Amphoterism

In chemistry, an amphoteric compound is a molecule or ion that can react both as an acid as well as a base.[1] Many metals (such as copper, zinc, tin, lead, aluminium, and beryllium) form amphoteric oxides or hydroxides. Amphoterism depends on the oxidation states of the oxide. Al2O3 is an example of an amphoteric oxide.

The prefix of the word 'amphoteric' is derived from a Greek prefix amphi-, which means both. In chemistry, an amphoteric substance is a substance that has the ability to act either as an acid or a base. Remember that acids donate protons (or accept electron pairs) and bases accept protons. Amphoteric substances can do either. So, we can think of an amphoteric substance as something like a double agent. Just like how a double agent can act as an ally or an enemy depending on the situation, the amphoteric substance can act either as an acid or a base, depending on what other substances it is reacting with.

Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Amphoteric oxides include lead oxide and zinc oxide, among many others.

One type of amphoteric species are amphiprotic molecules, which can either donate or accept a proton (H+). Examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water.

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.

Etymology

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.[2]

Amphiprotic molecules

According to the Brønsted-Lowry theory of acids and bases: acids are proton donors and bases are proton acceptors.[3] 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.

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− → CO32− + 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−

Not all amphoteric substances are amphiprotic

Although an amphiprotic species must be amphoteric, the converse is not true. For example, the metal oxide ZnO contains no hydrogen and cannot donate a proton. Instead it is a Lewis acid whose Zn atom accepts an electron pair from the base OH−. The other metal oxides and hydroxides mentioned above also function as Lewis acids rather than Brønsted acids.

Amphoteric oxides and hydroxides[4]

Amphoteric oxides

Zinc oxide (ZnO) reacts with both acids and with bases:

  • In acid: ZnO + H2SO4 → ZnSO4 + H2O
  • In base: ZnO + 2 NaOH + H2O → Na2[Zn(OH)4]

This reactivity can be used to separate different cations, such as zinc(II), which dissolves in base, from manganese(II), which does not dissolve in base.

Lead oxide (PbO):

  • In acid: PbO + 2 HCl → PbCl2 + H2O
  • In base: PbO + 2 NaOH + H2O → Na2[Pb(OH)4]

Aluminium oxide (Al2O3)

  • In acid: Al2O3 + 6 HCl→ 2 AlCl3 + 3 H2O
  • In base: Al2O3 + 2 NaOH + 3 H2O → 2 Na[Al(OH)4] (hydrated sodium aluminate)

Stannous oxide (SnO)

  • In acid : SnO +2 HCl ⇌ SnCl2 + H2O
  • In base : SnO +4 NaOH + H2O ⇌ Na4[Sn(OH)6]

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

Amphoteric hydroxides

Aluminium hydroxide is also amphoteric:

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

Beryllium hydroxide

  • with acid: Be(OH)2 + 2 HCl → BeCl2 + 2 H2O
  • with base: Be(OH)2 + 2 NaOH → Na2[Be(OH)4].[5]

See also

References

  1. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "amphoteric".
  2. ↑ Penguin Science Dictionary 1994, Penguin Books
  3. ↑ R.H. Petrucci, W.S. Harwood, and F.G. Herring, "General Chemistry" (8th edn, Prentice-Hall 2002), p.669
  4. ↑ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 173–4. ISBN 978-0130399137.
  5. ↑ CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes (program download required)
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