Dissociation of acids in water

Equilibrium isotope effects on the dissociation of acids in water and deuterium oxide [6]... 

A biologically useful definition of an acid is a molecule that acts as a proton (hydrogen ion) donor. A base is similarly defined as a proton acceptor. How readily acids or bases lose or gain protons depends on the chemical nature of the compoimds imder consideration. The degree of dissociation of acids in water, for example, ranges from essentially complete dissociation for a strong acid to practically no dissociation for a very weak acid, and any intermediate value is possible. 

Several theories have been proposed to answer the question What is an acid and what is a base One of the earliest, most significant of these theories was advanced in 1884 by Svante Arrhenius (1859-1927), a Swedish scientist, who stated that an acid is a hydrogen-containing substance that dissociates to produce hydrogen ions, and a base is a hydroxide-containing substance that dissociates to produce hydroxide ions in aqueous solutions. Arrhenius postulated that the hydrogen ions are produced by the dissociation of acids in water and that the hydroxide ions are produced by the dissociation of bases in water ... 

The dissociation of acids in water can be represented according to the following chemical equation ... 

Before we go on to define the acidity constant of a given organic acid in water, we need to introduce a thermodynamic convention for scaling such constants. We do this relative to H30+ in that we define the dissociation of H30+ in water to have a standard free-energy change ArG° = 0, which means that the equilibrium constant of this reaction is equal to 1 ... 

Figure 2.3 shows how the chemistry of dissolved arsenious acid varies with pH. An analogous graph for arsenic acid is in Figure 2.4. As expected, protonated species of the acids are more common under low pH conditions were H+ is abundant. For both weak acids, dissociation constants (Ka values) may be derived to describe their gain or loss of H+ with changing pH conditions (Table 2.10 (Faure, 1998), 119-120). For example, the following reaction involving the dissociation of H3ASO3 in water at 25 °C and 1 bar pressure has a dissociation constant (K ) of HP9 2 (Wolthers et al., 2005), 3490 ...

Chughtai, A., Marshall, R., and Nancollas, G.H. Complexes in calcium phosphate solutions. J. Phys. Chem. 72 208-211 (1968). Davies, C.W. The extent of dissociation of salts in water. Part VI. Some calcium salts of organic acids. J. Chem. Soc. 277-281 (1938). 

What energy terms are important in determining AH° for the dissociation of HX in water (Keep in mind that large, positive contributions to the value of AH° will tend to make AG° more highly positive, K3 smaller, and the acid weaker.) One important factor is certainly the H—X bond strength. Note from Table 19.8 that the H—F bond is much stronger than the other H—X bonds. This factor tends to make HF a weaker acid than the others. 

Liu et aL (1999a) studied the hydrolysis of CI2 in water, while Meijer and Sprik (1998a) investigated the reaction mechanism of water with formaldehyde in sulfuric acid. Laasonen and Klein (1997) studied the hydrolysis of HCl upon addition to water, and Meijer and Sprik (1998b) studied the addition of H2O to SO3 in solution. Finally, Trout and Parrinello (1998) evaluated the mechanism and free energy profile of the dissociation of H2O in water. 

The dissociation of acids in ethyl alcohol was discussed in a general manner in the third section of Chapter Four. The following statements were made at that time The dissociation constants of indicators which behaved like uncharged acids or anion acids were much smaller in alcohol than in water. This difference is much less marked if the acid form of the indicator is a cation. 

Concerning the idea of neutralization, Sumfleth [9] found that students think along the lines of acid-base equilibria After neutralization, sodium chloride solution contains the same amount of hydrochloric acid and sodium hydroxide solution with neutralization there exists equilibrium of acid and base . The reaction of solid sodium chloride with water to form a salt solution is correctly recognized as dissociation by only 15% of the students morethan35% assume an inversion of neutralization After the reaction of sodium chloride with water, the same amounts of acid and base are found in the solution . Considering the reaction of solid sodium phosphate and water and observing a basic solution, students do not hesitate to explain OH (aq) ions and Na3P04H+(aq) ions are produced by the reaction . In these examples, one can see that the term dissociation of salt in water seems to be very misunderstood [9]. 

Dissociation Constants of Acids in Water at Various Temperatures 74 Ed. 

The dissociation of hypochlorous acid depends on the pH. The unionized acid is present in greater quantities in acid solution, although in strongly acid solution the reaction with water is reversed and chlorine is Hberated. In alkaline solutions the hypochlorite ion OCL is increasingly Hberated as the pH is increased. The pH is important because unionized hypochlorous acid is largely responsible for the antimicrobial action of chlorine in water. Chlorine compounds are therefore more active in the acid or neutral range. The hypochlorites most commonly employed are sodium hypochlorite [7681-52-9] or calcium hypochlorite [7778-54-3]. 

Notice that the piO, value shown in Table 2.3 for water is 15.74, which results from the following calculation the Ka for any acid in water is the equilibrium constant /vet) for the acid dissociation multiplied by 55.4, the molar concentration of pure water. For the acid dissociation of water, we have... 

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