Dissociation of acids and bases

Figure 6. Experimental data on the effect of pK and degree of dissociation of acids and bases on their reverse osmosis separations in systems involving dilute aqueous solutions and cellulose acetate membranes ( 5, 16, 17,1...

This statement has its limitations. Ionic polymerization in hydrocarbons is always kinetically complicated, it often starts only after the addition of a polar compound (co-initiator), and it is affected by the aggregation of initiating and propagating particles. In strongly polar media, activation of initiator by dissociation of acids and bases is easy. Such solvent is simultaneously a reactive transfer agent. Propagation usually does not occur, and only low molecular products are formed. Exceptions can, of course, be found. During anionic polymerization of lactams in DMF, the solvent only increases the amount of dissociated initiator [27]. 

The dissociation of acids and bases. The basic properties c solvents. Since free protons are not present in solution, a dis solved acid will be dissociated electrolytically to an appreciabl degree only if the solvent possesses the power of accepting pro tons, or in other words, if the solvent has basic properties. Heno the dissociation of an acid is determined quantitatively not onl by its own acid strength but also by the basic strength of th solvent. 

Water, owing to the fact that it possesses both acid and base properties, is termed amphiprotic. The solvent properties of water are a consequence of this ability to either accept or donate protons. Water is the most commonly used solvent for acids and bases. These interactions promote solubility and dissociation of acids and bases. 

Dissociation of acids and bases is accompanied by a change in thermo(fynamic parameters. For example, the dissociation of acetic acid in H2O at 298 K is characterized by iJi = -0.46, FAS = -27.6, and AG = 27.14 kJ/mol. For an uncharged base (for example, H2O) the thermodynamic parameters are determined by the acid structure. Below we present these values T = 298 K) for several acids at equilibrium of the type... 

Dissociation of acids and bases depends very strongly on the solvent. The solvent SH ofren acts itself as an acid, and the strength of the dissolved base or acid depends on its acid-base properties. Acetonitrile is a weaker acid and base than water, its ion product being 3 10 . Therefore, in acetonitrile HBr, HCl, and H2SO4 dissociate incompletely, unlike their dissociation in water. In acetonitrile = 5.5 for HBr, 7.3 for H2SO4, and 8.9 for HCl. Thus, acetonitrile is a differentiating solvent. Carboxylic acids also dissociate in it to a lower extent than in water the difference between pAT of CH3CN and H2O is 14. 

Determination of the dissociation constants of acids and bases from the change of absorption spectra with pH. The spectrochemical method is particularly valuable for very weak bases, such as aromatic hydrocarbons and carbonyl compounds which require high concentrations of strong mineral acid in order to be converted into the conjugate acid to a measurable extent. 

This relationship between and Kb simplifies the tabulation of acid and base dissociation constants. Acid dissociation constants for a variety of weak acids are listed in Appendix 3B. The corresponding values of Kb for their conjugate weak bases are determined using equation 6.14. 

A schematic of the production of acid and base by electrodialytic water dissociation is shown in Fig. 22-61. The bipolar membrane is inserted in the ED stack as shown. Salt is fed into the center compartment, and base and acid are produced in the adjacent compartments. The bipolar membrane is placed so that the cations are paired with OH" ions and the anions are paired with H. Neither salt ion penetrates the bipolar membrane. As is true with conventional elec trodialysis, many cehs may be stacked between the anode and the cathode. 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

Scheme (b) includes reactions formerly described by a variety of names, such as dissociation, neutralisation, hydrolysis and buffer action (see below). One acid-base pair may involve the solvent (in water H30+ —H2OorH20 — OH ), showing that ions such as HsO+ and OH- are in principle only particular examples of an extended class of acids and bases though, of course, they do occupy a particularly important place in practice. It follows that the properties of an acid or base may be greatly influenced by the nature of the solvent employed. 

Ionic strength adjuster buffer 565, 570 Ionisation constants of indicators, 262, (T) 265 of acids and bases, (T) 832, 833, 834 see also Dissociation constants Ionisation suppressant 793 Iron(II), D. of by cerium(IV) ion, (cm) 546 by cerium(IV) sulphate, (ti) 382 by potassium dichromate, (ti) 376 by potassium permanganate, (ti) 368 see also under Iron... 

Using Environmental Examples to Teach About Acids. Acid-base reactions are usually presented to secondary students as examples of aqueous equilibrium (2). In their study of acids and bases, students are expected to master the characteristic properties and reactions. They are taught to test the acidity of solutions, identify familiar acids and label them as strong or weak. The ionic dissociation of water, the pH scale and some common reactions of acids are also included in high school chemistry. All of these topics may be illustrated with examples related to acid deposition (5). A lesson plan is presented in Table I. 

The first substantial constitutive concept of acid and bases came only in 1887 when Arrhenius applied the theory of electrolytic dissociation to acids and bases. An acid was defined as a substance that dissociated to hydrogen ions and anions in water (Day Selbin, 1969). For the first time, a base was defined in terms other than that of an antiacid and was regarded as a substance that dissociated in water into hydroxyl ions and cations. The reaction between an acid and a base was simply the combination of hydrogen and hydroxyl ions to form water. 

Segments 4 and 5 reflect the chemical equilibria of acid and base dissociation of Zn(OH)2, yielding Zn + and HZnO ions, respectively. 

The theory of electrolytic dissociation also provided the possibility for a transparent definition of the concept of acids and bases. According to the concepts of Arrhenius, an acid is a substance which upon dissociation forms hydrogen ions, and a base is a substance that forms hydroxyl ions. Later, these concepts were extended. 

In amphiprotic solvents protolysis of acids and bases takes place. This reaction is also termed solvolysis, classically dissociation. The degree of solvolysis depends on the nature of both the dissolved acid or base and on the solvent. 

It has already been mentioned several times that the strength of acids and bases is expressed by their dissociation constants KA and KB. This is... 

The dissociation constants of acids and bases are determined either exactly, by means of a suitable cell without liquid junction and without measuring the pH directly, or approximately on the basis of a pH measurement in a cell with liquid junction, the potential of which is reduced to a minimum with the help of a salt bridge. In the former case we shall use, for example, the cell... 

One of the characteristics of acids and bases is that they catalyze certain reactions. Many years ago, J. N. Bronsted studied the relationship between acid strength as measured by the dissociation constant and the rate of a reaction that is catalyzed by the acid. The relationship that Bronsted recognized can be written as... 

At the microscopic level, the Arrhenius theory defines acids as substances which, when dissolved in water, yield the hydronium ion (H30+) or H+(aq). Bases are defined as substances which, when dissolved in water, yield the hydroxide ion (OH). Acids and bases may be strong (as in strong electrolytes), dissociating completely in water, or weak (as in weak electrolytes), partially dissociating in water. (We will see the more useful Brpnsted-Lowry definitions of acids and bases in Chapter 15.) Strong acids include ... 

In Chapter 4, we introduced the concept of acids and bases. Acids and bases may be strong or weak. Strong acids completely dissociate in water and weak acids only partially dissociate. For example, consider two acids HC1 (strong) and CH3COOH (weak). If we add each to water to form aqueous solutions, the following reactions take place ... 

D Waldron-Edward. The microdetermination of acid and base dissociation constants by paper electrophoresis. J. Chromatogr. 20 556-562 (1965). 

The influence of neutral salts as well as of acids and bases on the swelling of gelatine which we have seen can be attributed to an apparent change in the solvation of the gel fibrils and may be interpreted in the light of Donnan s theory of the effect of a non-diffusible ion on the osmotic pressure differences between the two phases, is likewise to be noted in the alteration of the viscosity and alcohol precipitation values of protein solutions. From the considerations already advanced there should exist two well-defined maxima in the viscosity and alcohol precipitation curves when these properties are plotted as functions of the Ph, the maxima coinciding with the points of maximum dissociation of the salts... 

Since the side chains of a distinct amino acid do not show complete dissociation at common concentrations, these acids (when forming an anion during dissociation) and bases (when forming a cation) are called weak acids and bases, respectively, as opposed to the completely dissociated strong acids and bases, e.g., hydrochloric acid, trifluoroacetic acid, sodium hydroxide, or triethylammonium hydroxide. 

Determining the strength of acids and bases with the help of dissociation... 

Arrhenius subsequently expanded his theories to form one of the most widely used and straightforward definitions of acids and bases. Arrhenius said that acids are substances that form hydrogen (H+) ions when they dissociate in water, while bases cire substances that form hydroxide (OH ) ions when they dissociate in water. 

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