Ionization of polyprotic acids

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... 

Thus far we have considered only monoprotic weak acids. Acids that can furnish two or more hydronium ions per molecule are called polyprotic acids. The ionizations of polyprotic acids occur stepwise, that is, one proton at a time. An ionization constant expression can be written for each step, as the following example illustrates. Consider phosphoric acid as a typical polyprotic acid. It contains three acidic hydrogen atoms and ionizes in three steps. 

Calculations involving the ionization of polyprotic acids and bases present no great difficulties, provided that one makes a simplifying assumption, Because the second ionization constant is usually much less than the first, we can assume that the concentration of A , which is produced in the second ionization, is much less than the concentration of HA , which is produced in the first ionization. Another way of looking at it is that the first ionization accounts for most of the H+, only a little more being added by the second ionization. Calculations involving the first ionization can then be performed as if the acid were monoprotic, and the results of this calculation can then be used to find [A ]. Corresponding remarks apply to ionization of polyprotic bases. 

Write the ionization reactions and the expression for each ionization. Recall that ionization of polyprotic acids takes place one H+ at a time and that most of the H" is obtained from the first ionization. 

Here is a way to test the validity of the statement made on pages 761-762 in conjunction with the three key ideas governing the ionization of polyprotic acids. Determine the pH of 0.100 M succinic acid in two ways first by assuming that H30 is produced only in the first ionization step, and then by allowing for the possibility that some H30 is also produced in the second ionization step. Compare the results, and discuss the significance of your finding. 

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. 

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

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... 

Citric acid, HjCsHgOy, and vitamin C (ascorbic acid), H2C6H6O6, are polyprotic acids found in citrus fruits. Why are these acids classified as polyprotic acids Write balanced equations that show the stepwise ionization of these acids in water. 

A third group of salts that yield H30" ions in solutions consists of cations of strong bases and anions of polyprotic acids that still have one or more ioniz-able protons. For example, NaH2P04 yields an acidic solution because Na, the cation of a strong base, does not react, while H2P04, the first anion of the weak polyprotic acid H3PO4, is also a weak acid ... 

For many polyprotic acids is much larger than subsequent dissodation constants, in which case the H (aq) in the solution comes almost entirely from the first ionization reaction. As long as successive values differ by a factor of 10 or more, it is possible to obtain a satisfactory estimate of the pH of polyprotic acid solutions by treating the adds as if they were monoprotic, considering only JC i. 

Sulfuric acid is the type of polyprotic acid that can donate two protons per molecule, and it is therefore known as a diprotic acid. Ionizations of a monoprotic acid and a diprotic acid are shown in Figure 2.2. 

For all other polyprotic acids, each ionization is incomplete and is represented by an equation with a double arrow. TTie first, second, and third ionizations of phosphoric acid are represented as... 

In the case of a polyprotic acid, there is a value for each ionization, and the values decrease for each successive ionization. For example, the second ionization of phosphoric acid is represented... 

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. 

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. 

Acid strength is measured in terms of how easy it is for the H to ionize. The stronger the acid, the weaker the bond between the H atom and the rest of the acid molecule measured as Ka or, if the acid is polyprotic, KaU Kal. Ka3,. .. 

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 ... 

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. 

The dependence of the equilibrium constants of polyprotic weak acids and weak bases on ionic strength allows them to be treated in exactly the same manner as monoprotic weak acids and weak bases. The first, second, and third ionization constants for triprotic weak acids in terms of activity coefficient are given by ... 

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... 

---