Strong acids ionization constants

Table 10-3 gives acid ionization constants for several familiar acids at 25 C. The values for the strong acids are not well defined. Examine the Ions column and see how every acid yields a hydrogen ion and a complementary anion in solution. 

A strong acid is a substance that reacts completely with water, so that the acid ionization constant defined in Eq. (10) or (11) is effectively infinite. This situation can only be achieved if the conjugate base of the strong acid is very weak. A weak acid will be characterized by an acid ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (8) favors the existence of unreacted free acid. 

What would be the value of the acid-ionization constant for an acid that was so strong that not a single molecule remained un-ionized ... 

Because HCN is a weaker acid than HF, K , is smaller than K, and K is larger than 1. The eqnilibrium described by K lies strongly to the right. The net result is donation of by the stronger acid (HF) to the stronger base (CN ), to produce the weaker acid (HCN) and the weaker base (F ). This example illustrates how the magnitudes of acid ionization constants can be nsed to predict the direction of net hydrogen ion transfer in reactions between acids and bases in aqueous solution. 

Table 15.2 lists a number of organic acids with pfC values in aqueous solution between 0 (strong acid) and 14 (weak acid). Almost all of them are carboxylic acids with the characteristic — COOH functional group. Acid ionization constants... 

As long as the solubility of the generated salt is not exceeded, the concentration of [A ] generated by neutralization of HA by the strong base MA is calculated from knowledge of the acid ionization constant ... 

Acids ionize and produce hydrogen ions in aqueous solution. The strength of an acid depends on how completely it ionizes. Strong acids undergo essentially complete ionization, whereas weak acids are only partly ionized at equilibrium. Like other reversible reactions, the ionization of a weak acid can be represented by an equilibrium constant expression. The value of this expression, called the acid ionization constant, K, is a measure of the strength of a weak acid. 

Spinaceamines are fairly strong bases with two basic centres. The highest protonation constants correspond to the N-5 atom of SpA (8.90) and the N-5 atom of SpN (8.66), while the smaller constants characterize the basicity of the imidazole ring in the monocation of SpA (4.89) and SpN (4.96). The acid ionization constant of SpN (pKg) is 1.65 and is comparable with the pK of histidine (1.96) (73JCS(D)323). SpNs 517, 519, 563 and 564 substituted at an N-atom easily take up methyl halides to afford monoquaternary salts 559-562 at the N-5 atom. More stringent conditions are required to obtain diquaternary salts 565 (82MI5). 

Acid ionization constant Conjugate acid-base ( a). P-671 pair, p. 660 Base ionization constant Ion-product constant, p. 662 (K ), p. 678 Lewis acid, p. 697 Lewis base, p. 697 Strong acid, p. 666 Percent ionization, p. 677 Strong base, p. 667 pH, p. 663 Weak acid, p. 667 Salt hydrolysis, p. 689 Weak base, p. 668... 

Calculating the hydronium ion concentration in weak acid solutions isn t as straightforward as it is in strong solutions, because not all of the weak acid that dissolves initially has ionized. In order to calculate the hydronium ion concentration, you must use the equilibrium constant expression for the weak acid. Chapter 8 covers the K, expression that represents the equilibrium system. For weak acid solutions, you use a modified equilibrium constant expression called the K, — the acid ionization constant Take a look at the generalized ionization of some weak acid HA ... 

One way to distinguish between strong and weak acids is to look for an acid ionization constant (K value. If the acid has a Ka value, then it s weak. 

For acid-base complexes, the lone difference between a salt and a co-crystal is the location of the acidic H atom(s) in the crystal structure. While it is generally accepted that proton transfer will occur between an acid and base to form a salt in solution when the difference between their acid ionization constants (pXa of base - pXa of acid or ApXa) is greater than two or three units, crystallization may yield salts, co-crystals, or disordered solid forms that exhibit partial proton transfer when the ApXa is less, with the exact location of the acidic proton being strongly dependent on the specific crystal packing environment. Here, it must be understood that the value is a solution property that is not specifically defined in crystals and as such, cannot be transferred to the solid state in a general way. " ... 

For a strong acid, which ionizes completely in solution, the concentrations of ions are determined by the stoichiometry of the reaction from the initial concentration of acid. However, for a weak acid such as acetic acid, the concentrations of ions in solution are determined from the acid-ionization constant (also called the acid-dissociation constant), which is the equilibrium constant for the ionization of a weak acid. 

Classify each acid as strong or weak. If the acid is weak, write an expression for the acid ionization constant Ks. ... 

Hydrochloric acid, HCl, is one of a half-dozen strong acids, which means that its acid ionization constant is too large to measure accurately. We must find a way to handle the activity of unionized species such as HCl in spite of their unmeasurably small concentrations. Since aqueous HCl has an appreciable vapor pressure we assume that aqueous unionized HCl in an aqueous solution of HCl is at equilibrium with gaseous HCl. From the fundamental fact of phase equilibrium... 

Acids with > 10 are strong acids. Most organic acids have /T < 10 and are weak acids. Acid disso-ciation constants are often expressed as p.A values, where pAT = —log iCT. Note thatpK values increase as K decreases. Table 3.1 lists the acid ionization constants of some common acids. 

The acidity of hydrofluoric acid solutions vary with concentration owing to hydrogen-bond interactions of the fluoride ion. Dilute solutions are weakly acidic with an acid ionization constant Ka = 6.6 x 10 " (or pKa = 3.18), in contrast to corresponding solutions of the other hydrogen halides which are strong acids. Concentrated solutions of hydrogen fluoride are much more strongly acid than implied by this value, as shown by measurements of the Hammett acidity function Ho (or effective pH ). For 100%, HF has an Ho, estimated to be between -10.2 and -11, which is comparable to the value -12 for sulfuric acid. 

The equilibrium constant for the reaction of an acid with water is called the acid ionization constant (K ), equation (16.10), and that for the reaction of a base with water is called the base ionization constant (K, ), equation (16.13). Strong acids and strong bases have large ionization constants (Xa or Kb >i> 1) and are essentially completely ionized in water. Weak acids and weak bases have small ionization constants K or Xt, 1) and ionize to a limited extent in water. The extent to which acids or bases ionize in water is described in terms of either the degree of ionization (a) (equation 16.15) or the percent ionization (equation 16.16). For a weak acid or weak base, the degree of ionization increases with increasing dilution. 

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