Bronsted and Lowry

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... 

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) ... 

In 1923, two scientists, working independently, came up with an idea that would explain how substances that do not contain hydroxide ions could act as a base. A Danish scientist, Johannes Bronsted (1879-1947), and an English chemist named Thomas Lowry (1874-1936) both published papers about the same time stating that a base is any substance that accepts a proton (a hydrogen ion). Bronsted and Lowry s definitions explained how bases that do not contain the hydroxide ion work. Their definition also works for bases that do contain the hydroxide ion. 

If baking soda is added to hydrochloric acid, however, it does accept a hydrogen ion. So, according to Bronsted and Lowry s theories, baking soda is a base ... 

According to the Bronsted and Lowry definition, water itself can act as both an acid and a base as the result of autodissociation ... 

Any substance which increases the concentration of hydrogen ions in an aqueous solution above the level provided by the autodissociation of water itself is, by Bronsted and Lowry s definition, an acid. For pure water, the ionic product (Kw) suggests that at equilibrium at 25°C, the following holds true ... 

You need to know these Bronsted and Lowry definitions of an acid and a base. 

In 1923, Bronsted and Lowry defined an acid as a proton donor and a base as a proton acceptor. 

In the same year that Bronsted and Lowry proposed their definition for acids and bases, the American G. N. Lewis proposed an alternative definition based on the... 

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. 

In 1923, the same year in which Bronsted and Lowry defined acids and bases in terms of their proton donor/acceptor properties, the American chemist G. N. Lewis proposed an even more general concept of acids and bases. Lewis noticed... 

Different approaches are used to explain acid-base properties of substances. According to Bronsted and Lowry [16,17], an acid is a substance able to act as proton donor, while a base is a substance able to act as proton acceptor. According to Lewis [18], acids are the compounds that are able to accept electron pairs forming covalent bond, while bases are the compounds that act as donors of electron pair. 

Acids and bases were defined and described by early chemists, including Boyle, Lavoisier, Davy, Berzelius, Liebig, and Arrhenius. At the present time, depending on objectives, one of two definitions of acids and bases is likely to be accepted. These two definitions, by Bronsted and Lowry and by Lewis, were proposed about the same time. According to the Bronsted definition acids are substances having a tendency to lose a proton, and bases are those having a tendency to accept a proton. Thus, for an acid HA the acid-base half-reaction is... 

In Section 11.3 we introduced acids and bases, as defined by Arrhenius. An acid is a substance that when dissolved in water increases the concentration of hydro-nium ion (HaO ) above the value it takes in pure water. A base increases the concentration of hydroxide ion (OH ). In preparation for discussing acid-base equilibria we want to generalize these definitions to accommodate broader classes of compounds that are chemically similar to the familiar acids and bases. The first of these more general definitions was introduced by Bronsted and Lowry and the second by Lewis. 

Some definitions are appropriate here. In 1928, Bronsted and Lowry defined an acid as a substance that can give or donate a proton and a base as a substance that can receive or accept a proton (Butler 1964). Strong acids release a greater proportion of their protons than do weak acids. For example, above pH 4, the reaction... 

The most commonly used definitions of an acid and a base are those of Bronsted and Lowry. These definitions state that an acid is a substance which can donate a hydrogen ion, H, or a proton as it is commonly called, and that a base is a substance which can accept a proton. In the general reaction. 

At the same time, based on the proton theory by Bronsted and Lowry, Edward Armand Guggenheim (1901-1970) and John N. Scatchard (1892-1973) formed a specific ion interaction theory. At its basis is a concept of close interactions between individual ions, which are measured by values of interaction coefficients. This theory subsequently formed the foundation of the model of state of real water solution. 

Earlier in this chapter we considered Arrhenius s concept of acids and bases An acid is a substance that produces ions when dissolved in water, and a base is a substance that produces OH ions. Although these ideas are fundamentally correct, it is convenient to have a more general definition of a base, which covers substances that do not produce OH ions. Such a definition was provided by Bronsted and Lowry, who defined acids and bases as follows ... 

In 1923, two independent scientists, Johannes Bronsted and Thomas Lowry, defined acids to be proton donors and bases to be proton acceptors. Continuing to work with the concept of hydrogen in acids and bases, Bronsted and Lowry each independently defined a new set of conditions that has come to be known as the Br0nsted-Lowry model. 

The definition of pH (eqn (3.1)) was coined as early as 1909 by Sorensen of the Carlsberg breweries in Copenhagen. Sometime later in the 1920s, Bronsted and Lowry established the fundamental definitions of acidity and... 

In 1923 Bronsted and Lowry each developed an acid-base theory based on the central role of the proton. They defined an acid as a proton donor and a base as a proton acceptor. Thus, an acid-base reaction is one in which proton transfer occurs, i.e.. 

Classically acids are defined as substances that give off protons and bases are those that generate hydroxide ions when dissolved in water. These restrictive definitions are no longer adequate and have been modified or supplemented by others. Currently the most widely used acid-base definitions are due to Bronsted and Lowry, and Lewis. 

Bronsted and Lowry independently defined an acid as a proton donor. The classical acids can also be considered Bronsted acids. Upon ionization the Bronsted acids yield protons and conjugate bases. Conversely, Bronsted bases accept protons to form conjugate acids. 

As G. N. Lewis said, We frequently define an acid or a base as a substance whose aqueous solution gives, respectively, a higher concentration of hydrogen ion or hydroxide ion than that furnished by pure water. This is a very one-sided definition. In 1923, Bronsted and Lowry expanded the definitions of acids and bases to include species that do not involve solvent participation. According to the Bronsted-Lowry definition, an acid is any proton donor, whereas a base is any proton acceptor. This broader definition expanded acid-base theory to include gaseous species, such as HCI (g) and NH3 (g). It also allowed for the inclusion of acid-base reactions occurring in nonionizing solvents, such as benzene, as shown by Equation (14.6) ... 

The term acid is often used nowadays in a different sense, as first proposed by G. N. Lewis. There has been much controversy, sometimes immoderate in tone, about the relative merits of the Bronsted and Lowry definitions of acids. The question is essentially one of the convenience and consistency of verbal definitions, and not of any fundamental differences in the interpretation of experimental facts moreover, since the present book is about the proton in chemistry we shall have little occasion to mention non-protonic acids. However, a few comments seem desirable at this point. 

With the development of the definitions of acids and bases due to Bronsted and Lowry many solvents were found to which these definitions could be applied. It followed, therefore, that solvents were classified as proton containing solvents and proton-free solvents ". The former contain hydrogen in an ionizable form and the cations produced by auto-ionization are considered to be solvated protons. Typical proton-containing solvents are water, liquid ammonia, hydrazine, hydrogen cyanide, sulphuric acid, nitric acid, acetic and formic acids, alcohols and amides. Liquid hydrogen fluoride and the other hydrogen halides may also be included in this group of solvents. 

A more general definition was suggested independently by Bronsted and Lowry . Proton-transfer reactions are considered as responsible for boijh the self-ionization of the amphoteric solvent molecules and for most acid-base reactions in their solutions. Acids and bases are defined as proton donors and proton acceptors respectively and acid-base reactions are regarded as being due to proton transfer reactions (protolysis). The most significant difference from the Arrhenius definition is that the proton itself is neither acid nor base. Even the solvent molecules can act either as acids or bases, a phenomenon which is responsible for the autoprotolysis of the pure liquid solvents. 

Bronsted and Lowry proposed defining acids and bases in terms of their ability to transfer protons. According to their definition, an acid is a substance (molecule or km) that can donate a proton to another substance. Likewise, a base is a substance that can accept a proton. Thus, when HCl dissolves in water (Equation 16.3), HCl acts as a Bronsted-Lowry acid (it donates a proton to H2O), and H2O acts as a Bronsted-Lowry base (it accepts a proton from HCl). 

In the year 1923, two more theories defining acid-base character were proposed. The first theory, Bronsted and Lowry theory, is very satisfactory for understanding physiological processes and will therefore form the basis of all further discussions. The second theory, proposed by G. N. Lewis is much more general than the Bronsted - Lowry concept. A brief discussion of this theory is given in Box 1.1. 

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 ... 

Bronsted and Lowry generalised this further to a proton exchange reaction ... 

According to Bronsted and Lowry, a molecule or ion is an acid if, when the species is added to water, the concentration of H+ (or equivalently, H3O ) increases. Consider, for example, what happens when hydrochloric acid is added to water ... 

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