Chapter 19 Notes, Part II Acid/Base Theories

Three Definitions There are three ways to define acids and bases. This reflects the fact that science is always revising itself.

Arrhenius Acids/Bases This is the definition of acids and bases we saw earlier. An acid is a chemical that gives off hydrogen ions in solution. A base is a chemical that gives off hydroxide ions in solution.

Mono-, Di- and Triprotic Three ways to describe an acid: Monoprotic-gives off one H+ in solution Diprotic-gives off two H+ Triprotic-gives off three H+

Bronsted-Lowry Acids/Bases The Arrhenius definition is a good one, but does not encompass everything that shows acidic/basic qualities. To account for this, Johannes Bronsted and Thomas Lowry proposed a new idea.

Bronsted-Lowry Acids/Bases An acid is a hydrogen-ion (proton) donor. A base is a hydrogen-ion (proton) acceptor. A classic example is ammonia (NH3).

Conjugate Acids/Bases A conjugate acid is the particle that is formed when a base gains a hydrogen ion. A conjugate base is the particle that is formed when an acid had donated a hydrogen ion.

Conjugate Acids/Bases A conjugate acid/base pair consist of two substances related by the loss of a single hydrogen ion.

NH3 + H2O D NH4+ + OH- Which is the initial acid? Which is the base? What is the conjugate acid? What is the conjugate base?

HCl + H2O D H3O+ + Cl- Which is the initial acid? Which is the base? What is the conjugate acid? What is the conjugate base?

Amphoteric Substances Note that in one of the previous examples water was acidic and basic in the other. A substance that can act as an acid or a base is called amphoteric.

Lewis Acids/Bases Lewis came up with a third definition of acids and bases focusing on electrons. A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond.

Lewis Acids/Bases A Lewis base is a substance that can donate a pair of electrons to a covalent bond.