Reaction quotient definition

The equilibrium constant, K, is determined by the concentrations of reactants and products at equilibrium for a constant temperature. Therefore, of a reaction is always constant at a definite temperature. However, the value for reaction quotient Q is not constant. It is determined by the instantaneous concentrations of reactants and products. The reaction quotient expression is written as the same as expression for a reaction. For example, for the reaction ... 

Equations 27 and 28 permit a simple comparison to be made between the actual composition of a chemical system in a given state (degree of advancement) and the composition at the equilibrium state. If Q K, the affinity has a positive or negative value, indicating a thermodynamic tendency for spontaneous chemical reaction. Identifying conditions for spontaneous reaction and direction of a chemical reaction under given conditions is, of course, quite commonly applied to chemical thermodynamic principle (the inequality of the second law) in analytical chemistry, natural water chemistry, and chemical industry. Equality of Q and K indicates that the reaction is at chemical equilibrium. For each of several chemical reactions in a closed system there is a corresponding equilibrium constant, K, and reaction quotient, Q. The status of each of the independent reactions is subject to definition by Equations 26-28. 

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 ... 

Recall that for a system at equilibrium, AG = 0. This is the definition of thermodynamic equilibrium. Applying this definition to Equation 2.16 enables us to determine the precise ratio of reactant and product activities that lead to a perfect balance (equilibrium) between the reactant and product states in a chemical system. This specific value of the reaction quotient has a special name. It is known as the equilibrium... 

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. ... 

This equation is the link between tables of thermodynamic data (such as Table 4.2.1), which allow the evaluation of ArG°, and the equilibrium constant Kr of the reaction (sometimes also denoted as reaction quotient Qr), which is a function of the composition of the system in terms of concentration, molar fractions, and so on. The value and definition of Kr depends on the choice of the standard state and the ideality of the system, as shown subsequently for ideal and real gases, liquids, and gas-solid systems. 

Definitions. Define briefly (a) equilibrium, (b) equilibrium constant, (c) reaction quotient, (d) equilibrium condition, (e) heterogeneous equilibrium, (f) Le Chatelier principle. Choose a reaction and use it to illustrate each of these concepts. 

In many cases the functional or loading capacity is expressed in mmolg" polymer. For polymer-analogous derivatization reactions the definition of the degree of functionalization (DF) is useful. It is defined as the quotient of the content of new functional groups and that of original ftmctional groups [61]. 

To gauge the progress of a reaction relative to equilibrium, we use a quantity called the reaction quotient. The definition of the reaction quotient takes the same form as the definition of the equilibrium constant, except that the reaction need not be at equilibrium. So, for the general reaction ... 

What is the definition of the reaction quotient (Q) for a reaction What does Q measure ... 

The subscript eq on Q emphasizes that the equation, as written, applies only if the system has reached equilibrium. If we rearrange the expression above for In we get In Qeq = — ArG°/ RT. For a given reaction at a particular temperature, AfG° has a specific value, as demonstrated by Example 13-7. Therefore, In and also have certain fixed values once the reaction and the temperature are specified. Let s use the symbol K to represent the value of Qgq and call it the equilibrium constant. So, by definition, the equilibrium constant, K, represents the value of the reaction quotient, Q, at equilibrium. By replacing with K, we can write the equation above in the... 

If the reaction mixture is very dilute in the reactants and the products, the activity coefficients can all be approximated by unity. Then the last term on the right hand side of Eq. (2.20) vanishes, and the left hand side can be written as AG° = -RT n ATsolution, the equilibrium quotient becoming the equilibrium constant. Under ordinary conditions, however, the activity coefficient term must be taken into account, since there are solvent effects on all the terms on the right hand side except -RTIn K". The fact that different numbers of solvent molecules may specifically associate with the reactants and the products and that solvent molecules may be released or consumed in the reaction should not be included explicitly, since this effect is already covered by the terms in AG s of solvation of the reactants and products according to our definition of this concept. 

When we look carefully at Equations 18-11 and 18-12, we see that the constant is the electrode potential whenever the concentration quotient (actually, the activity quotient) has a value of 1. This constant is by definition the standard electrode potential for the half-reaction. Note that the quotient is always equal to 1 when the activities of the reactants and products of a half-reaction are unity. 

In spite of the outlined above formulation of chemical equilibrium problem in terms of rigorous thermodynamics (equilibrium constants defined as quotients of activities) which is well known and does not pose any special difficulty when it is compared with formulation in terms of conditional equilibrium constants (defined as quotients of concentrations), the former approach is not very popular, and many equilibrium constants of surface reactions reported in published papers were defined in terms of concentrations. Even praise of use of concentrations rather than activities in modeling of adsorption can be found in recent literature. Many publications do not address this question explicit, and then it is difficult to figure out how the equilibrium constants of surface species were defined K, or Accordingly, the equilibrium constants of surface species reported in tables of Chapter 4 constitute a mixture of constants defined in different ways (K, or The details regarding the definition of equilibrium constants can be found (but not always) in the original publications. 

There is one last mechanistic scenario that we want to cover. It relates to a fully established equilibrium with an intermediate. Remember that by definition the equilibrium constant for a reaction is the quotient of the forward and reverse rate constants. Therefore, the k] / fc ] term in Eq. 7.58 can be replaced by K, which represents the equilibrium constant for the formation of the intermediate. This leads to Eq. 7.60 for such a mechanism (see the Connections highlight on the next page for an example). 

---