The ability of a chemical to behave as both an acid and a base is called amphoterism, and this type of substance is known as an amphoteric substance. Generally, such a substance acts as an acid in the presence of a base, and it acts as a base in the presence of an acid.
Reactions of some amphoteric substances
The simplest example of an amphoteric substance is water. In the presence of an acid, it behaves as a base (proton acceptor); in the presence of a base, it behaves as an acid (proton donor). In particular, when hydrochloric acid (HCl) is dissolved in water, water acts as a base. The chemical reaction can be written as:
- H2O + HCl → H3O+ + Cl−
In its reaction with ammonia (NH3, a weak base), water acts as an acid, as follows:
- H2O + NH3 → NH4+ + OH−
- 2H2O → H3O+ + OH−
Zinc oxide (ZnO) is another amphoteric substance. Its behavior as an acid or base depends on the pH of the solution. In an acidic solution, it reacts as a base; in a basic solution, it reacts as an acid. The chemical reactions can be written as:
- ZnO + 2H+ → Zn2+ + H2O
- ZnO + H2O + 2OH- → [Zn(OH)4]2-
A third example of an amphoteric substance is aluminum hydroxide (Al(OH)3). It can react with hydrochloric acid or sodium hydroxide (NaOH), represented as follows:
- Al(OH)3 + 3HCl → AlCl3 + 3H2O
- Al(OH)3 + NaOH → NaAl(OH)4
Likewise, beryllium hydroxide (Be(OH)2) is amphoteric:
- Be(OH)2 + 2HCl → BeCl2 + 2H2O
- Be(OH)2 + 2NaOH → Na2Be(OH)4
Many amphoteric substances are also described as amphiprotic—that is, they can donate or accept a proton, thus acting as acids or bases. Water, amino acids, hydrogen carbonate (bicarbonate) ions, and hydrogen sulfate ions are examples of amphiprotic species. Because they can donate a proton, all amphiprotic substances contain a hydrogen atom.
It should be noted that all amphoteric substances are not amphiprotic. For instance, if a substance can accept or donate a pair of electrons, (thus acting as a Lewis acid or base), it would be amphoteric but not amphiprotic.
Reactions of an amphiprotic substance
A common example of an amphiprotic substance is the hydrogen carbonate ion. It can accept a proton, thus acting as a base; or it can donate a proton, thus acting as an acid. Its reactions with water can be written as follows:
- HCO3- + H2O → H2CO3 + OH-
- HCO3- + H2O → CO32- + H3O+
- ↑ According to the Brønsted-Lowry theory of acids and bases, acids are proton donors and bases are proton acceptors. In the Lewis theory of acids and bases, acids are electron pair acceptors and bases are electron pair donors.
- Brown, Theodore E., H. Eugene LeMay, and Bruce E. Bursten. 2005. Chemistry: The Central Science. 10th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 1110131704
- Chang, Raymond. 2006. Chemistry. 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031
- McMurry, John, and Robert C. Fay. 2004. Chemistry. 4th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0131402080
- Moore, John W., Conrad L. Stanitski, and Peter C. Jurs. 2002. Chemistry: The Molecular Science. New York: Harcourt College. ISBN 0030320119
- Oxlade, Chris. 2007. Acids and Bases. Chemicals in Action. Oxford: Heinemann Library. ISBN 1432900579
All links retrieved October 2, 2012.
- Amphoteric IUPAC Compendium of Chemical Terminology, Electronic version.
- Amphoterism Clackamas Community College.
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