Acetic acid, conjugate base pair

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 ... 

You are asked to make a buffer with a pH value close to 4 that would best resist an increase in pH. You can select one of the following acid-conjugate base pairs acetic acid-acetate. Kg = 1.8 x 10 ammonium ion-ammonia. Kg = 5.6 X 10 ° or benzoic acid-benzoate, Kg = 6.3 x 10 and you can mix them in the following acid-conjugate base ratios 1 1, 2 1, or 1 2. What combination would make the best buffer ... 

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. 

In this reaction, acetic acid donates a proton to produce its conjugate, the acetate ion, which is able to function as a proton acceptor. Ammonia accepts a proton to produce its conjugate, the ammonium ion, which can function as a proton donor. Two species that differ by the transfer of a proton are known as a conjugate pair. The conjugate acid of H20 is H30+, and the conjugate base of H20 is OH-. 

Acetate is a conjugate base because it can gain a hydrogen and become acetic acid again (the reverse of Equation (5.20)). Acetic acid and acetate ion (such as from sodium acetate) would constitute a conjugate acid-base pair. 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

This dissociation is an equilibrium reaction because it proceeds in both directions. Acetic acid is weak, so only a few ions dissociate. The position of equilibrium lies to the left, and the reverse reaction is favoured. In the reverse reaction, the hydronium ion gives up a proton to the acetate ion. Thus, these ions are an acid and a base, respectively, as shown in Figure 8.3. The acid on the left (CH3COOH) and the base on the right (CHsCOO") differ by one proton. They are called a conjugate acid-base pair. Similarly, H2O and are a conjugate acid-base pair. 

Conjugate acid-base pairs in the dissociation of acetic acid in water... 

Acids may be defined as proton donors and bases as proton acceptors. A proton donor and its corresponding proton acceptor make up a conjugate acid-base pair (Fig. 2-16). Acetic acid (CH3COOH), a proton donor, and the acetate anion (CII.COO ), the corre-... 

FIGURE 2-16 Conjugate acid-base pairs consist of a proton donor and a proton acceptor. Some compounds, such as acetic acid and ammonium ion, are monoprotic they can give up only one proton. Others are diprotic (H2C03 (carbonic acid) and glycine) or triprotic... 

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+. 

PH is a measure of the concentration of H+ in a solution. An acid is a proton donor, a base is a proton acceptor. Ionization of an acid yields its conjugate base, and the two are termed a conjugate acid-base pair, for example acetic acid (CH3COOH) and acetate (CH3COO ). The pK of an acid is the pH at which it is half dissociated. The Henderson-Hasselbach equation expresses the relationship between pH, ptC and the ratio of acid to base, and can be used to calculate these values. 

The species formed by the ionization of an acid is its conjugate base. Conversely, protonation of a base yields its conjugate acid. So, for example, acetic acid and acetate are a conjugate acid-base pair. 

Buffers An acid-base conjugate pair, such as acetic acid and acetate, is able to resist... 

In other words, the unshared electron pair of the base, acetate ion, is delocalized (spread over both oxygens) by resonance. This electron pair is stabilized and less available for bonding to the proton, which localizes this electron pair in the sigma bond and costs resonance energy. The most common effect of resonance on an acid-base reaction is to delocalize and stabilize the unshared electron pair of the conjugate base, resulting in a stronger acid. 

What you saw in the last section was an example of how the presence of certain ions in solution can regulate the pH of a system. In the acetic acid/sodium acetate example, we started with a weak acid and then added a salt containing the conjugate base of the weak acid to the solution. This pairing of the weak acid and the conjugate base modified the pH of the solution. This is the principle behind buffered solutions. If an acid was added to the solution containing the acetic acid/acetate conjugates, the hydronium ions from the acid would combine with the acetate ions to form more acetic acid. So, what should have amounted to an increase in hydronium ions in the solution was foiled by the acetate ions. 

Hydrolysis is an equilibrium between two conjugate acid-base pairs, in which water can play the part of a weak acid or a weak base. In the hydrolysis of acetate ions water acts as an acid ... 

Thus, reactions of carbonyl compounds with hydrogen peroxide and acids lead to products similar to those obtained by the ozonization of olefins (Section III), which also yields 1,2,4-trioxolans and 1,2,4,5-tetroxans. The similarity of the two reactions is understandable, since the intermediates )C+—OO- in the ozonization27a and )C+—OOH in the hydrogen peroxide reactions are related as a conjugate base-acid pair. A number of cyclic peroxides of structure 7 are prepared from bis(hydroperoxy)dialkyl peroxides (5) by reaction with lead(IV) acetate, as described by Criegee et al.la This reaction is also thought to involve a carbonium ion intermediate,31 which reacts with the second OOH group. 

The experimental verification of such a complex rate expression is usually carried out by varying the concentrations of a single conjugate acid-base pair at a time. In the mutarotation of glucose in water solution at 18°C, ko is found to be 0.0054, while for acetate ion, /cac- = 0.0265 for phenoxide ion, = 4.4 and for 011 it is 3800. For catalysis by ku is found to be 0.0040 liter/mole-min. It can be seen that the constants cover an exceedingly large range. 

An acid-base conjugate pair (such as acetic acid and acetate ion) has an important property it resists changes in the pH of a solution. In other words, it acts as a buffer. Consider the addition of OH to a solution of acetic acid (HA) ... 

The acetate ion is evidently a base. The acetic acid and the acetate ion are known as a conjugate acid-base pair. The term is applied generally to any two species related to each other as A is to B in the general equation. 

The Ad]q2 first step produces an ion pair that would be expected to have a p abH near 12. Our nearest model, choline p Ta = 13.9, is expected to be less acidic because it has an extra methylene (CH3)3N" CH2CH20H. The incoming nucleophile (conjugate base p a = 10.6) is only slightly less basic than the ion pair produced by the addition. The Adf 2 second step also removes the proton from acetic acid, p/STg = 4.8, thereby creating a weaker base, so both steps pass our crosschecks. 

By definition, a buffer solution resists changes in pH with dilution or with addition of acids or bases. Generally, buffer solutions are prepared from a conjugate acid/base pair, such as acetic acid/sodium acetate or ammonium chloride/ammonia. Chemists use buffers to maintain the pH of solutions at a relatively constant and predetermined level. You will find many references to buffers throughout this text. 

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