amphoterism

amphoterism, in chemistry, the property of certain substances of acting either as acids or as bases depending on the reaction in which they are involved. Many hydroxide compounds are amphoteric. For example, aluminum hydroxide, Al(OH)₃, reacts as a base with common acids to form salts, e.g., with sulfuric acid, H₂SO₄, to form aluminum sulfate, Al₂(SO₄)₃. It reacts as an acid with strong bases to form aluminates, e.g., with sodium hydroxide, NaOH, to form sodium aluminate, Na[Al(OH)₄(OH₂)₂]. Organic molecules that contain both acidic (e.g., carboxyl) and basic (e.g., amino) functional groups are usually amphoteric.

In chemistry, an amphoteric substance is one that can react as either an acid or base. The word is derived from the Greek prefix ampho- (αμφί-) meaning "both". Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloids have amphoteric oxides. Other examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water and ammonia.

Examples

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⁻

(It can do both at once: 2H₂O → H₃O⁺ + OH⁻)

Aluminium hydroxide is as well:

Base (neutralizing an acid): Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

Acid (neutralizing a base): Al(OH)₃ + NaOH → Na[Al(OH)₄]

Some other examples include:

• Beryllium hydroxide
• with Acid: Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O
• with Base: Be(OH)₂ + 2NaOH → Na₂Be(OH)₄

• Lead oxide
• with acid: PbO + 2HCl → PbCl₂ + H₂O
• with base: PbO + Ca(OH)₂ +H₂O → Ca²⁺[Pb(OH)₄]^2-

• Zinc oxide
• with acid: ZnO + 2HCl → ZnCl₂ + H₂O
• with base: ZnO + 2NaOH + H₂O → Na₂²⁺[Zn(OH)₄]^2-

Some elements not mentioned that are able to form amphoteric oxides: Si, Ti, V, Fe, Co, Ge, Zr, Ag, Sn, Au

See also

• Amphiprotic

References