1. Electrochemistry Oxidation-Reduction
Unit 14: The Chemistry-Electricity Connection
Unit 14: Electrochem
LPChem: Wz

2. Chemistry and Electricity
Electrochemistry
Chemistry and Electricity
Galvanic Cell: a spontaneous chemical reaction that causes electric current to be generated.
Electrolytic Cell: a nonspontaneous chemical reaction caused by the application of electric current.
LPChem: Wz
Unit 14: Electrochemistry

3. An electrochemical reaction involves the transfer of electrons:
Electrochemistry
An electrochemical reaction involves the transfer of electrons:
2 Na + Cl2  2 NaCl
2 Na0  2 Na1+
Cl2 0  2 Cl1-
LPChem: Wz
+ 2 e-
oxidation
2 e- +
reduction
The electrons produced here…
…Are used up here.
Unit 14: Electrochemistry

4. An electrochemical reaction involves the transfer of electrons:
Electrochemistry
An electrochemical reaction involves the transfer of electrons:
2 Na0  2 Na1+
Cl2 0  2 Cl1-
LPChem: Wz
Loss of Electrons is Oxidation
+ 2 e-
2 e- +
Gain of Electrons is Reduction
Unit 14: Electrochemistry

5. An electrochemical reaction involves the transfer of electrons:
Electrochemistry
An electrochemical reaction involves the transfer of electrons:
2 Na0  2 Na1+
Cl2 0  2 Cl1-
LPChem: Wz
Oxidation Is
Loss
(of electrons)
+ 2 e-
2 e- +
Reduction Is
Gain
(of electrons)
Unit 14: Electrochemistry

6. Redox Reaction (a complete reaction)
Electrochemistry
An electrochemical reaction involves the transfer of electrons:
2 Na0  2 Na1+
Cl2 0  2 Cl1-
Redox Reaction (a complete reaction)
LPChem: Wz
+ 2 e-
oxidation (half-reaction) +
2 e- +
reduction (half-reaction)
2 Na0
+ 2 e-
+ Cl20 
2 NaCl
+ 2 e-
Unit 14: Electrochemistry

7. Elements in their “elemental state” have a oxidation # of zero.
Electrochemistry
Transfer of electrons:
Redox Reaction 
LPChem: Wz
Elements in their “elemental state” have a oxidation # of zero.
oxidation +1 -1
2 Na0
+ Cl20 
2 NaCl
reduction
Elements in compounds have oxidation #s based on periodic table location.
Unit 14: Electrochemistry

8. Ions have an oxidation number equal to their charge.
Electrochemistry
Transfer of electrons:
2 Na Cl1-  2 NaCl
Not Redox. 
Redox Reaction 
LPChem: Wz
oxidation +1 -1
2 Na0
+ Cl20 
2 NaCl
Ions have an oxidation number equal to their charge.
reduction
+1 = +1 +1 -1 +1 -1
-1 = -1
Unit 14: Electrochemistry

9. Electrochemistry Zn + CuSO4  ZnSO4 + Cu Zn (0  +2) Cu (+2  0)
What element was oxidized?
What element was reduced?
Which element LOST electrons?
Which element GAINED electrons? -2 -2 +2 +2
LPChem: Wz
Zn (0  +2)
Cu (+2  0)
Zn (0  +2)
Cu (+2  0)
Unit 14: Electrochemistry

10. How do I make this reaction into a battery?
Galvanic Cells:
Zn + CuSO4  ZnSO4 + Cu
How do I make this reaction into a battery?
Physically separate the half- reactions
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
LPChem: Wz
Unit 14: Electrochemistry

11. Physically separate the half- reactions Zn  Zn2+ + 2e-
Galvanic Cells:
Physically separate the half- reactions
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
LPChem: Wz
Unit 14: Electrochemistry

12. Galvanic Cells:
When the half-reactions are separate, electrons produced by the oxidation must travel through a wire to the reduction.
LPChem: Wz e- e- e- e-
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
Unit 14: Electrochemistry

13. Galvanic Cells:
The salt bridge completes the circuit by allowing ions to transfer.
This is necessary to equalize the charge (so the beakers don’t end up charged).
LPChem: Wz
KCl
Cl- K+
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
Unit 14: Electrochemistry

14. Galvanic Cells: Anode Cathode At the Anode is Oxidation
Galvanic Cells are typically drawn in alphabetical order:
(This makes the electrons move left to right through the wire.)
LPChem: Wz
Anode
Cathode
Bridge
At the Anode is
Oxidation
KCl
At the Cathode is Reduction
Cl- K+
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
Unit 14: Electrochemistry

15. Galvanic Cells: The anode (+) attracts anions.
The cathode (-) attracts cations.
LPChem: Wz
Anode
Cathode
Bridge
At the Anode is
Oxidation
KCl
At the Cathode is Reduction
Cl- K+
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
Unit 14: Electrochemistry

16. Galvanic Cells:
If you put something in the middle of the wire, the electrons go through it on their way from anode to cathode.
Electricity!
LPChem: Wz
Separating half-reactions makes galvanic reactions into batteries.
KCl
Now let’s calculate its voltage!
Cl- K+
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
Unit 14: Electrochemistry

17. Voltage Voltage is the “push” behind electric current.
It depends on the two half-reactions used:
Because every element has different electron affinity, the voltage is different for each element.
LPChem: Wz
Zn  Zn2+ + 2e-
2e- + Cu2+  Cu
Unit 14: Electrochemistry

18. Voltage Standard Reduction Potential
Voltage (AKA Electrochem. Potential) is found on a chart like this.
LPChem: Wz
The chart is all REDUCTION half-reactions.
Unit 14: Electrochemistry

19. Voltage Standard Reduction Potential
The REDUCTION half-reactions from our battery was:
Cu2+ + 2e-  Cu
LPChem: Wz
Unit 14: Electrochemistry

20. Voltage Standard Reduction Potential
The chart is in order of potential/
voltage.
Eo (V)
Elements at the top are most desperate to reduce (gain electrons).
LPChem: Wz
Reactions with the most positive voltages are the most spontaneous reductions.
Reactions with the least positive (most negative) voltages are least spontaneous reductions.
Hydrogen is zero.
Unit 14: Electrochemistry

21. Voltage Standard Reduction Potential
If charts are always of reduction potential, how do we find the voltage for our oxidation half-reaction?
LPChem: Wz
If oxidation is the opposite of reduction, then the opposite of the original chart should do the trick!
Unit 14: Electrochemistry

22. Voltage Standard Reduction Potential
The OXIDATION half-reactions from our battery was:
Zn  Zn2+ + 2e-
Its opposite would be:
Zn2+ + 2e-  Zn
LPChem: Wz
Unit 14: Electrochemistry

23. Voltage Standard Reduction Potential The reduction potential of:
Zn2+ + 2e-  Zn
= V
Its opposite
Zn  Zn2+ + 2e-
would be worth the opposite voltage:
= 0.76 V
LPChem: Wz
Unit 14: Electrochemistry

24. + Voltage The two half-reactions used: Reduction: Oxidation:
Added together:
Complete Reaction =
2e- + Cu2+  Cu
0.34 V
LPChem: Wz +
0.76 V
Zn  Zn2+ + 2e-
1.10 V
Unit 14: Electrochemistry

25. Voltage
The further apart the two half reactions are on the chart, the greater the voltage of the overall reaction.
Voltage is a potential– like gravitational potential energy.
Larger separation between the half-reactions means the electrons have further to “fall.” More potential!
LPChem: Wz
Unit 14: Electrochemistry

26. Voltage
The anode (oxidation) half reaction will always need to be “flipped” because the chart is for reductions.
That means changing the sign on its voltage.
LPChem: Wz
Unit 14: Electrochemistry

27. Voltage
If the anode (oxidation) half-reaction were above the cathode half-reaction on the chart, what would happen??
Things don’t fall “up.”
No reaction would take place. Unless…
…external current were applied. (Electrolysis.)
LPChem: Wz
Unit 14: Electrochemistry