Bronsted—Lowry theory

The Bronsted-Lowry theory expands on this notion beyond solvents of just water, to cover others such as liquid ammonia, glacial acetic acid, anhydrous sulfuric acid, and all solvents containing hydrogen. Bases can accept protons but don t necessarily contain OH.  

For example, in liquid ammonia, the NHJ donates a proton, whereas the NH2 accepts a proton. 

When the chemical species differ in composition by a proton, they are called a conjugate pair. For every acid there is a conjugate base, and vice versa. 

In this reaction, the HCl is an acid because it donates protons, which forms free Cl as a base. 

In this reaction, the ammonium salt acts as the acid, because it donates the protons, whereas the sulfide ion is a base because it accepts the protons. This reaction is reversible and proceeds to produce the species that is weakest, such as HS and NHj. 

According to the Bronsted-Lowry theory of acids and bases, an acid is a proton donor and a base is a proton acceptor. The proton, which in this context means a hydrogen ion, H , is highly mobile and acids and bases in water are always in equihbrium with their deprotonated and protonated counterparts and hydro-nimn ions (H3O , 8). Thus, an acid HA, such as HCN, immediately estabhshes the equihbrium 

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. 

What exactly is meant by a hydrogen ion acceptor A hydrogen ion acceptor is a chemical species to which a hydrogen ion can attach and form a covalent bond. The most important examples, in addition to the Arrhenius bases (hydroxides), are ammonia (NHj), the carbonates, and the bicarbonates (Table 12.6). Examples of reactions in which hydrogen ions are accepted are found in Equations 12.17 through 12.21. 

Ammonia Sodium carbonate Potassium carbonate Caicium carbonate Sodium bicarbonate Potassium bicarbonate 

Notice that when hydroxides accept hydrogen ions, water is always the product. Water is not a product in the cases of ammonia, the carbonates, and the bicarbonates. However, in the case of carbonates and bicarbonates, water often does ultimately form. The carbonic acid, H2CO3, in Equations 12.20 and 12.21 is unstable and will spontaneously decompose into water and carbon dioxide. This reaction is often acconpanied by vigorous gas (COj) evolution and will be discussed in more detail in Section 12.7. 

The list of Bronsted-Lowry bases is actually much longer than the list that is given here. There are many anions like carbonate and bicarbonate that will form a covalent bond with hydrogen ions and form a weak acid like the carbonic acid. Thus the anion of any weak acid qualifies as a Bronsted-Lowry base. This list would include sulflde, cyanide, acetate, and anions of other carboxylic acids. 

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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 + ). 

Acids Bronsted-Lowry theory of, 21 common, concentration of, (T) 829 dissociation constants of, (T) 832 hard. 54 ionisation of, 20 Lewis, 22 polyprotic, 20... 

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. 

In the Bronsted-Lowry theory, the strength of an acid depends on the extent to which it donates protons to the solvent. We can therefore summarize the distinction between strong and weak acids as follows ... 

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 ... 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

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. 

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... 

Aluminosilicate glasses are used in certain AB cement formulations, and the acid-base balance in them is important. The Bronsted-Lowry theory cannot be applied to these aluminosilicate glasses it does not recognize silica as an acid, because silica is an aprotic acid. However, for most purposes the Bronsted-Lowry theory is a suitable conceptual framework although not of universal application in AB cement theory. 

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. 

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. 

In this reaction, the acetate ion is functioning as a base. On the other hand, Cl- has very littie tendency to function as a base because it comes from FI Cl, which is a very strong proton donor. According to the Bronsted-Lowry theory, the species remaining after a proton is donated is called the conjugate base of that proton donor. 

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. 

Lowry is best known to chemistry students through the tradition of eponymony, since the proton theory of acidity is known as the "Bronsted/Lowry theory" of proton donors. His most important experimental investigation likely was a long series of studies on optical rotatory dispersion.49 For our purposes, there is special interest in his discovery of mutarotation in camphor derivatives and his theory of dynamic tautomerism, which led him to an ionic theory of organic reaction mechanisms. 

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 ... 

Definition Term Arrhenius Theory Bronsted-Lowry Theory... 

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. 

Bronsted theory See Bronsted-Lowry theory. bron-steth, the-3-re brown lead oxide See lead dioxide. braun led ak.sTd ... 

You can find some Information about the Bronsted-Lowry theory of adds and bases at www. brlghtredbooks.net... 

Water can provide both H and OH H O H"+OH- According to Bronsted-Lowry theory, a water molecule can accept a proton, thereby becoming a hydronium ion. In this case, water is acting as a base (proton acceptor). H2O + H" H3O" 1 point given for correct Bronsted-Lowry concept of water being able to accept a proton resulting in a hydronium ion. 

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. 

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