Many reactions involve the transfer of a proton from an acid to a base.

  • Brønsted–Lowry acids are ($\rm HA(aq)$) proton donors:
    $\rm HA(aq) + H_2O(l) \rightarrow H_3O^+(aq) + A^–(aq)$
    A free proton, $\rm H^+$, cannot exist in aqueous solutions, as it immediately forms a coordinate bond with water and produces a hydronium ion, $\rm H_3O^+$. 
  • Brønsted–Lowry bases are proton acceptors:
    $\rm B(aq) + H_2O(l) \rightarrow BH^+(aq) + OH^–(aq)$
  • A conjugate acid–base pair differ by a single proton:
    $\rm HA + H_2O(l) \rightarrow H_3O^+(aq) + A^–(aq)$
    $\rm HA$ is the conjugate acid of $\rm A^–$, $\rm H_2O$ is the conjugate base of $\rm H_3O^+$
    $\rm B(aq) + H_2O(l) \rightarrow BH^+(aq) + OH^–(aq)$
    $\rm B(aq)$ is the conjugate base of $\rm BH^+(aq)$
    $\rm H_2O(l)$ is the conjugate acid of $\rm OH^–(aq)$
  • Amphiprotic species can act as both Brønsted–Lowry acids and bases.
    Water and the amino acids are important examples.
    Water as an acid: $\rm H_2O + B \rightarrow BH^+ + OH^–$
    Water as a base: $\rm HA + H_2O(l) \rightarrow H_3O^+(aq) + A^–(aq)$
  • Amphoteric species can react with both acids and bases.
    Zinc oxide is amphoteric as it reacts with both acids and bases:
    $\rm ZnO(s) + 2H^+(aq) \rightarrow Zn^{2+}(aq) + H_2O(l)$
    $\rm ZnO(s) + 2OH^–(aq) + H_2O(l)$ $\rm \rightarrow [Zn(OH)_4]^{2–}(aq)$
    But it is not amphiprotic as it cannot donate a proton.
    Zinc, aluminium and transition metals oxides and hydroxides are amphoteric.