Ampholytes are amphoteric molecules that contain both acidic and basic functional groups. For example, an amino acid H2N−RCH−CO2H has both a basic group −NH2 and an acidic group −COOH, and exists as several structures in chemical equilibrium:
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 zwitterionH3N+−RCH−COO−. 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 oxides 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 (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.[2]
Amphoteric is derived from the Greek word amphoteroi (ἀμφότεροι) meaning "both". Related words in acid-base chemistry are amphichromatic and amphichroic, both describing substances such as acid-base indicators which give one colour on reaction with an acid and another colour on reaction with a base.[3]
Amphiprotic moleculesedit
According to the Brønsted-Lowry theory of acids and bases, acids are proton donors and bases are proton acceptors.[4] An amphiprotic molecule (or ion) can either donate or accept a proton, thus acting either as an acid or a base. Water, amino acids, hydrogencarbonate ion (or bicarbonate ion) HCO−3, dihydrogen phosphate ion H2PO−4, and hydrogensulfate ion (or bisulfate ion) HSO−4 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.
Examplesedit
The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a hydronium ion H3O+, or else lose a proton to form a hydroxide ion OH−.[5]
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.
The bicarbonate ion, HCO−3, is amphoteric as it can act as either an acid or a base:
As an acid, losing a proton:
As a base, accepting a proton:
Note: in dilute aqueous solution the formation of the hydronium ion, H3O+(aq), is effectively complete, so that hydration of the proton can be ignored in relation to the equilibria.
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:
it is classified as amphoteric rather than amphiprotic.
Oxidesedit
Zinc oxide (ZnO) reacts with both acids and with bases:
This reactivity can be used to separate different cations, for instance zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.
^Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General chemistry: principles and modern applications (8th ed.). Upper Saddle River, NJ: Prentice Hall. p. 669. ISBN978-0-13-014329-7. LCCN 2001032331. OCLC 46872308.
^Skoog, Douglas A.; West, Donald M.; Holler, F. James; Crouch, Stanley R. (2014). Fundamentals of analytical chemistry (Ninth ed.). Belmont, CA. p. 200. ISBN978-0-495-55828-6. OCLC 824171785.{{cite book}}: CS1 maint: location missing publisher (link)
^CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes (program download required)
November 11, 2023
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In chemistry an amphoteric compound from Greek amphi both is a molecule or ion that can react both as an acid and as a base 1 What exactly this can mean depends on which definitions of acids and bases are being used One type of amphoteric species are amphiprotic molecules which can either donate or accept a proton H This is what amphoteric means in Bronsted Lowry acid base theory For example amino acids and proteins are amphiprotic molecules because of their amine NH2 and carboxylic acid COOH groups Self ionizable compounds like water are also amphiprotic Ampholytes are amphoteric molecules that contain both acidic and basic functional groups For example an amino acid H2N RCH CO2H has both a basic group NH2 and an acidic group COOH and exists as several structures in chemical equilibrium H 2 N RCH CO 2 H H 2 O displaystyle ce H2N RCH CO2H H2O H 2 N RCH COO H 3 O displaystyle ce lt gt H2N RCH COO H3O H 3 N RCH COOH OH displaystyle ce lt gt H3N RCH COOH OH H 3 N RCH COO H 2 O displaystyle ce lt gt H3N RCH COO 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 H3N RCH COO 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 oxides 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 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 2 Contents 1 Etymology 2 Amphiprotic molecules 2 1 Examples 3 Oxides 4 Hydroxides 5 See also 6 ReferencesEtymology editAmphoteric is derived from the Greek word amphoteroi ἀmfoteroi meaning both Related words in acid base chemistry are amphichromatic and amphichroic both describing substances such as acid base indicators which give one colour on reaction with an acid and another colour on reaction with a base 3 Amphiprotic molecules editAccording to the Bronsted Lowry theory of acids and bases acids are proton donors and bases are proton acceptors 4 An amphiprotic molecule or ion can either donate or accept a proton thus acting either as an acid or a base Water amino acids hydrogencarbonate ion or bicarbonate ion HCO 3 dihydrogen phosphate ion H2PO 4 and hydrogensulfate ion or bisulfate ion HSO 4 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 edit The water molecule is amphoteric in aqueous solution It can either gain a proton to form a hydronium ion H3O or else lose a proton to form a hydroxide ion OH 5 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 H 2 O H 2 O H 3 O OH displaystyle ce H2O H2O lt gt H3O OH nbsp The bicarbonate ion HCO 3 is amphoteric as it can act as either an acid or a base As an acid losing a proton HCO 3 OH CO 3 2 H 2 O displaystyle ce HCO3 OH lt gt CO3 2 H2O nbsp As a base accepting a proton HCO 3 H H 2 CO 3 displaystyle ce HCO3 H lt gt H2CO3 nbsp Note in dilute aqueous solution the formation of the hydronium ion 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 acids 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 ZnO s 2 OH H 2 O Zn OH 4 aq 2 displaystyle ce ZnO s 2OH H2O gt Zn OH 4 aq 2 nbsp This reaction is not covered by the Bronsted Lowry acid base theory Because zinc oxide can also act as a base ZnO s 2 H Zn aq 2 H 2 O displaystyle ce ZnO s 2H gt Zn 2 aq H2O nbsp it is classified as amphoteric rather than amphiprotic Oxides editZinc oxide ZnO reacts with both acids and with bases ZnO H 2 SO 4 acid ZnSO 4 H 2 O displaystyle ce ZnO overset acid H2SO4 gt ZnSO4 H2O nbsp ZnO 2 NaOH base H 2 O Na 2 Zn OH 4 displaystyle ce ZnO overset base 2 NaOH H2O gt Na2 Zn OH 4 nbsp This reactivity can be used to separate different cations for instance zinc II which dissolves in base from manganese II which does not dissolve in base Lead oxide PbO PbO 2 HCl acid PbCl 2 H 2 O displaystyle ce PbO overset acid 2 HCl gt PbCl2 H2O nbsp PbO 2 NaOH base H 2 O Na 2 Pb OH 4 displaystyle ce PbO overset base 2 NaOH H2O gt Na2 Pb OH 4 nbsp Aluminium oxide Al2O3 Al 2 O 3 6 HCl acid 2 AlCl 3 3 H 2 O displaystyle ce Al2O3 overset acid 6 HCl gt 2 AlCl3 3 H2O nbsp Al 2 O 3 2 NaOH base 3 H 2 O 2 Na Al OH 4 displaystyle ce Al2O3 overset base 2 NaOH 3 H2O gt 2 Na Al OH 4 nbsp hydrated sodium aluminate Stannous oxide SnO SnO 2 HCl acid SnCl 2 H 2 O displaystyle ce SnO overset acid 2 HCl lt gt SnCl2 H2O nbsp SnO 4 NaOH base H 2 O Na 4 Sn OH 6 displaystyle ce SnO overset base 4 NaOH H2O lt gt Na4 Sn OH 6 nbsp Vanadium dioxide VO2 VO 2 2 HCl acid VOCl 2 H 2 O displaystyle ce VO2 overset acid 2 HCl gt VOCl2 H2O nbsp 4 VO 2 2 NaOH base Na 2 V 4 O 9 H 2 O displaystyle ce 4 VO2 overset base 2 NaOH gt Na2V4O9 H2O nbsp 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 editAluminium hydroxide is also amphoteric Al OH 3 3 HCl acid AlCl 3 3 H 2 O displaystyle ce Al OH 3 overset acid 3 HCl gt AlCl3 3 H2O nbsp Al OH 3 NaOH base Na Al OH 4 displaystyle ce Al OH 3 overset base NaOH gt Na Al OH 4 nbsp Beryllium hydroxide Be OH 2 2 HCl acid BeCl 2 2 H 2 O displaystyle ce Be OH 2 overset acid 2 HCl gt BeCl2 2 H2O nbsp Be OH 2 2 NaOH base Na 2 Be OH 4 displaystyle ce Be OH 2 overset base 2 NaOH gt Na2 Be OH 4 nbsp 6 Chromium hydroxide Cr OH 3 3 HCl acid CrCl 3 3 H 2 O displaystyle ce Cr OH 3 overset acid 3 HCl gt CrCl3 3H2O nbsp Cr OH 3 NaOH base Na Cr OH 4 displaystyle ce Cr OH 3 overset base NaOH gt Na Cr OH 4 nbsp See also edit nbsp Wikimedia Commons has media related to Amphoteric oxides Ate complex Isoelectric point ZwitterionReferences edit IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 amphoteric doi 10 1351 goldbook A00306 Housecroft C E Sharpe A G 2004 Inorganic Chemistry 2nd ed Prentice Hall pp 173 4 ISBN 978 0 13 039913 7 Penguin Science Dictionary 1994 Penguin Books Petrucci Ralph H Harwood William S Herring F Geoffrey 2002 General chemistry principles and modern applications 8th ed Upper Saddle River NJ Prentice Hall p 669 ISBN 978 0 13 014329 7 LCCN 2001032331 OCLC 46872308 Skoog Douglas A West Donald M Holler F James Crouch Stanley R 2014 Fundamentals of analytical chemistry Ninth ed Belmont CA p 200 ISBN 978 0 495 55828 6 OCLC 824171785 a href Template Cite book html title Template Cite book cite book a CS1 maint location missing publisher link CHEMIX School amp Lab Software for Chemistry Learning by Arne Standnes program download required Retrieved from https en wikipedia org w index php title Amphoterism amp oldid 1165827510, wikipedia, wiki, book, books, library,
