Acid-base pair

In any acid-base equilibrium, both the forward reaction (to the right) and the reverse reaction (to the left) involve proton transfer. For example, consider the reaction of an acid HX with water  

In the forward reaction, HX donates a proton to H2O. Therefore, HX is the Bronsted-Lowry acid and H2O is the Bronsted-Lowry base. In the reverse reaction, the H3O ion donates a proton to the X ion, so H3O is the acid and X is the base. When the acid HX donates a proton, it leaves behind a substance, X , that can act as a base. Likewise, when H2O acts as a base, it generates H3O, which can act as an acid. 

In any acid—base (proton-transfer) reaction, we can identify two sets of conjugate acid—base pairs. For example, consider the reaction between nitrous acid and water  

Analyze We are asked to give the conjugate base for several acids and the conjugate acid for several bases. 

Plan The conjugate base of a substance is simply the parent substance minus one proton, and the conjugate acid of a substance is the parent substance plus one proton. 

At the beginning of this chapter, we introduced the concept of conjugate acids and conjugate bases. In this section, we examine the properties of conjugate acids and bases, independent of their parent compounds. 

According to the Bronsted-Lowry definition of acids and bases, when an acid donates a proton, what is left of the acid is itself a base—we call it the conjugate base of the acid. Likewise, when abase accepts a proton, it becomes an acid—the conjugate acid of the base. The idea of conjugate acid-base pairs is illustrated in the following reaction  

In this reaction, H PO is the conjugate base of HjPO, and H O is the conjugate acid of OH7 

The conjugate base of an acid is what is ieft when is removed from the acid. 

The conjugate acid of a base is what is formed when H+ is added to the base. 

Acids are listed from strongest to weakest, i.e., in order of decreasing strength. Bases are listed in order of increasing strength. 

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Most proton transfer reactions are fast they have been carefully studied by relaxation methods. A system consisting of a conjugate acid-base pair in water is a three-state cyclic equilibrium as shown in Scheme IV. [The symbolism is that used by Bemasconi. ... 

Another important acid-base process is the transfer of a proton from one solute acid-base pair to a second solute acid-base pair. This can take place via three pathways, shown in Scheme VI. 

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

For each of the following reactions, indicate the Brensted-Lowry adds and bases. What are the conjugate acid/base pairs ... 

Consider the weak acids in Table 13.2. Which acid-base pair would behest for a buffer at apH of... 

A buffer is prepared using the propionic acid/propionate (HC3H502/ C3H502 ) acid-base pair for which the ratio [HC3H502]/[C3H502 ] is 4.50. 

Write the equations for the reaction between each of the following acid-base pairs. For each reaction, predict whether reactants or products are favored (using the values of K given in Appendix 2). 

Scheme (b) includes reactions formerly described by a variety of names, such as dissociation, neutralisation, hydrolysis and buffer action (see below). One acid-base pair may involve the solvent (in water H30+ —H2OorH20 — OH ), showing that ions such as HsO+ and OH- are in principle only particular examples of an extended class of acids and bases though, of course, they do occupy a particularly important place in practice. It follows that the properties of an acid or base may be greatly influenced by the nature of the solvent employed. 

To use the above expression for measuring the strength of an acid, a standard acid-base pair, say A2-B2, must be chosen, and it is usually convenient to refer acid-base strength to the solvent. In water the acid-base pair H30 + -H20 is taken as the standard. The equilibrium defining acids is therefore ... 

To express the relative strengths of an acid and its conjugate base (a conjugate acid-base pair ), we consider the special case of the ammonia proton transfer equilibrium, reaction C, for which the basicity constant was given earlier (Kb = [NH4+l[OH ]/ NH3]). Now let s consider the proton transfer equilibrium of ammonia s conjugate acid, NH4+, in water ... 

In this expression, Ka is the acidity constant of a weak acid and Kh is the basicity constant of the conjugate base of that acid. The acid and base must form a conjugate acid-base pair (such as CH C00H/CH3C02 or NH4+/NH3). We can express Eq. 1 la in another way by taking logarithms of both sides of the equation ... 

FIGURE 10.17 As shown here for five conjugate acid-base pairs, the sum of the pKa of an acid (pink) and the p/Ch of its conjugate base (blue) is constant and equal to pKw, which is 14.00 at 25°C. 

TABLE P a 10.3 Conjugate Acid-Base Pairs Arranged by Strength Acid name Acid formula Base formula Base name PKb... 

Write the proton transfer equilibria for the following acids in aqueous solution and identify the conjugate acid-base pairs in each one (a) H2S04 (b) C6H5NH3+. anilinium ion ... 

Write the two proton transfer equilibria that show the amphiprotic character of (a) H2P04 (b) HC204, hydrogen oxalate ion. Identify the conjugate acid-base pairs in each equilibrium. 

A buffer is a mixture of a weak conjugate acid-base pair that stabilizes the pH... 

Relation between acidity and basicity constants of a conjugate acid—base pair ... 

The synthesis of three silaketenimines 105a-c prompted Tokitoh and Okazaki to calculate the optimized geometry of a model compound, PhiSiCNPh. This model reinforced that 105a-c are truly Lewis acid-base pairs, with the isocyanide donating its carbon lone pair to an empty p-orbital perpendicular to the lone pair... 

Water can act as an acid or a base, so there are two conjugate acid-base pairs for water H3 and H2 O are a conjugate acid-base pair, and H2 O and OH" are another conjugate acid-base pair. Example reinforces the structural relationships of conjugate acid-base pairs. 

The introduction of conjugate acid-base pairs completes our inventory of acids and bases, hi addition to strong bases, ammonia, and amines, the anions of weak acids act as bases. 

C17-0020. Draw ball-and-stick models for the conjugate acid-base pair of each weak acid and weak base in Section Exercise. ... 

C17-0039. Write a paragraph describing the conjugate acid-base pair and explaining how each interacts with water. C17-0040. Update your list of memory bank equations. 

This equation is exact, but it can be simplified by applying one of the key features of buffer solutions. Any buffer solution contains both members of a conjugate acid-base pair as major species. In other words, both the weak acid and its conjugate base are present in relatively large amounts. As a result, the change to equilibrium, x, is small relative to each initial concentration, and the equilibrium concentrations are virtually the same as the initial leq linitial " = linitial... 

The pH is close to the p of the conjugate acid-base pair, so this is a reasonable result. 

The pH of the buffer solution, both before and after adding the solid NaOH, is close to the p of the conjugate acid-base pair. Moreover, the pH increases when NaOH is added. The solution becomes more basic as a consequence of the added hydroxide anions. 

With a given weak acid, a buffer soiution can be prepared at any pH within about one unit of its p vaiue. Suppose, for exampie, that a biochemist needs a buffer system to maintain the pH of a soiution ciose to 5.0. What reagents shouid be used According to the previous anaiysis, the weak acid can have a p Z a between 4.0 and 6.0. As the p deviates from the desired pH, however, the soiution has a reduced buffer capacity. Thus, a buffer has maximum capacity when its acid has its p as ciose as possibie to the target pH. Tabie 18-1 iists some acid-base pairs often used as buffer soiutions. For a pH - 5.0 buffer, acetic acid (p Za — 4.75) and its conjugate base, acetate, wouid be a good choice. 

Because we know we are dealing with a buffer solution made from a specific conjugate acid-base pair, we can work directly with the buffer equation. We need to calculate the ratio of concentrations of conjugate base and acid that will produce a buffer solution of the desired pH. Then we use mole-mass-volume relationships to translate the ratio into actual quantities. 

A practical problem in solution preparation usually requires a different strategy than our standard seven-step procedure. The technician must first identify a suitable conjugate acid-base pair and decide what reagents to use. Then the concentrations must be calculated, using pH and total concentration. Finally, the technician must determine the amounts of starting materials. The technician needs a buffer at pH = 9.00. Of the buffer systems listed in Table 18-1. the combination of NH3 and NH4 has the proper pH range for the required buffer solution. 

C18-0053. From Table 18-1. select the best conjugate acid-base pairs for buffer solutions at pH 3.50 and 12.60. If you were going to add HCl solution as part of the buffer preparation, what other substance should you use in each case ... 

Acid-base reactivity is an important property of oxide catalysts, and its control is of interest in surface chemistry as well as being of importance in industrial applications. The exposed cations and anions on oxide surfaces have long been described as acid-base pairs. The polar planes of ZnO showed dissociative adsorption and subsequent decomposition of methanol and formic acid related with their surface acid-base properties[3]. Further examples related to the topic of acid-base properties have been accumulated to date[ 1,4-6]. 

Much more simply, the same result can be attained with bipolar membranes, membranes consisting of an anion- and cation-permeable (an anion- and cation-exchange) membrane laminated together. At such a membrane, when mounted between electrodes so that the cation-exchange layer faces the anode, water is split into and OH ions so that the acidic and alkaline solutions required for regeneration as above are produced at the respective surfaces of the bipolar membrane. When such membranes are suitably integrated into the sequence of membranes in the electrodialysis unit above, gas evolution at the electrodes is not needed the acid-base pair is produced with about half the power. 

According to this chemical equation, which of the following represents a conjugate acid-base pair ... 

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