Editing Amphoterism

{{Short description|Chemical property of reacting with either an acid or base}}
{{Acids and bases}}

In [[chemistry]], an '''amphoteric''' [[chemical compound|compound]] ({{etymology|el|amphoteros|both}}) is a [[molecule]] or [[ion]] that can react both as an [[acid]] and as a [[base (chemistry)|base]].<ref>{{GoldBookRef |title=amphoteric |file=A00306}}</ref> What exactly this can mean depends on [[acid–base reaction|which definitions of acids and bases]] are being used.


==Etymology and terminology==
'''Amphoteric''' is derived from the Greek word {{lang|el-Latn|amphoteroi}} ({{lang|el|ἀμφότεροι}}) meaning "both".  Related words in acid-base chemistry are '''amphichromatic''' and '''amphichroic''', both describing substances such as [[pH indicator|acid-base indicators]] which give one colour on reaction with an acid and another colour on reaction with a base.<ref>Penguin Science Dictionary 1994, Penguin Books</ref>

===Amphiprotism===
Amphiprotism is exhibited by compounds with both Brønsted acidic and basic properties.<ref>{{cite book |chapter=Amphiprotic (solvent) |doi=10.1351/goldbook.A00304 |chapter-url=https://goldbook.iupac.org/terms/view/A00304 |title=The IUPAC Compendium of Chemical Terminology |date=2008 }}</ref>  A prime example is H<sub>2</sub>O.
Amphiprotic molecules can either [[deprotonation|donate]] or [[protonation|accept]] a [[proton]] ({{chem2|H+}}). [[Amino acid]]s (and [[protein]]s) are amphiprotic molecules because of their [[amine]] ({{chem2|\sNH2}}) and [[carboxylic acid]] ({{chem2|\sCOOH}}) groups. 

===Ampholytes===
'''Ampholytes''' are [[zwitterion]]s<ref>{{cite book |title=Ampholyte |chapter=Ampholytes |date=2014 |doi=10.1351/goldbook.A00305 |chapter-url=https://goldbook.iupac.org/terms/view/A00305}}</ref> ‒ molecules or ions that contain both acidic and basic [[functional group]]s. [[Amino acid]]s {{chem2|H2N\sRCH\sCO2H}} have both a basic group {{chem2|\sNH2}} and an acidic group {{chem2|\sCOOH}}.  Often such species exists as several structures in [[chemical equilibrium]]:
<!--splits added to allow text wrapping for mobile-->
:{{chem2 | H2N\sCRH\sCO2H + H2O <-> H2N\sCRH\sCOO- + H3O+ <-> H3N+\sCRH\sCOOH + HO- <-> H3N+\sCRH\sCOO- + H2O }}

In approximately neutral [[aqueous solution]] (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is the [[zwitterion]] {{chem2|H3N+\sRCH\sCOO-}}. The pH at which the average charge is zero is known as the molecule's ''[[isoelectric point]]''. Ampholytes are used to establish a stable pH gradient for use in [[isoelectric focusing]].

[[Metal oxide]]s which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Many metals (such as [[zinc]], [[tin]], [[lead]], [[aluminium]], and [[beryllium]]) form amphoteric oxides or hydroxides. [[Aluminium oxide]] ({{chem2|Al2O3}}) is an example of an amphoteric oxide. Amphoterism depends on the [[oxidation states]] of the oxide. Amphoteric oxides include [[lead(II) oxide]] and [[zinc oxide]], among many others.<ref>{{Housecroft2nd|pages=173–4}}</ref>

==Amphiprotic molecules==
According to the [[Brønsted–Lowry acid–base theory|Brønsted-Lowry theory of acids and bases]], acids are proton donors and bases are proton acceptors.<ref>{{cite book |last1=Petrucci |first1=Ralph H. |last2=Harwood |first2=William S. |last3=Herring |first3=F. Geoffrey |date=2002 |title=General chemistry: principles and modern applications |url=https://archive.org/details/generalchemistry00hill |url-access=registration |edition=8th |location=Upper Saddle River, NJ |publisher=Prentice Hall |isbn=978-0-13-014329-7 |lccn=2001032331 |oclc=46872308 |page=[https://archive.org/details/generalchemistry00hill/page/669 669]}}</ref> An amphiprotic molecule (or ion) can either donate or accept a [[proton]], thus acting either as an [[acid]] or a [[base (chemistry)|base]]. [[Water]], [[amino acid]]s, [[bicarbonate|hydrogencarbonate]] ion (or bicarbonate ion) {{chem2|HCO3-}}, [[dihydrogen phosphate]] ion {{chem2|H2PO4-}}, and [[hydrogensulfate]] ion (or bisulfate ion) {{chem2|HSO4-}} are common examples of amphiprotic species. Since they can donate a proton, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric.

===Examples===
The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a [[hydronium]] ion {{chem2|H3O+}}, or else lose a proton to form a [[hydroxide]] ion {{chem2|OH-}}.<ref>{{cite book |url=https://www.worldcat.org/title/824171785 |title=Fundamentals of analytical chemistry |last1=Skoog |first1=Douglas A. |last2=West |first2=Donald M. |last3=Holler |first3=F. James |last4=Crouch |first4=Stanley R. |year=2014 |isbn=978-0-495-55828-6 |edition=Ninth |location=Belmont, CA |pages=200 |oclc=824171785}}</ref>

Another possibility is the [[molecular autoionization]] reaction between two water molecules, in which one water molecule acts as an acid and another as a base.
:{{chem2 | H2O + H2O <-> H3O+ + HO- }}

The [[bicarbonate]] ion, {{chem2|HCO3-}}, is amphoteric as it can act as either an acid or a base:
:As an acid, losing a proton: {{chem2 | HCO3- + OH- <-> CO3(2-) + H2O }}
:As a base, accepting a proton: {{chem2 | HCO3- + H+ <-> H2CO3 }}
Note: in dilute aqueous solution the formation of the [[hydronium ion]], {{chem2|H3O+(aq)}}, is effectively complete, so that hydration of the proton can be ignored in relation to the equilibria.

Other examples of inorganic polyprotic acids include anions of [[sulfuric acid]], [[phosphoric acid]] and [[hydrogen sulfide]] that have lost one or more protons. In organic chemistry and biochemistry, important examples include [[amino acid]]s and derivatives of [[citric acid]].

Although an amphiprotic species must be amphoteric, the converse is not true. For example, a metal oxide such as [[zinc oxide]], ZnO, contains no hydrogen and so cannot donate a proton. Nevertheless, it can act as an acid by reacting with the hydroxide ion, a base:
:{{chem2|ZnO + 2 OH- + H2O -> [Zn(OH)4](2-)}}
Zinc oxide can also act as a base:
:{{chem2|ZnO + 2H+ +  5 H2O -> [Zn(H2O)6](2+)}}

==Oxides==
[[Zinc oxide]] (ZnO) reacts both with acids and with bases:
*<chem>ZnO + \overset{acid}{H2SO4} -> ZnSO4 + H2O</chem>
*<chem>ZnO + \overset{base}{2 NaOH} + H2O -> Na2[Zn(OH)4]</chem>
This reactivity can be used to separate different [[cation]]s, for instance zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.

[[Lead(II) oxide|Lead oxide]] (PbO):
* <chem>PbO + \overset{acid}{2 HCl} -> PbCl2 + H2O</chem>
* <chem>PbO + \overset{base}{2 NaOH} + H2O -> Na2[Pb(OH)4]</chem>

[[Lead(IV) oxide|Lead oxide]] ({{chem2|PbO2}}):
* <chem>PbO2 + \overset{acid}{4 HCl} -> PbCl4 + 2H2O</chem>
* <chem>PbO2 + \overset{base}{2 NaOH} + 2H2O -> Na2[Pb(OH)6]</chem>

[[Aluminium oxide]] ({{chem2|Al2O3}}):
* <chem>Al2O3 + \overset{acid}{6 HCl} -> 2 AlCl3 + 3 H2O</chem>
* <chem>Al2O3 + \overset{base}{2 NaOH} + 3 H2O -> 2 Na[Al(OH)4]</chem> (hydrated [[sodium aluminate]])

[[Stannous oxide]] (SnO):
* <chem>SnO + \overset{acid}{2 HCl} <=> SnCl2 + H2O</chem>
* <chem>SnO + \overset{base}{4 NaOH} + H2O <=> Na4[Sn(OH)6]</chem>

[[Stannic oxide]] ({{chem2|SnO2}}):
* <chem>SnO2 + \overset{acid}{4 HCl} <=> SnCl4 + 2H2O</chem>
* <chem>SnO2 + \overset{base}{4 NaOH} + 2H2O <=> Na4[Sn(OH)8]</chem>

[[Vanadium(IV) oxide|Vanadium dioxide]] ({{chem2|VO2}}):
*<chem>VO2 + \overset{acid}{2 HCl} -> VOCl2 + H2O</chem>
*<chem>4 VO2 + \overset{base}{2 NaOH} -> Na2V4O9 + H2O</chem>

Some other elements which form amphoteric oxides are [[gallium]], [[indium]], [[scandium]], [[titanium]], [[zirconium]], [[chromium]], [[iron]], [[cobalt]], [[copper]], [[silver]], [[gold]], [[germanium]], [[antimony]], [[bismuth]], [[beryllium]], and [[tellurium]].

==Hydroxides==
[[Aluminium hydroxide]] is also amphoteric:<!--simplified reactions that are correct -->
*<chem>Al(OH)3 + \overset{acid}{3 HCl} -> AlCl3 + 3 H2O</chem>
*<chem>Al(OH)3 + \overset{base}{NaOH} -> Na[Al(OH)4]</chem>

[[Beryllium hydroxide]]:
*<chem>Be(OH)2 + \overset{acid}{2 HCl} -> BeCl2 + 2 H2O</chem>
*<chem>Be(OH)2 + \overset{base}{2 NaOH} -> Na2[Be(OH)4]</chem><ref>[http://webarchive.loc.gov/all/20020808162614/http://home.c2i.net/astandne/ CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes] (program download required)</ref>

[[Chromium(III) hydroxide|Chromium hydroxide]]:
*<chem>Cr(OH)3 + \overset{acid}{3 HCl} -> CrCl3 + 3H2O</chem>
*<chem>Cr(OH)3 + \overset{base}{NaOH} -> Na[Cr(OH)4]</chem>

==See also==
{{Commons category|Amphoteric oxides}}
* [[Ate complex]]
* [[Isoelectric point]]
* [[Zwitterion]]

==References==
{{Reflist}}

[[Category:Amphoteric compounds| ]]
[[Category:Acid–base chemistry]]
[[Category:Chemical properties]]
[[Category:General chemistry]]