R3.1.3 Amphiprotic species

Amphiprotic Species: Acting as Both Acids and Bases

What Does It Mean to Be Amphiprotic?

To understand amphiprotic species, let’s revisit the Brønsted–Lowry definition of acids and bases:

• Acids are proton (H⁺) donors.
• Bases are proton (H⁺) acceptors.

This dual behavior enables amphiprotic species to play critical roles in chemical and biological systems.

Water: The Quintessential Amphiprotic Species

1. Water (H₂O) is the most well-known amphiprotic species.
2. 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):$$\text{H₂O}+\text{NH₃}\to \text{NH₄⁺}+\text{OH⁻}$$
• 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):
  $$\text{H₂O}+\text{HCl}\to \text{H₃O⁺}+\text{Cl⁻}$$
• Here, water accepts a proton from HCl, forming the hydronium ion (H₃O⁺) and chloride ion (Cl⁻).

Hydrogencarbonate Ion (HCO₃⁻): A Buffering Superstar

1. The hydrogencarbonate ion (HCO₃⁻) is vital in biological buffer systems, such as the one that maintains the pH of human blood.
2. 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:
  $$\text{HCO₃⁻}\to \text{CO₃²⁻}+\text{H⁺}$$
• Here, HCO₃⁻ donates a proton to form the carbonate ion (CO₃²⁻).

Acting as a Base

• When HCO₃⁻ reacts with an acid, it accepts a proton:
  $$\text{HCO₃⁻}+\text{H⁺}\to \text{H₂CO₃}$$
• Here, HCO₃⁻ accepts a proton to form carbonic acid (H₂CO₃).

Hydrogen Sulfate Ion (HSO₄⁻): A Versatile Ion

1. The hydrogen sulfate ion (HSO₄⁻) is another amphiprotic species.
2. Its dual behavior makes it a key player in acid–base reactions.

Acting as an Acid

• When HSO₄⁻ reacts with a base, it donates a proton:
  $$\text{HSO₄⁻}\to \text{SO₄²⁻}+\text{H⁺}$$
• Here, HSO₄⁻ donates a proton to form the sulfate ion (SO₄²⁻).

Acting as a Base

• When HSO₄⁻ reacts with an acid, it accepts a proton:
  $$\text{HSO₄⁻}+\text{H⁺}\to \text{H₂SO₄}$$
• Here, HSO₄⁻ accepts a proton to form sulfuric acid (H₂SO₄).

Writing Equations for Amphiprotic Behavior

To demonstrate the amphiprotic nature of a species, follow these steps:

1. Identify the species and determine whether it can donate or accept a proton.
2. Write the equation for its reaction as an acid (proton donor).
3. Write the equation for its reaction as a base (proton acceptor).

Reflection