Reaction quotient

Overview
In chemistry, reaction quotient is a quantitative measure of the extent of reaction, the relative proportion of products and reactants present in the reaction mixture at some instant of time.

For a chemical mixture with certain initial concentrations of reactants and products, it is useful to know if the reaction will shift to the right/in the forward direction (increasing the concentrations of the products) or if it will shift to the left/in the reverse direction (increasing the concentrations of the reactants). Given a general equilibrium expression such as
 * kA + mB ... $$\rightleftharpoons$$ nC + pD ...

where A, B, C, and D are chemical species involved in this reaction and k, m, n, and p are the stoichiometric coefficients for the reaction, the reaction quotient, Q, is defined as :


 * $$Q = \frac{\left\{C_i\right\}^n \left\{D_i\right\}^p ...}{\left\{A_i\right\}^k \left\{B_i\right\}^m ...} $$

where the { Ai } denotes the instantaneous activity of the species A at a certain moment of time and so on for the other species. The reaction quotient is taken at a particular instant in time, not necessarily the moment when equilibrium is reached. The reaction quotient is directly related to Le Chatelier's Principle. For a reaction at chemical equilibrium, the equilibrium constant, K, may be defined as:


 * $$K = \frac{\left\{C\right\}^n \left\{D\right\}^p...}{\left\{A\right\}^k \left\{B\right\}^m...}$$

where {A} is the activity of the species A when the mixture is at equilibrium, etc. 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 (i.e. in the forward direction, and thus more products will form)


 * If Q > K : The reaction will shift to the left (i.e. in the reverse direction, and thus more reactants will form)


 * If Q = K : The reaction is at equilibrium

The relationship of reaction quotient Q with the instantaneous derivative of Gibbs energy (ΔG) and standard change of Gibbs energy (ΔGO) is given by ΔG = ΔGO + RT ln Q