Theory, Arrhenius Bronsted

In the previous sections, you learned about three acid-base theories Arrhenius, Bronsted-Lowry, and Lewis. The Bronsted-Lowry theory is especially useful for describing acid-base reactions that take place in aqueous solutions. This section will use the Bronsted-Lowry description to explore reactions between acids and bases. 

The theory of Bronsted (1923) and Lowry (1923a, b) is of more general applicability to AB cements. Their definition of an acid as a substance that gives up a proton differs little from that of Arrhenius. However, the same is not true of their definition of a base as a substance capable of accepting protons which is far wider than that of Arrhenius, which is limited to hydroxides yielding hydroxide ions in aqueous solution. These concepts of Bronsted and Lowry can be defined by the simple equation (Finston Rychtman, 1982) ... 

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. 

Skill 10.1 Analyzing acids and bases according to acid-base theories (i.e., Arrhenius, Bronsted-Lowry, Lewis)... 

A note on good practice The entities that are regarded as acids and bases are different in each theory. In the Lewis theory, the proton is an acid in the Bronsted theory, the species that supplies the proton is the acid. In both the Lewis and Bronsted theories, the species that accepts a proton is a base in the Arrhenius theory, the species that supplies the proton acceptor is the base (Fig. 10.61. 

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. 

Although the protonic theory is not confined to aqueous solutions, it does not cover aprotic solvents. The solvent system theory predates that of Bronsted-Lowry and represents an extension of the Arrhenius theory to solvents other than water. It may be represented by the defining equation ... 

Any text on acids and bases would not be deemed complete if mention were not made of the extended definition of acids and bases that is embodied in the Lowry-Bronsted theory. The theory basically proposed a more general definition of acids and bases to overpower the limitations of the theory arising from the Arrhenius concept. 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

A) In addition to the more modem Bronsted and Lewis theories, it is important not to forget the classic Arrhenius theory in its modern form, the so-called solvents theory, where it can be applied, i.e., with solvents that undergo self-dissociation in this form it was originally formulated in 1949 by Jander3 in Germany and is illustrated by the following reaction equations ... 

The Bronsted-Lowry definition of an acid is essentially the same as Arrhenius idea An acid is any substance that releases a hydrogen ion. Their idea has come to be known as the Bronsted-Lowry theory of acids and bases. 

The Br0nsted theory expands the definition of acids and bases to allow us to explain much more of solution chemistry. For example, the Brpnsted theory allows us to explain why a solution of ammonium chloride tests acidic and a solution of sodium acetate tests basic. Most of the substances that we consider acids in the Arrhenius theory are also acids in the Bronsted theory, and the same is true of bases. In both theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the ions, the weak acid, and the water. A similar statement can be made about weak bases ... 

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. 

Just after the First World War in 1923, Bronsted and Bjerrum in Denmark and Lowry in Great Britain jointly put forward a more acceptable and satisfactory theory of acids and bases which is devoid of objections earlier raised in Arrhenius definition. 

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 ... 

Definition Term Arrhenius Theory Bronsted-Lowry Theory... 

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... 

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. 

To account for the acidity of a substance, consider how it can produce H + ions in water (Arrhenius theory) and how it can act as a proton donor (Bronsted-Lowry theory). 

According to the Arrhenius theory, acids (HA) are substances that dissociate in water to produce H + (aq). Bases (MOH) are substances that dissociate to yield OH aq). The more general Bransted-Lowry theory defines an acid as a proton donor, a base as a proton acceptor, and an acid-base reaction as a proton-transfer reaction. Examples of Bronsted-Lowry acids are HC1, NH4+, and HSO4- examples of Bronsted-Lowry bases are OH-, F-, and NH3. 

This is the same as Bronsted s theory which was designed particularly for solutions. The concentration of the activated complex can be expressed in terms of the reactants and the equilibrium constant K. Also the heat of the reaction, AH, to give the activated complex, can be calculated approximately from the quantum theory or from the Arrhenius theory. Since AF= —RT In K and AF = AII — TAS, and since K can, in some cases, be calculated from known, fundamental constants, the entropy term remains the only unknown. Rodebush has long pointed out that the unknown quantity 5 in the formula k = se E/RT is related to an entropy term. As a first approximation it has been related to a collision frequency in bimolecular reactions and to a vibration frequency in unimolecu-lar reactions. Combining the two thermodynamic equations23... 

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

In 1923, two chemists working independently of each other, proposed a new theory of acids and bases. (See Figure 10.4.) Johannes Bronsted in Copenhagen, Denmark, and Thomas Lowry in London, England, proposed what is called the Brpnsted-Lowry theory of acids and bases. This theory overcame the problems related to the Arrhenius theory. 

Hydrogen was recognized as the essential element in acid by Davy after his work on hydrohalic acid. Theories of acid and base have played an important role ever since. The electrolytic dissociation theory of Arrhenius and Ostwald, the introduction of the pH scale for hydrogen-ion concentrations by Sorensen, the theory of acid-base titration and the use of indicators, and Bronsted s concept of acid and conjugate base as proton donors and acceptors are other landmarks in the recognition of hydrogen as an acid. 

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... 

In 1923, J. N. Bronsted (1879-1947) and T. M. Lowry (1874-1936) independently The Bronsted-Lowry theory is presented logical extensions of the Arrhenius theory. Bronsted s contribution was more especially useful fur reactions in thorough than Lowry s, and the result is known as the Bronsted theory or the Bron- aqueous solutions. It is widely used in sted-Lowry theory. medicine and in the biological... 

Many organic and biological reactions are acid-base reactions that do not lit within the Arrhenius or Bronsted-Lowry theories. Experienced chemists find the Lewis theory to be very usefol because so many other chemical reactions are covered by it. The less experienced sometimes find the theory less useful, but as their knowledge expands so does its utility. 

We usually prefer the Arrhenius or the Bronsted-Lowry theory when water or another protonic solvent is present. 

Arrhenius in 1887 was the first person to give a definition of an acid and a base. According to him, an acid is one that gives rise to excess of in aqueous solution, whereas a base gives rise to excess of OH in solution. This was modified by Bronsted-Lowry in 1923 such that a proton donor was defined as an acid and a proton acceptor as a base. They also introduced the familiar concept of the conjugate acid-base pair. The final refinement to the acid-base theory was completed by Lewis in 1923, who extended the concept that acid is an acceptor of electron pairs while base is a donor of electron pairs. 

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