Amphiprotic Species (DP IB Chemistry): Revision Note

Amphiprotic Species

• Species that can act both as proton donors and acceptors are called amphiprotic

Water as a Brønsted-Lowry acid

Diagram to show how water is amphiprotic

Lewis diagram for the reaction between water and ammonia

Water as a Brønsted-Lowry base

Diagram to show how water is amphiprotic

The diagram shows water acting as a Brønsted-Lowry base by accepting a proton from hydrochloric acid proton using its lone pair of electrons

Lewis diagram for the reaction between water and hydrochloric acid

What is the difference between amphiprotic and amphoteric?

• A compound that is amphoteric means it has both basic and acidic character

  • When the compound reacts with an acid, it shows that it has basic character

  • When it reacts with a base, it shows that it's acidic

  • An example of this is aluminium oxide which reacts with both hydrochloric acid and sodium hydroxide:

Al₂O₃ (s) + 6HCl (aq) → 2AlCl₃ (aq) + 3H₂O (l)

Al₂O₃ (s) + 2NaOH (aq) + 3H₂O (l) → 2NaAl(OH)₄ (aq)       

• When a compound is amphiprotic, it means it can act as a proton donor and as a proton acceptor

• Aluminium oxide is not amphiprotic, even though it is amphoteric

Amphiprotic versus Amphoteric Summary

Amphiprotic:

• Can both accept and donate a proton (H⁺)

• Amphiprotic substances act as both a proton donor and acceptor

• All amphiprotic substances are also amphoteric

Amphoteric:

• Can act as both an acid and a base

• Amphoteric substances can behave as acids or bases in a chemical reaction

• Not all amphoteric substances are amphiprotic