Understanding Acid-Base Theories: Arrhenius, Bronsted-Lowry, and Lewis Definitions
This chapter explores the three primary theories for defining acids and bases: Arrhenius, Bronsted-Lowry, and Lewis. The Arrhenius theory defines acids as substances that release hydrogen ions (H+) and bases as those that release hydroxide ions (OH-). The Bronsted-Lowry theory broadens this by defining acids as proton donors and bases as proton acceptors, introducing the concept of conjugate acid-base pairs. Finally, the Lewis theory shifts the focus to electron transfer, defining Lewis acids as electron pair acceptors and Lewis bases as electron pair donors.
Understanding Acid-Base Theories: Arrhenius, Bronsted-Lowry, and Lewis Definitions
E N D
Presentation Transcript
Chapter 19 Notes: Part II Acid/Base Theories
There are three ways to define acids and bases. • This reflects the fact that science is always revising itself.
1) Arrhenius Acids/Bases • An acid is a chemical that gives off hydrogen ions in solution. • A base is a chemical that gives off hydroxide ions in solution. Example: HCl + NaOH NaCl + H2O **BASICALLY…acids start with H+ and bases end with OH-!!!
Arrhenius Acids/Bases There are three ways to describe an acid: • Monoprotic-gives off one H+ in sol’n • 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 Bronstedand Thomas Lowryproposed a new idea.
2) Bronsted-Lowry Acids/Bases • An acid is a hydrogen-ion (proton) donor. • A base is a hydrogen-ion (proton) acceptor.
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 donates 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 NH4+ + OH- proton donator • Which is the initial acid? • Which is the initial base? • What is the conjugate acid? • What is the conjugate base? H2O NH3 NH4+ OH- proton acceptor formed after base gains H+ formed after acid donates H+
HCl + H2O H3O+ + Cl- • Which is the initial acid? • Which is the initial base? • What is the conjugate acid? • What is the conjugate base? HCl H2O H3O+ Cl-
Amphoteric Substances • Note that in one of the previous examples H2O was acidic and basic in the other. • A substance that can act as an acid or a base is called amphoteric.
3) Lewis Acids/Bases • A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond. • A Lewis base is a substance that can donate a pair of electrons to a covalent bond.
3 7 5 1 H x x N x x H x F B F x x x x H N H x x x x x x x x x x x H x F x x H x x x x x x F B x x F x x x x x x x x x x x F x x x x x x **MUST draw Lewis dot structures to determine whether a compound will gain or lose e-!!! • Ex… BF3 + NH3 F3BNH3 ACID (not happy) accept e- pair BASE (happy) donate e- pair
