Conjugate acid base pairs

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 HA with water  

In the forward reaction, HA donates a proton to H2O. Therefore, HA is the Bronsted-Lowry acid and H2O is the Bronsted-Lowry base. In the reverse reaction, the H3O ion 

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  

Likewise, for the reaction between NH3 and H2O (Equation 16.5), we have addH+ 

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

3 The diagrams show solutions of three different weak bases. List the bases in order of increasing value. 

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

6 Define and identify conjugate acid-base pairs. 

7 Given the formula of an acid or a base, write the formula of its conjugate base or acid. 

We noted that reaction between ammonia and water reaches equilibrium. The fact is that most acid-base reactions reach equilibrium. Accordingly, the general Brpnsted-Lowry acid-base proton-transfer reaction can be written with a double arrow to show that it is reversible. Look carefully at the reverse reaction, for which the arrow and the labels for the reactants are printed in red  

Is not HB+ donating a proton to A in the reverse reaction In other words, HB+ is an acid in the reverse reaction, and A is a base. From this we see that the products of any proton-transfer acid-base reaction are another acid and base for the reverse reaction. 

Let s apply these ideas to reaction of ammonia with water  

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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. ... 

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 ... 

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 ... 

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. 

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 ... 

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

Here are a few other examples of conjugate acid-base pairings ... 

Barthomeuf, D. (1984) Conjugate acid-base pairs in zeolites, J. Phys. Chem., 88, 42. 

The Ka and Kb of conjugate acid-base pairs are related through the Kw expression ... 

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. 

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