General Properties of Acids and Bases

Bases have a bitter taste and a slippery, soapy feel. Often the terms alkali, alkaline, lye, and caustic are associated with bases. Sodium hydroxide and potassium hydroxide have, for example, been referred to as caustic soda and caustic potash, respectively. The reason is that these compounds are caustic, corrosive compounds that can chemically attack a variety of materials, including plant and animal tissue. These strong bases are used in the production of soaps. 

Even to a person not educated in the properties of acids, the word acid conjures up thoughts of the possibility of serious chemical burns on skin. Acids in 

FIGURE 12.3 The structure of citric acid, the acid that gives citrus fruits their sour taste. 

The chemical properties that acids and bases exhibit are mostly due to the easy release of hydrogens (in the case of acids) and hydroxides (in the case of bases). The reason we study acids and bases together is that hydrogen ions and hydroxide ions react with each other to form water. 

In addition to the properties of acids and bases that affect the color of other chemicals, there are other important chemical properties of these con5)ounds. One is neutralization, which is the negation of the chemical properties of acids by their reaction with bases, and vice versa. In other words, it is the reaction of acids with bases. A base can be neutralized by an acid and an acid can be neutralized by a base. The reason that neutralization occurs is that hydrogen ions react with hydroxide ions to form water, as we discussed earlier. Water is one product of such a reaction, while salt is the other. This will be discussed in more detail in Section 12.7. 

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General Properties of Acids and Bases Bronsted Acids and Bases Acid-Base Neutralization Acid-Base Reactions Leading to Gas Formation... 

The Arrhenius theory accounts for the properties of many common acids and bases, but it has important limitations. For one thing, the Arrhenius theory is restricted to aqueous solutions for another, it doesn t account for the basicity of substances like ammonia (NH3) that don t contain OH groups. In 1923, a more general theory of acids and bases was proposed independently by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. According to the Bronsted-Lowry theory, an acid is any substance (molecule or ion) that can transfer a proton (H + ion) to another substance, and a base is any substance that can accept a proton. In short, acids are proton donors, bases are proton acceptors, and acid-base reactions are proton-transfer reactions ... 

In 1923, the same year in which Bronsted and Lowry defined acids and bases in terms of their proton donor/acceptor properties, the American chemist G. N. Lewis proposed an even more general concept of acids and bases. Lewis noticed... 

Acids are defined classically as substances which liberate hydrogen ions in aqueous solutions, while bases are substances which dissociate in water with the formation of hydroxyl ions. This formulation of the concept of acids and bases had developed chiefly on the basis of acidic and basic properties displayed in aqueous solutions. When an attempt was made to establish a more general theory of acids and bases, the old formulation appeared in many ways to be one-sided and incomplete. 

In each case, the atom to which the proton becomes attached possesses at least one unshared pair of electrons. This characteristic property of OH, NH3, and other Brpnsted bases suggests a more general definition of acids and bases. 

Of the many theories that have been proposed through the years to explain the properties of acids and bases, the Bronsted-Lowry, or proton transfer theory, and the older, more general Lewis theory are most generally useful. 

New Chapter 1 has been retitled Structure Deter mines Properties to better reflect its purpose and has been rewritten to feature a detailed treatment of acids and bases Rather than a review of what stu dents learned about acids and bases m general chemistry Sections 1 12-1 17 discuss acids and bases from an organic chemistry perspective... 

It was not until the last decade of the nineteenth century that chemists had an adequate theoretical description of acid and bases. Until then, most acids and bases were classified according to their general properties. Chemists knew acids and bases displayed contrary properties, and these were adequate for identifying many acids and bases. Some of these properties are listed in Table 13.1. 

The work of Brpnsted and Pedersen (23) on the catalytic decomposition of nitramide and the kinetic studies of Lowry and Faulkner (24) on the mutarotation of tetramethylglucose led to the formulation of a more general viewpoint on acids and bases which logically showed that the hydrogen ion and hydroxyl ion were not the unique carriers of acid and basic properties. An acid was defined as any substance capable of donating a proton, and a base any substance capable of accepting a proton. In accordance with this definition (Lowry, 25 Brpnsted, 26), the following substances are typical acids and bases ... 

Introduction - Liposomes are vesicles composed of one or more lipid bilayers completely surrounding an internal aqueous space. They are usually composed of phospholipids either in pure form or In combination with other amphipathic molecules such as sterols, long chain bases or acids, or membrane proteins. The structure of liposomes varies from large (0.5->5y) multllamellar vesicles to small ( 300 A) unilamellar vesicles.2,3 More recently, new methods have been reported describing the formation of unilamellar vesicles of intermediate size. >5.6 xhe general properties of liposomes and their interaction with various macromolecules have been described in several reviews. ... 

The theory of acids and bases, the Bronsted-Lowry theory, was dependent on the presence of proton (H ) to show acidic or basic properties, which may not always be the case hence in 1923, G. N. Lewis introduced a theory of acids and bases with a more general definition of acid-base reactions by examining what happens when an ion combines with an OH ion to form water. 

In Chapter 4 we defined a Brpnsted acid as a snbstance capable of donating a proton, and a Brpnsted base as a substance that can accept a proton. These definitions are generally suitable for a discussion of the properties and reactions of acids and bases. 

Throughout his career as a chemist Ostwald followed the general approach of applying physical measurements and mathematical reasoning to chemical issues. One of his major research topics was the chemical affinities of acids and bases. To that end, he studied the points of equilibria in reaction systems where two acids in an aqueous solution compete with each other for a reaction with one base and vice versa. Because chemical analysis would have changed the equilibria, he skillfully adapted the measurement of physical properties, such as volume, refractive index, and electrical conductivity, to that problem. From his extensive data he derived for each acid and base a characteristic affinity coefficient independent of the particular acid-base reactions. 

However, the protolytic theory cannot explain the distinctly acid or base properties of numerous substances which are not able to either split-off or accept a proton. This stimulated G. N. Lewis (1923) to a different generalization of the notion of acids and bases. According to the Lewis theory a base is a substance which is the donor of a free electron pair, whereas, acid can bond a free electron pair of another particle and thus, it is its acceptor. Neutralization of an acid by base is conditioned by the formation of coordination (donor-acceptor) bond. The Lewis theory is of importance particularly in the chemistry of coordination compounds where all central... 

As another example, the Brpnsted theory of acids and bases replaced the older Arrhenius theory because of its more general applicability (e.g., it applied also to non-aqueous solutions and even to gases). Attempts have been made to familiarise secondary school students with principles of quantum mechanics and thermodynamics, albeit often in a simplified and sometimes distorted form. It was expected that this would save much time because formulas and physical and chemical properties of specific compounds could then simply be derived from theoretical principles instead of having to be memorised. It was decided that many aspects of descriptive chemistry could be left out when students were provided with a small handbook covering most of the relevant facts and formulas. 

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