Polyprotic acids ionization

Polyprotic acids ionize in steps, each having its own value for the dissociation constant. 

All polyprotic acids ionize in steps. The three ionizations of phosphoric acid are described by these equations. 

Describe how polyprotic acids ionize in steps and how to calculate concentrations of all species in solutions of polyprotic acids... 

Several common acids have more than one ionizable hydrogen ion (Table 1). Each successive hydrogen ion in these polyprotic acids ionizes less readily. For example, sulfuric acid is a strong acid because of the complete ionization of the first hydrogen ion. 

In Section 15.4, we discussed that some acids, called polyprotic acids, contain two or more ionizable protons. Recall that sulfurous acid (H2SO3) is a diprotic acid containing two ionizable protons and phosphoric acid (H3PO4) is a triprotic acid containing three ionizable protons. Typically, a polyprotic acid ionizes in successive steps, each with its own A"a. For example, sulfurous acid ionizes in two steps ... 

Generally, polyprotic acids ionize in successive steps, with the value of Kg becoming smaller for each step. 

From a chemical point of view, the common amino acids are all weak polyprotic acids. The ionizable groups are not strongly dissociating ones, and the degree of dissociation thus depends on the pH of the medium. All the amino acids contain at least two dissociable hydrogens. 

Certain weak acids are polyprotic they contain more than one ionizable hydrogen atom. Such acids ionize in steps, with a separate equilibrium constant for each step. Oxalic acid, a weak organic acid sometimes used to remove bloodstains, is diprotic ... 

An acidic salt is a salt that contains an ionizable hydrogen atom. It is the product formed when less than the amount of base required for complete neutralization (reaction) reacts with a polyprotic acid ... 

Because the K for HOC1 is more than 1000 times that of HOI, the pH in the solution is due only to the ionization of HOC1, following the same train of thought as for polyprotic acids. 

If the acid contains more than one ionizable hydrogen, the acid is known as a polyprotic acid. Specifically, diprotic acids, such as sulfuric acid, H2S04, contain two ionizable hydrogens and triprotics, such as phosphoric acid, H3P04, contain three ionizable hydrogens. 

Table 10-5 gives ionization data for four series of polyprotic acids. The integer in parentheses after the name denotes which hydrogen is being ionized, where (1) is the first and most easily ionized hydrogen. 

Five of the acids listed in Table 13-1 are polyprotic acids, i.e., acids with more than one acidic H. For example, phosphoric acid ionizes in three steps ... 

The secondary ionization constant of polyprotic acids is always smaller than the primary (K2 < K1) the tertiary, K3, is even smaller and so on. 

Just as in the ionization of polyprotic acids, so in the hydrolysis of their salts, the reaction proceeds in successive stages. The extent of the second stage is generally very small compared with the first. This is particularly true in this case, where H2Cr04 is quite a strong acid with respect to its first ionization and much weaker in the second ionization. The equation of interest is... 

When dealing with polyprotic acids, the exact solution for pFl calculations quickly becomes very daunting. Flowever, Le Chatelier s principle can come to our rescue. The first ionization is much more complete than the second, because the first K3 is larger than the second. The hydrogen ion produced by the first ionization is also a product of the second ionization. Thus, the increase in hydrogen ion concentration pushes the second equilibrium back to the left, suppressing the acid ionization of H,P()4. Therefore, with polyprotic acids, we can usually just consider the first acid ionization and ignore the subsequent acid ionizations. 

Write the steps in the complete ionization of the following polyprotic acids. 

For polyprotic acids such as H3PO4 or H3As04, there is usually a factor of approximately 105 difference in successive Ka values. Phosphoric acid has dissociation constants that have the values Kal = 7.5 x 10-3, Ka2 = 6.2 x Itr8, and Ka3 = 1.0 x 10-12. This is because the first proton comes from a neutral molecule, the second from a -1 ion, and the third from a -2 ion. As a result of electrostatic attraction, it is energetically less favorable to remove H+ from species that are already negative. When considering the first and second ionization... 

Acids that contain more than one ionizable hydrogen atom per molecule are called polyprotic acids. These acids ionize in steps. The second (or third) proton has a much lower dissociation constant than does the prior proton because it is harder to remove a hydrogen ion the more negatively charged the Brpnsted acid (Table 19.4). Also, the prior ionization produces hydronium ions that repress the further ionization, in accord with LeChatelier s principle. Any acid ionizes less in the presence of a stronger acid (see Example 19.20). Thus, the hydronium ion in a solution of a polyprotic acid comes mainly from the first step in the ionization. 

Polyprotic acids (Section 19.5) ionize stepwise, and the hydronium ion from each step represses the ionization of later steps. The second (and third) steps are essentially weaker because it is harder to remove a proton from a negatively charged species than from a neutral one. However, polyprotic acids follow the usual rules of equilibrium. For example, LeChatelier s principle can be used to predict and explain their behavior. 

Successive Ionizations of a Polyprotic Acid. A polyprotic acid has several acid constants, corresponding to dissociation of successive hydro-... 

Table 7.3 pK values at 25 °C of some acids and bases (upper section) and some large organic zwitterions (lower section) commonly used in buffer solutions. For polyprotic acids, where more than one proton my dissociate, the p/fa values are given for each ionization step. Only the trivial acronyms of the larger molecules are provided their full names can be obtained from the catalogues of most chemical suppliers... 

In the case of a polyprotic acid for which the individual ionizations are well separated (ideally, by at least 3 log units), values for the individual constants can be calculated from data points in the appropriate regions of the titration curve. If the individual ionizations overlap, the Bjerrum fi (n-bar) method may be used. This mathematical approach was introduced by Bjerrum for the calculation of stability constants of metal-ligand complexes, but it can also be applied to the determination of proton-ligand equilibrium constants. 

An intermediate ion of a polyprotic acid when dissolved in water undergoes both ionization as an acid (reaction 1) and ionization as a base or hydrolysis (reaction 2). 

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