Mass-action expression quotient

Many systems are not at equilibrium. The mass action expression, also called the reaction quotient, Q, is a measure of how far a system is from equilibrium and in what direction the system must go to get to equilibrium The reaction quotient has the same form as the equilibrium constant, K, but the concentration values put into Q are the actual values found in the system at that given moment. 

The reaction quotient is sometimes called the mass action expression. 

Reaction quotient, Q The mass action expression under any set of conditions (not necessarily equiUbrium) its magnitude relative to determines the direction in which reaction must occur to establish equilibrium. 

Strictly constant values for this quotient are obtained only at constant temperature and pressure and by use of the thermodynamic activity for each term in the mass action expression (2-1). This is related to molarity in the solution by... 

The reaction quotient Is sometimes The reaction quotient, Q, for the general reaction is given as follows, called the mass action expression. 

Reaction quotient (in precipitation reactions) (Qsp) The mass action expression that applies to the dissolution of a slightly soluble compound this has the same algebraic form as that for Ksp, except that the concentrations are not necessarily equilibrium ones. 

The particular ratio of concentration terms that we write for a given reaction is called the reaction quotient (Q, also known as the mass-action expression). For the reversible breakdown of N2O4 to NO2, the reaction quotient, which is based directly on the balanced equation, is... 

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Note that molar concentrations must be used in the expression for K and that K equals the reaction quotient Q only at equilibrium (Fig. 1-8). Equation [1-12], the expression of the equilibrium state of a reaction, is known as the mass action law and is a direct consequence of the minimization of Gibbs free... 

This expression can be substituted for H+ in the mass-action quotients to give... 

Plan Using the law of mass action, we write each expression as a quotient having the product concentration terms in the numerator and the reactant concentration terms in the denominator. Each concentration term is raised to the power of its coefficient in the balanced chemical equation. 

Depending on the values chosen to write the compositions of the multi-component phases, the expression of the reaction quotient may take a variety of forms, and therefore there are also different possible forms for the law of mass action. 

This expression is known as the law of mass action. Like the equilibrium constant K, Q is subscripted with a c to indicate that the quotient is defined in terms of concentrations. For the N2O4-NO2 system, this approach gives the same equilibrium expression as we got from our kinetics approach [N02]eq/[N204leq. But the law of mass action was developed empirically from countless observations of many different reactions—long before the principles of kinetics were developed. Additionally, it applies not only to elementary reactions, but also to more complex reactions that occur via a series of steps. Furthermore, the law of mass action enables us to write the equilibrium expression for any reaction for which we know the balanced equation. Knowing the equilibrium expression for a reaction, we can use equilibrium concentrations to calculate the value of the equilibrium constant. 

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