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.

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 ... $ightleftharpoons$ 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 [cite book | author=Zumdahl, Steven; Zumdahl, Susan | title=Chemistry 6th Edition| publisher=Houghton Mifflin Company | year=2003 | id=ISBN 0-618-22158-1] :

:$Q\; =\; frac\{left\{C\_i\; ight\}^n\; left\{D\_i\; ight\}^p\; ...\}\{left\{A\_i\; ight\}^k\; left\{B\_i\; ight\}^m\; ...\}$

where the { "A"_i } denotes the "instantaneous" activity [Under certain circumstances (see chemical equilibrium) each activity term such as {"A"} may be replaced by a concentration term, ["A"] . Both the reaction quotient and the equilibrium constant are then concentration quotients.] 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:

:$K\; =\; frac\{left\{C\; ight\}^n\; left\{D\; ight\}^p...\}\{left\{A\; ight\}^k\; left\{B\; ight\}^m...\}$

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 (ΔG^O) is given by Δ"G" = Δ"G"^O + RT ln "Q"

ee also

*Chemical equilibrium
*Le Chatelier's Principle
*Equilibrium Constant
*Activity
*Activity coefficient

External links

* Reaction quotient tutorial I [http://elchem.kaist.ac.kr/vt/chem-ed/courses/equil/intro/reactquo.htm Link]
* Reaction quotient tutorial II [http://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Reaction_Quotient.htm Link]
* Reaction quotient tutorial III [http://www.cartage.org.lb/en/themes/sciences/chemistry/Miscellenous/Helpfile/Equilibrium/ReactionQuotient.htm Link]

References