Acid-base theories Bronsted-Lowry theory

The theory of acids and bases, the Bronsted-Lowry theory, was dependent on the presence of proton (H ) to show acidic or basic properties, which may not always be the case hence in 1923, G. N. Lewis introduced a theory of acids and bases with a more general definition of acid-base reactions by examining what happens when an ion combines with an OH ion to form water. 

Thus the acids of the Bronsted-Lowry theory may be viewed as Lewis acid-base adducts in which the acidic component is (for example, can be viewed as... 

There are substances other than Arrhenius bases that turn litmus paper blue, feel slippery in water solution, or neutralize acids. In 1923, Johannes Bronsted, a Danish chemist, and Thomas Lowry, an English chemist, presented a broader definition of acids and bases. The Bronsted-Lowry theory states that an acid is a proton donor and a base is a proton acceptor. This definition of an acid is similar to that in the Arrhenius theory (a hydrogen ion is a proton), and thus all Arrhenius acids are also Bronsted-Lowry acids, and vice versa. However, the Bronsted-Lowry definition of a base is broader. There are substances other than Arrhenius bases that accept hydrogen ions. In other words, the hydroxide ion is a hydrogen ion acceptor, but there are other anions that also do this. 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

This more general view of acids and bases is named the Bronsted-Lowry theory after the two scientists who proposed it, J. N. Brpnsted and T. M. Lowry. 

The Bronsted-Lowry theory of acids and bases referred to in Section 10.7 can be applied equally well to reactions occurring during acid-base titrations in non-aqueous solvents. This is because their approach considers an acid as any substance which will tend to donate a proton, and a base as a substance which will accept a proton. Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end points when titrations are carried out in non-aqueous media. An additional advantage is that many substances which are insoluble in water are sufficiently soluble in organic solvents to permit their titration in these non-aqueous media. 

In the Bronsted-Lowry theory, any acid (HB) is considered to dissociate in solution to give a proton (H + ) and a conjugate base (B ) whilst any base (B) will combine with a proton to produce a conjugate acid (HB + ). 

The term proton in these definitions refers to the hydrogen ion, H+. An acid is a species containing an acidic hydrogen atom, which is a hydrogen atom that can be transferred as its nucleus, a proton, to another species acting as a base. The same definitions were proposed independently by the English chemist Thomas Lowry, and the theory based on them is called the Bronsted-Lowry theory of acids and bases. 

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 ... 

We saw in Section J that a salt is produced by the neutralization of an acid by a base. However, if we measure the pH of a solution of a salt, we do not in general find the neutral value (pH = 7). For instance, if we neutralize 0.3 M CHjCOOH(aq) with 0.3 M NaOH(aq), the resulting solution of sodium acetate has pH = 9.0. How can this be The Bronsted-Lowry theory provides the explanation. According to this theory, an ion may be an acid or a base. The acetate ion, for instance, is a base, and the ammonium ion is an acid. The pH of a solution of a salt depends on the relative acidity and basicity of its ions. 

Bronsted-Lowry theory A theory of acids and bases involving proton transfer from one species to another. 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

Characteristics of the reactions described so far lead to several conclusions regarding acids and bases according to the Bronsted-Lowry theory. 

Base strength refers to the relative tendency to produce OH- ions in aqueous solution by (1) the dissociation of soluble metal hydroxides or (2) by ionization reactions with water using Arrhenius theory. A more general definition, applying Bronsted-Lowry theory, is that base strength is a measure of the relative tendency to accept a proton from any acid. 

At the microscopic level, acids are defined as proton (H ) donors (Bronsted-Lowry theory) or electron-pair acceptors (Lewis theory). Bases are defined as proton (H+) acceptors (Bronsted-Lowry theory) or electron-pair donors (Lewis theory). Consider the gas-phase reaction between hydrogen chloride and ammonia ... 

The limitations of the Arrhenius theory of acids and bases are overcome by a more general theory, called the Bronsted-Lowry theory. This theory was proposed independently, in 1923, by Johannes Br0nsted, a Danish chemist, and Thomas Lowry, an English chemist. It recognizes an acid-base reaction as a chemical equilibrium, having both a forward reaction and a reverse reaction that involve the transfer of a proton. The Bronsted-Lowry theory defines acids and bases as follows ... 

Unlike the Arrhenius theory, the Bronsted-Lowry theory of acids and bases can explain the basic properties of ammonia when it dissolves in water. See Figure 8.4. 

The above reaction depicts water as an Arrhenius acid and base. Treating water in terms of the Bronsted-Lowry theory, a more appropriate reaction would be... 

In the same year that Bronsted and Lowry proposed their definition of acids and bases, an American chemist named Gilbert Lewis proposed an alternative definition that not only encompassed Bronsted-Lowry theory but also accounted for acid-base reactions in which a hydrogen ion isn t exchanged. Lewis s definition relies on tracking lone pairs of electrons. Under his theory, a base is any substance that donates a pair of electrons to form a coordinate covalent bond with another substance, while an acid is a substance that accepts that electron pair in such a reaction. As we explain in Chapter 5, a coordinate covalent bond is a covalent bond in which both of the bonding electrons are donated by one of the atoms forming the bond. 

The Bronsted-Lowry theory provides the answer. According to this theory, an ion may be an acid or a base. The acetate ion, for instance, is a base, and the ammonium ion is an acid. Therefore, it is hardly surprising that a solution of a salt is acidic or basic. 

The Arrhenius theory accounts for the properties of many common acids and bases, but it has important limitations. For one thing, the Arrhenius theory is restricted to aqueous solutions for another, it doesn t account for the basicity of substances like ammonia (NH3) that don t contain OH groups. In 1923, a more general theory of acids and bases was proposed independently by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. According to the Bronsted-Lowry theory, an acid is any substance (molecule or ion) that can transfer a proton (H + ion) to another substance, and a base is any substance that can accept a proton. In short, acids are proton donors, bases are proton acceptors, and acid-base reactions are proton-transfer reactions ... 

Account for the acidic properties of nitrous acid (HN02) in terms of the Arrhenius theory and the Bronsted-Lowry theory, and identify the conjugate base of HNO2. 

The Lewis theory of acids and bases is more general than Bronsted-Lowry theory, but Bronsted-Lowry s definition is used more frequently. The terms "acid" and "base" most often refer to Bronsted acids and bases, and the term "Lewis acid" is usually reserved for chemicals like BF3 that are not Bronsted acids. 

Bronsted acid-base theory — In 1923, Bron-sted and, independently of him, Lowry published essentially the same theory of acids and bases which can be applied not only to water as a solvent but also to all other - protic solvents, as well as to proton transfer reactions in gases. An acid is defined as a proton donor, i.e.,... 

Feb. 22,1879, Varde, Denmark - Dec. 17,1947, Copenhagen, Denmark) Ph.D. Copenhagen 1908, since 1908 Professor of Chemistry (the 3rd chair, i.e., the chair of Physical Chemistry at the Univ. of Copenhagen). 1926/27 visiting Professor at Yale Univ., New Haven, Connecticut, USA. Famous for his work on chemical reaction kinetics, chemical affinity, indicators, and thermodynamics of solutions. He could explain the effect of activity coefficients on reaction rates in solutions. In 1923 he developed independently of - Lowry, and - Bjerrum a new -> acid-base theory, the so-called Bronsted acid-base theory. 

Arrhenius and Bronsted-Lowry theories of acids and bases... 

According to the Bronsted-Lowry theory, there is only one requirement for an acid-base reaction. One substance must provide a proton, and another substance must receive the same proton. In other words, an acid-base reaction involves the transfer of a proton. 

The idea of proton transfer has major implications for understanding the nature of acids and bases. According to the Bronsted-Lowry theory, any substance can behave as an acid, but only if another substance behaves as a base at the same time. Similarly, any substance can behave as a base, but only if another substance behaves as an acid at the same time. 

For example, consider the reaction between hydrochloric acid and water shown in Figure 10.5. In this reaction, hydrochloric acid is an acid because it provides a proton (H+) to the water. The water molecule receives the proton. Therefore, according to the Bronsted-Lowry theory, water is a base in this reaction. When the water receives the proton, it becomes a hydronium ion (H30+). Notice the hydronium ion on the right side of the equation. 

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