Polyprotic Acids
Chemistry: IB Diploma Higher Level, topic 6.9.8
Polyprotic Acids
Acids are classified by the number of hydronium ions per molecule that they can give up in a reaction. Polyprotic acids give up more than one hydronium ion.
Monoprotic acids
- Acids such as HCl, HNO3, and HCN that contain one ionizable hydrogen atom in each molecule are called monoprotic acids.
- Their reactions with water are:
- HCl(aq) + H2O(l) ⟶ H3O+(aq) + Cl−(aq)
- HNO3(aq) + H2O(l) ⟶ H3O+(aq) + NO3-(aq)
- HCN(aq) + H2O(l) ⇌ H3O+(aq) + CN-(aq)
- Even though it contains four hydrogen atoms, acetic acid, CH3CO2H, is also monoprotic because only the hydrogen atom from the carboxyl group (COOH) reacts with bases.
Diprotic acids
- Diprotic acids contain two ionizable hydrogen atoms per molecule. Ionization of such acids occurs in two steps.
- The first ionization always takes place to a greater extent than the second ionization.
- For example, sulfuric acid, a strong acid, ionizes as follows:
- First ionization:
- H2SO4(aq) + H2O(l) ⇌ H3O+(aq) + HSO4-(aq) 𝐾a1 = more than 102;
- Second ionization:
- HSO4-(aq) + H2O(l) ⇌ H3O+(aq) + SO42- (aq) 𝐾a2 = 1.2 × 10-2
Titration curves
- The stepwise ionization of diprotic acids can be seen on titration curves as there will be two equivalence points.
- Using the titration of H2SO4 with LiOH as an example:
- At the first equivalence point, all the H3O+ ions from the first ionization have reacted with OH- ions from LiOH.
- By the second one, all H3O+ ions have reacted with the OH- ions from LiOH and been neutralized.
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