1. Redox in Action: Voltaic cells
How is a battery
Like a waterfall?
Electrons are pulled “downhill” through a wire creating electric current
Water is
pulled down
by gravity

2. Voltaic Cells
Section A

3. Redox in Action: Voltaic cells
How is a battery
Like a waterfall?
Pairing two different metals will always create an electron flow. Lets see how it works!
In this electrochemical cell, copper ions (right side)
pull electrons away from the zinc atoms(left side)

4. Redox in Action: Voltaic cells
How is a battery
Like a waterfall?
Pairing two different metals will always create an electron flow. Lets see how it works!
A metal conductor wire between the two electrodes provides the path for electrons to flow.

5. Which Reaction will occur?
Cu(s) + ZnSO4(aq)  Zn(s) + CuSO4(aq)
Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq) e-
Notice that the reactions above are the reverse of one another
The reaction that occurs depends on the relative positions of Cu and Zn on reference table J.

6. Which Reaction will occur?
Cu(s) + ZnSO4(aq)  Zn(s) + CuSO4(aq) +2
Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq) e-
2 e-
Once oxidation states are identified you can see that zinc atoms lose electrons to the copper ions.

7. Which Reaction will occur?
Cu(s) + ZnSO4(aq)  Zn(s) + CuSO4(aq) +2
Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq) e-
2 e-
During this reaction the Zn and CuSO4 decreases,
While the Cu metal and ZnSO4 increases.
(No surprise since reactants get used up, while products get produced!)

8. The Daniell cell: a Voltaic Cell
Zn + CuSO4  Cu + ZnSO4 Zn Cu
ZnSO4
CuSO4
In an electrochemical cell, the two metal electrodes, are placed in aqueous solutions of their ion salts and connected by a strand of wire.

9. The Daniell cell: a Voltaic Cell+2
Zn + CuSO4  Cu + ZnSO4 e- Zn
We assign the zinc electrode as negative since it is the source of electrons
We assign the copper electrode as positive since it is missing the electrons. – + Cu – + – +
ZnSO4
CuSO4
Electrons flow from the zinc electrode to the copper electrode, where they are picked up by Cu2+ ion from the solution.

10. The Daniell cell: a Voltaic CellZn – + Cu – + – +
ZnSO4
CuSO4
Oxidation: Zn0  Zn+2
+ 2 e-
Zinc loses electrons (oxidation – recall LEO?)

11. The Daniell cell: a Voltaic CellZn – + Cu
ANODE
“AN OX” – + – +
ZnSO4
CuSO4
Oxidation: Zn0  Zn+2
+ 2 e-
The electrode where oxidation occurs is called the “anode”. THINK “An Ox”

12. The Daniell cell: a Voltaic CellZn – + Cu
ANODE
“AN OX” – + – +
ZnSO4
CuSO4
Oxidation: Zn0  Zn+2
+ 2 e-
Reduction: Cu  Cu0
+ 2 e-
Cu2+ gains electrons at the copper electrode (reduction – GER)

13. The Daniell cell: a Voltaic CellZn – + Cu
ANODE
“AN OX”
CATHODE
“RED CAT” – + – +
ZnSO4
CuSO4
Oxidation: Zn0  Zn+2
+ 2 e-
Reduction: Cu  Cu0
+ 2 e-
This electrode is called the cathode.
Think Red Cat

14. The Daniell cell: a Voltaic CellZn – + Cu
ANODE
“AN OX”
CATHODE
“RED CAT” – + – +
ZnSO4
CuSO4
Oxidation: Zn0  Zn+2
+ 2 e-
Reduction: Cu  Cu0
+ 2 e-
Together we remember this as “RED CAT, AN OX. Reduction at the cathode, anode is for oxidation.

15. The Daniell cell: a Voltaic CellZn – + Cu
ANODE
“AN OX”
CATHODE
“RED CAT” – + – +
ZnSO4
CuSO4
Oxidation: Zn0  Zn+2
+ 2 e-
Reduction: Cu  Cu0
+ 2 e-
Electrons will flow spontaneously from “A to C”
From the Anode to the Cathode.

16. Recall that reactants (Zn) decrease?
Recall that products (Cu) increase?
Zn + CuSO4  Cu + ZnSO4
Na2SO4 Zn Cu
SO4-2 e-
Salt
bridge _
Zn electrode
Reacts so
its mass
decreases
Cu is product
So its mass
increases + _ +
Cu2+
Zn2+
SO4-2 _
SO4-2 +
Cu2+ ions react and leave to form Cu (s)
Zn2+ ions produced dissolve into (aq)
Salt Bridge – allows flow of spectator ions in opposite direction of electron flow to maintain balance of charge

17. Electrons flow from anode to cathode An- Ions flow is opposite

18. e- Practice: e- Al  Al3+ + 3e- Cu2+ + 2e-  Cu RED CAT AN OX
Ion flow-
Anode e-
LEO
AnOx
Anode
GER
RedCat
Cathode
Cathode
Draw arrows in the diagram to show direction of electron flow, and the ion flow
Write the half reactions for the oxidation and reduction which occurs.
Label the anode and cathode
What happens to the mass of the Cu electrode? The mass of the Al electrode?
Al  Al3+ + 3e-
Cu2+ + 2e-  Cu
RED CAT AN OX
Cu is produced – increases Al is a reactant, decreases

23. +2 +2
Which element is losing and which is gaining?

24. LEO
LIP

25. Which element is losing and which is gaining?

26. What change happens to lead?

27. Porous cup (cup within a cup) Voltaic cell
Instead of a salt bridge, ions can migrate through the wall of the inner cup

28. Porous cup setup

29. Voltaic cells (aka “batteries) :
Are Spontaneous Redox reactions
Are Exothermic: energy is released
during a chemical change
Convert Chemical energy to Electrical energy
Electrons flow
“downhill”

30. Electrolytic Cells
Section B

31. Voltaic cells (aka “batteries) :
In voltaic cells
Electrons flow
“downhill”

32. M.C. Escher
Can water flow uphill?

34. It can if its pumped using outside energy!

35. Electrochemistry / electrochemical cells
Voltaic cells (aka “batteries) :
Are Spontaneous Redox reactions
Are Exothermic: energy is released
during a chemical change
Convert Chemical energy  Electrical energy
Can electrons flow uphill?
They can if we use
an outside energy source!

36. Electrochemistry / electrochemical cells
Voltaic cells (aka “batteries) :
Are Spontaneous Redox reactions
Are Exothermic: energy is released
during a chemical change
Convert Chemical energy  Electrical energy
Electrolytic cells:
“splitting with electricity”

37. For electrolysis of salt we’ll take a container and fill it with liquid salt.
Na+1
Cl-1
Na+1
Cl-1
Cl-1
Na+1
Cl-1
Na+1
Na+1

38.
Battery
Next we’ll add two electrodes and connect them to a power source. + -
Na+1
Cl-1
Na+1
Cl-1
Na+1
Cl-1
Cl-1
Na+1
Na+1

39.
Battery
When electric current is applied the ions begin to migrate. + -
Na+1
Cl-1
Na+1
Cl-1
Na+1
Cl-1
Cl-1
Na+1
Na+1

40.
Battery
Na+ ions migrate toward the negative, while Cl- migrate toward the positive + -
Na+1
Cl-1
Na+1
Cl-1
Na+1
Cl-1
Cl-1
Na+1
Na+1

41.
Battery e-
Once at the positive side, Cl- ions give up their electrons and are oxidized to elemental chlorine. + -
Cl-1
Na+1
Cl2o
Cl-1
Na+1
Cl-1
Na+1
Cl2o
Na+1
Cl-1
Na+1

42.
Battery e-
While at the negative side sodium gains electrons an is reduced back to its elemental state. + -
Na0
Na+1
Cl2o
Na+1
Na0
Na0
Na+1
Cl2o
Na+1
Na0
Na0
Na+1

43.
Battery
If you’ve been paying attention you can see that NaCl has been split apart into Na and Cl2 e- + -
Na0
Cl2o
Na0
The equation is
NaCl  Na + Cl2
Na0
Cl2o
Na0
Na0

44. Electrolytic cell (electrolysis of liquid sodium chloride)
Battery
NaCl(l) 
Na0(s) + Cl20(g)
Two electrodes are placed into liquid salt and connected to a source of electricity

45. Electrolytic cell (electrolysis of Fused sodium chloride)
NaCl(l) 
Na0(s) + Cl20(g)
e-1
e-1
Battery
The Battery sends electrons
to the cathode, reducing
Na+ ions to Na0
The Battery pulls electrons
oxidizing Cl- ions to Cl0
at the anode.
Cl- ions are pulled to the positive electrode
Na+ ions are pulled to the negative electrode

46. Electrolytic cell (electrolysis of Fused sodium chloride)
NaCl(l) 
Na0(s) + Cl20(g)
e-1
e-1
Battery
Cl- ions give up their electrons at the anode:
An Ox: loss of electrons
Na+ ions gain electrons at the cathode: Red Cat: Gain of electrons.
Oxidation
Cl-  Cl 0 + e-
Reduction:
Na+ + e-  Na 0

47. Hydrogen gas (and oxygen) is obtained by hydrolysis

48. 1. What ion will be attracted to the anode? The cathode?
Practice
The diagram at left represents a cell for the electrolysis of fused Al2O3
1. What ion will be attracted to the anode? The cathode?
2. Write the balanced half reaction that occurs at the anode.
3. Which electrode will increase in mass? Why?
Anion = O-2
Cation = Al3+
GER: Al e-  Al0
LEO: O-2  O + 2 e-
2Al2O3(l)  4Al (s) + 3O2(g)
Cathode: Al – metal deposited as its produced

49. Voltaic cells:
Spontaneous Redox reactions
Exothermic: energy is released
during a chemical change
Chemical energy  electrical energy
Electrolytic cells: “splitting with electricity”
Non-spontaneous Redox reaction
Endothermic – outside energy
drives a chemical change
Electrical energy  Chemical energy

55. (an application of electrolytic cells)
Electroplating
(an application of electrolytic cells)

56. Electroplating (an application of electrolytic cells)
Object
to be plated
Connected to
negative
Source of silver
Oxidation:
Ag0(s)  Ag+(aq) + e-
Reduction:
Ag+(aq) + e-  Ag0(s)
Anode
Cathode
Object
to be plated
Increases in mass
Silver mass
decreases

57. The diagram represents a cell for electroplating silver.
Practice
The diagram represents a cell for electroplating silver.
Is this cell a voltaic or electrolytic cell? Why?
2. Which object is the cathode?
3. Write the half reaction that occurs at the cathode.
4. What happens to the concentration of Ag+(aq) while the cell operates?
Electrolytic: energy source drives non-spontaneous reaction
Spoon (RedCat: silver is reduced at cathode, plates the spoon)
GER: Ag+ + e-  Ag0
RTS (Silver ions enter from anode, lost at cathode)