Amphiprotic Species: Acting as Both Acids and Bases
Definition
Amphiprotic species
An amphiprotic species is a chemical species that can either donate a proton (acting as an acid) or accept a proton (acting as a base), depending on the reaction conditions.
This dual behavior enables amphiprotic species to play critical roles in chemical and biological systems.Example
Common Examples of Amphiprotic Species
- Water (H₂O)
- Hydrogencarbonate ion (HCO₃⁻)
- Hydrogen sulfate ion (HSO₄⁻)
- Amphiprotic species are always involved in proton transfer reactions.
- Look for the presence of H⁺ to identify their potential roles!
Water: The Quintessential Amphiprotic Species
- Water (H₂O) is the most well-known amphiprotic species.
- It can act as both an acid and a base, depending on its reaction partner.
Water Acting as an Acid
- When water reacts with a base, it donates a proton (H⁺), acting as an acid.
- For example, in its reaction with ammonia (NH₃):$$
\text{H₂O} + \text{NH₃} \rightarrow \text{NH₄⁺} + \text{OH⁻}
$$ - Here, water donates a proton to ammonia, forming the ammonium ion (NH₄⁺) and hydroxide ion (OH⁻).
Water Acting as a Base
- When water reacts with an acid, it accepts a proton (H⁺), acting as a base.
- For example, in its reaction with hydrogen chloride (HCl):
$$
\text{H₂O} + \text{HCl} \rightarrow \text{H₃O⁺} + \text{Cl⁻}
$$ - Here, water accepts a proton from HCl, forming the hydronium ion (H₃O⁺) and chloride ion (Cl⁻).
- Water’s role depends on the strength of the other reactant.
- With a strong acid, water acts as a base.
- With a strong base, water acts as an acid.
Hydrogencarbonate Ion (HCO₃⁻): A Buffering Superstar
- The hydrogencarbonate ion (HCO₃⁻) is vital in biological buffer systems, such as the one that maintains the pH of human blood.
- Its ability to act as both an acid and a base makes it a key player in maintaining pH balance.
Acting as an Acid
- When HCO₃⁻ reacts with a base, it donates a proton:
$$
\text{HCO₃⁻} \rightarrow \text{CO₃²⁻} + \text{H⁺}
$$
