Reaction quotient defined

The first term, AG°, is the change in Gibb s free energy under standard-state conditions defined as a temperature of 298 K, all gases with partial pressures of 1 atm, all solids and liquids pure, and all solutes present with 1 M concentrations. The second term, which includes the reaction quotient, Q, accounts for nonstandard-state pressures or concentrations. Eor reaction 6.1 the reaction quotient is... 

Traditional chemical kinetics uses notation that is most satisfactory in two cases all components are gases with or without an inert buffer gas, or all components are solutes in a Hquid solvent. In these cases, molar concentrations represented by brackets, are defined, which are either constant throughout the system or at least locally meaningful. The reaction quotient Z is defined as... 

At any point during the preceding reaction, a relationship may be defined called the reaction quotient, Q. It has the following form ... 

The reaction quotient Qc is defined in the same way as the equilibrium constant Kc except that the concentrations in Qc are not necessarily equilibrium values. 

The value of the equilibrium constant for a reaction makes it possible to judge the extent of reaction, predict the direction of reaction, and calculate equilibrium concentrations (or partial pressures) from initial concentrations (or partial pressures). The farther the reaction proceeds toward completion, the larger the value of Kc. The direction of a reaction not at equilibrium depends on the relative values of Kc and the reaction quotient Qc, which is defined in the same way as Kc except that the concentrations in the equilibrium constant expression are not necessarily equilibrium concentrations. If Qc Kcr net reaction goes from left to right to attain equilibrium if Qc > Kc/ net reaction goes from right to left if Qc = Kc/ the system is at equilibrium. 

The IP is defined in the same way as Ksp/ except that the concentrations in the expression for IP are initial concentrations (that is, arbitrary concentrations at time f), not necessarily equilibrium concentrations. Thus, the IP is actually a reaction quotient Qc (Section 13.5), but the term ion product is more descriptive because the equilibrium constant expression isn t a quotient. 

That is, the particular function of the three concentrations defined by Q (reaction quotient) is always the same at equilibrium. This is true even though any individual concentration may vary by as much as a magnitude of 10. This unifying principle allows the calculation of conditions at equilibrium under virtually any set of conditions. 

Equation (12.10) is known as the Nernst equation, where R is the gas constant (8.314 J mol K ), T the temperature in Kelvin, Q the reaction quotient as defined in Eq. (12.8), T the Faraday constant (9.65 x 10" C mol ) and n being the number of electrons involved during the oxido-reduction reaction. By using log (to the base 10) instead of the natural logarithm, replacing the variables with their numerical values and fixing the temperature at 25 °C (298 K), the Nernst equation becomes ... 

Now, if we define A G in a similar way as A G°, using the reaction quotient (instead of the equilibrium constant), given by... 

The equilibrium constant, K, and the reaction quotient, Q, are expressed fundamentally in terms of activities, /. To describe equilibrium or disf qui-librium in terms of concentrations, we must bring in the relationships discussed in Section 2.3, namely, activity = concentration X activity coefficient. In each phase and for each type of species, a reference state defines the numerical scale... 

The reaction quotient Q for a general gas-phase reaction is defined as... 

Suppose a solution is prepared by mixing one soluble salt, such as AgN03, with a solution of a second, such as NaCl. Will a precipitate of very slightly soluble silver chloride form To reach an answer, the reaction quotient Q that was defined in connection with gaseous equilibria (see Section 14.6) is used. The initial reaction quotient Qo, when the mixing of the solutions is complete but before any reaction occurs, is... 

For the moment, consider only reactions involving chemicals dissolved in water. The preceding equations can be combined with the definition of the reaction quotient, Eq. [1-10], to define an equilibrium constant, K, that applies to the final expected chemical composition of the system ... 

Define reaction quotient. How does it differ from equilibrium constant ... 

The quotient of activities and partial pressure which appears as a logarithmic term in the expression for AG is one which is of the same algebraic form as that appearing in the equihbrium constant. It, however, relates to a situation when the reaction is not at equilibrium. It must be clearly distinguished from the quotient defining the equihbrium constant which describes a situation when reaction is at equilibrium. It is for this reason that a subscript eq is always added to the quotient describing equihbrium values. 

Plan We have to And the composition of the equilibrium mixture, in other words, the equilibrium concentrations. As always, we use the balanced equation to write the reaction quotient. We find the initial [CO] and [H2O] from the given amounts (0.250 mol of each) and volume (0.125 L), use the balanced equation to define x and set up a reaction table, substitute into 2o and solve for x, from which we calculate the concentrations. 

Defining the equilibrium constant in terms of the reaction quotient (544) ... 

This approach can be formalized by defining a quantity called the reaction quotient. The reaction quotient, Q, is a number obtained by substituting reactant and product concentrations or partial pressures at any point during a reaction into an equilibrium-constant expression. Therefore, for the general reaction... 

But how does this response offset the applied stress When we began our discussion of the equihbrium constant in Section 12.3, we first defined the reaction quotient, Q, and then considered the equilibrium constant as a special case of this ratio. Thus at equihbrium, we have the relationship, Q = K But if we change the concentrations fi-om their equilibrium values, Q will no longer be equal to K Immediately following an increase in the concentration of a reactant, as in Figure 12.7, Q must become smaller than K. So to establish a new equilibrium, the value of the ratio must increase. Thus the system must respond by increasing the con-... 

At any point of a reaction, the ratio of products to reactants, in this case [B]/[A], is defined by the reaction quotient Q. Formally, and more generally, we can define Q as... 

Define the reaction quotient, Q. Distinguish between Q andiC... 

The for a half-reaction is the potential of that reaction versus the standard hydrogen electrode, with all species at unit activity. Most reduction potentials are not determined under such conditions, so it is expedient to define a formal reduction potential. This is a reduction potential measured under conditions where the reaction quotient in the Nernst equation is one and other nonstandard conditions are described solvent, electrolyte, pN, and so on. Formal reduction potentials are represented by °. Reduction potentials determined by cyclic voltammetry are usually formal potentials. The difference between standard and formal potentials is not expected to be great. Other definitions of the formal potential are offered. ... 

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