Ionization of strong acid

Most of this chapter and Chapter 17 deal with equilibrium and the af lication of the principles of equilibrium to a variety of reaction types. In the context of our discussion of acids and bases, however, it is necessaiy to review the ionization of strong acids and the dissociation of strong bases. These reactions generally are not treated as equilibria but rather as processes that go to completion. This makes the determination of pH fra- a solution of strong acid or strong base relatively simple. 

Why are ionizations of strong acids and strong bases generally not treated as equilibria ... 

Example The pK s for the first and second ionizations of sulfuric acid are —48 and 2 0 respectively Sulfuric acid (HOSO2OH) is a strong acid hydrogen sulfate ion (H0S020 ) is a weak acid... 

Solutions of alkah metal and ammonium iodides in Hquid iodine are good conductors of electricity, comparable to fused salts and aqueous solutions of strong acids. The Hquid is therefore a polar solvent of considerable ionising power, whereas its own electrical conductivity suggests that it is appreciably ionized, probably into I" and I (triodide). Iodine resembles water in this respect. The metal iodides and polyiodides are bases, whereas the iodine haHdes are acids. 

The acidic character of acids depends on the availability ofhydrogen ions in their solution. An acid X3 is said to be stronger than another acid X2 if, in equimolar solutions, X3 provides more hydrogen ions than does X2. This will be possible provided that the degree of dissociation of X3 is greater than that of X2. Based on the Arrhenius theory of electrolytic dissociation, solutions may be classified in the manner shown in Figure 6.1. If the ionization of an acid is almost complete in water, the acid is said to be a strong acid, but if the... 

According to Le Chatelier s principle, what does the H30+ concentration from the ionization of an acid (strong or weak) do to the ionization of water ... 

Reverse-phase chromatography is used mainly for the separation of nonionic substances because ionic, and hence strongly polar, compounds show very little affinity for the non-polar stationary phase. However, ionization of weak acids (or weak bases) may be suppressed in solvents with low (or high) pH values. The effect of such a reduction in the ionization is to make the compound more soluble in the non-polar stationary phase but the pH of the solvent must not exceed the permitted range for bonded phases, i.e. pH 2-8. 

Nitric acid-trifluoroacetic anhydride mixtures are used extensively for nitrolysis and N-nitration reactions (see Chapters 5 and 6). The same is not true for aromatic nitrations. This reagent contains trifluoroacetyl nitrate, which can ionize to nitronium and trifluoroacetate ions in the presence of strong acid. 

The ionization of the acid depends not only on the basicity of the solvent, but also on its dielectric constant and its ion-solvating ability. The dependence of the acidity and basicity constants of a compound on the basicity and acidity, respectively, of the solvents, leads to a distinction between leveling and differentiating solvents. When the solvent is a stronger base than water, its leveling effect will apply also to weaker acids. Similarly, strong bases will also have equal basicities in sufficiently acidic solvents. All bases stronger than the HO ion are adjusted to the basicity of this ion in water. 

T. M. Lowry represented the strong acidity developed when, say, hydrogen chloride is mixed with water by assuming that the reaction can be symbolized HCl-j-H2CMCr - 0H 3, which, expressed in terms of the electron theory of atomic structure, means that the hydrogen nucleus does not readily attach itself to an octet too fully occupied by other atoms to leave room for an additional nucleus the water provides the required acceptor for the hydrogen nucleus, and the ionization of the acid involves the transfer of a portion from one octet to another ... 

Equations 12.15 and 12.16 have been tested on a series of acidic, basic, and neutral molecules. For benzoic acid (log P = 1.85, p/<"a = 4.2) in rat colon (pH = 6.8), the calculated log D value from Equation 12.15 is 1.28, considerably lower than the log P value. Note that the ionization of benzoic acid decreases the lipophilicity of the molecule because the charged species strongly favors the water layer (Scheme 12.4). 

The value of Ka or Kb (pKa or pKb) determines the extent of ionization of an acid or base. We think of strong acids or bases as completely ionized (or nearly so). This generally implies Ka or Kb greater than 1 (negative pKa or pKb). The pKa values for several acids are given in Table 13-1. 

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... 

This answer simply won t work because the [H+] cannot possibly be more than the beginning molar concentration of the acid. Apparently the amount of chloroacetate ion is greater than the equivalent amount of base added. This fact is related to the relatively strong acidity of the acid and to the appreciable ionization of the acid, even before the titration begins. Mathematically, this is taken into account by an equation of electroneutrality, according to which there must be equal numbers of cations and anions in solution. 

Strong acids completely ionize in water. This means every mole of strong acid falls apart into an equal number of moles of hydrogen ions. Therefore, [H+] is equal to the formal concentration of the acid. 

What about sulfuric acid Well H2S04 is certainly a strong acid. In fact, it is a much stronger acid than HC1. Like all diprotic acids, sulfuric acid dissociates in a stepwise manner. The first proton is very, very acidic. Kz for the first ionization is very large, something on the order of 1010, so the first ionization of sulfuric acid in water is 100% complete. 

As will be seen, the large amount of quantitative structure-activity relationship (QSAR) modeling that has been carried out for soil sorption has almost exclusively involved nonionic organic compounds. For strongly ionizing and inorganic chemicals, no QSARs are available. However, Bintein and Devillers (1994) developed a soil sorption QSAR that incorporated correction factors for ionization of weak acids and bases. 

The total heat effect, 12,000 calories, produced by the action of one equivalent of acetic acid and one equivalent of ammonium hydroxide, is the sum of the heat effects of the three separate reactions, and we should expect the value to be different from the value of the neutralization of a strong acid and a strong base. Since 13,700 calories must be generated by the formation of 1 mole of water, the difference between this value and 12,000, or 1,700 calories, must have been absorbed in the ionizing of the acid and the base. 

Strong acids and bases ionize completely in solution, while weak acids and bases partially ionize. The partial ionization of weak acids and bases creates solutions in which an equilibrium is established. 

The heat of neutralization of strong acids and bases is usually about 13 7 Cal., and the enhanced value for lithium fluoride may be attributed to the heat evolved during neutralization by the ionization of the weak hydrofluoric acid. 

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