Acid and base conjugate

Figure 1 The tt-electron densities in imidazole, its conjugate acid and base, and pyrazole...

Explain why, in the study of an acid-base equilibrium, we observe absorption peaks of both species (conjugate acid and base) when using electronic absorption spectroscopy, but only a single peak by NMR. 

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

Pulse radiolysis. Consider the reaction shown in Eq. (11-58). Show that H and e q are related as conjugate acid and base. Calculate the pKa of H. ... 

EXAMPLE 10.1 Sample exercise Writing the formulas of conjugate acids and bases... 

Because conjugate acids and bases are in equilibrium in solution, we use the equilibrium constant for proton transfer between the solute and the solvent as an indicator of the strength of an acid or a base. For example, for acetic acid in water,... 

Write the formulas for conjugate acids and bases (Example 10.1). 

Buffers are often prepared with equal molar concentrations of the conjugate acid and base. In these equimolar solutions,... 

This very simple result makes it easy to make an initial choice of a buffer we just select an acid that has a pfC, close to the pH that we require and prepare an equimolar solution with its conjugate base. When we prepare a buffer for pH > 7 we have to remember that the acid is supplied by the salt, that the conjugate base is the base itself, and that the pKa is that of the conjugate acid of the base (and hence related to the pKh of the base by pR l + pKh = p/conjugate acid and base have unequal concentrations—such as those considered in Examples 11.1 and 11.2—are buffers, but they may be less effective than those in which the concentrations are nearly equal (see Section 11.3). Table 11.1 lists some typical buffer systems. 

Step 4 Find the molarities of the conjugate acid and base in solution. Use J1 = /V, where V is the total volume of the solution, V = Vanalvtc + Vtltrjllt. 

Step 5 Use an equilibrium table to find the H.O concentration in a weak acid or the OH concentration in a weak base. Alternatively, if the concentrations of conjugate acid and base calculated in step 4 are both large relative to the concentration of hydronium ions, use them in the expression for /<, or the Henderson—Hasselbalch equation to determine the pH. In each case, if the pH is less than 6 or greater than 8, assume that the autoprotolysis of water does not significantly affect the pH. If necessary, convert between Ka and Kh by using Kw = KA X Kb. 

FIGURE 11.10 The stoichiometric point of an acid base titration may be detected by the color change of an indicator. Here we see the colors of solutions containing a few drops of phenolphthalein at (from left to right) pH of 7.0, 8.5, 9.4 (its end point), 9.8, and 12.0. At the end point, the concentrations of the conjugate acid and base forms of the indicator are equal... 

Heisenberg uncertainty principle If the location of a particle is known to within an uncertainty Ax, then the linear momentum parallel to the x-axis can he known only to within an uncertainty Ap, where ApAx > till. Henderson-Hasselbalch equation An approximate equation for estimating the pH of a solution containing a conjugate acid and base. See also Section 11.2. Henry s constant The constant kH that appears in Henry s law. 

C17-0076. Write aii the conjugate acid-base equiiibrium expressions that appiy to an aqueous soiution of each of the substances in Probiem, and identify each conjugate acid and base. 

EXAMPLE 20.1. Write an equilibrium equation for the reaction of NH, and H 0. and label each of the conjugate acids and bases. 

Equilibria involving acids and bases are discussed from within the Lowry-Br0nsted theory, which defines an acid as a proton donor and a base as a proton acceptor (or abstracter ). The additional concept of pH is then introduced. Strong and weak acids are discussed in terms of the acidity constant Ka, and then conjugate acids and bases are identified. 

The answer is in the expression for the ionization constant, K l or Kb, where the ratio of the conjugate acid and base concentrations is found. In the case of a weak monoprotic acid, HA, we have the following ... 

When water dissociates into ions, we can write the following equation and identify the conjugate acid and base ... 

Pairs of conjugated acids and bases are always involved in proton exchange reactions (see p.30). The dissociation state of an acid-base pair depends on the concentration. Usually, it is not this concentration itself that is expressed, but its negative decadic logarithm, the pH value. The connection between the pH value and the dissociation state is described by the Henderson-Hasselbalch equation (below). As a measure of the proton transfer potential of an acid-base pair, its pKa value is used—the negative logarithm of the acid constant Kg (where a stands for acid). 

V. Experimental Determination of the Structure of Charged Species (Conjugated Acids and Bases)... 

The use of cross-polarization/magic angle spinning NMR may well become the most powerful method to determine the structure of conjugated acid and bases in the solid state. 

Acetate is a base because it can accept a proton to make acetic acid. Methylammonium ion is an acid because it can donate a proton and become methylamine. Acetic acid and the acetate ion are said to be a conjugate acid-base pair. Methylamine and methylammonium ion are likewise conjugate. Conjugate acids and bases are related to each other by the gain or loss of one H+. 

Conjugate acids and bases are related by the gain or loss of one proton. In these structures, a solid wedge is a bond coming out of the plane of the page and a dashed wedge is a bond to an atom behind the page. 

A buffered solution resists changes in pH when acids or bases are added or when dilution occurs. The buffer is a mixture of an acid and its conjugate base. There must be comparable amounts of the conjugate acid and base (say, within a factor of 10) to exert significant buffering. 

The Henderson-Hasselbalch equation tells us the pH of a solution, provided we know the ratio of the concentrations of conjugate acid and base, as well as p/f, for the acid. If a solution is prepared from the weak base B and its conjugate acid, the analogous equation is... 

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