THREE DEFINITIONS

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THREE DEFINITIONS

• Lewis (broadest definition)
• Bronsted-Lowry
• Arrhenius (most specific definition)

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1. Arrhenius Acids/Bases

• Acids substances that can donate protons (H
  ions)
• Bases substances that can donate hydroxide ions
  (OH- ions).

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Example of Arrhenius Acids/Bases

• HCl H2O ? H3O Cl-
• HCl donates proton therefore an acid
• NaOH H2O ? Na OH-
• NaOH donates an OH- therefore a base

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How Many Hs Can an Acid Relinquish?

• Substance must be in aqueous form.
• It depends. For water to be able to ionize a H,
  the H must be part of a polar bond (bonded to a
  very electronegative element)

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How Many Hs Can an Acid Relinquish?

• HCl(g) H2O(l) ? H3O(aq) Cl-(aq)
• HCl is monoprotic an acid that contains only
  one ionizable hydrogen
• CH4 not acidic Hs arent in polar bonds
• Acetic (Ethanoic) acid (CH3COOH)
• H O
• HCCOH
• H

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How Many Hs Can an Acid Relinquish?

• Diprotic acid any acid that contains two
  ionizable protons
• (H2SO4 Sulfuric Acid)
• Triprotic acid any acid that contains three
  ionizable protons.
• (H3PO4 Phosphoric Acid)

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Broader Definition Needed

• Not all substances that behave like bases
  contained OH- ions.
• EXAMPLES
• Compounds containing carbonate ions (CO32-) often
  act like bases (i.e. Na2CO3)
• NH3 is a common base with no OH- present

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2. Bronsted-Lowry Acid/Bases

• Acid proton (H) donor
• Base proton (H) acceptor

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Conjugate Acid-Base Pairs

• NH3 H2S ? NH4 HS
• Base acid ? acid base
• Notice above that what serves as the acid in the
  forward reaction (the hydrogen donator) acts as a
  base in the reverse reaction (hydrogen acceptor).
  
• HS is the conjugate base for the acid H2S.
• NH4 is the conjugate acid for the base NH3.
• Both of these pairs are considered conjugate
  acid-base pairs two substances that are related
  by the loss or gain of a single hydrogen ion.

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Two-Face Substances . . .

• Some substances can act as both an acid and as a
  base. Such substances are termed amphoteric.
  Water is an example
• NH3(aq) H2O(l) ? NH4(aq) OH-(aq)
• HCl(g) H2O(l) ? H3O(aq) Cl-(aq)

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3. Lewis Acid/Bases

• Acid substance that can accept a pair of
  electrons to form a covalent bond
• Base substance that can donate a pair of
  electrons to form a covalent bond

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3. Lewis Acid/Bases

• Useful when describing coordination compounds
  i.e. compounds with central metal atom or ion
  with several identical groups attached to it.
  Metal is acid, groups attached are base.
• EXAMPLE
• NH3 (the base) attached to Ag, Pt2, Zn2, and
  Co3 (the acids)
• H OH- ? H2O Lewis Acid Lewis Base
• Lewis definition fits, but this combination of
  compounds not normally considered in acid base
  chemistry, so we will concentrate on the
  Bronsted-Lowry definition.