Amphiprotic substances play a crucial role in acid-base chemistry, demonstrating unique properties that allow them to act as both acids and bases. This dual nature makes them particularly interesting in chemical reactions and biological systems.
What Are Amphiprotic Substances?
An amphiprotic substance (also known as an ampholyte) is a chemical species that can both donate and accept protons (H⁺), depending on the reaction conditions. This behavior is fundamental to understanding intermediate species in acid-base reactions.
Key Characteristics
The behavior of an amphiprotic substance depends on what it reacts with:
When reacting with strong acids, it acts as a base (proton acceptor)
When reacting with strong bases, it acts as an acid (proton donor)
Common Examples and Their Reactions
Hydrogen Carbonate Ion (HCO₃⁻)
The hydrogen carbonate ion is a classic example of an amphiprotic substance. It participates in two key equilibria:
Acting as a base (with acids):
HCO3−(aq)+H3O+(aq)→H2CO3(aq)+H2O(l)
Acting as an acid (with bases):
HCO3−(aq)+OH−(aq)→CO32−(aq)+H2O(l)
Determining Acidic or Basic Behavior
The behavior of amphiprotic substances depends on their relative acid (Ka) and base (Kb) dissociation constants:
When Ka > Kb: The solution becomes acidic
When Kb > Ka: The solution becomes basic
For hydrogen carbonate in water:
HCO3−(aq)+H2O(l)⇌CO32−(aq)+H3O+(aq)Ka=5.0×10−11HCO3−(aq)+H2O(l)⇌H2CO3(aq)+OH−(aq)Kb=2.0×10−8
Water as an Amphiprotic Substance
Water itself is amphiprotic, participating in self-ionization:
H2O(l)+H2O(l)⇌H3O+(aq)+OH−(aq)
The ionic product of water (Kw) at 25°C is:
Kw=[H3O+][OH−]=1.0×10−14