The Arrhenius Definition

Arrhenius s definition of an acid recognized the importance of the hydrogen ion, H. Arrhenius said that a chemical species is an acid if it can dissociate in aqueous solution and yield H+ ions to the solution. H+ ions were responsible for the sour taste of acids, for the effect of acids on litmus paper, and for the reaction of acids with metals and sedimentary materials like limestone. Examples of Arrhenius acids included hydrochloric acid (HCl), nitric acid (HNOj), sulfuric acid (H SO ), acetic acid (HC HjOj), phosphoric acid (H PO ), and oxalic acid (H CjOj. 

Arrhenius went on to say that a chemical species is a base if it can dissociate in aqueous solution and yield hydroxide ions (OH ) to the solution. OH ions are responsible for the bitter taste and slippery feel of bases, the effect of basic substances on Utmus paper, and the reaction of bases with acids to form salts. Examples of Arrhenius bases included NaOH, KOH, Ba(OH)2, Ca(OH), and Mg(OH).  

The Arrhenius definition of acids and bases explains neutraUzation reactions. In neutralization reactions, an acid reacts with a base to produce a salt plus water. For example, the following equation shows the reaction between HCl and NaOH  

Arrhenius would have said that the driving force behind the tendency for acids and bases to react with each other is the strong affinity H+ and OH ions have for each other. Given the opportunity, H and OH ions will always react to form water molecules. 

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 

In the 1880s, the Swedish chemist Svante Arrhenius proposed the following molecular definitions of acids and bases. 

Arrhenius definition states that an acid is a substance that produces H ions in solution. These H ions associate with H2O to form H30 ions. 

Acid—An acid produces H ions in aqueous solution. 

HCl is a covalent compound and does not contain ions. However, in water it ionizes to form ions and Cl aq) ions. The H ions are highly reactive. In 

What are the main characteristics of the molecules and ions that exhibit acid and base behavior In this chapter, we examine three different definitions the Arrhenius definition, the Br0nsted-Lowry definition, and the Lewis definition. Why three definitions, and which one is correct As Huheey noted in the quotation that opens this chapter, no single definition is correct. Rather, each definition is useful in a given instance. We discuss the Lewis definition of acids and bases in Section 15.11 here we discuss the other two. 

According to Arrhenius, an acid is a substance that increases ions in an aqueous solution. A base is a substance that increases OH ions in an aqueous solution. The Arhhenius definitions explain the reactions involving acids that contain acidic hydrogens, and bases that contain basic hydroxyl groups (e.g., metal hydroxides). But the definitions are not able to explain die behavior of acids and bases of other types, such as in nonaqueous solutions. 

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According to the Arrhenius definitions an acid ionizes m water to pro duce protons (H" ) and a base produces hydroxide ions (HO ) The strength of an acid is given by its equilibrium constant for ionization m aqueous solution... 

Achiral (Section 7 1) Opposite of chiral An achiral object is supenmposable on its mirror image Acid According to the Arrhenius definition (Section 1 12) a substance that ionizes in water to produce protons Accord mg to the Br0nsted-Lowry definition (Section 1 13) a sub stance that donates a proton to some other substance According to the Lewis definition (Section 1 17) an electron pair acceptor... 

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923, when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key process responsible for the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... 

When chemists see a pattern in the reactions of certain substances, they formulate a definition of a class of substance that captures them all. The reactions of the substances we call acids and bases are an excellent illustration of this approach. The pattern in these reactions was first identified in aqueous solutions, and led to the Arrhenius definitions of acids and bases (Section J). However, chemists discovered that similar reactions take place in nonaqueous solutions and even in the absence of solvent. The original definitions had to be replaced by more general definitions that encompassed this new knowledge. 

The Bronsted definition also includes the possibility that an ion is an acid (an option not allowed by the Arrhenius definition). For instance, a hydrogen carbonate ion, HC03, one of the species present in natural waters, can act as an acid and lose a proton, and the resulting carbonate ion is removed by precipitation if suitable cations are present (Fig. 10.2) ... 

The Bronsted definitions of acids and bases are more general than the Arrhenius definitions they also apply to species in nonaqueous solvents and even to gas-phase reactions. For example, when pure acetic acid is added to liquid ammonia, proton transfer takes place and the following equilibrium is reached ... 

The Arrhenius definition is not suitable for AB cements for several reasons. It cannot be applied to zinc oxide eugenol cements, for these are non-aqueous, nor to the metal oxychloride and oxysulphate cements, where the acid component is not a protonic acid. Indeed, the theory is, strictly speaking, not applicable at all to AB cements where the base is not a water-soluble hydroxide but either an insoluble oxide or a silicate. 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

According to the Arrhenius definition of acids and bases, acids are substances that produce hydrogen ions (H+) in solution, and bases are substances that produce hydroxide ions (OH ) in solution. When an acid and a base combine, the hydrogen ions from the acid react with the hydroxide ions from the base to form water—a neutralization reaction. 

The Arrhenius definition implies that acids contain hydrogen ions and bases con-... 

Table 13.2 illustrates the presence of hydrogen in acids. It is also apparent that bases contain hydroxide ions, but the weak base ammonia seems to be an exception. Ammonia illustrates one of the shortcomings of the Arrhenius definition of acids and bases specifically, bases do not have to contain the hydroxide ion to produce hydroxide in aqueous solution. When ammonia dissolves in water, the reaction is represented by ... 

This reaction shows that the hydroxide ions come from ammonia pulling a hydrogen away from water resulting in the formation of OH . Therefore, a compound does not have to contain hydroxide to be a base. In addition to this limitation, the Arrhenius definition limits acids and bases to aqueous solutions. 

Peruse Table 16-1 for a list of common acids and bases, noting that all the acids in the list contain a hydrogen at the beginnings of their formulas and that most of the bases contain a hydroxide. The Arrhenius definition of acids and bases is straightforward and works for many common acids and bases, but it s limited by its narrow definition of bases. 

You no doubt noticed that some of the bases in Table 16-1 don t contain a hydroxide ion, which means that the Arrhenius definition of acids and bases can t apply. When chemists realized that several substances behaved like bases but didn t contain a hydroxide ion, they reluctantly acknowledged that another determination method was needed. Independently proposed by Johannes Bronsted and Thomas Lowry in 1923 and therefore named cifter both of them, the Bronsted-Lowry method for determining acids and bases accounts for those pesky non-hydroxide-containing bases. 

Use the Arrhenius definition to identify the acid or base in each reaction and explain how you know. 

Brpnsted-Lowry definition does not differ appreciably from the Arrhenius definition of hydrogen ions (acids) and hydroxide ions (bases) ... 

They have a pH below 7. The definition of pH is minus the power of the hydronium ion concentration. If the pH of a solution is 6, then the hydronium ion concentration is 10 M and an acid is present. According to the Arrhenius definition of an acid, an acid is a substance that dissociates in water to produce hydrogen ions, H. ... 

While there are many definitions of acids and bases, probably the Arrhenius definition is the most operational one. 

In the Arrhenius definitions for acids and bases, the strength of the acid or base is determined by the degree of ionization. For example, HC1 is considered a strong acid because when it is placed in water it completely ionizes to H+ and Cl-. The strongest acids and bases are all Arrhenius acids and bases. 

While the Arrhenius definitions focus on the H+ and OH- ions, the Bransted-Lowry definitions focus on the behavior of protons—that is, the transfer of a proton from one substance to another. 

A Bronsted-Lowry base is a substance that accepts a proton from another substance. This is a significant change from the Arrhenius definition. Arrhenius bases possess an OH, whereas Bronsted-Lowry bases need not. In the example below, notice how ammonia increases the concentration of hydroxide ion in the resulting solution without donating a OH- ion. It does so by accepting a proton from water ... 

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 ... 

The Arrhenius definition was an important contribution to understanding many acids and bases, but it does not explain why a compound such as ammonia (NH3) neutralizes acids, even though it has no hydroxide ion in its molecular formula. In Section 1-13 we discuss a more versatile theory of acids and bases that will include ammonia and a wider variety of organic acids and bases. 

The strength of a Brpnsted-Lowry acid is expressed as it is in the Arrhenius definition, by the extent of its ionization in water. The general reaction of an acid (HA) with water is the following ... 

The Br0nsted-Lowry definition of acids and bases does not replace the Arrhenius definition, but extends it. The Bronsted-Lowry definition of acids and bases requires you to take a closer look at the reactants and products of an acid-base reaction. In this case, acids and bases are not easily defined as having hydronium and hydroxide ions. Instead, you are asked to look and see which substance has lost a proton and which has gained the very same proton that was lost. 

E While the Arrhenius definition of an acid says that an acid yields hydro-nium ions as the only positive ions in solution, the Bronsted-Lowry definition says that acids are proton donors. 

C The Lewis definition of acids states that acids are electron pair acceptors while bases are electron pair donors. Choices A, D, and E show the Arrhenius definition whereas choice B shows the Bronsted-Lowry definition. 

In Chapter 8, we learned the Arrhenius definition of acids and bases—that an acid is a snbstance that can increase the concentration of ions in water and a base is a snbstance that can increase the concentration of OH ions in water. In Chapter 18, we learned about equilibrium systems. This chapter extends both of these concepts in discussing acid-base equilibria in aqueous solutions, which are extremely important to biological as well as chemical processes. 

NHj contains no hydroxide ions yet when you put it in water it increases the hydroxide ion concentration. To account for examples such as this we have what is known as the Bronsted-Lowry (Johannes Bronsted and Thomas Lowry) theory of acids and bases. In this definition an acid is any substance that donates protons in a reaction and a base is any substance that accepts protons in a reaction. This is basically (no pun intended) the same as the Arrhenius definition of acids because if a substance readily donates protons in a reaction it will also increase the hydronium ion concentration when placed in water. The definition of a base, though, is expanded to include any substance that readily accepts protons in a reaction, and that substance doesn t have to contain hydroxide ions. Let s continue with our above example. There, NH contributed to the Off concentration when dissolved in water. But NH3 is also a proton acceptor (which we have defined as a base) because it picks up a proton to become as in... 

The Arrhenius definition of an acid a substance that produces HH ions in solution. 

Describe the distinctive properties of strong and weak acids, and relate their properties to the Arrhenius definition of an acid. 

Both strong and weak bases generate hydroxide ions when they dissolve in water, as Figure 6 shows. This property is the basis of the Arrhenius definitions of a base. 

The definitions of Arrhenius acid and Arrhenius base given earlier in this book are variants of the definitions of acid and base originally proposed by Arrhenius in the late 19th century. One drawback that the Arrhenius definitions have is that they are limited to aqueous solutions HCl, for instance, should be considered an acid whether it is in the form of a pure gas or in aqueous solution. Another limitation is that the Arrhenius definition cannot classify substances that sometimes act as acids and sometimes act as bases. 

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