Oxidation Numbers

1
Oxidation Numbers

• Elements have an oxidation number of zero.
• Monatomic ions have an oxidation number the same
  as the charge.
• Hydrogen is 1 and oxygen is -2.
• The sum of the oxidation numbers in a compound is
  zero.
• The sum of the oxidation numbers in a polyatomic
  ion is the charge on the ion.

2
Definitions

• Oxidation
• Loss of electrons
• Increase in oxidation number
• Reduction
• Gain of electrons
• Decrease in oxidation number

3

• Cu2 Zn(s) ? Cu(s) Zn2
• Cu2 gains 2 electrons reduced
• Zn(s) loses 2 electrons oxidized
• LEO GER

4

• Cu2 is reduced and is the OXIDIZING agent
• Zn(s) is oxidized and is the REDUCING agent

5
Na Cl2 ? NaCl
6
Balancing Redox Equations

• Identify reduction and oxidation half reactions
  (Write these out).
• Multiply equations so that the number of
  electrons transferred is equal.
• Balance in acidic solution by adding H to the
  side with the electrons and H2O to the other.
• For basic solution, add OH- to both sides of the
  equation (same number as H).
• H OH- ? H2O (Cancel water)

7
Cr NO3- ? Cr3 NO
8
Cr NO3- ? Cr3 NO

• Acidic
• Cr 4H NO3 ? Cr3 NO 2H2O
• Basic
• Cr 2H2O NO3 ? Cr3 NO 4OH-

9
Al MnO4- ? Al3 Mn2
10
Al MnO4- ? Al3 Mn2

• Acidic
• 5Al 3MnO4- 24H
• ? 3Mn2 5Al3 12H2O
• Basic
• 5Al 3MnO4- 12H2O
• ? 3Mn2 5Al3 24OH-

11
Oxidation-Reduction Reactions

• Elements have different abilities to gain or lose
  electrons.
• We can harness this energy by putting two
  elements in a system and collecting the electrons
  as they move in the favored direction.
  (Batteries)

12
Activity (Electromotive) Series

• Elements at top like to lose electrons
• Electrons will flow to the less active metal ion
  producing an electric current
• Elements above hydrogen react with acids to
  generate hydrogen gas
• Elements below hydrogen do not react in the same
  way
• (see web site for activity series)