The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (the hydrogen cation, or H+). This theory is a generalization of the Arrhenius theory. Definitions of acids and bases In the Arrhenius theory, acids are defined as substances that dissociate in aqueous solution to give H+ (hydrogen ions), while bases are defined as substances that dissociate in aqueous solution to give OH− (hydroxide ions). In 1923 physical chemists Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in England both independently proposed the theory that carries their names. In the Brønsted–Lowry theory acids and bases are defined by the way they react with each other, which allows for greater generality. The definition is expressed in terms of an equilibrium expression acid + base ⇌ conjugate base + conjugate acid. With an acid, HA, the equation can be written symbolically as: HA + B <=> A- + HB+ The equilibrium sign, ⇌, is used because the reaction can occur in both forward and backward directions. The acid, HA, can lose a proton to become its conjugate base, A−. The base, B, can accept a proton to become its conjugate acid, HB+. Most acid–base reactions are fast, so the components of the reaction are usually in dynamic equilibrium with each other. Aqueous solutions Consider the following acid–base reaction: CH3 COOH + H2O <=> CH3 COO- + H3O+ Acetic acid, , is an acid because it donates a proton to water () and becomes its conjugate base, the acetate ion (). is a base because it accepts a proton from and becomes its conjugate acid, the hydronium ion, (). 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, acetate is the base of the reverse reaction and hydronium ion is the acid. H3O+ + CH3 COO- <=> CH3COOH + H2O One hallmark of the Brønsted–Lowry theory in contrast to Arrhenius theory is that it does not require an acid to dissociate. Amphoteric substances