Chapter 20 Notes, Part II

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Chapter 20 Notes, Part II

• Acid/Base Theories

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Three Definitions

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

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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.

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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

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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.

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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).

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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.

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Conjugate Acids/Bases

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

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NH3 H2O D NH4 OH-

• Which is the initial acid?
• Which is the base?
• What is the conjugate acid?
• What is the conjugate base?

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HCl H2O D H3O Cl-

• Which is the initial acid?
• Which is the base?
• What is the conjugate acid?
• What is the conjugate base?

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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.

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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.

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Lewis Acids/Bases

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