Weak Acids and Their Equilibrium Constants

Many solutes behave as weak acids that is, they react reversibly with water to form H3O+ ions. Using HB to represent a weak acid, its Bronsted-Lowry reaction with water is 

Typically, this reaction occurs to a very small extent usually, fewer than 1% of the HB molecules are converted to ions. 

Most weak acids fall into one of two categories  

Molecules containing an ionizable hydrogen atom. This type of weak acid was discussed in Chapter 4. There are literally thousands of molecular weak acids, most of them organic in nature. Among the molecular inorganic weak acids is nitrous acid  

Cations. The ammonium ion, NH4+, behaves as a weak acid in water a 0.10 M solution of NH4CI has a pH of about 5. The process by which the NH4+ ion lowers the pH of water can be represented by the (Bronsted-Lowry) equation  

Universal indicator is deep red in strongly acidic solution (upper left). It changes to yellow and green at pH 6 to 8, and then to deep violet in strongly basic solution (lower right). 

The color change of hydrangeas is exactly the opposite of that for litmus. 

Influence of acidic or basic soil. Hydrangeas grown in strongly acidic soil (below pH 5) are blue. When they are grown in neutral or basic soil, the flowers are rosy pink. 

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The carbon-metal bonds of organolithium and organomagnesium compounds have appreciable carbamomc character Carbanions rank among the strongest bases that we 11 see m this text Their conjugate acids are hydrocarbons—very weak acids indeed The equilibrium constants for ionization of hydrocarbons are much smaller than the s for water and alcohols thus hydrocarbons have much larger pA s... 

Equilibrium Constants for Weak Acids and Their Conjugate Bases... 

Weak acids and bases exist in equilibrium with their ions, and their equilibrium constants (Ka) are small. The position of equilibrium is measured by the equilibrium constant [equation (4-10)] and Kt. [equation (4-12)]. The dis-... 

Equation 2-7 is more commonly called the Henderson-Ha.sselbalch equation and is the basis for most calculations involving weak acids and bases. It is used to calculate the pH of solutions of weak acids, weak ba.ses. and buffers consisting of weak acids and their conjugate bases or weak bases and their conjugate acids. Because the pK is a modified equilibrium constant, it corrects for the fact that weak acids do itnl completely react with water. 

Buffers help maintain a relatively constant hydrogen ion concentration. The most common buffers consist of weak acids and their conjugate bases. A buffered solution can resist pH changes because an equilibrium between the buffer s components... 

The polar O-H bond of alcohols makes them weak acids. By the Bronsted-Lowry definition, acids are hydrogen ion donors and bases are hydrogen ion acceptors in chemical reactions. Strong acids are 100% ionized in water and weak acids are only partially ionized. Weak acids establish an equilibrium in water between their ionized and unionized forms. This equilibrium and the strength of an acid is described by the acidity constant, Ka. Ka is defined as the concentrations of the ionized forms of the acids (H30+ and A-) divided by the un-ionized form... 

In 1923, Broensted was the first to develop an acid-base concept that was no longer related to substances, but rather to the function of particles. Acids are proton donors and are capable, with suitable reaction partners, to donate protons to base particles or proton acceptors, i.e. protolysis or proton transfer reaction. For example, HC1 molecules, as acid particles, transfer protons when colliding with H20 molecules (see Fig. 7.3). In this sense, the proton donors of pure sulfuric acid are H2S04 molecules, the acid particles of the sulfuric acid solution are the hydronium ions (or also the hydrogen sulfate ions in concentrated solutions). In weak acids, the protolysis equilibrium is to be considered, equilibria and their constants are well defined. 

A weak acid ionizes only slightly in solution, perhaps only to a few percent, and in solution between the molecular acid is in equilibrium with its ions. There are thousands of known weak acids and five of the more common ones are listed below with the equation for their equilibrium in solution. The equilibrium constant for the ionization of weak acids is symbolized Ka, and is called the acid-ionization constant. 

The acidity or basicity of a solution is frequently an important factor in chemical reactions. The use of buffers of a given pH to maintain the solution pH at a desired level is very important. In addition, fundamental acid-base equihbria are important in understanding acid-base titrations and the effects of acids on chemical species and reactions, for example, the effects of complexation or precipitation. In Chapter 6, we described the fundamental concept of equilibrium constants. In this chapter, we consider in more detail various acid-base equilibrium calculations, including weak acids and bases, hydrolysis, of salts of weak acids and bases, buffers, polyprotic acids and their salts, and physiological buffers. Acid-base theories and the basic pH concept are reviewed first. 

The compounds at the beginning of Table 4.2 are very strong acids. Their equilibrium constants are very large and cannot be measured accurately because the concentrations of the reactants are extremely small. The equilibrium constants for these compounds are determined by some indirect method and only approximate values can be obtained. Because the pKg values cannot be determined very precisely, they are listed without any figures right of the decimal place. A similar problem occurs with the extremely weak acids at the end of the table. 

In Table 21.18, the distribution constants of the adds and their association constants in the aqueous phase are also listed. All the adds are quite weak, and all except HAA more or less strongly prefer the organic phase. Though these properties of the acids obviously influence the extraction equilibrium, there is no immediate correlation between the values of Xp, or Xj (H), and those of Kg. The specific interaction between ligand and metal ion is the paramount factor. 

Weak Acids and Weak Bases—A weak acid or weak base ionizes to a limited extent in water. The extent of their ionization can be related to the ionization constants Ka and Kt, or their logarithmic equivalents pKa = -log Ka and pKt, = -log Kt, (Table 16.4) by setting up and solving an equilibrium calculation. Calculations involving ionization equilibria are in many ways similar to those introduced in Chapter 15, although some additional considerations are necessary for polyprotic acids. 

As the titration begins, mostly HAc is present, plus some H and Ac in amounts that can be calculated (see the Example on page 45). Addition of a solution of NaOH allows hydroxide ions to neutralize any H present. Note that reaction (2) as written is strongly favored its apparent equilibrium constant is greater than lO As H is neutralized, more HAc dissociates to H and Ac. As further NaOH is added, the pH gradually increases as Ac accumulates at the expense of diminishing HAc and the neutralization of H. At the point where half of the HAc has been neutralized, that is, where 0.5 equivalent of OH has been added, the concentrations of HAc and Ac are equal and pH = pV, for HAc. Thus, we have an experimental method for determining the pV, values of weak electrolytes. These p V, values lie at the midpoint of their respective titration curves. After all of the acid has been neutralized (that is, when one equivalent of base has been added), the pH rises exponentially. 

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

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