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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. Arrhenius acids produce hydrogen ions in a solution, while bases provide hydroxide ions. In contrast, the Bronsted-Lowry theory introduces the concepts of proton donors and acceptors, expanding our understanding of acidity and basicity. Finally, the Lewis theory focuses on electron pairs, defining acids as electron acceptors and bases as electron donors. This evolution in acid-base definitions showcases the dynamic nature of scientific inquiry.

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Understanding Acid-Base Theories: Arrhenius, Bronsted-Lowry, and Lewis Definitions

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  1. Chapter 19 Notes: Part II Acid/Base Theories

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

  3. 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-!!!

  4. 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+

  5. 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.

  6. 2) Bronsted-Lowry Acids/Bases • An acid is a hydrogen-ion (proton) donor. • A base is a hydrogen-ion (proton) acceptor.

  7. 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.

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

  9. 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+

  10. 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-

  11. 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.

  12. 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.

  13. 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

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