Amphoterism

"Amphoteric" redirects here.

In chemistry, an amphoteric substance is one that can react as either an acid or base.

Examples

Examples include amino acids, proteins, and water. Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloid have amphoteric oxides.

For example, 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.

There are many other examples of chemical compounds which are also amphoteric, for the simplest example water:

BASE (Proton Acceptor): H₂O + HCl → H₃O⁺ + Cl⁻

ACID (Proton Donor): H₂O + NH₃ → NH₄⁺ + OH⁻

(Indeed, it can do both at once: 2H₂O → H₃O⁺ + OH⁻)

Aluminium hydroxide is as well:

Acid: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

Base: Al(OH)₃ + NaOH → NaAl(OH)₄

Beryllium hydroxide is also amphoteric:

Acid: Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O

Base: Be(OH)₂ + 2NaOH → Na₂Be(OH)₄

Amphiprotic substances

In chemistry, a substance is described as amphiprotic if it can both donate or accept a proton, thus acting either like an acid or a base (according to Brønsted-Lowry theory of acids and bases: acids are proton donors and bases are proton acceptors. In Lewis theory of acids and bases; acids are electron pair acceptors and bases are electron pair donors). Water, amino acids, hydrogen carbonate ions and hydrogen sulfate ions are common examples of amphiprotic species. Since they can donate an electron, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric. Amphoteric substances, however, are not necessarily amphiprotic.

Examples

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. Water is the most common example of an amphiprotic substance:

Basic: H₂O + HCl → H₃O⁺ + Cl^-

Acidic: H₂O + NH₃ → NH₄⁺ + OH^-

See also

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

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).

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