m, n, and p are the coefficients. And the expression for the reaction quotient is:

where the square brackets denote the concentrations of the various chemical species involved in the reaction in molarity, and the ' i ' denotes the instantaneous concentration at a certain period of time. This is crucial in understanding the idea of the Reaction Quotient. The Reaction Quotient is taken at a particular instant, this is at a time before equilbrium is reached. This page refers to concentration in the chemical sense. ...

This reaction quotient is directly related to the Law of Mass Action and the Le Chatelier Principle. For a reaction in chemical equilibrium, we define the equilibrium constant as: Chemical equilibrium is the state in which a chemical reaction proceeds at the same rate as its reverse reaction; the rates of the forward and reverse reactions are equal, and the concentration of the reactants and products stop changing. ... In chemistry, Le Chateliers principle can be used to predict the effect of a change in conditions on a chemical equilibrium. ...

By comparing the values of Q and K, one can determine whether the reaction will shift to the right, to the left, or if the concentrations will remain the same (equilibrium).

• If Q < K : The reaction will shift to the right
• If Q > K : The reaction will shift to the left
• If Q = K : The reaction is at equilibrium

See Also

• Chemical equilibrium
• Le Chatelier's Principle
• Equilibrium Constant

Chemical equilibrium is the state in which a chemical reaction proceeds at the same rate as its reverse reaction; the rates of the forward and reverse reactions are equal, and the concentration of the reactants and products stop changing. ... In chemistry, Le Chateliers principle can be used to predict the effect of a change in conditions on a chemical equilibrium. ... In chemistry, the equilibrium constant is a theoretically-based number that helps chemists determine the concentration of various reactants or products in a reaction where chemical equilibrium exists. ...

References

• Zumdahl, Steven; Zumdahl, Susan. Zumdahl Chemistry, 6th Edition.