8.1.2 Conjugate Acid-Base Pairs

• A Brønsted-Lowry acid is a species that can donate a proton
• A Brønsted-Lowry base is a species that can accept a proton
• In a reaction equilibrium the products are formed at the same rate as the reactants are used

CH₃COOH (aq) + H₂O (l)         ⇌    CH₃COO^- (aq)          +         H₃O⁺ (aq)

         acid                 base                    conjugate base             conjugate acid

• The reactant CH₃COOH is linked to the product CH₃COO^- by the transfer of a proton from the acid to the base
• Similarly, the H₂O molecule is linked to H₃O⁺ ion by the transfer of a proton
• These pairs are therefore called conjugate acid-base pairs
• A conjugate acid-base pair is two species that are different from each other by a H⁺ ion
  • Conjugate here means related
  • In other words, the acid and base are related to each other by one proton difference

• Species that can act both as proton donors and acceptors are called amphiprotic
  • Eg. water as a Brønsted-Lowry acid

The diagram shows water acting as a Brønsted-Lowry acid by donating a proton to ammonia which accepts the proton using its lone pair of electrons

The Lewis diagram for the reaction of water with ammonia to show how water acts as a Brønsted-Lowry acid and ammonia as a Brønsted-Lowry base

• Eg. water as a Brønsted-Lowry base

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

The Lewis diagram for the reaction of water with hydrochloric acid to show how water acts as a Brønsted-Lowry base and ammonia as a Brønsted-Lowry 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 aluminum 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 Table

Answer

The correct option in B

• • A conjugate acid-base pair differ only by an H⁺ ion