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{{Acids_and_Bases}}
An '''acid-base reaction''' is a [[chemical reaction]] that occurs between an [[acid]] and a [[base (chemistry)|base]]. Several concepts exist which provide alternative definitions for the reaction mechanisms involved and their application in solving related problems. Despite several similarities in definitions, their importance becomes apparent as different methods of analysis when applied to acid-base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. Historically, the first of these scientific concepts of acids and bases was provided by the [[France|French]] [[chemist]] [[Antoine Lavoisier]], circa [[1776]].<ref name="lavoisier_1">Miessler, L. M., Tar, D. A., (1991) p166 - Table of discoveries attributes Antoine Lavoisier as the first to posit a scientific theory in relation to [[oxyacid]]s.</ref>

==Common acid-base theories==
=== Lavoisier definition ===
Since Lavoisier's knowledge of [[strong acids]] was mainly restricted to [[oxyacid]]s, which tend to contain central atoms in high [[oxidation number|oxidation states]] surrounded by oxygen, such as [[Nitric acid|HNO<sub>3</sub>]] and [[Sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]], and since he was not aware of the true composition of the hydrohalic acids, HCl, HBr, and HI, he defined acids in terms of their containing ''[[oxygen]]'', which in fact he named from Greek words meaning "acid-former" (from the [[Greek language|Greek]] οξυς (''oxys'') meaning "acid" or "sharp" and γεινομαι (''geinomai'') or "engender"). The Lavoisier definition was held as absolute truth for over 30 years, until the 1810 article and subsequent lectures by [[Humphry Davy|Sir Humphry Davy]] in which he proved the lack of oxygen in [[hydrogen sulfide|H<sub>2</sub>S]], [[hydrogen telluride|H<sub>2</sub>Te]], and the [[hydrogen halide|hydrohalic acids]].

=== Liebig definition ===
This definition is proposed by [[Justus von Liebig]] circa [[1838]],<ref name="liebig_1">Miessler, L. M., Tar, D. A., (1991) p166 - table of discoveries attributes Justus von Liebig's publication as 1838</ref> based on his extensive works on the chemical composition of [[organic acid]]s. This finished the doctrinal shift from oxygen-based acids to hydrogen-based acids, started by Davy. According to Liebig, an acid is a hydrogen-containing substance in which the hydrogen could be replaced by a metal.<ref name=meyers_156>Meyers, R. (2003) p156</ref> Liebig's definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition.<ref name="liebig_2">H. L. Finston and A. C. Rychtman, A New View of Current Acid-Base Theories, John Wiley & Sons, New York, 1982, pp. 140-146.</ref>

=== Arrhenius definition ===
[[Image:Arrhenius2.jpg|thumb|right|Devised after the metal-replacement theory of [[Justus von Liebig]] and despite some modifications by later theories, the Arrhenius (pictured) concept remains a simple scientific definition of acid-base reaction character.<ref name="miessler_165">Miessler, L. M., Tar, D. A., (1991) p165</ref> ]]

The Arrhenius definition of acid-base reactions is a more simplified acid-base concept devised by [[Svante Arrhenius]], which was used to provide a modern definition of bases that followed from his work with [[Friedrich Wilhelm Ostwald]] in establishing the presence of ions in aqueous solution in [[1884]], and led to Arrhenius receiving the [[Nobel prize|Nobel prize in chemistry]] in [[1903]] for "''recognition of the extraordinary services ... rendered to the advancement of chemistry by his electrolytic theory of dissociation''"<ref name="miessler_165" />

As defined at the time of discovery, acid-base reactions are characterized by Arrhenius acids, which [[dissociation constant|dissociate]] in aqueous solution form hydrogen or the later-termed [[oxonium]] (H<sub>3</sub>O<sup>+</sup>) ions,<ref name="miessler_165" /> and Arrhenius bases which form hydroxide (OH<sup>-</sup>) ions. More recent [[IUPAC]] recommendations now suggest the newer term "hydronium"<ref>Murray, K. K., Boyd, R. K., et al. (2006) -- Please note that in this document, there is no reference to deprecation of "oxonium", which is also still accepted as it remains in the IUPAC Gold book, but rather reveals preference for the term "Hydronium".</ref> be used in favor of the older accepted term "oxonium"<ref name="iupac_gold">International Union of Pure and Applied Chemistry, et al. (2006), "''Oxonium Ions''"</ref> to illustrate reaction mechanisms such as those defined in the Brønsted-Lowry and solvent system definitions more clearly, with the Arrhenius definition serving as a simple general outline of acid-base character<ref name="miessler_165" /> More succinctly, the Arrhenius definition can be surmised as;

{{cquote|''Arrhenius acids form hydrogen ions in aqueous solution with Arrhenius bases forming hydroxide ions.''}}

The ''universal aqueous acid-base definition'' of the Arrhenius concept is described as the formation of water from hydrogen and hydroxide ions, or hydronium ions and hydroxide ions produced from the dissociation of an acid and base in aqueous solution (2 H<sub>2</sub>O → OH<sup>-</sup> + H<sub>3</sub>O<sup>+</sup>  )<ref>The use of H<sup>+</sup> is as shorthand for H<sub>3</sub>O; 2 H<sub>2</sub>O → H<sub>3</sub>O<sup>+</sup> + OH<sup>-</sup> = H<sub>2</sub>O → H<sup>+</sup> + OH<sup>-</sup> </ref>, which leads to the definition that in Arrhenius acid-base reactions, a salt and water is formed from the reaction between an acid and a base --<ref name="miessler_165" /> in more simple scientific definitions, this form of reaction is called a [[Neutralization|Neutralization reaction]].

:acid<sup>+</sup> + base<sup>-</sup> → salt + water

The positive ion from a base can form a salt with the negative ion from an acid. For example, two [[mole (unit)|mole]]s of the base [[sodium hydroxide]] (NaOH) can combine with one mole of sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) to form two moles of [[water]] and one mole of [[sodium sulfate]].

:2NaOH + H<sub>2</sub>SO<sub>4</sub> → 2 H<sub>2</sub>O + Na<sub>2</sub>SO<sub>4</sub>

=== Brønsted-Lowry definition ===
{{main|Brønsted-Lowry acid-base theory}}
The Brønsted-Lowry definition, formulated independently by its two proponents [[Johannes Nicolaus Brønsted]] and [[Martin Lowry]] in [[1923]] is based upon the idea of [[protonation]] of bases through the [[deprotonation|de-protonation]] of acids -- more commonly referred to as the ability of acids to "donate" hydrogen ions (H<sup>+</sup>) or [[proton (physics)|protons]] to bases, which "accept" them.<ref name="miessler_167">Miessler, L. M., Tar, D. A., (1991), p167-169 -- According to this page, the original definition was that "acids have a tendency to lose a proton"</ref> In contrast to the Arrhenius definition, the Brønsted-Lowry definition refers to the products of an acid-base reaction as conjugate acids and bases to refer to the relation of one proton, and to indicate that there has been a reaction between the two quantities, rather than a "formation" of salt and water, as explained in the Arrhenius definition.<ref name="miessler_165" /><ref name="miessler_167" />

It defines that in reactions, there is the donation and reception of a proton, which essentially refers to the removal of a hydrogen ion bonded within a compound and its reaction with another compound,<ref name="Clayden_1">Clayden, J., Warren, S., et al. (2000), p182-184</ref> and not the removal of a proton from the nucleus of an atom, which would require inordinate amounts of energy not attainable through the simple dissociation of acids. In differentiation from the [[Arrhenius]] definition, the Brønsted-Lowry definition postulates that for each acid, there is a conjugate acid and base or "''conjugate acid-base pair''" that is formed through a complete reaction, which also includes water, which is [[amphoteric]]:<ref name="Clayden_1" /><ref name="miessler_165" /><ref name="miessler_167" />

[[Image:Bronsted-lowry-3d-explanation-diagram.png|600px|center]]
<br />
{| align="center"
|
::AH + B → BH<sup>+</sup> + A<sup>-</sup>
::''General formula for representing Brønsted-Lowry reactions.''
<br />
::HCl (aq) + H<sub>2</sub>O → H<sub>3</sub>O<sup>+</sup> (aq) + Cl<sup>-</sup> (aq)
::''[[Hydrochloric acid]] completely reacts with water to form the hydronium and chloride ions ''
<br />
::CH<sub>3</sub>COOH + NH<sub>3</sub> → NH<sub>4</sub><sup>+</sup> + CH<sub>3</sub>COO<sup>-</sup>
::''[[Acetic acid]] reacts incompletely with [[ammonia]], no hydronium ions being produced''
<br />
|}

=== Lewis definition ===
{{see|Lewis acid|Lewis base}}
The Lewis definition of acid-base reactions, devised by [[Gilbert N. Lewis]] in [[1923]]<ref name="lewis_1">Miessler, L. M., Tar, D. A., (1991) p166 - Table of discoveries attributes the date of publication/release for the Lewis theory as 1923.</ref> is an encompassing theory to the Brønsted-Lowry and solvent-system definitions<ref name="lewis_2" /> with regards to the premise of a donation mechanism,<ref name="lewis_2" /> which conversely attributes the donation of electron pairs from bases and the acceptance by acids, rather than protons or other bonded substances<ref name="lewis_2">Miessler, L. M., Tar, D. A., (1991) p170-172</ref> and spans both aqueous and non-aqueous reactions.<ref name="lewis_2" />

: Ag<sup>+</sup> + 2 :NH<sub>3</sub> → [H<sub>3</sub>N:Ag:NH<sub>3</sub>]<sup>+</sup>
:''A silver cation reacts as an acid with [[ammonia]] which acts as an electron-pair donor, forming an ammonia-silver adduct''

In reactions between [[Lewis acid]]s and [[Lewis base|base]]s, there is the formation of an adduct<ref name="lewis_2" /> when the highest occupied molecular orbital ([[HOMO]]) of a molecule, such as NH<sub>3</sub> with available lone electron pair(s) donates lone pairs of electrons to the electron-deficient molecule's lowest unoccupied molecular orbital ([[LUMO]]) through a [[co-ordinate covalent bond]]; in such a reaction, the HOMO-interacting molecule acts as a base, and the LUMO-interacting molecule acts as an acid.<ref name="lewis_2" /> In highly-polar molecules, such as [[boron trifluoride]] (BF<sub>3</sub>),<ref name="lewis_2" /> the most [[electronegativity|electronegative]] element pulls electrons towards its own orbitals, providing a more positive charge on the less-electronegative element and a difference in its electronic structure due to the axial or equatorial orbiting positions of its electrons, causing repulsive effects from ''lone pair-bonding pair'' (Lp-Bp) interactions between bonded atoms in excess of those already provided by ''bonding pair-bonding pair'' (Bp-Bp) interactions.<ref name="lewis_2" /> Adducts involving metal ions are referred to as co-ordination compounds.<ref name="lewis_2" />

=== Solvent-system definition ===

This definition is based on a generalization of the earlier Arrhenius definition to all autodissociating solvents.
In all such solvents there is a certain concentration of a positive species, '''solvonium cations''' and negative species, '''solvate anions''', in equilibrium with the neutral solvent molecules. For example:

: 2H<sub>2</sub>O {{unicode|&#8652;}} H<sub>3</sub>O<sup>+</sup> ([[hydronium]]) + OH<sup>-</sup> ([[hydroxide]])
: 2NH<sub>3</sub> {{unicode|&#8652;}} NH<sub>4</sub><sup>+</sup> ([[ammonium]]) + NH<sub>2</sub><sup>−</sup> ([[amide]])
or even some aprotic systems
: N<sub>2</sub>O<sub>4</sub> {{unicode|&#8652;}} NO<sup>+</sup> ([[nitrosonium]]) + NO<sub>3</sub><sup>−</sup> ([[nitrate]])
: 2SbCl<sub>3</sub> {{unicode|&#8652;}} SbCl<sub>2</sub><sup>+</sup> (dichloroantimonium) + SbCl<sub>4</sub><sup>-</sup> (tetrachloroantimonate)

A solute causing an increase in the concentration of the solvonium ions and a decrease in the solvate ions is an '''acid''' and one causing the reverse is a '''base'''. Thus, in liquid [[ammonia]], KNH<sub>2</sub> (supplying NH<sub>2</sub><sup>-</sup>) is a strong base, and NH<sub>4</sub>NO<sub>3</sub> (supplying NH<sub>4</sub><sup>+</sup>) is a strong acid. In liquid [[sulfur dioxide]] (SO<sub>2</sub>), [[thionyl]] compounds (supplying SO<sup>2+</sup>) behave as acids, and [[sulfites]] (supplying SO<sub>3</sub><sup>2−</sup>) behave as bases.

Here are some nonaqueous acid-base reactions in liquid ammonia
: 2NaNH<sub>2</sub> (base) + Zn(NH<sub>2</sub>)<sub>2</sub> ([[amphiphilic]] amide) → Na<sub>2</sub>[Zn(NH<sub>2</sub>)<sub>4</sub>]
: 2NH<sub>4</sub>I (acid) + Zn(NH<sub>2</sub>)<sub>2</sub> ([[amphiphilic]] amide) → [Zn(NH<sub>3</sub>)<sub>4</sub>)]I<sub>2</sub>
Nitric acid can be a base in liquid sulfuric acid:
: HNO<sub>3</sub> (base) + 2H<sub>2</sub>SO<sub>4</sub> → NO<sub>2</sub><sup>+</sup> + H<sub>3</sub>O<sup>+</sup> + 2HSO<sub>4</sub><sup>-</sup>
And things become even stranger in the aprotic world, for example in liquid N<sub>2</sub>O<sub>4</sub>:
: AgNO<sub>3</sub> (base) + NOCl (acid) → N<sub>2</sub>O<sub>4</sub> + AgCl 

Since solvent-system definition depends on the solvent as well as on the compound itself, the same compound can change its role depending on the choice of the solvent. Thus, HClO<sub>4</sub> is a strong acid in water, a weak acid in acetic acid, and a weak base in fluorosulfonic acid.

==Other acid-base theories==
=== Usanovich definition ===

The most general definition is that of the Russian chemist ''Mikhail Usanovich'', and can basically be summarized as defining an acid as anything that accepts negative species or donates positive ones, and a base as the reverse. This tends to overlap the concept of [[redox]] (oxidation-reduction), and so is not highly favored by chemists. This is because redox reactions focus more on physical electron transfer processes, rather than bond making/bond breaking processes, although the distinction between these two processes is somewhat ambiguous.

=== Lux-Flood definition ===

This definition, proposed by German chemist [[Hermann Lux]]<ref>Franz, H. (1966), p4</ref><ref name=lux>
{{cite journal|title="Säuren" und "Basen" im Schmelzfluss: die Bestimmung. der Sauerstoffionen-Konzentration|first=Hermann|last=Lux|authorlink=Hermann Lux|journal=Ztschr. Elektrochem|date=1939|volume=45|issue=4|pages=303–309}}</ref> in [[1939]], further improved by [[Håkon Flood]] circa [[1947]]<ref name=flood>{{cite journal|title=The Acidic and Basic Properties of Oxides|last=Flood|first=H.|authorlink=Håkon Flood|coauthors=Forland, T.|journal=Acta Chem. Scand.|date=1947|volume=1|pages=592}}</ref> and now commonly used in modern [[geochemistry]] and [[electrochemistry]] of molten salts, describes an '''acid''' as an oxide ion acceptor and a '''base''' as an oxide ion donor. For example:
: MgO (base) + CO<sub>2</sub> (acid) → MgCO<sub>3</sub>
: CaO (base) + SiO<sub>2</sub> (acid) → CaSiO<sub>3</sub>
: NO<sub>3</sub><sup>-</sup> (base) + S<sub>2</sub>O<sub>7</sub><sup>2-</sup> (acid) → NO<sub>2</sub><sup>+</sup> + 2SO<sub>4</sub><sup>2-</sup><ref name=drago>{{cite journal|title=The Synthesis of Oxyhalides Utilizing Fused-Salt Media|first=Russel S.|last=Drago|coauthors=Whitten, Kenneth W.|journal=[[Inorg. Chem.]]|date=1966|volume=5|issue=4|pages=677 - 682|doi=10.1021/ic50038a038}}</ref>

=== Pearson definition ===
{{main|HSAB concept}}
In [[1963]]<ref name=pearson>{{cite journal|title=Hard and Soft Acids and Bases|last=Pearson|first=Ralph G.|journal= [[J. Am. Chem. Soc.]] |date=1963| volume= 85 |issue=22|pages=3533 - 3539|doi=10.1021/ja00905a001}}</ref> Ralph Pearson proposed an advanced qualitative concept known as [[HSAB concept|Hard Soft Acid Base principle]], later made quantitative with help of [[Robert Parr]] in [[1984]]. 'Hard' applies to species which are small, have high charge states, and are weakly polarizable. 'Soft' applies to species which are large, have low charge states and are strongly polarizable. Acids and bases interact and the most stable interactions are hard-hard and soft-soft. This theory has found use in both organic and inorganic chemistry.

==See also==
* [[Electron configuration]]
* [[Lewis structure]]
* [[Resonance structure]]
* [[Protonation]] and [[Deprotonation]]
* [[Nucleophilic substitution]] and [[Redox reaction]]s
* [[Acid-base titration]]

== Notes ==
<div class="references-small" style="-moz-column-count:2; column-count:2;">
<references />
</div>

== References ==
<div class="references-small" style="-moz-column-count:2; column-count:2;">
# Miessler, L. M., Tar, D. A., (1991) "''Inorganic Chemistry''" 2<sup>nd</sup> ed. Pearson Prentice-Hall
# Clayden, J., Warren, S., et al. (2000) "''Organic Chemistry''" Oxford University Press
# Meyers, R. (2003) "''The Basics of Chemistry''" Greenwood Press
# {{cite journal|title="Säuren" und "Basen" im Schmelzfluss: die Bestimmung. der Sauerstoffionen-Konzentration|first=Hermann|last=Lux|authorlink=Hermann Lux|journal=Ztschr. Elektrochem|date=1939|volume=45|issue=4|pages=303–309}}
# '''Translated as''': Lux, Hermann: “Acids” and “bases” in a fused salt bath: the determination of oxygen-ion. In: Journal of Electrochemistry, Vol 45 (1939), S. 303–309
# {{cite journal|title=The Synthesis of Oxyhalides Utilizing Fused-Salt Media|first=Russel S.|last=Drago|coauthors=Whitten, Kenneth W.|journal=[[Inorg. Chem.]]|date=1966|volume=5|issue=4|pages=677 - 682|doi=10.1021/ic50038a038}}
# H. L. Finston and A. C. Rychtman, A New View of Current Acid-Base Theories, John Wiley & Sons, New York, 1982, pp. 140-146.
# {{cite journal|title=Solubility of Water Vapor in Alkali Borate Melts|last=Franz|first=H.|date=1966|journal=J. Am. Ceram. Soc.|volume=49|issue=9|pages=473–477}}
# International Union of Pure and Applied Chemistry (2006) ''IUPAC Compendium of Chemical Terminology, Electronic version'' Retrieved from International Union of Pure and Applied Chemistry on [[9 May]] [[2007]] on URL http://goldbook.iupac.org/O04379.html
# Murray, K. K., Boyd, R. K., et al. (2006) "''Standard definition of terms relating to mass spectrometry recommendations''" International Union of Pure and Applied Chemistry.
</div>

==External links==
*[http://www.acid-base.com Acid-Base Tutorial]
*[http://www.anaesthesiamcq.com/AcidBaseBook/ABindex.php Acid-base Physiology: an on-line text]
*[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Acids_Bases.html John W. Kimball's online Biology book section of acid and bases.]
*[http://dbhs.wvusd.k12.ca.us/webdocs/AcidBase/Early-Acid-Base.html Lavoisier, Davy, and Liebig theories at the ChemTeam Tutorials]

[[Category:Acid-base chemistry]]

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