Amphoterism

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"Amphoteric" redirects here. For other uses, see Amphoteric (disambiguation).
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In chemistry, an amphoteric substance is one that can react as either an acid or base.[1] The word is derived from the Greek prefix ampho- (αμφί-) meaning "both". Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloids have amphoteric oxides or hydroxides.

Another class of amphoteric substances are amphiprotic molecules which 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.

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[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
  • Lead oxide
    • with acid: PbO + 2HCl → PbCl2 + H2O
    • with base: PbO + Ca(OH)2 +H2O → Ca2+[Pb(OH)4]2-
  • Zinc oxide
    • with acid: ZnO + 2HCl → ZnCl2 + H2O
    • with base: ZnO + 2NaOH + H2O → Na22+[Zn(OH)4]2-

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

[edit] Amphiprotic molecules

An amphiprotic molecule (or ion) can both donate or accept a proton, thus acting either as an acid or a base (according to the Brønsted-Lowry theory of acids and bases: acids are proton donors and bases are proton acceptors.[4] 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- + H2O → H2CO3 + OH-

or as an acid:

HCO3- + H2O → CO32- + H3O+

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] References

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 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. ^ International Union of Pure and Applied Chemistry. "amphoteric". Compendium of Chemical Terminology Internet edition.
  2. ^ C.E. Housecroft and A.G. Sharpe, "Inorganic Chemistry" (2nd edn, Pearson 2005), p.173-4
  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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