Oxidation Reduction Reactions

1
Oxidation Reduction Reactions
2
Re-dox Reactions

• Oxidation is the loss of electrons
• Reduction is a gain of electrons
• Leo goes Grr!
• Lose Electrons Oxidation
• Gain electrons Reduce

3
Redox Reactions

• Many reactions involve (or seem to involve) a
  transfer of electrons.
• Such reactions always involve both
• Oxidation ? Loss of electrons
• Na ? Na e-
• Reduction ? Gain of electrons
• Cl e- ? Cl-

4
Determine a Redox Reaction

• To determine if what you have is a redox
  reaction, first rewrite the equation to include
  the oxidation number for each element
• The oxidation number is the charge an element has
  or appears to have when combining to form
  compounds

5
Oxidizing/Reducing Agents

1. The element that was oxidized is part of the
   reducing agent.
2. The element that was reduced is part of the
   oxidizing agent.

6
Assigning Oxidation Numbers

1. All pure elements and homogeneous molecules 0
2. Elements in group IA 1
3. Elements in group IIA 2
4. Ag, Zn2, Al3 (unless it is a pure metal)
5. In binary compounds the second element anion
   charge
6. Oxygen is almost always -2 unless it is
   O2(g)
7. Hydrogen is almost always 1 unless it is
   H2(g)
8. The total charge of a compound is always 0

7
A Sample Problem

• What are the oxidation numbers of the elements
  in Na2SO4?

Na 1 (times 2 atoms) 2 O -2 (times 4
atoms) -8 2 -8 -6 If the
compound must 0, then S must
6
8
Give the oxidation number for

1. S in Na2SO3
2. Mn in KMnO4
3. N in Ca(NO3)2
4. C in Na2CO3
5. N in NO2-
6. S in SO4-2
7. S in H2S2O7
8. Fe in Fe(C2H3O2)2

4 7 5 4 3 6 6 2
9
Ammonia plus Oxygen

• NH3 O2 ? NO H2O
• Is this equation balanced?
• No, it does not matter
• Step 1 Rewrite the equation with the charges
• N3-H3 O02 ? N2O2- H2O2-

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Ammonia plus Oxygen

• Step 2 Determine the changes in the charges of
  each element
• N3-H3 O02 ? N2O2- H2O2-
• Nitrogen changes from 3- to 2
• it lost an electron and was oxidized
• Oxygen changes from 0 to 2-
• it gains electrons and so is reduced

11
Last Step

• Step 3 If there is a species oxidized and one
  reduced, then it is a redox equation