Water, acid-base behavior

Walden, Paul, 360 Walden inversion. 359-360 Wang resin, solid-phase peptide synthesis and. 1037 Water, acid-base behavior of, 50 dipole moment of, 39 electrostatic potential map of. 53 nucleophilic addition reactions of, 705-706 pKaof, 51-52... 

Acid-Base Behavior. The relative acidity-basicity of the filler, generally determined by measuring the pH value of a slurry of a specific mass of filler in 100 mL of deionized water, can influence the behavior of a filler in some systems. For example, the curing behavior of some elastomers is sensitive to the pH value of carbon black. 

Both our original prediction about the effect of ionization energy on acid-base behavior and the trend which we have observed in the first three elements lead us to expect that the hydroxide or oxide of silicon should not be basic, but perhaps should be weakly acidic. This is in fact observed. Silicon dioxide, Si02, can exist as a hydrated solid containing variable amounts of water,... 

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923, when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key process responsible for the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... 

Hydronium cations, neutral water molecules, and hydroxide anions illustrate the effect of charge on acid-base behavior. 

An early attempt to provide a framework to observations on the chemistry of substances that react in water to produce acids or bases was provided by S. A. Arrhenius. At that time, the approach was limited to aqueous solutions, and the definitions of an acid and a base were given in these terms. Of course we now know that acid-base behavior is not limited to these cases, but it applies much more broadly. If we consider the reaction between gaseous HC1 and water,... 

Organic solvents can also be classified according to their ability to accept or transfer protons (i.e., their acid-base behavior) [20,21]. Amphiprotic solvents possess donor as well as acceptor capabilities and can undergo autoprotolysis. They can be subdivided into neutral solvents that possess approximately equal donor and acceptor capabilities (water and alcohols), acidic solvents with predominantly proton donor properties (acetic acid, formic acid), and basic solvents with primarily proton acceptor characteristics (formamide, N-methylformamide, and N,N-dimethylformamide). Aprotic solvents are not capable of autoprotolysis but may be able to accept protons (ACN, DMSO, propylene carbonate). Inert solvents (hexane) neither accept nor donate protons nor are they capable of autoprotolysis. 

The "equilibrium boxes" for the solvents (Fig. 10-1) indicate the range over which differentiation occurs outside the range of a particular solvent, all species are leveled. For example, water can differentiate species (i.e., they are weak adds and bases) with pKa s from about 0 to 14 (such as acetic acid). Ammonia, on the other hand, behaves the same toward acetic acid and sulfuric acid because both lie below the differentiating limit of —12. The extent of these ranges is determined by the autoionization constant of the solvent (e.g, —14 units for water). The acid-base behavior of several species discussed previously may be seen to correlate with Fig. 10.1.11... 

Hammett and Deyrup, in 1932, were the first to propose a method of determiningquantitatively acid-base behavior in water-strong acid mixtures.15 "17 In order to understand their contribution, we begin with the general expression for the equilibrium constant for the dissociation of an acid (compare Equation 3.16) ... 

The major biological solvent is water, and the acid-base behavior of dissolved molecules is intimately linked with the dissociation of water. Water is a weak electrolyte capable of dissociating to a proton and a hydroxyl ion. In this process, the proton binds to an adjacent water molecule to which it is hydrogen-bonded (Chap. 4) to form a hydronium ion (H30+) ... 

Just as the cation produced by dissociation of water (H30+) is the acidic species in aqueous solutions, the NH4+ ion is the acidic species in liquid ammonia. Similarly, the amide ion, NH2, is the base in liquid ammonia just as OH- is the basic species in water. Generalization to other nonaqueous solvents leads to the solvent concept of acid-base behavior. It can be stated simply as follows A substance that increases the concentration of the cation characteristic of the solvent is an acid, and a substance that increases the concentration of the anion characteristic of the solvent is a base. Consequently, NH4C1 is an acid in liquid ammonia, and NaNH2 is a base in that solvent. Neutralization becomes the reaction of the cation and anion characteristic of the particular solvent to produce unionized solvent. For example, in liquid ammonia the following is a neutralization ... 

Bates, R. G., Robinson, R. A., Acid-Base Behavior in Methanol-Water... 

Z. Pawlak, Solvent effects on acid-base behavior. Acidity constants of eight pro-tonated substituted pyridines in (acetonitrile + water), /. Chem. Thermodynam. 19 (1987), 443 47. 

Solvents can change the acid-base behavior of solutes. Compare the acid-base properties of dimethylamine in water, acetic acid, and 2-butanone. 

Chapters 10 and 11 describe the special properties of liquid water. Because of its substantial dipole moment, water is especially effective as a solvent, stabilizing both polar and ionic solutes. Water is not only the solvent, but also participates in acid-base reactions as a reactant. Water plays an integral role in virtually all biochemical reactions essential to the survival of living organisms these reactions involve acids, bases, and ionic species. In view of the wide-ranging importance of these reactions, we devote the remainder of this chapter to acid-base behavior and related ionic reactions in aqueous solution. The Bronsted-Lowry definition of acids and bases is especially well suited to describe these reactions. 

Values of p/f =-log at 25°C for some important dissolved acids and bases in natural waters are given in Table 5.1. In the table Fe " and AP" have been written as aquocomplexes, to show the role of water in their acid-base behavior. The convention is generally not to write the waters of hydration. Notice that the species HSO4, H2PO4, Fe0H(H20), HCO3, HPO, and H SiOi can act either as acids or as bases. Thus they are called ampholytes or amphiprotic. 

Acid-Base Behavior of the Element Oxides Metals are also distinguished from nonmetals by the acid-base behavior of their oxides in water ... 

Summarize the acid-base behavior of the main-group metal and nonmetal oxides in water. How does oxide acidity in water change down a group and across a period ... 

Before we discuss the next major definition of acid-base behavior, let s examine a crucial property of water that enables us to quantify [H30 J in any aqueous system water is an extremely weak electrolyte. The electrical conductivity of tap water is due almost entirely to dissolved ions, but even water that has been repeatedly distilled and deionized exhibits a tiny conductance. The reason is that water itself dissociates into ions very slightly in an equilibrium process known as autoionization (or self-ionization) ... 

Table 18.3 displays the acid-base behavior of the various types of salts in water. 

In which of the three concepts of acid-base behavior discussed in the text can water be a product of an acid-base reaction In which is it the only product ... 

Arrhenius ocid-bose definition A model of acid-base behavior in which an acid is a substance that has H in its formula and produces in water, and a base is a substance that has OH in its formula and produces OH" in water. (579)... 

Transition metal hydrides, which are weakly basic as isolated molecules, are expected to display acidic properties in solution. With an appropriate choice of solvent we are thus able to induce Umpolung of the acid-base behavior of certain transition metal hydrides. The break-even point of a TMH in water would be reached with 3.7. This relatively low value indicates that most transition metal hydrides will dissociate protons in water. 

We can use these principles to explain the acid-base behavior of oxides when they are dissolved in water. For example, when a covalent oxide such as sulfur trioxide is dissolved in water, an acidic solution results because sulfuric acid is formed ... 

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