Polyprotic Acids

Polyprotic Acids Overview

• Definition: Polyprotic acids are acids that can donate more than one proton (H+) per molecule in a solution.

Ionization Stages

• First Ionization: Represents the initial dissociation of the acid in water.
• Subsequent Ionizations: Each further dissociation involves the transfer of additional protons and typically has a progressively smaller ionization constant (Ka).

Examples of Polyprotic Acids

1. Sulfuric Acid (H2SO4)

   • First Ionization:
     • H2SO4 + H2O → H3O+ + HSO4− (Strong Acid)
   • Second Ionization:
     • HSO4− + H2O ⇌ H3O+ + SO4²−
     • Ka2 ≈ 1.2 × 10⁻²

2. Carbonic Acid (H2CO3)

   • First Ionization:
     • H2CO3 + H2O ⇌ H3O+ + HCO3−
     • Ka1 ≈ 4.3 × 10⁻⁷
   • Second Ionization:
     • HCO3− + H2O ⇌ H3O+ + CO3²−
     • Ka2 ≈ 4.7 × 10⁻¹¹

3. Phosphoric Acid (H3PO4)

   • First Ionization:
     • H3PO4 + H2O ⇌ H3O+ + H2PO4−
     • Ka1 ≈ 7.5 × 10⁻³
   • Second Ionization:
     • H2PO4− + H2O ⇌ H3O+ + HPO4²−
     • Ka2 ≈ 6.2 × 10⁻⁸
   • Third Ionization:
     • HPO4²− + H2O ⇌ H3O+ + PO4³−
     • Ka3 ≈ 4.2 × 10⁻¹³

Uniqueness of Polyprotic Acids

• Each successive ionization generally has a weaker tendency to donate protons compared to the previous ionization, indicated by decreasing values of Ka.
• This behavior illustrates the dynamic nature of acid-base equilibria.

Applications in Chemistry

• Polyprotic acids play significant roles in buffering systems, metabolic processes, and various industrial applications.

Conclusion

• Understanding polyprotic acids is crucial for predicting the behavior of acids in various chemical and biological reactions. Their ability to donate multiple protons makes them unique and important in chemistry.