Reaction quotient

Description

In chemistry, a reaction quotient: Qr is a function of the activities or concentrations of the chemical species involved in a chemical reaction. In the special case that the reaction is at equilibrium the reaction quotient is constant and equal to the equilibrium constant which appears in the expression of the law of mass action.

A general chemical reaction in which α moles of a reactant A and β moles of a reactant B react to give σ moles of a product S and τ moles of a product T can be written as:

αA + βB <=>σS + τT

The reaction is written as an equilibrium even though in many cases it may appear that all of the reactants on one side have been converted to the other side. When a mixture of A and B is made up and the reaction is allowed to occur, the reaction quotient, Qr, is defined as shown here.

Related formulas

Variables

Qrreaction quotient (dimensionless)
Stinstantaneous activity of species S at time t (dimensionless)
σmoles of a reactant S (dimensionless)
Ttinstantaneous activity of species T at time t (dimensionless)
τmoles of a reactant T (dimensionless)
Atinstantaneous activity of species A at time t (dimensionless)
αmoles of a reactant A (dimensionless)
Btinstantaneous activity of species B at time t (dimensionless)
βmoles of a reactant B (dimensionless)