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Reaction quotient

 

 
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Reaction quotient
 
 
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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, 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 concentration
Concentration

In chemistry, concentration is the measure of how much of a given chemical substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent....
s of reactants and products
Product (chemistry)

A product is a substance that forms as a result of a biological- or chemical reaction. While the end product of some chemical reactions may be the result of a relatively rapid reaction, nanoseconds to seconds, chemical equilibrium in complex systems may require years or even centuries to be established....
, 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).






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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, 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 concentration
Concentration

In chemistry, concentration is the measure of how much of a given chemical substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent....
s of reactants and products
Product (chemistry)

A product is a substance that forms as a result of a biological- or chemical reaction. While the end product of some chemical reactions may be the result of a relatively rapid reaction, nanoseconds to seconds, chemical equilibrium in complex systems may require years or even centuries to be established....
, 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 ... 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 :

where the denotes the instantaneous activity
Activity (chemistry)

In chemical thermodynamics activity is a measure of the ?effective concentration? of a species in a mixture. By convention, it is a dimensionless quantity....
  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
Le Châtelier's principle

In chemistry, Le Chatelier's Principle, also called the Le Chatelier-Braun principle, can be used to predict the effect of a change in conditions on a chemical equilibrium....
. For a reaction at chemical equilibrium, the equilibrium constant
Equilibrium constant

For a general chemical equilibriumthe equilibrium constant can be defined bywhere is the activity of the chemical species A etc . It is conventional to put the activities of the products in the numerator and those of the reactants in the denominator....
, K, may be defined as:

where 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 (?rG) and standard Gibbs energy change (?rGO) is given by

?rG = ?rGO + RT ln Q

See also

  • Chemical equilibrium
    Chemical equilibrium

    In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
  • Le Chatelier's Principle
    Le Châtelier's principle

    In chemistry, Le Chatelier's Principle, also called the Le Chatelier-Braun principle, can be used to predict the effect of a change in conditions on a chemical equilibrium....
  • Equilibrium Constant
    Equilibrium constant

    For a general chemical equilibriumthe equilibrium constant can be defined bywhere is the activity of the chemical species A etc . It is conventional to put the activities of the products in the numerator and those of the reactants in the denominator....
  • Activity
    Activity (chemistry)

    In chemical thermodynamics activity is a measure of the ?effective concentration? of a species in a mixture. By convention, it is a dimensionless quantity....
  • Activity coefficient
    Activity coefficient

    An activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture the interactions between each pair of chemical species are the same and, as a result, properties of the mixtures can be expressed directly in terms...


External links

  • Reaction quotient tutorial I
  • Reaction quotient tutorial II
  • Reaction quotient tutorial III