Acids and bases

• Acid

  A substance that dissociates in water to produce H+ ions
• Strong acid

  A substance that almost completely dissociates in water to give hydrogen ions
• Weak acid

  A substance that only slightly dissociates in water to give hydrogen ions
• Base

  A substance that dissociates in water to produce OH- ions
• Strong base

  A substance that almost completely dissociates in water to give hydroxide ions
• Weak base

  A substance that only slightly dissociates in water to give hydroxide ions
• Hydronium ion (H3O+)


  The ion formed when H+ reacts with water
• Hydrogen ions (H+) do not exist independently in solution, they form hydronium ions (H3O+)
• Arrhenius theory of acids and bases

  Acids dissociate in water to produce H+ ions, bases dissociate in water to produce OH- ions
• The Arrhenius theory is limited to aqueous solutions
• Brønsted-Lowry theory of acids and bases

  Acids are proton donors, bases are proton acceptors
• The Brønsted-Lowry theory is not limited to aqueous solutions
• Water can act as both an acid and a base (amphoteric/amphiprotic)
• Acid-base reaction in liquid ammonia

  1. HCl donates a proton to NH3

  2. HCl is the acid, NH3 is the base
• Ethanoic acid (acetic acid)

  CH3COOH, a weak acid found in vinegar
• Ethanoic acid dissociation

  CH3COOH acts as an acid, donating a proton, H2O acts as a base, accepting the proton
• Bases produce hydroxide ions in water but not all bases produce hydroxide ions, eg. there are no hydroxide ions in NH3, so it is impossible for NH3 to produce OH- ions
• Substances that would not be classified as acids or bases in the Arrhenius theory (e.g. NH3) can be classified as acids or bases in the Bronsted-Lowry theory, i.e. the Bronsted-Lowry definitions broaden the range of species that can be defined as acids and bases
• The Arrhenius theory does not take the evidence of hydronium ions into account but the Bronsted-Lowry theory does
• The Arrhenius theory cannot explain how a substance can act as both an acid and a base (as an amphoteric substance) but the Bronsted-Lowry theory explains this in terms of proton transfer
• The two theories (Arrhenius and Bronsted-Lowry) are not in conflict with each other. The Bronsted-Lowry theory adds to the Arrhenius theory and improves our understanding of reactions involving acids and bases
• Methyl orange

  Acts as a weak acid in aqueous solution
• Dissociation of methyl orange in aqueous solution

  1. Undissociated form (HX) is red

  2. Dissociated form (X-) is yellow
• Ethanoic acid (acetic acid)

  Acid found in vinegar
• Reaction of ethanoic acid with water

  CH3COOH + H2O ⇌ CH3COO- + H3O+
• CH3COOH donates a proton


  CH3COO- accepts a proton
• Conjugate acid-base pair

  Any pair consisting of an acid and a base that differ by one proton
• CH3COOH and CH3COO- are a conjugate acid-base pair
• H2O and H3O+ are a conjugate acid-base pair
• Neutralisation

  Reaction between an acid and a base to form a salt and water
• Examples of neutralisation in everyday life

  • Medicine
  • Agriculture
  • Environmental protection
  • Miscellaneous (toothpaste, baking soda, vinegar, shampoo)