Equilibrium constant acid dissociation

Apparent forward (or backward) activation rate constant Equilibrium constant Acidity dissociation equilibrium constant Formal acidity dissociation equilibrium constant... 

Each dissociation step has an associated equilibrium constant (acid dissociation constant), and it is general for polybasic acids that Aia(l) > K 2), and so on it is more difficult to remove H from an anion than from a neutral species. Values of equilibrium constants may be temperature-dependent, and the inclusion of the temperature to which... 

Acid dissociation constant (Section 1 12) Equilibrium constant for dissociation of an acid... 

In contrast to the reactions of the cycloamyloses with esters of carboxylic acids and organophosphorus compounds, the rate of an organic reaction may, in some cases, be modified simply by inclusion of the reactant within the cycloamylose cavity. Noncovalent catalysis may be attributed to either (1) a microsolvent effect derived from the relatively apolar properties of the microscopic cycloamylose cavity or (2) a conformational effect derived from the geometrical requirements of the inclusion process. Kinetically, noncovalent catalysis may be characterized in the same way as covalent catalysis that is, /c2 once again represents the rate of all productive processes that occur within the inclusion complex, and Kd represents the equilibrium constant for dissociation of the complex. 

For acids that can be studied in aqueous solution, we measure the strenfitfr. by the magnitude of the equilibrium constant for dissociation, Ka. This quantity is defined by first writing the equilibrium constant K a for Reaction 3.15, using... 

To graph the curves representing [HA] and [A-], a mathematical expression of each as a function of [H+] (a function of the master variable) is needed. The appropriate equation for [HA] is derived by combining the equilibrium constant for dissociation of a weak acid [equation (2.10)] with the mass balance equation [equation (2.13)] to yield... 

Dibasic acids dissociate in two steps, and both dissociation equilibria can be characterized by separate dissociation equilibrium constants. The dissociation of the dibasic acid H2A can be represented by the following two equilibria ... 

The enthalpy change of some reactions can be measured directly, but for those that do not go to completion (as is common in acid-base reactions), thermodynamic data from reactions that do go to completion can be combined using Hess s law to obtain the needed data. For example, the enthalpy and entropy of ionization of a weak acid, HA, can be found by measming (1) the enthalpy of reaction of HA with NaOH, (2) the enthalpy of reaction of a strong acid (such as HCl) with NaOH, and (3) the equilibrium constant for dissociation of the acid (usually determined from the titration curve). 

Note that [H2O] does not appear in the denominator of either equation because the concentration of water is so large relative to the concentration of the weak acid or base that the dissociation does not alter [H2O] appreciably (see Feature 9-2). Just as in the derivation of the ion-product constant for water, [H2O] is incorporated into the equilibrium constants and Dissociation constants for weak acids are found in Appendix 3. 

To understand of specific phenomena on the basis of a few simple principles. Examples (i) the concept of the equilibrium constant for dissociation of acetic acid, (ii) the effect of surface potential on the surface reaction enthalpy. 

The degree of ionization of weak polyelectrolytes depends on the degree of neutralization, a, and therefore on the pH. To monitor the ionization behavior of the weak polyelectrolytes, one can measure the pH while adding strong acid or base solution. The degree of neutralization a can be calculated from the added amount of acid or base. An important value is the pKa or pKb, which denotes the negative decadic logarithm of equilibrium constant of dissociation of acids or protonation of bases. 

If A° can be found and A is known for various stoichiometric concentrations, then values of a can be found at the various concentrations, and from these the equilibrium constant for dissociation into ions for the specihc electrolyte can be found. This provides an alternative route to Ks for acid/base equilibria. 

HCl is a much better proton donor than H3O+. Consequently the forward reaction predominates, the reverse reaction is inconsequential, and hydrochloric acid is termed a strong acid. As we learned in Chapter 8, reactions in which the forward reaction is strongly favored have large equilibrium constants. The dissociation of hydrochloric acid is so favorable that we describe it as 100% dissociated and use only a single forward arrow to represent its behavior in water ... 

The tendency of the acid (HA) to dissociate and donate a hydrogen ion to solution is denoted by its K, the equilibrium constant for dissociation of a weak acid (Equation 4.4). The higher the K, the greater is the tendency to dissociate a proton. 

The pKa of a Bronsted acid provides a description of the tendency of the acid to donate a proton in an aqueous (water-based) solution. A lower pKa is associated with a stronger tendency to donate protons, and thus a stronger Bronsted acid. The pKa is calculated mathematically as the negative logarithm (base 10) of the equilibrium constant for dissociation of a proton from the acid in water. 

Any quantitative measure of the acidity of organic acids or bases involves measuring the equilibrium concentrations of the various components in an acid-base equilibrium. The strength of an acid is then expressed by an equilibrium constant. The dissociation (ionization) of acetic acid in water is given by the following equation ... 

Dissociation Constant For an acid, the equilibrium constant JCa for the dissociation of the acid into its conjugate base and a proton. For a complex of two biomolecules, the equilibrium constant for dissociation into the component molecules. 

Equilibrium constant for dissociation of first proton from acid. 

C. Aqueous Equilibrium Constants 1. Dissociation Constants for Acids at 25 °C... 

Kg is the dissociation constant for an acid. It is the equilibrium constant forthe dissociation of a weak acid, HA(aq) ... 

Tables of calculated equilibrium constants for dissociation reactions of the monocarboxylic, dicarboxylic, hydroxy, and aromatic acids listed in Table 1...

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

The acid dissociation constant has the same form m Brpnsted-Lowry as m the Arrhenius approach but is expressed m the concentration of H30" rather than The concentration terms [H30" ] and [H" ] are considered equivalent quantities m equilibrium constant expressions... 

All these facts—the observation of second order kinetics nucleophilic attack at the carbonyl group and the involvement of a tetrahedral intermediate—are accommodated by the reaction mechanism shown m Figure 20 5 Like the acid catalyzed mechanism it has two distinct stages namely formation of the tetrahedral intermediate and its subsequent dissociation All the steps are reversible except the last one The equilibrium constant for proton abstraction from the carboxylic acid by hydroxide is so large that step 4 is for all intents and purposes irreversible and this makes the overall reaction irreversible... 

The equilibrium constant for this reaction is called an acid dissociation constant, K-, and is written as... 

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