A more satisfactory theory was proposed in 1923 by the Danish chemist Johannes Brønsted and independently by Thomas Lowry, a British chemist known as the Brønsted-Lowry Theory. Their theory states that an acid is a proton (hydrogen ion, H+) donor and a base a proton acceptor. Although the acid must still contain hydrogen, the Brønsted-Lowry theory does not need an aqueous medium. For example, liquid ammonia, which acts as a base in aqueous solution, can act as an acid in the absence of water by transferring a proton to a base and forming the amide anion (negative ion) NH2-: NH3 + base⇄NH2- + base + H+
The Brønsted-Lowry definition of acids and bases also explains why a strong acid displaces a weak acid from its compounds (and likewise for strong and weak bases). Here acid-base reactions are viewed as a competition for protons. In terms of a general chemical equation, the reaction of Acid (1) with Base (2) Acid (1) + Base (2)⇄Acid (2) + Base (1)results in the transfer of a proton from Acid (1) to Base (2). In losing the proton, Acid (1) becomes its conjugate base, Base (1). In gaining a proton, Base (2) becomes its conjugate acid, Acid (2). The equilibrium represented by the equation above may be displaced either to the left or to the right, and the actual reaction will take place in the direction that produces the weaker acid-base pair. For example, hydrogen chloride (HCl) is a strong acid in water because it readily transfers a proton to water to form a hydronium ion: HCl + H2O⇄H3O+ + Cl-The equilibrium lies mostly to the right because the conjugate base of HCl, Cl-, is a weak base, and H3O+, the conjugate acid of H2O, is a weak acid.
In contrast, hydrogen fluoride, HF, is a weak acid in water because it does not readily transfer a proton to water: HF + H2O⇄H3O+ + F-This equilibrium lies mostly to the left because H2O is a weaker base than F-, and because HF is a weaker acid (in water) than H3O+. The Brønsted-Lowry theory also explains why water can be amphoteric, that is, why it can serve as either an acid or a base. Water serves as a base in the presence of an acid that is stronger than water (such as HCl), in other words, an acid that has a greater tendency to dissociate than does water: HCl + H2O⇄H3O+ + Cl-Water can also serve as an acid in the presence of a base that is stronger than water (such as ammonia) according to Brønsted-Lowry Theory: NH3 + H2O⇄NH4+ + OH-