Defining Oxidation and Reduction

1
Defining Oxidation and Reduction

• Oxidation is the loss of electrons
• Reduction is the gain of electrons
• Ex. Consider the following reaction
• Zn(s) CuSO4(aq) ? Cu(s) ZnSO4(aq)
• The total ionic equation is
• Zn(s) Cu2 (aq) SO42-(aq) ? Cu(s) Zn2 (aq)
  SO42-(aq)
• The net ionic equation is
• Zn(s) Cu2 (aq) ? Cu(s) Zn2 (aq)

2
Defining Oxidation and Reduction
gains electrons

• Zn(s) Cu2 (aq) ? Cu(s) Zn2 (aq)
• In this reaction
• Zn atoms lose electrons ? oxidized ? zinc ions
• Cu ions gain electrons ? reduced ? Cu atoms
• In this case oxidation and reduction happen in
  the same reaction so the reaction is called an
  oxidation-reduction reaction or redox reaction.

loses electrons
3
Oxidizing and Reducing Agents

• Oxidizing Agents a reactant that oxidizes
  another reactant, accepts electrons (causes the
  other reactant to lose electrons)
• Reducing Agents a reactant that reduces another
  reactant, donates electrons (causes the other
  reactant to gain electrons)
• Zn(s) Cu2 (aq) ? Cu(s) Zn2 (aq)
• Zn is the reducing agent ? donates electrons ?
  undergoes oxidation
• Cu2 is the oxidizing agent ? accepts electrons ?
  undergoes reduction

4
Practice Problems

• Pg. 653 2.

5
Half-Reactions

• To show the transfer of electrons in a chemical
  reaction we often write the oxidation and
  reduction separately.
• Half-reaction a balanced chemical reaction that
  shows the number of electrons involved in either
  oxidation or reduction
• Both half-reactions are required to represent a
  redox reaction.

6
Representing Half-Reactions

• For the reaction of Zn with copper(II)sulfate
• The net ionic equation is
• Zn(s) Cu2(aq) ? Cu(s) Zn2(aq)
• The oxidation half-reaction is
• Zn(s) ? Zn2 (aq) 2e-
• The reduction half-reaction is
• Cu2(aq) 2e- ? Cu(s)
• In half-reactions, the charges and atoms are
  balanced using coefficients

7
Half Reactions Continued

• Half-reactions always come in pairs
• 1 oxidation half-reaction
• 1 reduction half-reaction
• Why? We need both an electron donor and an
  electron acceptor

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Disproportionation Reactions

• Occur when a single element undergoes both
  oxidation and reduction in the same reaction
• Some of the reactant molecules are oxidized, some
  are reduced
• Ex. Disproportionation of a copper(I) solution
• 2Cu(aq) ? Cu(s) Cu2(aq)
• Oxidation half-reaction Cu(aq) ? Cu2(aq) 1e-
• Reduction half-reaction Cu (aq) 1e- ? Cu(s)

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Practice Problems/ Section Review

• Practice Problems
• Complete Pg. 656 8, 9