Arrhenius model of acids and bases

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. 

The Arrhenius theory of acid-base behavior satisfactorily explained reactions of protonic acids with metal hydroxides (hydroxy bases). It was a significant contribution to chemical thought and theory in the latter part of the nineteenth century. The Arrhenius model of acids and bases, although limited in scope, led to the development of more general theories of acid-base behavior. They will be considered in later sections. 

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 ... 

Compare the Arrhenius and Bronsted-Lowry models of acids and bases. 

Hydrogen fluoride is an acid according to both the Arrhenius and Brpnsted-Lowry definihons. All of the acids and bases that fit the Arrhenius definihon of acids and bases also fit the Brpnsted-Lowry dehihtion. But what about bases such as ammoiha, that cannot be considered bases according to the Arrhenius definihon because they lack a hydroxide group Does the Brpnsted-Lowry model explain why they are bases ... 

Compare what you have learned about the Arrhenius model and the Br0nsted-Lowry model of acids and bases. It should be clear to you that all substances classified as acids and bases by the Arrhenius model are classified as acids and bases by the Br0nsted-Lowry model. In addition, some substances not classified as bases by the Arrhenius model are classified as bases by the Br0nsted-Lowry model. 

Arrhenius model (p. 597) A model of acids and bases states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution and a base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in aqueous solution. 

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, while bases produce hydroxide ions. At the time, 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 by the Danish chemist Johannes Bronsted (1879-1947) and the English chemist Thomas Lowry (1874-1936). In terms of the Bronstcd-Lowry model, an acid is a proton (H ) donor, and a base is a proton acceptor. For example, when gaseous FICI dissolves in water, each FICI molecule donates a proton to a water molecule and so qualifies as a Bronsted-Lowry acid. The molecule that accepts the proton, in this case water, is a Bronsted-Lowry base. 

Arrhenius concept of acids and bases (16.1) Bronsted-Lowry model (16.1) conjugate acid (16.1) conjugate base (16.1) conjugate acid-base pair (16.1)... 

Acid-base models. As indicated before, students encounter difficulties in making a clear distinction between the Arrhenius model about acids and bases as substances and the Bronsted model about acids and bases as particles (see subsection on difficulties with models). They mix parts of each model which leads to the use of a hybrid acid-base model. Because of these difficulties and the broader application range of the Bronsted model, you can reflect on the possibility to teach the Bronsted model only and, afterwards, to present the Arrhenius model as an interesting historical model (Hawkes, 1992). 

Mental models are thus produced of both acid concepts those of Arrhenius and of Broensted. The Arrhenius concept explains some phenomena in the area of acids and bases, for instance, the neutralization of solutions of strong acids and bases. Terms like weak acids or derived concepts like acid constants or buffer already reach the limits of Arrhenius concept. 

We have seen that the first successful conceptualization of acid-base behavior was proposed by Arrhenius. This useful but limited model was replaced by the more general Br0nsted-Lowry model. An even more general model for acid-base behavior was suggested by G. N. Lewis in the early 1920s. A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. Another way of saying this is that a Lewis acid has an empty atomic orbital that it can use to accept (share) an electron pair from a molecule that has a lone pair of electrons (Lewis base). The three models for acids and bases are summarized in Table 14.10. 

We have just seen some of the properties of acids and bases. In this section we examine two different models that explain the molecular basis for acid and base behavior the Arrhenius model and the Bronsted-Lowry model. The Arrhenius model, which was developed earlier, is more limited in its scope. The Bronsted-Lowry model was developed later and is more broadly applicable. 

We began our definitions of acids and bases with the Arrhenius model. We then saw how the Br0nsted-Lowry model, by introducing the concept of a proton donor and proton acceptor, expanded the range of substances that we consider acids and bases. We now introduce a third model, which further broadens the range of substances that we can consider acids. This third model is the Lewis model, named after G. N. Lewis, the American chemist who devised the electron-dot representation of chemical bonding (Section 9.1). While the Br0nsted-Lowry model focuses on the transfer of a proton, the Lewis model focuses on the transfer of an electron pair. Consider the simple acid-base reaction between the ion and NH3, shown here with Lewis structures ... 

The Lewis bonding model with its electron pairs can be used to define a more general kind of acid-base behavior of which the Arrhenius and Bronsted-Lowry definitions are special cases. A Lewis base is any species that donates lone-pair electrons, and a Lewis acid is any species that accepts such electron pairs. The Arrhenius acids and bases considered so far fit this description (with the Lewis acid, H, acting as an acceptor toward various Lewis bases such as NH3 and OH , the electron pair donors). Other reactions that do not involve hydrogen ions can still be considered Lewis acid-base reactions. An example is the reaction between electron-deficient BF3 and electron-rich NH3 ... 

Acid-Base Concepts. In the lesson, after becoming familiar with many phenomena on acids and bases as substances, the question is raised as to whether the substance-related Arrhenius concept should be taught, or the particle-related Broensted concept - or the genetic development of both concepts in the form of historically oriented lessons (see Fig. 7.2). Tests on electrical conductivity of solutions of strong acids and bases (see E7.7) confirms that acidic solutions contain H + (aq) ions and basic solutions contain OH (aq) ions. After stating the existence of these ions, one can discuss model drawings and emphasize that the (aq) symbol denotes the complete separation of the ions by hydration (see Figs. 7.10 and 7.11). 

Arrhenius ocid-bose definition A model of acid-base behavior in which an acid is a substance that has H in its formula and produces in water, and a base is a substance that has OH in its formula and produces OH" in water. (579)... 

Arrhenius acids and bases represent a subset of a more generalized model of acid-base theory known as solvent theory. In solvent theory, an acid is defined as any substance that increases the concentration of the cationic species that results from autoionization of the solvent, whereas a base increases the concentration of the anionic species from autoionization. Table 14.1 lists the ion products for some of the more common solvents that undergo autoionization. 

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. 

Although the Arrhenius model is useful in explaining many acidic and basic solutions, it has some shortcomings. For example, ammonia (NH3) and sodium carbonate (Na2C03) do not contain a hydroxide group, yet both substances produce hydroxide ions in solution and are well-known bases. Sodium carbonate is the compound that causes the alkalinity of Lake Natron, Tanzania, which is shown in Figure 18.5. Clearly, a model that includes all bases is needed. 

Arts. The Arrhenius model focuses on the acid or base molecule only, in that an acid is defined as a molecule which dissociates in water to produce protons, and a base is defined as a molecule which dissociates in water to produce hydroxyl ions. In the Brpnsted-Lowry model, the solvent assumes a central role. This theory proposes that an acid is a compound which can donate protons to a base, and consequently, a base is a compound which can accept the acid s donated proton. Therefore, an acid cannot behave as an acid in the absence of a base. Water assumes a central role in these processes, since in the presence of an acid, it acts as a base, accepting the acid s proton. Furthermore, in the presence of a base, it can donate a proton, and act as an acid. 

The Arrhenius model is still widely used, and substances that are called acid are usually acids in the Arrhenius sense. Since citric acid, for example, produces hydrogen ions in solution, it is therefore an acid. Inddenlally, it also tastes sour and is edible at the same time. The same is true for tartaric add. The most commonly consumed form of tartaric add is wine, but it is mixed with alcohol there, which may have a significant health efled. In the Arrhenius concept, a substance can be called acid or base without further specifications (bases decrease the concentration of hydrogen ions). In the remaining two theories, strictly speaking, a substaiKe cannot be called acid or base. All that can be said is that a certain substance behaves as an add (or base) in a certain chemical reaction with another reactarrt. 

The Arrhenius concept is important in that it has provided us with the first mechanistic approach to acid - base behaviour and has been instrumental for the development of more sophisticated theories. TTiere are, however, two major shortcomings in the Arrhenius model. 

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