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Acids as Proton Donors

Define acids as proton donors and describe the ionisation of acids in water



  • The Lowry-Bronsted theory defines an acid as a substance that acts as a proton donor.
  • When an acid molecule is in water, it can ionise, releasing a proton, or hydrogen ion, and forming a negative ion.
  • The proton, or hydrogen ion, is donated to a water molecule, forming a hydronium ion:
  • For example, in the case of hydrochloric acid:
  • Acids can be classified according to their potential for proton donation per molecule:
    • Monoprotic acids: Acids that can release only one proton, or hydrogen ion, per molecule.
    • Diprotic acids: Acids that can release up to two protons, or hydrogen ions, per molecule.
    • Triprotic acids: Acids that can release up to three protons, or hydrogen ions, per molecule.


Ionisation/Dissociation of acetic acid in water (click to enlarge)

Easychem - Acids as Proton Donors



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