Oxidation and Reduction

1
Oxidation and Reduction

• Chapters 20 21

2
Oxidation vs Reduction

• Oxidation A substance loses electrons
• Reduction A substance gains electrons
• 2Al(s)0 3CuCl2(aq) ? 2AlCl3(aq) 3Cu(s)0
• Al(s)0 ? Al(aq)3 aluminum oxno increases
• Cu(aq)2 ? Cu(s)0 copper oxno decreases

3
Whats really happening

• 2Al(s)0 3CuCl2(aq) ? 2AlCl3(aq) 3Cu(s)0
• 2Al(s)0 ? 2Al(aq)3 6e- Al oxno increases
• 3Cu(aq)2 6e- ? Cu(s)0 Cu oxno decreases
• These are called half reactions. Notice the
  number of electrons lost is the same as the
  number gained.

4
What????

• Oxidation increase in oxygen atoms, increase in
  oxno, loss in electrons
• Reduction loss of oxygen atoms, decrease in
  oxno, gain in electrons
• Remember
• L.E.O. says G.E.R.
• Loss of electrons oxidation
• Gain electrons reduction

5
Agents

• The oxidizing agent causes oxidation of another
  substance. Example copper
• Cu(aq)2 ? Cu(s)0
• The reducing agent causes reduction of another
  substance. Example aluminum
• Al(s)0 ? Al(aq)3

6
Activity Series of Metals (p. 668)
7
When the the reaction happens, electrons move
from Al to Cu

• 2Al(s)0 ? 2Al(aq)3 6e-
• 3Cu(aq)2 6e- ? Cu(s)0
• This electron flow can be measured as voltage!
• We will see how later.

8
Types of Redox Reactions

• Direct Combination
• S O2 ? SO2
• Decomposition
• HgO ? 2Hg O2
• Single Replacement
• Cu(s) 2AgNO3(aq) ? Cu(NO3)2(aq) 2Ag(s)
• Cu(s) 2Ag(aq) ? Cu(aq) 2Ag(s) (net ionic)
• But Cu(s) ZnCl2(aq) ? NR
• Cu(s) Zn2(aq) ? No reaction (due to relative
  reactivity rank)

9
Balancing Redox Equations

• Some equations are are difficult to balance by
  inspection or trial and error that worked up
  until now.
• The fundamental principle is that the number of
  electrons lost in the oxidation process must
  equal the number of electrons gained in the
  reduction process.

10
Electrochemical cells

• Use redox reactions to either produce or use
  electricity.

11
Voltaic Cells

• In late 1700s Italian physician Luigi Galvani
  twitched frog legs by connecting two metals.
  Italian scientist Alessandro Volta
  concluded the two metals in the presence of water
  produce electricity.

12
Voltaic Cells

• Zn(s) CuSO4(aq) ? ZnSO4(aq) Cu(s)
• Zn(s) ? Zn2(aq) 2e- oxidation
• Cu2(aq) 2e- ? Cu(s) reduction
• Half Cell- Zn (anode)
• Pushes e- to Cu
• (cathode)

13
Voltaic Cell

• Electrons move spontaneously from the anode (-)
  to the cathode ()
• The salt bridge allows
• Electrons to move freely
• Without mixing solutions.

14
Cell Potential

• Ability to move e- through a wire from one
  electrode to another is the electrical or cell
  potential. It is measured in volts (v)
• For Example A Zn-Cu cell with 1 M solutions
  produces 1.10 volts.
• Here is how it works

15
Standard Reduction Potentials (p. 693)
16
Standard Electrode Potentials

• Ecell Eoxidation Ereduction
• Ecell0 sum of the oxidation potential (Eox0)
  plus reduction potential (Ered0)
• The standard state conditions are noted with the
  0.
• E0 are determined by measuring half cell
  potential differences.
• Zn(s) ? Zn2(aq) 2e- E0 ox 0.76 V
• Zn2(aq) 2e- ? Zn(s) E0 red - 0.76 V

17
Calculating Cell Potentials

• Zn(s) ? Zn2(aq) 2e- E0 ox 0.76 V
• Cu2(aq) 2e- ? Cu(s) E0 red 0.34 V
• Total Voltage (Ecell) 1.10 Volts
• Practice Problems 1 and 2 on P. 696.

18
Thats it for this

• Electrifying lecture!