In chemistry, reaction quotient: Qr 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.
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, Qr, is defined as [1]:
where the { At } 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:
where {Aeq} is the activity of the species A when the mixture is at equilibrium, etc. By comparing the values of Qr 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 Qr < K : The reaction will shift to the right (i.e. in the forward direction, and thus more products will form)
- If Qr > K : The reaction will shift to the left (i.e. in the reverse direction, and thus more reactants will form)
- If Qr = K : The reaction is at equilibrium
The relationship of reaction quotient Qr with the instantaneous derivative of Gibbs energy (ΔrG) and standard Gibbs energy change (ΔrGO) is given by
ΔrG = ΔrGO + RT ln Qr
See also
External links
- Reaction quotient tutorial I Link
- Reaction quotient tutorial II Link
- Reaction quotient tutorial III Link
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.
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