Amphoterism

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"Amphoteric" redirects here. For other uses, see Amphoteric (disambiguation).

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In chemistry, an amphoteric or amphiprotic substance is a compound that can react as an acid as well as a base.^[1] The word is derived from the Greek word amphoteroi (ἀμφότεροι) meaning "both". Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloids have amphoteric oxides or hydroxides. Amphoteric substances can either donate or accept a proton. Examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water and ammonia.
Ampholytes are amphoteric molecules that contain both acidic and basic groups and will exist mostly as zwitterions in a certain range of pH. 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.

Contents [hide] 1 Amphoteric oxides and hydroxides[2] 2 Amphiprotic molecules 2.1 Examples 3 See also 4 References

[edit] Amphoteric oxides and hydroxides^[2]

Zinc oxide (ZnO) reacts differently depending on the pH of the solution:

• In acids: ZnO + 2H⁺ → Zn²⁺ + H₂O
• In bases: ZnO + H₂O + 2OH^- → [Zn(OH)₄]^2-

This effect can be used to separate different cations, such as zinc from manganese.
Aluminium hydroxide is as well:

• Base (neutralizing an acid): Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O
• Acid (neutralizing a base): Al(OH)₃ + NaOH → Na[Al(OH)₄]

Some other examples include:

• Beryllium hydroxide
  • with Acid: Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O
  • with Base: Be(OH)₂ + 2NaOH → Na₂Be(OH)₄

• Aluminium oxide
  • with acid: Al₂O₃ + 3 H₂O + 6 H₃O⁺(aq) → 2 [Al(H₂O)₆]³⁺(aq)
  • with base: Al₂O₃ + 3 H₂O + 2 OH^-(aq) → 2 [Al(OH)₄]^-(aq)

• Lead oxide
  • with acid: PbO + 2HCl → PbCl₂ + H₂O
  • with base: PbO + Ca(OH)₂ +H₂O → Ca²⁺[Pb(OH)₄]^2-

Some other elements which form amphoteric oxides: Si, Ti, V, Fe, Co, Ge, Zr, Ag, Sn, Au^[3]

[edit] Amphiprotic molecules

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, hydrogen carbonate ions and hydrogen sulfate ions 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.

[edit] Examples

A common example of an amphiprotic substance is the hydrogen carbonate ion, which can act as a base:
HCO₃^- + H₃O⁺ → H₂CO₃ + H₂O
or as an acid:
HCO₃^- + OH^- → CO₃^2- + H₂O
Thus, it can effectively accept or donate a proton.
Water is the most common example, acting as a base when reacting with an acid such as hydrogen chloride
H₂O + HCl → H₃O⁺ + Cl^-,
and acting as an acid when reacting with a base such as ammonia:
H₂O + NH₃ → NH₄⁺ + OH^-

[edit] See also

• Zwitterion
• Isoelectric point
• Ate complex

[edit] References

This June 2009 includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (June 2009)

1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006-) "amphoteric".
2. ^ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. ISBN 978-0130399137. 
3. ^ CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes (program download required)
4. ^ R.H. Petrucci, W.S. Harwood, and F.G. Herring, "General Chemistry" (8th edn, Prentice-Hall 2002), p.669

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Categories: Chemical properties | Acid-base chemistry