Definition of acids and bases Arrhenius

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. 

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. 

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 ... 

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. 

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. 

Compare the Bronsted-Lowry definitions of acids and bases with the Arrhenius definitions of acids and bases. 

In this chapter we use the Arrhenius definition of acids and bases. 

Because both dihydrogen phosphate and hydrogen carbonate (and other substances like them) can be either Bronsted-Lowry acids or bases, they cannot be described as a Bronsted-Lowry acid or base except with reference to a specific acid-base reaction. For this reason, the Arrhenius definitions of acids and bases are the ones used to categorize isolated substances on the stockroom shelf A substance generates either hydronium ions, hydroxide ions, or neither when added to water, so it is always either an acid, a base, or neutral in the Arrhenius sense. Hydrogen carbonate is an Arrhenius base because it yields hydroxide ions when added to water. Dihydrogen phosphate is an Arrhenius acid because it generates hydronium ions when added to water. 

Ammonia—a Bronsted-Lowry base All of the acids and bases that fit the Arrhenius definition of acids and bases also fit the Bronsted-Lowry definition. But some other substances that lack a hydroxide group and, therefore, cannot be considered bases according to the Arrhenius definition can be classified as acids according to the Bronsted-Lowry model. One example is ammonia (NHsj.When ammonia dissolves in water, water is a Bronsted-Lowry acid in the forward reaction. Because the NH3 molecule accepts a H+ ion to form the ammonium ion (NH4+), ammonia is a Bronsted-Lowry base in the forward reaction. 

Use the Arrhenius definition of acids and bases to write the net ionic equation for the reaction of an acid with a base. 

The traditional Arrhenius definition of acids and bases derives from the early experiments of Arrhenius and Ostwald on the theory of electrolytic dissociation, specifically as it applies to the autoionization of water given by Equation (I4.1). At 298 K, = 1.0 X 10 and the equilibrium concentrations of H3O+ and OH are identical. 

Apart from this modification, the Arrhenius definitions of acid and base are still valid and useful today, as long as we are talking about aqueous solutions. However, the Arrhenius concept of acids and bases is so intimately tied to reactions that take place in water that it has no good way to deal with acid-base reactions in nonaqueous solutions. For this reason, we concentrate in this chapter on the Bronsted-Lowry definitions of acids and bases, which are more useful to us in our discussion of reactions of organic compounds. 

Although the Arrhenius definition of acids and bases works in many cases, it cannot easily explain why some substances act as bases even though they do not contain OH . The Arrhenius definition also does not apply to nonaqueous solvents. A second definition of acids and bases, called the Bronsted-Lowry definition, introduced in 1923, applies to a wider range of acid-base phenomena. This definition focuses on the transfer of H ions in an acid-base reaction. Since an H ion is a proton—a hydrogen atom with its electron taken away—this definition focuses on the idea of a proton donor and a proton acceptor. 

This definition is known as the Arrhenius definition of acids and bases, and although it describes a great deal of acid-base chemistry, it does not apply in all... 

For most uses, scientists found the Arrhenius definition of acids and bases to be adequate. However, as scientists further investigated acid-base behavior, they found that some substances acted as acids or bases when they were not in a water solution. Because the Arrhenius definition requires that the substances be aqueous, the definitions of acids and bases had to be revised. 

A major improvement on Arrhenius definition of acids and bases came in 1923 when Johannes N. Bronsted and Thomas M. Lowry independently proposed a more general theory. Their theory states that an acid-base reaction involves a proton transfer the acid is the proton donor and the base is the proton acceptor. This theory emphasizes the interdependence of an acid and a base one cannot react without the other. The stronger the acid, the more readily it donates a proton... 

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 definitions (of acids and bases) (700) hydronium ion (701) Brpnsted-Lowry definitions (of acids and bases) (701)... 

The Arrhenius definition of acids and bases states that in an aqueous solution, an acid produces hydrogen ions and a base produces hydroxide ions. 

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