Acids and Bases The Bronsted-Lowry View

Acids and bases are a big part of organic chemistry, but the emphasis is much different from what you may be familiar with from your general chemistry course. Most of the attention in general chemistry is given to numerical calculations pH, percent ionization, buffer problems, and so on. Some of this returns in organic chemistry, but mostly we are concerned with acids and bases as reactants, products, and catalysts in chemical reactions. We ll start by reviewing some general ideas about acids and bases. 

According to the theory proposed by Svante Arrhenius, a Swedish chemist and winner of the 1903 Nobel Prize in Chemistry, an acid is a substance that ionizes to give protons when dissolved in water a base ionizes to give hydroxide ions. 

A more general theory of acids and bases was devised by Johaimes Brdnsted (Denmark) and Thomas M. Lowry (England) in 1923. In the Br0nsted-Lowry approach, an acid is a proton donor, and a base is a proton acceptor. The reaction that occurs between an acid and a base is proton transfer. 

In the equation shown, the base uses an unshared pair of electrons to remove a proton from an acid. The base is converted to its conjugate acid, and the acid is converted to its conjugate base. A base and its conjugate acid always differ by a single proton. Likewise, an acid and its conjugate base always differ by a single proton. 

In the Br0nsted-Lowry view, an acid doesn t dissociate in water it transfers a proton to water. Water acts as a base. 

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The old definition of neutralization states that an acid and a base react with each other to form salt and water. The Bronsted-Lowry concept offers a much broader view of the process of neutralization. According to this concept, neutralization is a process of proton transfer from an acid to a base. Neutralization need not result in the formation of a recognizable salt and may not Involve water. 

This more general view of acids and bases is named the Bronsted-Lowry theory after the two scientists who proposed it, J. N. Brpnsted and T. M. Lowry. 

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. 

HCl + H2O HsO + Clin view of the disadvantages, it can be questioned whether it is worth tr dng to extend the ideas of acid and base in the way that Lowry and Bronsted do. Chemistry might be better served if the classification of molecules and ions according to their ability to accept or donate protons - which is undoubtedly a very useful classification - were to be divorced Ifom the classification of substances into acids and bases, and be given a terminology of its own. In fact, such terminology lies readily to hand, the terms proton donor and proton acceptor being entirely suitable for this purpose. 

The acid-base concept, either in the limited interpretation of the Bronsted-Lowry theory or in the more general sense of the Lewis theory, is one of the most useful classification schemes in chemistry and a tool for systematizing the relationship between structure and reactivity. In this section we examine how this general concept can be applied to solid surfaces [29], From the most general (Lewis) point of view, a surface is classified as acidic (electron acceptor) or basic (electron donor) according to the direction of net electron transfer that results in the formation of new chemical bonds with an adsorbed molecule. 

In conclusion, we have two complementary definitions of Bronsted-Lowry acids and bases. From one point of view, we say that an acid is a substance that increases the concentration of H+ (or H3O+) in a solution, and a base is a substance that increases the concentration of OH. From the other point of view, we say an acid is a proton... 

The nucleophile, the ammonia molecule, is also acting as a base, while the electrophile, the proton, is also acting as an acid. So a base may also be thought of as a nucleophile, because both are electron rich species and seek positive centres. The above reaction between the ammonia and the proton may be classified as a Bronsted-Lowry acid/base reaction. It may also be classed as a Lewis acid/base reaction, depending on whether one views the ammonia as a proton acceptor or as a donor of a lone pair of electrons. 

The classic acid-base reaction between hydrogen ions and hydroxide ions to produce water can be viewed from the Arrhenius, Bronsted-Lowry, and Lewis definitions. Assign each reactant as an acid or a base under each definition and briefly justify your assignments. 

Ideas about adds and bases (or alkalis) date back to ancient times. The word acid is derived from the Latin acidus (sour). Alkali (base) comes from the Arabic al-qali, referring to the ashes of certain plants from which alkaline substances can be extracted. The acid-base concept is a major theme in the history of chemistry. In this section, we emphasize the view proposed by Svante Arrhenius in 1884 but also introduce a more modern theory proposed in 1923 by Thomas Lowry and by Johannes Bronsted. 

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