Conjugated base pair

Polarization of the solvent medium rather than that of the acid-conjugate base pair better explains the observed influence of fluorination on acidity in solution. These interactions can be attributed to the dipole-mediated assistance to solvent organization.33... 

In order to avoid proliferation of tables, it iscus tomary to report only one constant for each conjugate acid-conjugate base pair. The reader may easily verify thatif acid A is a stroneer acid an H B. the conjugate base of A will be a weaker base than the conjugal nf R—... 

For each of the compounds in the Acid-Base Properties of Salts activity (eChapter 15.14), write the reaction that takes place when the compound is placed in water. Identify the conjugate base pairs in each reaction. 

Example In the following reactions, identify the acid/conjugate base pair and the... 

The equilibrium reaction for this acid/conjugate base pair is ... 

A plot of dissociation fraction against pH for an acid-conjugate base pair is shown in Figure 1. 

Relative Strengths of Acid-Conjugate Base Pairs... 

BRONSTED CONCEPT OF CONJUGATE ACTD-CONJUGATE BASE PAIRS... 

The ionization of water itself can be considered in two ways (1) as a simple dissociation to yield and OH" ions and (2) in terms of Bronsted conjugate acid-conjugate base pairs. In either case, it is obvious that waier is amphoteric—it yields both H and OH" ions it can both donate and accept protons. 

Note that water produces two conjugate id-conjugate base pairs ... 

The two conjugate acid-conjugate base pairs involved are R-NH /R-NHj and HOH/OH", The ionization can be described by an ionization constant,... 

Figure 3.3 Species concentration as a function of pH for H3PO4, with pAai = 2.2, pAa2 = 7.2, pAa3 = 12.2. Note how each acid and conjugate base concentration varies with pH and that the concentrations of each coupled acid/conjugate base pair cross (become equal) at their related pAa.

Weak acid/conjugate base pairs In the Brpnsted-Lowry view, solute pairs that differ from one another by one proton. 

BUFFERING CAPACITY The capacity of a buffer to maintain a specific pH depends on two factors (1) the molar concentration of the acid-conjugate base pair and (2) the ratio of their concentrations. Buffering capacity is directly proportional to the concentration of the buffer components. In other words, the more molecules of buffer present, the more H+ and OH ions can be absorbed without changing the pH. The concentration of the buffer is defined as the sum... 

PHOSPHATE BUFFER Phosphate buffer consists of the weak acid-conjugate base pair H2P04 /HP042 (Figure 3.19) ... 

The shaded band indicates the pH range over which the weak acid-conjugate base pair H2P047HP042 functions effectively as a buffer. 

The extent of the ionization step depends on the relative strength of the conjugate acid-conjugate base pairs. The amphiprotic properties of the solvent have an essential effect on the equilibrium constant of this reaction step. The extent of the dissociation step is influenced by the polarity of the solvent, increasing with the dielectric constant of the solvent. In water, all products of acid-base reactions of moderate to low concentrations are essentially completely dissociated into solvated ions (Pecsok et al., 1976). The dissociation step is suppressed by addition or substitution with cosolvents of lower polarity, e.g., alcohols in aqueous formulations. The ion-pair aggregates may have absorption spectra different from the dissociated species. Thus, the amphiprotic properties and polarity (expressed as the dielectric constant) of the solvent are essential for the acid-base equilibrium of the drug and thus the absorption spectrum of the compound. This subject is further discussed in Section 14.2.3. 

Consider the buffer solution prepared by mixing together acetic acid—HC2H3O2—and sodium acetate—C2H3O2- containing Na+ as a spectator ion (See Skill 2.1b). The equilibrium reaction for this acid/conjugate base pair is ... 

Write balanced chemical equations for the two successive ionizations of carbonic acid in water. Identify the conjugate-base pair in each of the equations. 

An important relationship exists between the Ka value of an acid and the Kb value for its conjugate base. Using hydrocyanic acid, HCN, and the cyanide ion, CN , as an example of an acid-conjugate base pair, we can write equations for the donation of a proton to water by HCN and for the acceptance of a proton from water by CN ,... 

We can plot the pC-pH diagrams for these two acid-conjugate base pairs on the same diagram (Fig. 4-8). Because = x.ac is the same for each pair, some of the lines are superimposed. The proton condition is... 

From an examination of the equilibrium constants in Table 4-1, determine which acid-conjugate base pair should be used to buffer a solution at (i) pH 4.7 and (ii) pH 7. What ratio of acid to conjugate base should be used in eaqh of these buffers ... 

If no weak acid-conjugate base pair were present, derivation by the above procedure would have yielded the buffer intensity attributable to water ... 

More than one acid-conjugate base pair may be used to buffer a solution. A derivation similar to the previous equations yields the equations applicable to such a solution,... 

The maximum amount of ammonia that will be used in the experiments is 12.5 mg as N/liter (0.89 X 10 moles/liter). Select an appropriate acid-conjugate base pair and determine the concentration of it that will control the pH to within 0.5 units of 8.0 during the reaction. Neglect ionic strength effects the temperature = 25°C. Determine the buffer intensity of this solution. (Note There is a detailed discussion of the breakpoint chlorination reaction in Chapter 7.)... 

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