My watch list
my.chemeurope.com

# Reaction quotient

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.

Concepts in
Chemical Equilibria
Acid dissociation constant
Binding constant
Chemical equilibrium
Dissociation constant
Distribution coefficient
Distribution ratio
Equilibrium constant
Equilibrium unfolding
Equilibrium stage
Liquid-liquid extraction
Phase diagram
Phase rule
Reaction quotient
Relative volatility
Solubility equilibrium
Stability constant
Thermodynamic equilibrium
Theoretical plate
Vapor-liquid equilibrium
edit

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 [1]:

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

where the { Ai } denotes the instantaneous activity[2] 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