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 equal to the equilibrium constant.
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 is in equilibrium with σS + τT
The reaction is written as an equilibrium even though in many cases it may appear to have gone to completion. When a mixture of A and B is made up and the reaction is allowed to occur, the reaction quotient, Qr, is defined as:

where {Xt} denotes the instantaneous activity of a species X at time, t. A compact general definition is (where Пj is the product across all j-indexed variables, and idem for Пi):

where the numerator is a product of reaction product activities, a j, each raised to the power of a stoichiometric coefficient, ν j, and the denominator is a similar product of reactant activities. All activities refer to a time t.
As the reaction proceeds (assuming that there is no energy barrier to the reaction) with the passage of time the species' activities and hence the reaction quotient change. Eventually the activities become constant and the mixture is then said to be at equilibrium. An equilibrium constant, K, can be expressed symbolically as
- K = Qr (t=∞)
though equilibrium will be effectively reached in a finite time in most reactions.
The following video is about the reaction quotient which is being explained to you by a lecturer of a leading educational institute and his name is Mr. Lalit Sardana (IIT-JEE AIR 243). He has been working in the field of education since past many years and has been playing a key role in getting students into IITs and NITs.
He has explained the students all the important topics like product of concentration of products,product of concentration of reactants, mass of action ratio. The students who refer to this video will be able to understand things effectively and thus score better marks in their 11th and 12th class examinations. The video is also useful for competitive exams like JEE Mains, JEE Advanced, VITEEE, MPPET, AIPMT, AFMC, BITSAT,etc.
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