Weak acids and bases

Weak acids and bases cannot dissociate completely. They undergo the same type of dissociation as that of strong acids and bases, but the extent of dissociation is very little compared to strong acid or strong base dissociations. 

A typical way to represent the dissociation of a weak acid is shown below  

In this reaction, the protons from the weak acid are transferred to the water molecules. The acid-ionization constant (AT) of this reaction is shown below  

We encounter weak acids every day. An example is carbonated water that contains dissolved carbon dioxide, and is called carbonic acid. Fmits like lemons and oranges contain citric acid which is also a weak acid. The list goes on and on. 

Weak Acids and Bases Ionization of a Weak Acid 

AQA N solution of a strong acid such as hydrochloric acid is 0.1 N in hydrogen ion, since this acid is very nearly com.pletely dissociated into ions except in very concentrated solutions. But a 0.1 N solution of acetic acid contains hydrogen ions in much smaller concentration, as is seen by testing with indicators, observing the rate of attack of metals, or simply by tasting. Acetic acid is a weak acid the acetic acid molecules hold their protons so firmly that not all of them are transferred to water molecules to form hydronium ions. Instead, there is an equilibrium reaction. 

In general, for an acid HA in equilibrium with ions H and A the equilibrium expression is 

The constant characteristic of the acid, is called its acid constant or ionization constant. 

Values of acid constants are found experimentally by measuring the pH of solutions of the acids. A table of values is given later in this chapter (Section 12-8). 

Many acids, including HCN, do not react completely in water. We learned in Section 3.3 that these compounds are weak acids. A weak acid and water react to produce a conjugate acid-base system in which the acid and base on the right are 

Stronger than their conjugate acid and base on the left. This means that equilibrium will favor the left-hand side, and only a small percentage of the acid molecules ionize. We can write a general equation for the dissociation of any weak acid, denoted here as HA  

The water in the equilibrium reaction is a pure liquid, of course. So just like the pure solids in solubility equilibria, its concentration is constant. So [H2O] is dropped from the expression, and the resulting iit is called an acid ionization constant, K  

Acid ionization constants are shown for some common acids at 25°C. Larger values of indicate stronger acids. 

Many students have misconceptions about pH. For example, pH can he less than 0 or greater than 14. 

When a strong acid or base undergoes a complete ionization in solution, the concentrations of the newly formed ions can be understood using basic stoichiometry principles. This is because essentially all of the acid is converted to ions. With weaker acids and bases, equilibrium is established between the ions, much like the equilibria studied in the last chapter. The concentrations of the ions must be determined by using an equilibrium constant, K. The equilibrium constants used to describes acid-base equilibria are in the same form as Kc from the last chapter. Well use the dissociation of acetic acid to begin our description of the new equilibrium constant. 

The equation that describes the ionization of acetic acid in water takes the form  

If you were going to determine the equilibrium constant, Kc, for this reaction, you would set it up as follows  

Water is omitted from the expression because its concentration is not likely to be affected by this equilibrium. When the equilibrium constant, Kc, is written for an acid-base equilibrium, it is known as the acid-dissociation constant, Ka. The generic equation for the 

You ve already seen how to calculate the pH of a solution containing a strong acid. This was relatively easy because the acid completely dissociated. That meant for every mole of acid, you generated a mole of hydrogen ions (for monoprotic acids). With weak acids, the process is not as simple, but you will notice that it is very similar to the calculations you did in the last chapter for Kc. Well review the procedure using the equilibrium of acetic acid in water, shown below  

Let s review the meaning of the acid dissociation constant, K.r for the acid HA  

A weak acid is one that is not completely dissociated. That is, Reaction 9-3 does not go to completion. For a base, B, the base hydrolysis constant, Kb, is defined by the reaction 

A weak base is one for which Reaction 9-4 does not go to completion. pK is the negative logarithm of an equilibrium constant  

As K increases, pK decreases, and vice versa. Comparing formic and benzoic acids, we see that formic acid is stronger, with a larger Ka and smaller pKa, than benzoic acid. 

The acid HA and its corresponding base, A , are said to be a conjugate acid-base pair, because they are related by the gain or loss of a proton. Similarly, B and BH+ are a conjugate pair. The important relation between Ka and Kb for a conjugate acid-base pair, derived in Equation 6-35, is 

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They permit clear cut distinctions between strong and weak acids and bases A strong acid is one that is stronger than Conversely a weak acid is one that... 

Gordus, A. A. Ghemical Equilibrium IV. Weak Acids and Bases, /. Chem. Educ. 1991, 68, 397-399. 

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

Svante Arrhenius, like Berzelius, was born in Sweden and spent his entire professional career there. According to Arrhenius, the concept of strong and weak acids and bases came to him on May 13,... 

For very weak or slightly ionised electrolyes, the expression a2/( 1 — a) V = K reduces to a2 = KV or a = fKV, since a may be neglected in comparison with unity. Hence for any two weak acids or bases at a given dilution V (in L), we have a1 = y/K1 V and a2 = yjK2V, or ol1/ol2 = Jk1/ /K2. Expressed in words, for any two weak or slightly dissociated electrolytes at equal dilutions, the degrees of dissociation are proportional to the square roots of their ionisation constants. Some values for the dissociation constants at 25 °C for weak acids and bases are collected in Appendix 7. 

Solutions which prevent the hydrolysis of salts of weak acids and bases. If the precipitate is a salt of weak acid and is slightly soluble it may exhibit a tendency to hydrolyse, and the soluble product of hydrolysis will be a base the wash liquid must therefore be basic. Thus Mg(NH4)P04 may hydrolyse appreciably to give the hydrogenphosphate ion HPO and hydroxide ion, and should accordingly be washed with dilute aqueous ammonia. If salts of weak bases, such as hydrated iron(III), chromium(III), or aluminium ion, are to be separated from a precipitate, e.g. silica, by washing with water, the salts may be hydrolysed and their insoluble basic salts or hydroxides may be produced together with an acid ... 

Fluoride ion, and weak acids and bases do not interfere, but nitrate, nitrite, perchlorate, thiocyanate, chromate, chlorate, iodide, and bromide do. Since analysis of almost all boron-containing compounds requires a preliminary treatment which ultimately results in an aqueous boric acid sample, this procedure may be regarded as a gravimetric determination of boron. 

A reaction with a rate constant that conforms to Eq. (10-21)—particularly to the feature that the catalysts are H+ and OH-, and not weak acids and bases—is said to show specific acid-base catalysis. This phenomenon is illustrated by the kinetic data for the hydrolysis of methyl o-carboxyphenyl acetate16 (the methyl ester of aspirin— compare with Section 6.6) ... 

A catalytic system may contain active components other than H30+, H2O, and OH-. Weak acids and bases may also be efficient catalysts. These include, of course, both components of the buffer. Their contributions are in addition to the three terms seen before. If they are designated as BH+ and B, the rate constant is... 

Weak acids and bases are, generally speaking, less effective catalysts than H+ and OH at the same concentrations. Proton transfer occurs in all acid-base catalysis, regardless of the detailed mechanism (this aspect is considered in the next section). It is only... 

In this part of the chapter, we develop a quantitative measure of the strengths ol weak acids and bases. We then use this information to explore how acid strength is related to molecular structure. 

The rest of this chapter is a variation on a theme introduced in Chapter 9 the use of equilibrium constants to calculate the equilibrium composition of solutions of acids, bases, and salts. We shall see how to predict the pH of solutions of weak acids and bases and how to calculate the extent of deprotonation of a weak acid and the extent of protonation of a weak base. We shall also see how to calculate the pH of a solution of a salt in which the cation or anion of the salt may itself be a weak acid or base. 

Buffer capacity also depends on the relative concentrations of weak acid and base. Broadly speaking, a buffer is found experimentally to have a high capacity for acid when the amount of base present is at least 10% of the amount of acid. Otherwise, the base is used up quickly as strong acid is added. Similarly, a buffer has a high capacity for base when the amount of acid present is at least 10% of the amount of base, because otherwise the acid is used up quickly as strong base is added. 

Because the concentrations of ions in a solution of a sparingly soluble salt are low, we assume, just as we did for solutions of weak acids and bases (Section 10.7), that we can approximate Ksp by... 

Before leaving the subject of polarity and in relation to uptake and distribution, mention should be made of weak acids and bases. The complicating factor here is that they exist in solution in different forms, the balance between which is dependent on pH. The different forms have different polarities, and thus different values. In other words, the values measured are pH-dependent. Take, for example, the plant growth regulator herbicide 2,4-D. This is often formulated as the sodium or potassium salt, which has high water solubility. When dissolved in water, however, the following equilibrium is established ... 

We express the relative strengths of weak acids and bases in terms of their dissociation constants. Shown... 

In the last two sections we have described several different categories of strong and weak acids and bases. Table 17-5 summarizes the characteristics of these species. 

The key to understanding the titrations of weak acids and bases is to be familiar with the species in solution and the dominant equilibrium at each point along the titration curve. Example reinforces these qualitative features. 

For ionizable molecules, the membrane permeability, P (Pc in cellular models), depends on pH of the bulk aqueous solution. The maximum possible Pm is designated Pq, the intrinsic permeabiUty of the uncharged species. For monoprotic weak acids and bases, the relationship between P and Pq may be stated in terms of the fraction of the uncharged species,, as Pm= Pofo, i-e. ... 

Silica-based columns are only stable in the range of pH 2 to 8, so ion suppression is particularly useful for weak acids and bases. The advent of... 

Arrhenius postulated in 1887 that an appreciable fraction of electrolyte in water dissociates to free ions, which are responsible for the electrical conductance of its aqueous solution. Later Kohlrausch plotted the equivalent conductivities of an electrolyte at a constant temperature against the square root of its concentration he found a slow linear increase of A with increasing dilution for so-called strong electrolytes (salts), but a tangential increase for weak electrolytes (weak acids and bases). Hence the equivalent conductivity of an electrolyte reaches a limiting value at infinite dilution, defined as... 

Case B is illustrated in Fig. 2.18 for a few different weak acids and bases at 25° C. The pH curves were obtained on the basis of the relevant dissociation constants. For instance, for acid HA we simply write... 

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