Bronsted-Lowry Acid-Base Model

In Chapter 4 we considered an acid to be a substance that produces an excess of H+ ions in water. A base was similarly defined to be a substance that forms excess 0H ions in water solution. This approach, first proposed by Svante Arrhenius in 1884, is a very practical one, but it has one disadvantage. It severely limits the number of reactions that qualify as acid-base. 

The model of acids and bases 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 

A Bronsted-Lowry acid-base reaction might be represented as 

The species HB and HA act as Bronsted-Lowry acids in the forward and reverse reactions, respectively A and B act as Bronsted-Lowry bases. 

The Bronsted-Lowry model introduces some new terminology. 

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Boyle s law The gas law stating that, at constant temperature and amount of gas, the volume occupied by a gas is inversely proportional to the applied (external) pressure V - IP. (144) Bronsted-Lowry acid-base definition A model of acid-base behavior based on proton transfer, in which an acid and a base are defined, respectively, as species that donate and accept a proton. (587) buffer (See acid-base buffer.)... 

Note that Bronsted-Lowry acid-base reactions (proton donor-proton acceptor reactions) are encompassed by the Lewis model. For example, the reaction between a proton and an ammonia molecule, that is. 

The ability of certain chemical compounds to lose or gain protons has been an active area of research since the formulation the concept of pH in 1909 [1] and the appearance of the Bronsted-Lowry acid-base theory in 1923. According to Bron-sted and Lowry an acid is a compound that can donate a proton, whereas a base is a compound that can accept a proton. The dissociation of a proton from an acid in solution can be modeled by a simple equilibrium constant... 

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 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. 

The real value of the Lewis model for acids and bases is that it covers many reactions that do not involve Bronsted-Lowry acids. For example, consider the gas-phase reaction between boron trifluoride and ammonia ... 

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

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. 

Perhaps you will not be surprised, then, you to learn that an even more general model of acids and bases was proposed by American chemist G. N. Lewis (1875-1946). Recall that Lewis developed the electron-pair theory of chemical bonding and introduced Lewis structures to keep track of the electrons in atoms and molecules. He applied his electron-pair theory of chemical bonding to acid-base reactions. Lewis proposed that an acid is an ion or molecule with a vacant atomic orbital that can accept (share) an electron pair. A base is an ion or molecule with a lone electron pair that it can donate (share). According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Note that the Lewis model includes all the substances classified as Bronsted-Lowry acids and bases and many more. 

Note that the SO3 molecule, a Lewis acid, accepts an electron pair from the ion, a Lewis base. The Arrhenius, Bronsted-Lowry, and Lewis acid-base models are summarized in Table 18-2. 

The Lewis concept of acids and bases includes proton-transfer reactions all Bronsted-Lowry bases (proton acceptors) are also Lewis bases, and all Bronsted-Lowry acids (proton donors) are also Lewis acids. The Lewis model, however, is more general in that it is not restricted to proton-transfer reactions. 

Boron, a Group 3A element, has three electrons in its valence shell, and after forming single bonds with three fluorine atoms to give BFj, boron still has only sbc electrons in its valence shell. Because it has an empty orbital in its valence shell and can accept two electrons into it, boron trifluoride is electron deficient and, therefore, a Lewis add. In forming the O—B bond, the oxygen atom of diethyl ether (a Lewis base) donates an electron pair and boron accepts the electron pair. The reaction between diethyl ether and boron trifluoride is dassified as an acid-base reaction according to the Lewis model, but because there is no proton transfer involved, it is not classified as an add-base reaction by the Bronsted-Lowry model. Said another way, all Bronsted-Lowry acids are protic acids Lewis adds maybe protic acids or aprotic acids. 

We have seen that the Br0nsted-Lowry model extends the Arrhenius picture of acid-base reactions considerably. However, the Bronsted-Lowry model is restricted in one important respect. It can be applied only to reactions involving a proton transfer. For a species to act as a Bronsted-Lowry acid, it must contain an ionizable hydrogen atom. 

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. 

Today, when chemists use the words acid or base they refer to a model developed independently by Bronsted, Lowry, and Bjerrum. Since the most explicit statement of this theory was contained in the writings of Br /nsted, it is most commonly known as the Bronsted acid-base theory. 

Bronsted-Lowry definition (model) a model proposing that an acid is a proton donor, and a base is a proton acceptor. (7.1)... 

The Bronsted-Lowry model The Danish chemist Johannes Brpnsted and the English chemist Thomas Lowry independently proposed a more inclusive model of acids and bases—a model that focuses on the hydrogen ion (H+). In the Bronsted-Lowry model of acids and bases, an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor. 

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 ... 

A large number of definitions for acids and bases have been introduced, but the 1923 definitions of J.N. Bronsted and T.M. Lowry are the most useful for discussions of ionic equilibria in aqueous systems. According to the Bronsted-Lowry model, an acid is a substance capable of donating a proton to another substance, such as water ... 

Bronsted-Lowry model Conjugate acid Conjugate base... 

According to the Bronsted-Lowry model, the general reaction that occurs when an acid is dissolved in water can best be represented as an acid (HA) donating a proton to a water molecule to form a new acid (the conjugate acid) and a new base (the conjugate base). 

Use the Bronsted-Lowry model to label the acid-base pairs in the following equation for the ionization of water ... 

What is an acid in the Bronsted-Lowry model What is a base ... 

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 Bronsted-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... 

Arrhenius concept Bronsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant Section 14.2 strong acid weak acid diprotic acid oxyacids organic acids carboxyl group monoprotic acids amphoteric substance autoionization... 

Notice that all substances classified as acids and bases by the Arrhenius model are classified as acids and bases by the Bronsted-Lowry model. 

In this reaction, the FI" ion is the Lewis acid. Its vacant Is orbital accepts an electron pair from the F ion. The fluoride ion is the Lewis base. It donates a lone electron pair to form the hydrogen-fluorine bond in HF. Note that this reaction also conforms to the Bronsted-Lowry model of acids and bases because H+ can be considered a hydrogen-ion donor and F a hydrogen-ion acceptor. 

Bronsted-Lowry model (p. 638) A model of acids and bases in which an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor. 

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)... 

Acids or bases in water are commonly described by two different models. Arrhenius postulated that acids produce H+ ions in aqueous solutions and that bases produce OH ions. The Bronsted-Lowry model is... 

According to the Bronsted-Lowry model, an acid is a "proton donor" and a base is a "proton acceptor." Explain. 

Within the Bronsted-Lowry model, certain pairs of molecules are described as a conjugate acid-base pair. The two species in a conjugate acid-base pair differ by a proton only. A base is said to have a conjugate acid, and an acid is said to have a conjugate base. 

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