Editing Acid–base reaction (section)

===Brønsted–Lowry definition===
{{Main|Brønsted–Lowry acid–base theory}}
{{Infobox
|data1= [[File:Johannes Brønsted.jpg|100px]] [[File:Thomas Martin Lowry2.jpg|100px]]
|data2= [[Johannes Nicolaus Brønsted]] and [[Martin Lowry|Thomas Martin Lowry]]}}

The Brønsted–Lowry definition, formulated in 1923, independently by [[Johannes Nicolaus Brønsted]] in Denmark and [[Martin Lowry]] in England,<ref>{{cite journal |last= Brönsted |first= J.N. |date= 1923 |title= Einige Bemerkungen über den Begriff der Säuren und Basen |trans-title=Some observations about the concept of acids and bases |journal= Recueil des Travaux Chimiques des Pays-Bas |volume= 42 |issue= 8 |pages= 718–728|doi= 10.1002/recl.19230420815 }}</ref><ref>{{cite journal |last= Lowry |first= T.M. |date= 1923 |url= https://archive.org/stream/ost-chemistry-chemistryindustr01soci/chemistryindustr01soci#page/n65/mode/2up |title= The uniqueness of hydrogen |journal= [[Journal of the Society of Chemical Industry]] |volume= 42 |issue= 3 |pages= 43–47|doi= 10.1002/jctb.5000420302 }}</ref> is based upon the idea of [[protonation]] of bases through the [[deprotonation]] of acids – that is, the ability of acids to "donate" hydrogen ions ({{chem2|H+}}){{snd}} otherwise known as [[hydron (chemistry)|proton]]s{{snd}} to bases, which "accept" them.<ref name="miessler_167">{{harvnb|Miessler|Tarr|1991|pp=167–169}} – According to this page, the original definition was that "acids have a tendency to lose a proton"</ref><ref group=note>"Removal and addition of a proton from the nucleus of an atom does not occur{{snd}} it would require very much more energy than is involved in the dissociation of acids."</ref>

An acid–base reaction is, thus, the removal of a hydrogen ion from the acid and its addition to the base.<ref name="Clayden_1">{{harvnb|Clayden|Greeves|Warren|Wothers|2015|pp=182–184}}</ref> The removal of a hydrogen ion from an acid produces its ''[[conjugate acid|conjugate base]]'', which is the acid with a hydrogen ion removed. The reception of a proton by a base produces its ''[[conjugate acid]]'', which is the base with a hydrogen ion added.

Unlike the previous definitions, the Brønsted–Lowry definition does not refer to the formation of salt and solvent, but instead to the formation of ''conjugate acids'' and ''conjugate bases'', produced by the transfer of a proton from the acid to the base.<ref name="miessler_165"/><ref name="miessler_167"/> In this approach, acids and bases are fundamentally different in behavior from salts, which are seen as electrolytes, subject to the theories of [[Peter Debye|Debye]], [[Lars Onsager|Onsager]], and others. An acid and a base react not to produce a salt and a solvent, but to form a new acid and a new base. The concept of neutralization is thus absent.<ref name=review1940/> Brønsted–Lowry acid–base behavior is formally independent of any solvent, making it more all-encompassing than the Arrhenius model. The calculation of [[pH]] under the Arrhenius model depended on alkalis (bases) dissolving in water ([[aqueous solution]]). The Brønsted–Lowry model expanded what could be [[pH]] tested using insoluble and soluble solutions (gas, liquid, solid).

The general formula for acid–base reactions according to the Brønsted–Lowry definition is:
<math chem display=block>\ce{HA + B -> BH+ + A-}</math>
where HA represents the acid, B represents the base, {{chem2|BH+}} represents the conjugate acid of B, and {{chem2|A-}} represents the conjugate base of HA.

For example, a Brønsted–Lowry model for the dissociation of [[hydrochloric acid]] (HCl) in [[aqueous solution]] would be the following:
<math chem display=block>\underset{\text{acid}}{\ce{HCl_{\,}}} \ + \ \underset{\text{base}}{\ce{H2O}} \quad \ce{<=>} \quad \underset{\text{conjugate } \atop \text{acid }}{\ce{H3O+}} \ + \underset{\text{conjugate} \atop \text{base}}{\ce{Cl_{\,}-}}</math>

The removal of {{chem2|H+}} from the {{chem2|HCl}} produces the [[chloride]] ion, {{chem2|Cl-}}, the conjugate base of the acid. The addition of {{chem2|H+}} to the {{chem2|H2O}} (acting as a base) forms the [[hydronium]] ion, {{chem2|H3O+}}, the conjugate acid of the base.

Water is [[amphoterism|amphoteric]]{{snd}} that is, it can act as both an acid and a base. The Brønsted–Lowry model explains this, showing the dissociation of water into low concentrations of hydronium and [[hydroxide]] ions:
<math chem display=block>\ce{H2O + H2O <=> H3O+ + OH-}</math>

This equation is demonstrated in the image below:
[[File:Bronsted lowry 3d diagram.png|600px|center]]

Here, one molecule of water acts as an acid, donating an {{chem2|H+}} and forming the conjugate base, {{chem2|OH-}}, and a second molecule of water acts as a base, accepting the {{chem2|H+}} ion and forming the conjugate acid, {{chem2|H3O+}}.

As an example of water acting as an acid, consider an aqueous solution of [[pyridine]], {{chem2|C5H5N}}.
<math chem display=block>\ce{C5H5N + H2O <=> [C5H5NH]+ + OH-}</math>

In this example, a water molecule is split into a hydrogen ion, which is donated to a pyridine molecule, and a hydroxide ion.

In the Brønsted–Lowry model, the solvent does not necessarily have to be water, as is required by the [[#Arrhenius definition|Arrhenius Acid–Base model]]. For example, consider what happens when [[acetic acid]], {{chem2|CH3COOH}}, dissolves in [[liquid ammonia]].
<math chem display=block>\ce{CH3COOH + NH3 <=> NH4+ + CH3COO-}</math>

An {{chem2|H+}} ion is removed from acetic acid, forming its conjugate base, the [[acetate]] ion, {{chem2|CH3COO-}}. The addition of an {{chem2|H+}} ion to an ammonia molecule of the solvent creates its conjugate acid, the ammonium ion, {{chem2|NH4+}}.

The Brønsted–Lowry model calls hydrogen-containing substances (like {{chem2|HCl}}) acids. Thus, some substances, which many chemists considered to be acids, such as {{chem2|SO3}} or {{chem2|BCl3}}, are excluded from this classification due to lack of hydrogen. [[Gilbert N. Lewis]] wrote in 1938, "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]]."<ref name=review1940/> Furthermore, {{chem2|KOH}} and {{chem2|KNH2}} are not considered Brønsted bases, but rather salts containing the bases {{chem2|OH(-)}} and {{chem|NH|2|−}}.