Reactivity 3.1.3 – Amphiprotic Species

An amphiprotic species is a substance that can act as both a Brønsted–Lowry acid (proton donor) and a Brønsted–Lowry base (proton acceptor).

This dual behavior depends on the reaction conditions and the other species present in the system.

Key Characteristics

• They must contain at least one hydrogen atom that can be donated as \( \text{H}^+ \) and also a lone pair to accept a proton.
• Their role in a reaction (acid or base) is determined by the nature of the other reactant (whether it is a stronger acid or base).

Common Amphiprotic Species

• \( \text{H}_2\text{O} \): can donate \( \text{H}^+ \) to become OH⁻, or accept \( \text{H}^+ \) to become \( \text{H}_3\text{O}^+ \)
• \( \text{HSO}_4^- \): can donate a proton to form \( \text{SO}_4^{2-} \), or accept a proton to form \( \text{H}_2\text{SO}_4 \)
• \( \text{HCO}_3^- \): can donate \( \text{H}^+ \) to form \( \text{CO}_3^{2-} \), or accept a proton to form \( \text{H}_2\text{CO}_3 \)

Water acting as a Brønsted–Lowry acid:

• \( \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{OH}^- + \text{NH}_4^+ \)
• Water donates a proton (H⁺) to ammonia → acts as an acid.

Water acting as a Brønsted–Lowry base:

• \( \text{H}_2\text{O} + \text{HCl} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^- \)
• Water accepts a proton (H⁺) from hydrochloric acid → acts as a base.

Amphiprotic Behavior in Equilibrium

• In acid–base equilibria, amphiprotic species can appear on both sides of the equation, forming conjugate acid–base pairs with different reactants.
• Their ability to shift between roles makes them essential in buffer systems and many biological processes.

Interpreting and Formulating Equations with Amphiprotic Species

• To demonstrate the amphiprotic nature of a species, write two equations:
• One where it acts as a Brønsted–Lowry acid (donates a proton).
• One where it acts as a Brønsted–Lowry base (accepts a proton).
• 

Step-by-Step Process

• Identify the amphiprotic species (e.g., \( \text{HCO}_3^- \), \( \text{H}_2\text{O} \)).
• Choose one species it could donate a proton to (weaker base), and one it could accept a proton from (stronger acid).
• Write both reactions and identify the conjugate pairs.

Difference between Amphiprotic and Amphoteric:

• Amphiprotic: A species is amphiprotic if it can both donate a proton (act as an acid) and accept a proton (act as a base) in different reactions. This is strictly within the Brønsted–Lowry theory of acids and bases.

Examples:

• • \( \text{H}_2\text{O} \): can become \( \text{OH}^- \) (acid) or \( \text{H}_3\text{O}^+ \) (base)
  • \( \text{HCO}_3^- \): can become \( \text{CO}_3^{2-} \) or \( \text{H}_2\text{CO}_3 \)

• Amphoteric: A species is amphoteric if it can act as either an acid or a base, but this definition is broader — it includes reactions outside proton transfer, such as Lewis acid–base behavior.

 Examples:

• • Metal oxides like \( \text{Al}_2\text{O}_3 \): react with both acids and bases but do not involve proton transfer
  • \( \text{Zn(OH)}_2 \): dissolves in both acidic and basic solutions

Note:

All amphiprotic species are amphoteric (because proton transfer is one kind of acid–base behavior), but not all amphoteric species are amphiprotic (since some do not involve H⁺).