Bronsted definition of acids and bases

The Bronsted definitions of acids and bases are more general than the Arrhenius definitions they also apply to species in nonaqueous solvents and even to gas-phase reactions. For example, when pure acetic acid is added to liquid ammonia, proton transfer takes place and the following equilibrium is reached ... 

An important implication of the Bronsted definitions of acids and bases is that the same substance may be able to function as both an acid and a base. For example, we have seen that a water molecule accepts a proton from an acid molecule (such as HC1 or HCN) to form an H30+ ion. So water is a base. I lowever, a water molecule can donate a proton to a base (such as O2- or NH3) and become an OH ion. So water is also an acid. We describe water as amphiprotic, meaning that an H20 molecule can act both as a proton donor and as a proton acceptor. 

The nature of catalysis in homogeneous systems has been the subject of a considerable amount of research. A catalyst is any substance which affects the rate of reaction but is not consumed in the overall reaction. From thermodynamic principles we know that the equilibrium constant for the overall reaction must be independent of the mechanism, so that a catalyst for the forward reaction must also be one for the reverse reaction. In aqueous solution, a large number of reactions are catalyzed by acids and bases for our purposes we shall employ the Bronsted definition of acids and bases as proton donors and acceptors, respectively. Catalysis by acids and bases involves proton transfer either to or from the substrate. For example, the dehydration of acetaldehyde hydrate is subject to acid catalysis [20], probably by the mechanism (II). 

An extension of the Bronsted definition of acids and bases is the concept of the conjugate add-base pair, which can be defined as an acid and its conjugate base or a base and its conjugate acid. The conjugate base of a Bronsted acid is the species that remains when one proton has been removed from the acid. Conversely, a conjugate acid results from the addition of a proton to a Bronsted base. 

According to tiie Bronsted definition of acids and bases, solutes release hydrogen ions and hydroxide ions, respectively. 

The Bronsted definitions of acids and bases are not restricted to species in aqueous solution. In fact, Bronsted acid-base reactions sometimes take place in the gas phase. For example, in the reaction between HCl and NH3 gases, HCl acts as the Bronsted acid, donating its proton to NH3, which, by accepting the proton, acts as a Bronsted base. The products of this proton transfer are the chloride ion (Cl ) and the armnonium ion (NH4), which subsequently combine to form the ionic solid ammoninm chloride. 

A useful definition of acids and bases is that independently introduced by Johannes Bronsted (1879-1947) and Thomas Lowry (1874-1936) in 1923. In the Bronsted-Lowry definition, acids are proton donors, and bases are proton acceptors. Note that these definitions are interrelated. Defining a base as a proton acceptor means an acid must be available to provide the proton. For example, in reaction 6.7 acetic acid, CH3COOH, donates a proton to ammonia, NH3, which serves as the base. 

The Lewis definition of acids and bases is broader and more encompassing than the Bronsted-Lowry definition because it s not limited to substances that donate or accept just protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an electron pair. The donated electron pair is shared between the acid and the base in a covalent bond. 

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 process responsible for 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 ... 

Bronsted-Lowry definition A definition of acids and bases in terms of the ability of molecules and ions to participate in proton transfer. 

Any reaction in which a proton is transferred from one substance to another is an acid-base reaction. More specifically, the proton-transfer view is known as the Bronsted-Lowiy definition of acids and bases. In an acid-base reaction, an acid donates a proton, and a base accepts that proton. Any species that can give up a proton to another substance is an acid, and any substance that can accept a proton from another substance is a base. The production of two water molecules from a hydroxide anion (a base) and a hydronium ion (an acid) is just one example of an acid-base reaction acids and bases are abundant in chemistry. 

KolthoflF, I. M. (1944). The Lewis and Bronsted-Lowry definitions of acids and bases. Journal of Physical Chemistry, 48, 51-7. 

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. 

According to the Bronsted-Lowry definition of acids and bases, an acid is a proton donor. The particle that is left over after an acid donates its proton, however, can now accept a proton and,... 

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 ... 

Thus, a reducing agent donates electrons, while an oxidizing agent receives them. The Bronsted-Lowry definitions of acid and base specify that... 

Things are a little more complicated than implied by this statement. Molecules that donate protons and molecules that accept them are formally known as Bronsted acids and bases. There is a more general definition of acid and base— Lewis acids and bases—in which a Lewis acid is any molecule having the ability to accept a pair of electrons and a Lewis base is any molecule having the ability to donate a pair of electrons to a Lewis acid. The Bronsted concept will suffice for all our purposes. 

Lewis defined a base as an electron pair donor and an acid as an electron pair acceptor. Lewis electron pair donor was the same as Bronsted-Lowry s proton acceptor, and therefore, was an equivalent way of defining a base. Lewis acids were defined as a substance with an empty valence shell that could accommodate a pair of electrons. This definition broadened the Bronsted-Lowry definition of an acid. The three definitions of acids and bases are summarized in Table 13.3. 

You no doubt noticed that some of the bases in Table 16-1 don t contain a hydroxide ion, which means that the Arrhenius definition of acids and bases can t apply. When chemists realized that several substances behaved like bases but didn t contain a hydroxide ion, they reluctantly acknowledged that another determination method was needed. Independently proposed by Johannes Bronsted and Thomas Lowry in 1923 and therefore named cifter both of them, the Bronsted-Lowry method for determining acids and bases accounts for those pesky non-hydroxide-containing bases. 

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 practice, it s much simpler to use the Arrhenius or Bronsted-Lowry definition of acid and base, but you ll need to use the Lewis definition when hydrogen ions aren t being exchanged. You can pick and choose among the definitions when you re asked to identify the acid and base in a reaction. 

Brensted-Lowry acid proton donor Bronsted-Lowry base proton acceptor J. N. Bronsted of the University of Copenhagen published his definition of acids and bases in 1923. 

What are the Bronsted-Lowry definitions of acid and base ... 

Which definition of acids and bases is more universal the Bronsted-Lowry definition or the Lewis definition ... 

In 1923 G. N. Lewis proposed a definition of acids and bases somewhat different from that of Bronsted 106... 

These ideas were rather limiting since they only applied to aqueous solutions. There were situations where acid-base reactions were taking place in solvents other than water, or even in no solvent at all. This problem was addressed in 1923 by the Danish chemist Johannes Bronsted (1879-1947) and the English chemist Thomas Lowry (1874-1936) when they independentiy proposed a more general definition of acids and bases, and the study of acids and bases took a great step forward. This theory became known as the Bronsted-Lowry theory of acids and bases. 

General definitions of acids and bases were given by G. N. Lewis as an extension of the concept of the electron-pair covalent bond an acid is an electron-pair acceptor, a base an electron-pair donor. All Lewis bases are Bronsted bases—an unshared electron pair is required to accept a proton. H+ is an acid in the Lewis sense since it can accept an electron pair from a base such as NH3, but LLO and NH4... 

Bronsted-Lowry and Lewis definitions of acids and bases are reviewed in Sec. 7.6. Alcohols are comparable in acidity to water, but phenols are much more acidic. This increased acidity is due to charge delocalization (resonance) in phenoxide ions. Electron-withdrawing groups, such as -F and -N02, increase acidity, through either an inductive or a resonance effect, or both. 

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. 

There are several definitions of acids and bases. According to the Bronsted-Lowrv definition, an acid is a proton donor and a base is a proton acceptor. Any compound that has a hydrogen can potentially act as a Bronsted-Lowry acid (although the strength of the acid can vary enormously). Therefore, H—A is used as a general representation for an acid. To accept a proton, most bases... 

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 ... 

BRONSTED-LOWRY AND LEWIS DEFINITIONS OF ACIDS AND BASES... 

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