Reaction quotient Information & Reaction quotient Links at HealthHaven.com

In chemistry, reaction quotient: Q_r is a function of the extent of reaction: ξ, 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
Buffer solution
Chemical equilibrium
Chemical stability
Complex dissociation constant
Dissociation constant
Distribution coefficient
Distribution ratio
Equilibrium constant
Equilibrium unfolding
Equilibrium stage
Liquid-liquid extraction
Phase diagram
Predominance diagram
Phase rule
Reaction quotient
Relative volatility
Solubility equilibrium Molar solubility
Stability constant
Stability constants of complexes
Thermodynamic equilibrium
Theoretical plate
Vapor-liquid equilibrium
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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 ...

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_r, is defined as ^[1]:

$Q_r = \frac{\left\{C_t\right\}^n \left\{D_t\right\}^p ...}{\left\{A_t\right\}^k \left\{B_t\right\}^m ...}$

where the { A_t } denotes the instantaneous activity^[2] of the species A at a certain moment of time (t) 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:

$K = \frac{\left\{C_{eq}\right\}^n \left\{D_{eq}\right\}^p...}{\left\{A_{eq}\right\}^k \left\{B_{eq}\right\}^m...}$

where {A_eq} is the activity of the species A when the mixture is at equilibrium, etc. By comparing the values of Q_r 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_r < K : The reaction will shift to the right (i.e. in the forward direction, and thus more products will form)

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

If Q_r = K : The reaction is at equilibrium

The relationship of reaction quotient Q_r with the instantaneous derivative of Gibbs energy (Δ_rG) and standard Gibbs energy change (Δ_rG^O) is given by

Δ_rG = Δ_rG^O + RT ln Q_r

[edit] See also

[edit] External links

Reaction quotient tutorial I Link
Reaction quotient tutorial II Link
Reaction quotient tutorial III Link

[edit] References

^ Zumdahl, Steven; Zumdahl, Susan (2003). Chemistry 6th Edition. Houghton Mifflin Company. ISBN 0-618-22158-1.
^ 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.

[0] Zumdahl, Steven; Zumdahl, Susan (2003). Chemistry 6th Edition. Houghton Mifflin Company. ISBN 0-618-22158-1.

[1] 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.

[1]

[2]