Conjugate Acid-Base Pairs

Amphoteric Substances

Amphoteric substances can act as both acids and bases. This means they can donate or accept a proton ( $ H^+ $ ) depending on the reaction conditions.

Key Characteristics:

• Can react with both acids and bases.
• Often contain a molecule with both acidic and basic functional groups.
• The behavior depends on the relative strength of the acid and base it is reacting with.

Examples:

• Water ( $ H_2O $ ): $ H_2O + H^+ \rightleftharpoons H_3O^+ $ (acts as a base) ; $ H_2O \rightleftharpoons H^+ + OH^- $ (acts as an acid)
• Amino acids: Contain both carboxyl (-COOH) groups (acidic) and amino (- $ NH_2 $ ) groups (basic).
• Hydrogen carbonate ion ( $ HCO_3^- $ ): $ HCO_3^- + H^+ \rightleftharpoons H_2CO_3 $ (acts as a base) ; $ HCO_3^- \rightleftharpoons H^+ + CO_3^{2-} $ (acts as an acid)
• Aluminum hydroxide ( $ Al(OH)_3 $ ): Reacts with acids to form salts and water, and with bases to form aluminates.

Reactions:

• With Acids: An amphoteric substance acts as a base, accepting a proton. For example: $ Al(OH)_3(s) + 3HCl(aq) \rightarrow AlCl_3(aq) + 3H_2O(l) $
• With Bases: An amphoteric substance acts as an acid, donating a proton. For example: $ Al(OH)_3(s) + NaOH(aq) \rightarrow NaAl(OH)_4(aq) $

Acid-Base Reactions Brønsted-Lowry Theory Conjugate Acid-Base Pairs]]

Further points to consider:

• The pH at which an amphoteric substance exhibits acidic or basic character.
• How the structure of the molecule influences its amphoteric nature.
• The equilibrium constants for the acid and base reactions of amphoteric substances.