Amphoterism

Water can behave as an acid or a base and is therefore called an amphoteric substance.

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. Examples of amphoteric substances include water, amino acids, and proteins. Many metals (such as zinc, tin, lead, aluminum, and beryllium) and most metalloids have amphoteric oxides.

Reactions of some amphoteric substances

The simplest example of an amphoteric substance is water. When hydrochloric acid (HCl) is dissolved in water, water behaves as a base (proton acceptor). The chemical reaction can be written as:

H₂O + HCl → H₃O⁺ + Cl⁻

Water can also behave as an acid (proton donor), such as when it reacts with ammonia (NH₃), as follows:

H₂O + NH₃ → NH₄⁺ + OH⁻

Furthermore, water molecules can exchange protons with one another. In this case, water behaves as both an acid and a base simultaneously.

2H₂O → H₃O⁺ + OH⁻

Zinc oxide (ZnO) behaves as an acid or a base, depending 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-

This property can be used to separate different cations, such as zinc from manganese.

Aluminum hydroxide (Al(OH)₃) is another amphoteric substance, as seen by its ability to react with hydrochloric acid and sodium hydroxide (NaOH):

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

Amphiprotic substances

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 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, however, that amphoteric substances are not necessarily 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 is the hydrogen carbonate ion, which can act as a base:

HCO₃^- + H₂O → H₂CO₃ + OH^-

Or as an acid:

HCO₃^- + H₂O → CO₃^2- + H₃O⁺

Thus, it can effectively accept or donate a proton.

See also

• Acid
• Base (chemistry)
• Brønsted-Lowry Theory
• Oxide
• Water

References

ISBN links support NWE through referral fees

• Brown, Theodore E., H. Eugene LeMay, and Bruce E Bursten (2005). Chemistry: The Central Science (10th Edition). Upper Saddle River, NJ: Prentice Hall. ISBN 0131096869 (ISBN-13: 978-0131096868).
• McMurry, J., & Fay, R. C. (2004). Chemistry (4th ed.). Upper Saddle River, NJ: Prentice Hall. ISBN 0131402080.
• Moore, J. W., Stanitski, C. L., & Jurs, P. C. (2002). Chemistry The Molecular Science. New York: Harcourt College. ISBN: 0030320119
• Oxlade, Chris (2002). Acids and Bases (Chemicals in Action). Heinemann Library. ISBN 1588101940 (ISBN-13: 978-1588101945).

External links

• Amphoteric IUPAC Compendium of Chemical Terminology, Electronic version.

• Amphoterism Clackamas Community College.

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