The Nature of Acids and Bases

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Acids and bases were first discussed in Section 4.2. 

Acids were first recognized as a class of substances that taste sour. Vinegar tastes sour because it is a dilute solution of acetic add citric acid is responsible for the sour taste of a lemon. Bases, sometimes called alkalis, are characterized by their bitter taste and slippery feel. Commercial preparations for unclogging drains are highly basic. 

The first person to recognize the essential nature of acids and bases was Svante Arrhenius. Based on his experiments with electrolytes, Arrhenius postulated that 

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Acids have the following general properties a sour taste, the ability to dissolve many metals, the ability to turn blue litmus paper red, and the ability to neutralize bases. Table 15.1 lists some common acids. 

You can find hydrochloric acid in most chanistry laboratories. In industry, it is used to clean metals, to prepare and process some foods, and to refine metal ores. As we just discussed in Section 15.1, hydrochloric acid is also the main component of stomach acid. 

I The formula for acetic acid can also he I written as CH3COOH. 

Hydrochloric acid (HCl) Metal cleaning food preparation ore refining primary component of stomach acid 

Sulfuric acid (H2SO4) Fertilizer and explosives manufacturing dye and glue production automobile batteries electroplating of copper 

The first person to recognize the essential nature of acids and bases was Svante Arrhenius. Based on his experiments with electrolytes, Arrhenius postulated that acids produce hydrogen ions in aqueous solution, and bases produce hydroxide ions. At the time of its discovery the Arrhenius concept of acids and bases was a major step forward in quantifying acid—base chemistry, but this concept is limited because it applies only to aqueous solutions and allows for only one kind of base—the hydroxide ion. A more general definition of acids and bases was suggested independently by the Danish chemist Johannes N. Bronsted (1879-1947) and the English chemist Thomas M. Lowry (1874-1936) in 1923. In terms of the Bronsted—Lowry definition, an acid is a proton (H+) donor, and a base is a proton acceptor. For example, when gaseous HCl dissolves in water, each HCl molecule donates a proton to a water molecule, and so HCl qualifies as a Bronsted-Lowry acid. The molecule that accepts the proton—water in this case—is a Bronsted-Lowry base. 

To understand how water can act as a base, we need to recognize that the oxygen of the water molecule has two unshared electron pairs, either of which can form a covalent bond with an H+ ion. When gaseous HCl dissolves, the following reaction occurs  

Note that the proton is transferred from the HCl molecule to the water molecule to form H30+, which is called the hydronium ion. 

The general reaction that occurs when an acid is dissolved in water can best be represented as 

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The model of acids and bases used in this chapter is a somewhat more general one developed independently by Johannes Bronsted (1879-1947) in Denmark and Thomas Lowry (1874-1936) in England in 1923. The Bronsted-Lowry model focuses on the nature of acids and bases and the reactions that take place between them. Specifically, it considers that—... 

The nature of the association between cement-forming cation and anion is important. As we shall see from theoretical considerations of the nature of acids and bases in section 2.3, these bonds are not completely ionic in character. Also while cement-forming cations are predominantly a-... 

In 1887, Arrhenius published a theory to explain the nature of acids and bases. It is called the Arrhenius theory of acids and bases. 

The idea of proton transfer has major implications for understanding the nature of acids and bases. According to the Bronsted-Lowry theory, any substance can behave as an acid, but only if another substance behaves as a base at the same time. Similarly, any substance can behave as a base, but only if another substance behaves as an acid at the same time. 

The Nature of Acids and Bases. The alchemists observed that many different substances when dissolved in water give solutions with certain properties in common, shell as acidic taste and the property of reacting with metals such as zinc with liberation of hydrogen. These substances were classed as acids. It is now known that the acidic properties of the solutions are due to the presence of hydrogen ion H+, in concentration greater than in pure water. 

In a wider interpretation of the nature of acids and bases (Bronsted-Lowry Theory), HA and A are said to be conjugate they form a conjugate acid-base pair. Here, HA is the conjugate acid of A , and A is the conjugate base of HA. 

The shortcomings of the Arrhenius theory led chemists to seek other explanations for the nature of acids and bases. The Br0nsted-Lowry theory was introduced independently in 1923 by the Danish chemist Johannes Nicolaus Brqnsted and the English chemist Thomas Martin Lowry, stating that any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. Their theory explained the behaviour of all of the acids and bases covered by the Arrhenius theory, but also was able to resolve some of the problems with that theory. That is, they were able to explain why some salts are acidic and basic (due to salt hydrolysis) and why no free protons are found in the solutions of some acids. 

Figure B.1.19. Influence of the nature of acids and bases on the corrosion of 1100 H14 [34].

Theories on the nature of acids and bases are presented in most general chemistry texts [139]. The Arrhenius theory proposes that an acid is a material that releases protons (H ), and a base is a material that provides hydroxyl (OH ) ions in solution. It follows that the combination of these ions produces water ... 

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