Equilibrium constants for ionization

According to the Arrhenius definitions an acid ionizes m water to pro duce protons (H" ) and a base produces hydroxide ions (HO ) The strength of an acid is given by its equilibrium constant for ionization m aqueous solution... 

The carbon-metal bonds of organolithium and organomagnesium compounds have appreciable carbamomc character Carbanions rank among the strongest bases that we 11 see m this text Their conjugate acids are hydrocarbons—very weak acids indeed The equilibrium constants for ionization of hydrocarbons are much smaller than the s for water and alcohols thus hydrocarbons have much larger pA s... 

It is always important to keep in mind the relative nature of substituent effects. Thus, the effect of the chlorine atoms in the case of trichloroacetic acid is primarily to stabilize the dissociated anion. The acid is more highly dissociated than in the unsubstituted case because there is a more favorable energy difference between the parent acid and the anion. It is the energy differences, not the absolute energies, that determine the equilibrium constant for ionization. As we will discuss more fully in Chapter 4, there are other mechanisms by which substituents affect the energy of reactants and products. The detailed understanding of substituent effects will require that we separate polar effects fiom these other factors. 

Equilibrium constants for ionization reactions are usually called ionization or dissociation constants, often designated Ka. The dissociation constants of some acids are given in Figure 2-16. Stronger acids, such as phosphoric and carbonic acids, have larger dissociation constants weaker acids, such as monohydrogen phosphate (Ill Of ), have smaller dissociation constants. 

The underlying assumption in this concept is the absence of steric effects. Using the rate and equilibrium constants for ionization of benzoic acid as references, Hammett found that equilibrium constants for a variety of reactions showed a linear relationship with o and defined the LFER. Data for these equilibria are typically graphed as illustrated in Figure 5.4. 

Because ionization constants are equilibrium constants for ionization reactions, their values indicate the extents to which weak electrolytes ionize. At the same concentrations, acids with larger ionization constants ionize to greater extents (and are stronger acids) than acids with smaller ionization constants. From Table 18-4, we see that the order of decreasing acid strength for these five weak acids is... 

As first approximations, equilibrium constants for ionization of SOH and for Na and CF binding on quartz and corundum were assumed equal to values selected by Davis et ai (27) for amorphous silica and by James and Parks (28) for Y-AI2O3, respectively. If necessary, the equilibrium constants were adjusted to fit experimental data for systems more closely approximating those in which surfactant adsorption data were available. The adjusted equilibrium constants are summarized in Table I. Sources of data and details of fitting procedure are described below. 

The acid dissociation constant is the equilibrium constant for ionization of an acid and is expressed in negative log units. 

Most acids are weak acids, which ionize only to a limited extent in water. At equilibrium, aqueous solutions of weak acids contain a mixture of nonionized acid molecules, ions, and the conjugate base. Examples of weak acids are hydrofluoric acid (HF), acetic acid (CH3COOH), and the ammonium ion (NH4). The limited ionization of weak acids is related to the equilibrium constant for ionization, which we will study in the next section. 

The strength of an acid is given by its equilibrium constant for ionization in aqueous solution ... 

The outstanding properly of each acid-base system is its pH curve the one shown in Fig. 2.12 is typical of a base neutralized by an acid. The shape of the curve is related to the equilibrium constants for ionization of the acid and base, and the concentrations of each of the ions. But the basis of the coordinate system is logarithmic, in that pH is defined as the negative logarithm of the hydrogen-ion concentration, in gram-ions/ liter ... 

The strength of an acid with the general formula HA is given by the equilibrium constant for ionization, which is obtained from the equation for ionization. 

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