Bronsted-Lowry acid-base concept

Kauffman GB. The Bronsted-Lowry acid-base concept. J Chem Ed 1988 85 28-31. 

George B. Kauffman, "The Bronsted-Lowry Acid-Base Concept," ]. Chem. Educ., Vol. 65, 1988, 28-31. 

KEY CONCEPT PROBLEM 15.3 For the following reaction in aqueous solution, identify the Bronsted-Lowry acids, bases, and conjugate acid-base pairs ... 

The concepts of acid-base equilibria were developed in this chapter for aqueous solutions (in aqueous solutions, water is the solvent and is intimately involved in the equilibria). However, the Bronsted-Lowry acid-base theory can be extended easily to other solvents. One such solvent that has been investigated in depth is liquid ammonia, NH3. 

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... 

When water acts as a Bronsted-Lowry acid, it donates a proton to another species, thereby converting to the hydroxide ion. HjO + H O OH + H3O" base acid conjugate conjugate base acid 1 point given for correct Bransted-Lowry concept of water being able to donate a proton, resulting in a hydroxide ion. 

Acid-Base Concepts The Bronsted-Lowry Theory 613... 

The Lewis concept deals with the behavior of electron pairs in chemical reactions. The same electron pairs we looked at when we discussed molecular geometry (see Chapter 7) can be involved in many reactions. Substances that can form a covalent bond by accepting an electron pair from another substance are known as Lewis acids. Substances that can form a covalent bond by donating an electron pair to another substance are known as Lewis bases. Be careful that you don t mix these up with the Bronsted-Lowry acids and bases. It is easy to do since the words donate and accept are used, except they are associated with the opposite species (Bronsted-Lowry acids donate protons, while Lewis acids accept electron pairs). 

A vast generalization beyond the Bronsted—Lowry acids and bases concepts is the concept of a Lewis29 base (an electron pair donor) and a Lewis acid (an electron pair acceptor). This concept has been used extensively in all branches of chemistry. In physical organic chemistry, quantities of the type pA = —logio[A] have used extensively to study reactivities—for example, in the Hammett equation. 

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. 

All compounds that are acids and bases in the Arrhenius concept are also acids and bases in the Bronsted-Lowry scheme. Ammonia gas, NH3fgJ, is very soluble in water, quickly engaging in an equilibrium that produces hydroxide ion making the solution basic. In doing so, water acts as a Bronsted-Lowry acid, donating a proton to ammonia, leaving the remainder of the water molecule, the hydroxide ion, behind. 

Bronsted-Lowry concept (14) An acid-base concept that defines an acid as a proton donor and a base as a proton acceptor. Neutralization is the transfer of a proton from an acid to a base. 

Skill 25.1 Compare the Arrhenius, Bronsted-Lowry, and Lewis concepts of acids and bases. 

The acid-base concept, either in the limited interpretation of the Bronsted-Lowry theory or in the more general sense of the Lewis theory, is one of the most useful classification schemes in chemistry and a tool for systematizing the relationship between structure and reactivity. In this section we examine how this general concept can be applied to solid surfaces [29], From the most general (Lewis) point of view, a surface is classified as acidic (electron acceptor) or basic (electron donor) according to the direction of net electron transfer that results in the formation of new chemical bonds with an adsorbed molecule. 

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. 

In the first part of this chapter, we will discuss the Arrhenius, the Bronsted-Lowry, and the Lewis concepts of acids and bases.The Bronsted-Lowry and Lewis concepts apply to nonaqueous as well as aqueous solutions and also enlarge on the Arrhenius concept in other ways.This chapter expands on what you learned in Chapter 4 about acids and bases. 

The meaning of the terms acid and base has changed in the course of the development of chemical science. Even now, they are not uniformly standardized. For interpretation of phenomena in aqueous solution, the acid-base concept of Bronsted and Lowry has proved very useful. It is the basis of the following treatment. Following this concept, acids are characterized by their function in releasing protons, whereas bases are able to accept protons. This means that, as a precondition for an acid-base reaction, an acid as well as a base must be present. Only protons can be subject to transfer from one partner to another. Acid-base reactions always follow a scheme like this ... 

Ideas about adds and bases (or alkalis) date back to ancient times. The word acid is derived from the Latin acidus (sour). Alkali (base) comes from the Arabic al-qali, referring to the ashes of certain plants from which alkaline substances can be extracted. The acid-base concept is a major theme in the history of chemistry. In this section, we emphasize the view proposed by Svante Arrhenius in 1884 but also introduce a more modern theory proposed in 1923 by Thomas Lowry and by Johannes Bronsted. 

On the above basis it is, in principle, unnecessary to treat the strength of bases separately from acids, since any protolytic reaction involving an acid must also involve its conjugate base. The basic properties of ammonia and various amines in water are readily understood on the Bronsted-Lowry concept. 

The Bronsted-Lowry concept of acids and bases4 makes it unnecessary to distinguish between acid and base indicators emphasis is placed upon the charge types of the acid and alkaline forms of the indicator. The equilibrium between the acidic form InA and the basic form InB may be expressed as ... 

The Bronsted-Lowry theory focuses on the transfer of a proton from one species to another. However, the concepts of acids and bases have a much wider significance than the transfer of protons. Even more substances can be classified as acids or bases under the definitions developed by G. N. Lewis ... 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

This concept covers most situations in the theory of AB cements. Cements based on aqueous solutions of phosphoric acid and poly(acrylic acid), and non-aqueous cements based on eugenol, alike fall within this definition. However, the theory does not, unfortunately, recognize salt formation as a criterion of an acid-base reaction, and the matrices of AB cements are conveniently described as salts. It is also uncertain whether it covers the metal oxide/metal halide or sulphate cements. Bare cations are not recognized as acids in the Bronsted-Lowry theory, but hydrated... 

Each of the three definitions expands our concept of acids and bases. Arrhenius basic definition is adequate for understanding many of the properties of acids and bases. It is important to recognize, though, that acids and bases are not fixed labels that can be applied to a substance. Bronsted-Lowry and Lewis showed that acid-base characteristics are dependent on the reactions that take place between substances. A... 

Generalizing from the data, one notes the following not unexpected results about 10% of the course is devoted to the introductory concepts of stoichiometry and chemical equations more than 20% of the course is devoted to the study of structure of matter and chemical bonding acids and bases (Bronsted-Lowry) and equilibrium are the most extensively covered topics among the remaining content areas. 

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 concept underlying 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 ... 

The base in the Bronsted-Lowry concept is any substance that can accept the proton it can even be the solvent. A Bronsted-Lowry base has an electron pair (a lone pair) that will accept the proton. The point that needs to be stressed here is that the proton is involved in both the definition of an acid (donates a proton) and a base (1accepts a proton). 

The Bronsted-Lowry concept looks at the equilibrium reaction and ties the acid on the left to a base on the right, called a conjugate acid-base pair or, more simply, a conjugate pair. Suppose we were to consider an acid reacting with a compound in equilibrium with the acid s anion and the products, as... 

Note that HA and B are not necessarily neutral. They could be ions that are capable of acting as an acid or a base. This is one of the features of the Bronsted-Lowry concept that broadens the definitions of acids and bases over the Arrhenius concept—there are many more substances that can behave as acids or bases. Further, we can write the reaction including the solvent, water in this case, and the associated Ka. 

Definition According to Bronsted-Lowry concept an acid is a molecule that can donate a proton and a base is a molecule that can accept that proton. 

The Bronsted-Lowry concept of acids and bases will be discussed in detail in Chapter 7. 

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