Amphoterism

In chemistry, an amphoteric compound is a molecule or ion that can react both as an acid and 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. Acids donate protons (or accept electron pairs) and bases accept protons; amphoteric substances can do either.

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.^[2]

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.^[3]

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 ion (bicarbonate ion) and hydrogen sulfate ion (bisulfate ion) 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

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]

References

^ 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, N.J: Prentice Hall. p. 669. ISBN 978-0-13-014329-7. LCCN 2001032331. OCLC 46872308. {{cite book}}: Invalid |ref=harv (help)
^ CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes (program download required)

[1] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "amphoteric". doi:10.1351/goldbook.A00306

[2] Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 173–4. ISBN 978-0-13-039913-7.

[3] Penguin Science Dictionary 1994, Penguin Books

[4] Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General chemistry: principles and modern applications (8th ed.). Upper Saddle River, N.J: Prentice Hall. p. 669. ISBN 978-0-13-014329-7. LCCN 2001032331. OCLC 46872308. {{cite book}}: Invalid |ref=harv (help)

[5] CHEMIX School & Lab - Software for Chemistry Learning, by Arne Standnes (program download required)

[1]

[2]

[3]

[4]

[5]