Brønsted–Lowry acid–base theory

Aqueous solutions

Consider the following acid–base reaction:

${\displaystyle {\text{CH}}_3^{}\text{COOH}+{\text{H}}_2^{}\text{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{CH}}_3^{}{\text{COO}}^{-}+{\text{H}}_3^{}{\text{O}}^{+}}$

Acetic acid, CH₃COOH, is an acid because it donates a proton to water (H₂O) and becomes its conjugate base, the acetate ion (CH₃COO⁻). H₂O is a base because it accepts a proton from CH₃COOH and becomes its conjugate acid, the hydronium ion (H₃O⁺).

The reverse of an acid–base reaction is also an acid–base reaction, between the conjugate acid of the base in the first reaction and the conjugate base of the acid. In the above example, ethanoate is the base of the reverse reaction and hydronium ion is the acid.

${\displaystyle {\text{H}}_3^{}{\text{O}}^{+}+{\text{CH}}_3^{}{\text{COO}}^{-}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{CH}}_3^{}\text{COOH}+{\text{H}}_2^{}\text{O}}$

One feature of the Brønsted–Lowry theory in contrast to Arrhenius theory is that it does not require an acid to dissociate.

Amphoteric substances

The essence of Brønsted–Lowry theory is that an acid is only such in relation to a base, and vice versa. Water is amphoteric as it can act as an acid or as a base. In the image shown at the right one molecule of H₂O acts as a base and gains H⁺ to become H₃O⁺ while the other acts as an acid and loses H⁺ to become OH⁻.

Another example is illustrated by substances like aluminium hydroxide, Al(OH)₃.

${\displaystyle \stackrel{(\text{acid})}{\text{Al}{(\text{OH})}_3^{}}+{\text{OH}}^{-}\stackrel{-\rightharpoonup }{\leftharpoondown -}\text{Al}{(\text{OH})}_4^{-}}$

${\displaystyle 3{\text{H}}^{+}+\stackrel{(\text{base})}{\text{Al}{(\text{OH})}_3^{}}\stackrel{-\rightharpoonup }{\leftharpoondown -}3{\text{H}}_2^{}\text{O}+{\text{Al}}_{(\text{aq})}^{3+}}$

Non-aqueous solutions

The hydrogen ion, or hydronium ion, is a Brønsted–Lowry acid when dissolved in H₂O and the hydroxide ion is a base because of the autoionization of water reaction

${\displaystyle {\text{H}}_2^{}\text{O}+{\text{H}}_2^{}\text{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{H}}_3^{}{\text{O}}^{+}+{\text{OH}}^{-}}$

An analogous reaction occurs in liquid ammonia

${\displaystyle {\text{NH}}_3^{}+{\text{NH}}_3^{}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{NH}}_4^{+}+{\text{NH}}_2^{-}}$

Thus, the ammonium ion, NH+4, in liquid ammonia corresponds to the hydronium ion in water and the amide ion, NH−2 in ammonia, to the hydroxide ion in water. Ammonium salts behave as acids, and metal amides behave as bases.

Some non-aqueous solvents can behave as bases, i.e. accept protons, in relation to Brønsted–Lowry acids.

${\displaystyle \text{HA}+\text{S}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{A}}^{-}+{\text{SH}}^{+}}$

where S stands for a solvent molecule. The most important of such solvents are dimethylsulfoxide, DMSO, and acetonitrile, CH₃CN, as these solvents have been widely used to measure the acid dissociation constants of carbon-containing molecules. Because DMSO accepts protons more strongly than H₂O the acid becomes stronger in this solvent than in water. Indeed, many molecules behave as acids in non-aqueous solutions but not in aqueous solutions. An extreme case occurs with carbon acids, where a proton is extracted from a C−H bond.

Some non-aqueous solvents can behave as acids. An acidic solvent will make dissolved substances more basic. For example, the compound CH₃COOH is known as acetic acid since it behaves as an acid in water. However, it behaves as a base in liquid hydrogen fluoride, a much more acidic solvent.

${\displaystyle {\text{CH}}_3^{}\text{COOH}+2\text{HF}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{CH}}_3^{}\text{C}{(\text{OH})}_2^{+}+{\text{HF}}_2^{-}}$

Comparison with Lewis acid–base theory

In the same year that Brønsted and Lowry published their theory, G. N. Lewis created an alternative theory of acid–base reactions. The Lewis theory is based on electronic structure. A Lewis base is a compound that can give an electron pair to a Lewis acid, a compound that can accept an electron pair. Lewis's proposal explains the Brønsted–Lowry classification using electronic structure.

${\displaystyle \text{HA}+\text{B}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{A}}^{-}+{\text{BH}}^{+}}$

In this representation both the base, B, and the conjugate base, A⁻, are shown carrying a lone pair of electrons and the proton, which is a Lewis acid, is transferred between them.

Lewis later wrote "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen." In Lewis theory an acid, A, and a base, B, form an adduct, AB, where the electron pair forms a dative covalent bond between A and B. This is shown when the adduct H₃N−BF₃ forms from ammonia and boron trifluoride, a reaction that cannot occur in water because boron trifluoride is destroyed by water.

${\displaystyle 4{\text{BF}}_3^{}+3{\text{H}}_2^{}\text{O}⟶\text{B}{(\text{OH})}_3^{}+3{\text{HBF}}_4^{}}$

The reaction above illustrates that BF₃ is an acid in both Lewis and Brønsted–Lowry classifications and shows that the theories agree with each other.

Boric acid is recognised as a Lewis acid because of the reaction

${\displaystyle \text{B}{(\text{OH})}_3^{}+{\text{H}}_2^{}\text{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}\text{B}{(\text{OH})}_4^{-}+{\text{H}}^{+}}$

In this case the acid does not split up but the base, H₂O, does. A solution of B(OH)₃ is acidic because hydrogen ions are given off in this reaction.

There is strong evidence that dilute aqueous solutions of ammonia contain minute amounts of the ammonium ion

${\displaystyle {\text{H}}_2^{}\text{O}+{\text{NH}}_3^{}⟶{\text{OH}}^{-}+{\text{NH}}_4^{+}}$

and that, when dissolved in water, ammonia functions as a Lewis base.

Comparison with the Lux–Flood theory

The reactions between oxides in the solid or liquid states are excluded in the Brønsted–Lowry theory. For example, the reaction

${\displaystyle 2\text{MgO}+{\text{SiO}}_2^{}⟶{\text{Mg}}_2^{}{\text{SiO}}_4^{}}$

is not covered in the Brønsted–Lowry definition of acids and bases. On the other hand, magnesium oxide acts as a base when it reacts with an aqueous solution of an acid (instead, the reaction can be considered a Lewis acid–base reaction).

${\displaystyle 2{\text{H}}^{+}+\text{MgO}(\text{s})⟶{\text{Mg}}^{2+}(\text{aq})+{\text{H}}_2^{}\text{O}}$

Dissolved silicon dioxide, SiO₂, has been predicted to be a weak acid in the Brønsted–Lowry sense.

${\displaystyle {\text{SiO}}_2^{}(\text{s})+2{\text{H}}_2^{}\text{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}\text{Si}{(\text{OH})}_4^{}(\text{solution})}$

${\displaystyle \text{Si}{(\text{OH})}_4^{}\stackrel{-\rightharpoonup }{\leftharpoondown -}\text{Si}{(\text{OH})}_3^{}{\text{O}}^{-}+{\text{H}}^{+}}$

According to the Lux–Flood theory, oxides like MgO and SiO₂ in the solid state may be called acids or bases. For example, the mineral olivine may be known as a compound of a basic oxide, MgO, and silicon dioxide, SiO₂, as an acidic oxide. This is important in geochemistry.