Editing Acid (section)

===Brønsted–Lowry acids{{anchor|Brønsted acids}}===
{{Main|Brønsted–Lowry acid–base theory}}

[[File:Acetic-acid-dissociation-3D-balls.png|thumb|350px|alt=Acetic acid, CH<sub>3</sub>COOH, is composed of a methyl group, CH<sub>3</sub>, bound chemically to a carboxylate group, COOH. The carboxylate group can lose a proton and donate it to a water molecule, H<sub>2</sub>0, leaving behind an acetate anion CH<sub>3</sub>COO- and creating a hydronium cation H<sub>3</sub>O. This is an equilibrium reaction, so the reverse process can also take place.|[[Acetic acid]], a [[weak acid]], donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the [[acetate]] ion and the [[hydronium]] ion. Red: oxygen, black: carbon, white: hydrogen.]]

While the Arrhenius concept is useful for describing many reactions, it is also quite limited in its scope. In 1923, chemists [[Johannes Nicolaus Brønsted]] and [[Thomas Martin Lowry]] independently recognized that acid–base reactions involve the transfer of a proton. A '''Brønsted–Lowry acid''' (or simply Brønsted acid) is a species that donates a proton to a Brønsted–Lowry base.<ref name="Ebbing" /> Brønsted–Lowry acid–base theory has several advantages over Arrhenius theory. Consider the following reactions of [[acetic acid]] (CH<sub>3</sub>COOH), the [[organic acid]] that gives vinegar its characteristic taste:

:{{chem2|CH3COOH + H2O <-> CH3COO- + H3O+}}
:{{chem2|CH3COOH + NH3 <-> CH3COO− + NH4+}}

Both theories easily describe the first reaction: CH<sub>3</sub>COOH acts as an Arrhenius acid because it acts as a source of H<sub>3</sub>O<sup>+</sup> when dissolved in water, and it acts as a Brønsted acid by donating a proton to water. In the second example CH<sub>3</sub>COOH undergoes the same transformation, in this case donating a proton to ammonia (NH<sub>3</sub>), but does not relate to the Arrhenius definition of an acid because the reaction does not produce hydronium. Nevertheless, CH<sub>3</sub>COOH is both an Arrhenius and a Brønsted–Lowry acid.

Brønsted–Lowry theory can be used to describe reactions of [[molecule|molecular compounds]] in nonaqueous solution or the gas phase. [[Hydrogen chloride]] (HCl) and ammonia combine under several different conditions to form [[ammonium chloride]], NH<sub>4</sub>Cl. In aqueous solution HCl behaves as [[hydrochloric acid]] and exists as hydronium and chloride ions. The following reactions illustrate the limitations of Arrhenius's definition:
# H<sub>3</sub>O{{su|p=+|b=(aq)}} + Cl{{su|p=−|b=(aq)}} + NH<sub>3</sub> → Cl{{su|p=−|b=(aq)}} + NH{{su|b=4|p=+}}<sub>(aq)</sub> + H<sub>2</sub>O
# HCl<sub>(benzene)</sub> + NH<sub>3(benzene)</sub> → NH<sub>4</sub>Cl<sub>(s)</sub>
# HCl<sub>(g)</sub> + NH<sub>3(g)</sub> → NH<sub>4</sub>Cl<sub>(s)</sub>

As with the acetic acid reactions, both definitions work for the first example, where water is the solvent and hydronium ion is formed by the HCl solute. The next two reactions do not involve the formation of ions but are still proton-transfer reactions. In the second reaction hydrogen chloride and ammonia (dissolved in [[benzene]]) react to form solid ammonium chloride in a benzene solvent and in the third gaseous HCl and NH<sub>3</sub> combine to form the solid.