DEFINING ACIDS AND BASES

This theory was a milestone in the development of acid-base concepts it was the first to define acids and bases in terms other than that of a reaction between them and the first to give quantitative descriptions. However, the theory of Arrhenius is far more narrow than both its predecessors and its successors and, indeed, it is the most restrictive of all acid-base theories. 

The limitations of the Arrhenius theory of acids and bases are overcome by a more general theory, called the Bronsted-Lowry theory. This theory was proposed independently, in 1923, by Johannes Br0nsted, a Danish chemist, and Thomas Lowry, an English chemist. It recognizes an acid-base reaction as a chemical equilibrium, having both a forward reaction and a reverse reaction that involve the transfer of a proton. The Bronsted-Lowry theory defines acids and bases as follows ... 

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... 

All the reactions discussed in the previous section could be described as acid/base phenomena, defining acids and bases quite liberally. The importance of ionic equilibria in aqueous solution was recognised in the 1880s by Arrhenius, who proposed that acids were sources of H+(aq) while bases were sources of OH-(aq), and it was soon realised that this definition was closely related to the self-dissociation of water ... 

Lewis has defined acids and bases in a general way as electron acceptors and donors, respectively (21). Accordingly, a compound or element capable of accepting electrons (electron seeking) is termed a Lewis acid. Conversely a compound or element capable of giving (or sharing electrons) is a Lewis base. In other words, those elements which are deficient in electrons --that is, have unfilled electron shells -- will seek out those elements carrying extra electrons (lone pairs). This is the basis for the Lewis concept of "acid -base interaction. Thus ... 

In the 1880s, Svante Arrhenius defined acids and bases as substances that ionize in solution to produce H+ and OH- ions, respectively. We still often think of acids and bases in these terms, but the definitions can be misleading. For example, an aqueous solution of ammonia, NH3, contains ammonium and hydroxide ions, NH4... 

In 1923, Brpnsted and Lowry defined acids and bases on the basis of the transfer of protons. A Brpnsted-Lowry acid is any species that can donate a proton, and a Brpnsted-Lowry base is any species that can accept a proton. These definitions also include all the Arrhenius acids and bases because compounds that dissociate to give H30+ are proton donors, and compounds that dissociate to give OH are proton acceptors. (Hydroxide ion accepts a proton to form H20.)... 

There are also a few reactions that can help define acids and bases. Here are some examples ... 

Understand the various operational and conceptual methods for defining acids and bases. 

In 1923, Johannes N. Brpnsted (1879-1947) and Thomas M. Lowry (1874-1936) independently defined acids and bases in a different way from the Arrhenins definitions. The resulting theory is sometimes called the Brpnsted-Lowry theory, bnt more often is referred to as just the Brpnsted theory. The Brpnsted theory extends the definitions of acid and base in a way that explains more than the Arrhenins definitions can explain. According to this theory, a Brpnsted acid is a proton donor, and a Brpnsted base is a proton acceptor. 

Great. Now if we only knew what acids and bases are There are a number of ways to define acids and bases, and Til give you two of them here. Before doing that, I need to remind you of how water interacts with other chemicals. Water molecules are polar, with one end being positive and the other negative. See Figure 4.6. 

Refer to the previous chapter, where I define acids and bases. Acids are proton donors and bases are proton accepters. 

Earlier in this chapter we considered Arrhenius s concept of acids and bases An acid is a substance that produces H+ ions when dissolved in water, and a base is a substance that produces OH- ions. Although these ideas are fundamentally correct, it is convenient to have a more general definition of a base, which covers substances that do not produce OH- ions. Such a definition was provided by Bransted and Lowry, who defined acids and bases as follows ... 

Table 11.1 lists the names and formulas of a number of important acids. Acids and bases have been known and characterized since ancient times. Chemical description and explanation of their properties and behavior has progressed through several stages of sophistication and generality. A broadly applicable modern treatment is presented in Chapter 15. Here, we introduce titrations using the treatment of the Swedish chemist Svante Arrhenius, who defined acids and bases by their behavior when dissolved in water. 

Define acid and base and be able to cite an example of each. 

Macroscopic Properties of Acids and Bases Defining Acids and Bases A Submicroscopic Look Acid Ionization... 

The net-ionic equation clearly shows that the core of neutralization is the combining of those ions that define acids and bases to form neutral water. 

A major problem with Arrhenius s acid-base theory is that some substances, like ammonia, produce basic solutions and react with acids, but do not contain hydroxide ions. In 1923 Johannes Bronsted, a Danish chemist, and Thomas Lowry, an English chemist, independently proposed a new way to define acids and bases. An acid donates hydrogen ions (also called a proton donor) a base accepts hydrogen ions (also called a proton acceptor). These definitions not only explain all the acids and bases covered by Arrhenius s theory, they also explain the basicity of ammonia and ions such as carbonate, and phosphate, P04 ... 

Acid-Base Chemistry Acid-base reactions represent another important area of chemistry with applications in engineering, and again we have integrated our coverage into appropriate areas of the text. Initially, we define acids and bases in conjunction with the introduction to solutions in Chapter 3. Simple solution stoichiometry is presented in Chapter 4. Finally, a more detailed treatment of acid-base chemistry is presented in the context of equilibria in Chapter 12. 

Another way to define acids and bases was first proposed by G. N. Lewis. A Lewis acid is a substance that can accept an electron pair, and a Lewis base is a substance that can donate an electron pair. According to this definition, a proton is considered to be a Lewis acid because it can accept an electron pair from a donor (a Lewis base) to fill its Is shell. 

General chemistry defines acids and bases in terms of their ionization in water. The chemistry discussed in this book will show that many organic compounds are insoluble in water. Although the acetic acid and ethanol discussed in the... 

Bronsted and Lowry proposed defining acids and bases in terms of their ability to transfer protons. According to their definition, an acid is a substance (molecule or km) that can donate a proton to another substance. Likewise, a base is a substance that can accept a proton. Thus, when HCl dissolves in water (Equation 16.3), HCl acts as a Bronsted-Lowry acid (it donates a proton to H2O), and H2O acts as a Bronsted-Lowry base (it accepts a proton from HCl). 

From the experiment that we have discussed above, one can safely conclude that acid-base reactions are a function of ionization principle. Thus, based on ionization principle, Arrhenius defined acids and bases. These definitions are elaborated below. 

The Swedish chemist Svante Arrhenius framed the first successful concept of acids and bases. He defined acids and bases in terms of the effect these substances have on water. According to Arrhenius, acids are substances that increase the concentration of H ion in aqueous solution, and bases increase the concentration of OH ion in aqueous solution. But many reactions that have characteristics of acid-base reactions in aqueous solution occur in other solvents or without a solvent. For example, hydrochloric acid reacts with aqueous ammonia, which in the Arrhenius view is a base because it increases the concentration of OH ion in aqueous solution. The reaction can be written... 

These descriptions of acids and bases lasted hundreds of years, but they told alchemists nothing about the elements of which acids or bases were made. Even after the discovery of hydrogen by the English chemist Henry Cavendish (1731-1810) in 1766, it still took chemists over a century to begin to define acids and bases in terms of their chemical compositions. The first to do so was the Swedish chemist Svante Arrhenius (1859-1927), who received the Nobel prize in chemistry in only the third year in which Nobel prizes were awarded (1903). 

Although Arrhenius is justly credited with an attempt to define acids and bases in terms of their chemical compositions, he fell short of explaining the behavior of substances that turn litmus red but which do not contain H+ ions in their formulas, or substances that turn Htmus blue but which do not contain OH ions in their formulas. It was a good start, but a more... 

Here s how the Bronsted-Lowry theory defines acids and bases ... 

The Arrhenius theory similarly attempts to define acids and bases as isolated species in solution giving rise, respectively, to hydrogen ions and hydroxide ions. While this view accounts for some properties of acids and bases and their reactions, it cannot begin to explain acidic and basic characteristics in non-aqueous media. Acidic and basic properties are, in fact, consequent upon interaction with the solvent and until such interaction has taken place the properties are not shown. 

Acid-base models. You can indicate that in the 17 century Boyle described acids as substances with a sour taste and bases ( alkahs ) as substances that have the ability to neutralise them. At the end of the 19 century Arrhenius used his work on electrolytic dissociation to revise this model by defining acids and bases as substances that produce LL ions (acids) or OH" ions (bases) in an aqueous solution. In the 1920s a more general model was introduced by Bronsted (and also by Lowry) who defined acids and bases as particles that donate protons (acids) or accept protons (bases). Their model exceeds the acid-base model to solvents other than water such as ammonia. In the same period of time this model was extended by Lewis who defined acids as electron pair acceptors and bases as electron pair donors. This extension also comprises of reactions that do not involve ions. 

The development of the theory of solvent systems was begun by Franklin in 1905. Reasoning from formal analogy to the hydrogen ion-hydroxyl ion theory he defined acids and bases in liquid ammonia. According to his theory, if water ionizes into hydronium and hydroxyl ions, liquid ammonia must ionize into ammonium and amide ions ... 

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