Reaction quotient

The reaction quotient, Q, is the ratio of product activities to reactant activities, each raised to its stoichiometric coefficient. It describes a reaction mixture’s composition at a particular moment.

Equilibrium constant: A constant value of the reaction quotient for a reaction at equilibrium under specified conditions. Comparing Q with this equilibrium value reveals whether the mixture is at equilibrium.
Chemical activity: A dimensionless effective concentration that accounts for nonideal interactions in a chemical system. Activities, rather than raw concentrations, define Q thermodynamically.
Reaction quotient in concentration form: An approximation to the reaction quotient that substitutes molar concentrations for activities. It is commonly used for dilute solutions when activity corrections are small.
Equilibrium constant expression: The product-over-reactant activity expression evaluated specifically at equilibrium. Unlike Q at any moment, this expression has the equilibrium value K.
Chemical equilibrium: A dynamic state in which forward and reverse reaction rates are equal, leaving macroscopic composition unchanged. At equilibrium, the reaction quotient equals the equilibrium constant.
Stoichiometric coefficient: The numerical factor specifying the relative amount of a substance in a balanced chemical equation. Each coefficient becomes an exponent on the corresponding activity in Q.
Nernst equation: An equation relating an electrochemical cell potential to its standard potential and reaction composition. The reaction quotient gives the composition-dependent term in the equation.
Mass action: A relationship connecting reaction rates or equilibrium composition to powers of reactant and product concentrations or activities. Its equilibrium form motivates the product-and-reactant powers used in Q.
Le Chatelier's principle: The principle that a system at equilibrium shifts in response to a disturbance in ways that oppose it. Changes in composition alter Q and prompt a shift toward a new equilibrium.
Chemical reaction: A process in which substances are transformed into other substances through rearrangement of atoms and bonds. A reaction equation specifies which activities belong in the quotient.

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