Ionization of acids and bases in water

A Most laboratory pH meters can be read to the nearest 0.01 unit. Some pH meters for research work can be read to 0.001 unit, but unless unusual precautions are taken, the reading of the meter may not correspond to the true pH. 

The color of thymol blue indicator, which is present in both solutions, depends on the pH of the solution. 

HCI occurs to a greater extent than does the ionization of CH3C(X)H, an indication that HCI is a much stronger acid than CH3COOH. Tfiis conclusion is reflected in the placements of HCI and CH3COOH in Table 16.2 which ranks a number of acids and bases in order of increasing acid or base strengths. This 

The strength of an acid or a base is quantified by the value of the equilibrium constant for the reaction describing its ionization in water. As discussed on page 737, a monoprotic Bronsted-Lowry acid may be represented by the 

TABLE 16.2 Relative Strengths of Some Common Bronsted-Lowry Acids and Bases 

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As a result of the ionization of acids and bases in water, the essential reaction in each case can be shown as... 

The behavior of acids and bases in water is influenced considerably by the fact that water ionizes into hydrogen and hydroxide ions ... 

The equilibrium constant of a reversible dissociation is called the dissociation constant. The term dissociation is also applied to ionization reactions of acids and bases in water for example HCN + H2O H3O+ + CN-which is often regarded as a straightforward dissociation into ions HCN H+ + CN-... 

The HCl separates to form H+ and Ch. Ionization in water will be discussed with the chemistry of acids and bases in Chapter 9. 

Even if the (fictitious) values of acid and base in aqueous solution at a concentration of 1 mol m (cp. Sect. 6.2) are used, the proton exchange with the solvent water should be completely inhibited. That means that the acetic acid HAc in the solution in question should only exist as molecule (and not in the ionized form) the acetate ion Ac , however, should only exist as ion. 

As with K,y and K , the subscript b in Ky denotes that the equilibrium constant refers to a particular type of reaction, namely the ionization of a weak base in water. The constant Ky, the base-dissociation constant, always refers to the equilibrium in which a base reacts with H2O to form the corresponding conjugate acid and OH. ... 

The Br0nsted theory expands the definition of acids and bases to allow us to explain much more of solution chemistry. For example, the Brpnsted theory allows us to explain why a solution of ammonium chloride tests acidic and a solution of sodium acetate tests basic. Most of the substances that we consider acids in the Arrhenius theory are also acids in the Bronsted theory, and the same is true of bases. In both theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the ions, the weak acid, and the water. A similar statement can be made about weak bases ... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

D. Sykora, E. Tesarova, and D. W. Armstrong, Practical Considerations of the Influence of Organic Modifiers on the Ionization of Analytes and Buffers in Reversed-Phase LC, LCGC 2002,20, 974 G. W. Tindall, Mobile-Phase Buffers. I. The Interpretation of pH in Partially Aqueous Mobile Phases, LCGC 2002,20, 102 S. Espinosa, E. Bosch, and M. Roses, Acid-Base Constants of Neutral Bases in Acetonitrile-Water Mixtures, Anal. Chim. Acta 2002,454, 157. 

Before continuing on to the last definition of acids and bases, it will be helpful to consider the definitions for strong and weak acids within the context of the Bronsted-Lowry model of acids and bases. The definitions are really an extension of the Arrhenius ideas. In the Arrhenius definitions, strong acids and bases were those that ionize completely. Most Bronsted-Lowry acids and bases do not completely ionize in solution, so the strengths are determined based on the degree of ionization in solution. For example, acetic acid, found in vinegar, is a weak acid that is only about 1 percent ionized in solution. That means that when acetic acid, HC2H302, is placed in water, the reaction looks like ... 

Section 19.1 discusses the Brpnsted theory of acids and bases, which extends the concepts of add and base beyond aqueous solutions and also explains the acidic or basic nature of solutions of most salts. Dissociation constants, the equilibrium constants for the reactions of weak acids or bases with water, are introduced in Section 19.2. The concept of the ionization of covalent compounds is extended to water itself in Section 19.3, which also covers pH, a scale of acidity and basicity. Section 19.4 describes buffer solutions, which resist change in their acidity or basicity even when some strong acid or base is added. Both the preparation and the action of buffer solutions are explained. Section 19.5 discusses the equilibria of acids containing more than one ionizable hydrogen atom per molecule. 

Because the reactions of Br0nsted acids and bases with water are equilibrium reactions, we can write equilibrium constant expressions for these ionizations or dissociations. For example, the dissociation of nitrous acid in water and its equilibrium constant expression are as follows ... 

Definition of Acids and Bases. —The old definitions of an acid as a substance which yields hydrogen ions, of a base as one giving hydroxyl ions, and of neutralization as the formation of a salt and water from an acid and a base, are reasonably satisfactory for aqueous solutions, but there are serious limitations when non-aqueous media, such as ethers, nitro-compounds, ketones, etc., are involved. As a result of various studies, particularly those on the catalytic influence of un-ionized molecules of acids and bases and of certain ions, a new concept of acids and bases, generally associated with the names of Br nsted and of Lowry, has been developed in recent years. According to this point of view an acid is defined as a substance with a tendency to lose a proton, while a base is any substance with a tendency to gain a proton the relationship between an acid and a base may then be written in the form... 

FIGURE 1.1 loiiization of acids and bases. An acid is defined as a chemical that dissociates and donates a proton to water. A base is defined as a chemical that can accept a proton. The double arrows indicate that the ionization process occurs in the forward and backward directions. The term equilibrium means that the rate of the forward reaction is equal to die rate of the backward reaction, and that no net accumulation of products or reactants occurs over time. 

The usual solvent for acids and bases is water. Water produces equal numbers of H+ ions and OH ions in a process known as self-ionization. In selfionization, two water molecules react to form a hydronium ion (H30 ) and a hydroxide ion according to this equihbrium. 

The Arrhenius model of acids and bases If pure water itself is neutral, how does an aqueous solution become acidic or basic The first person to answer this question was the Swedish chemist Svante Arrhenius, who in 1883 proposed what is now called the Arrhenius model of acids and bases. The Arrhenius model states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution. A base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in aqueous solution. Some household acids and bases are shown in Figure 19-3. 

As an example of the Arrhenius model of acids and bases, consider what happens when hydrogen chloride gas dissolves in water. HCl molecules ionize to form H+ ions, which make the solution acidic. 

Water not only serves as the solvent in solutions of acids and bases, it also plays a role in the formation of the ions. In aqueous solutions of acids and bases, water sometimes acts as an acid and sometimes as a base. You can think of the self-ionization of water as an example of water assuming the role of an acid and a base in the same reaction. 

Look at the bottles of acid and base solutions in Figure 19-13. They are labeled with the number of moles of molecules or formula units that were dissolved in a liter of water (M) when the solutions were made. Each of the bottles contains a strong acid or base. Recall from Section 19.2 that strong acids and bases are essentially 100% ionized. That means that this reaction for the ionization of HCI goes to completion. 

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