1. Oxidation and Reduction Reactions

2. Oxidation (Read only) Original definition:
When substances combined with oxygen.
Ex:
All combustion (burning) reactions
CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
All “rusting” reactions
4Fe(s) + 3O2(g) 2Fe2O3(s)

3. Reduction (Read Only) Original Definition:
Reaction where a substance “gave up” oxygen.
Called “reductions” because they produced
products that were “reduced” in mass because
gas escaped.
Ex:
2Fe2O3(l) + 3C(s) 4Fe(l) + 3CO2(g)

4. Oxidation/Reduction Deals with movement of ELECTRONS
during a chemical reaction.
(Oxygen doesn’t have to be present)

5. Electron Transfer Reactions
Oxidation: LOSS of one or more electrons.
Reduction: GAIN of one or more electrons

6. Electron Transfer Reactions
Oxidation & reduction always occur together.
Electrons travel from what is oxidized towards
what is reduced.
One atom loses e-, the other gains e-

7. Redox Reactions: ALWAYS involve changes in charge
A competition for electrons between atoms!

8. Remember!!

9. Or…Remember

10. Conservation of “Charge”
Total electrons lost = Total electrons gained

11. Oxidizing/Reducing Agents
Oxidizing Agent:
substance reduced
Gains electrons
Reducing Agent:
substance oxidized
Loses electrons
The “Agent” is the “opposite”

12. Assigning Oxidation Numbers
Practice Problems
Animation of Oxidation and Reduction

13. Identify What is Changing in Charge
What is oxidized and reduced?
What are the oxidizing and reducing agents?
Ex:
3Br AlI3 2AlBr I2

14.
3Br AlI3 2AlBr I2
Br2 is reduced and is the oxidizing agent
I-1 is oxidized and is the reducing agent

15. What is oxidized and reduced?
What are the oxidizing and reducing agents?
Mg + CuSO4 MgSO4 + Cu
2K + Br2 2KBr
Cu + 2AgNO Cu(NO3) Ag
NOTE:
Atoms in a polyatomic ion DO NOT change in charge!

16. 0 +2 +2 0 Mg + CuSO4 MgSO4 + Cu 0 0 +1 -1 2K + Br2 2KBr
Mg + CuSO4 MgSO4 + Cu
Mg oxidized (reducing agent)
Cu+2 reduced (oxidizing agent)
2K + Br2 2KBr
K oxidized (reducing agent)
Br2 reduced (oxidizing agent)
Cu + 2AgNO Cu(NO3) Ag
Cu oxidized (reducing agent)
Ag+1 reduced (oxidizing agent)

17. Redox or Not Redox (that is the question…)
Redox Reactions: must have atoms changing in
charge.
Not all reactions are redox.
Easy way to spot a redox reaction!!!
Look for elements entering and leaving
compounds.

18. Is it Redox? Look for Changes in Charge!
Are elements entering and leaving compounds?
Synthesis:
Ex: 2H2 + O H2O
Decomposition:
Ex: 2KClO3 2KCl + 3O2

19. Is it Redox? Synthesis: YES 0 0 +1 -2 Ex: 2H2 + O2 2H2O
Ex: 2H2 + O H2O
Decomposition: YES
Ex: 2KClO3 2KCl + 3O2

20. Is it Redox? Combustion: CH4 + 2O2 CO2 + 2H20 Single Replacement:
Zn + CuCl2 ZnCl2 + Cu

21. Is it Redox? Combustion: YES -4 +1 0 +4 -2 +1 -2 CH4 + 2O2 CO2 + 2H20
CH O2 CO H20
Single Replacement: YES
Zn + CuCl2 ZnCl2 + Cu

22. Is it Redox?
Double Replacement:
AgNO3 + LiCl AgCl + LiNO3

23. Is it Redox? Double Replacement: NO!!!!
Ions switch partners, but don’t change in charge
AgNO3 + LiCl AgCl + LiNO3
Remember charges of atoms inside polyatomic ions
do not change!

24. Writing Half Reactions
Redox Reactions are composed of two parts
or half reactions.
Half Reactions Show:
Element being oxidized or reduced.
Change in charge
# of electrons being lost or gained

25. Writing Half Reactions
2Na + F2 2NaF
Oxidation: Na Na+1 + 1e-
or 2Na 2Na e-
Note: e- are “lost” (on the right of arrow)
Reduction: F e- F-1
or F e- 2F-1
Note: e- are “gained” (on the left of arrow)

26. Ox’s Have Tails!!
Oxidation Half reactions always have “tails” of electrons
Na Na+1 + 1e-

27.
Zn + CuCl2 ZnCl2 + Cu
Ox: Zn Zn e-
Red: Cu e- Cu

28. Balancing Simple Redox Rxns
Must be:
Balanced for Mass
ATOMS balance
Balanced for Charge
Total e- Lost = Total e- Gained

29. Balancing Harder Redox Reactions (Honors)

30. Oxidation Number Method (Balancing in Acid Solution)
Find ox #’s and use brackets to connect elements
changing in charge.
Balance atoms changing in charge
Find total e- involved in each change
If necessary balance e- by multiplication
Balance all other atoms except H and O
Balance oxygen by adding H2O to side deficient
Balance hydrogen by adding H+1 to side deficient
Check for balance with respect to atoms and charge.

31. Half Reaction Method (Ion/Electron Method) (In acid solution)
Separate equation into two “basic” half reactions
Balance all atoms except H and O
Balance oxygen by adding H2O
Balance hydrogen by adding H+1
Balance charge by adding electrons to more positive side
If necessary balance e- by multiplication
Add together half reactions and simplify
Check for balance of atoms and charge

32. Applications of Redox Reactions
Corrosion of Metals
the metal gets oxidized
forming metal oxides on
the surface
Prevention: Use paint, oil,
plating or attach to negative
terminal of a battery.
Gold doesn’t rust…Why?

33. Photograph Development involves oxidation and
reduction of silver atoms and ions

34. Bleach acts on
stains by oxidizing
them, getting reduced
in the process
Explosives form
neutral gases like N2
from compounds!

35. Reactivity of Metals Reference Table J Metals Higher on Table J
are more ‘active”
It is easier for more “active”
metals to be oxidized
or lose electrons.

36. Cu0(s) + AgNO3(aq) Ag0(s) + CuNO3(aq)
Copper replaces silver!
Cu0(s) + AgNO3(aq) Ag0(s) + CuNO3(aq)
Ag0(s) + CuNO3(aq) wouldn’t happen!!!

37. Reactivity of Nonmetals
Reference Table J
Nonmetals higher on Table J
are more “active”
It is easier for more “active”
nonmetals to “gain” electrons
and be reduced.

38. Electrochemical Cells (Batteries)
Chemical reaction that produces electricity.
Called “voltaic cells” as they produce voltage
This happens SPONTANEOUSLY.

39. Moving Electrons = Electricity
Electrons given off by oxidized substance
travel towards substance being reduced.
Traveling electrons move
through “external circuit”
where they do work.

40. How do the Electrons Move?
Batteries often contain 2 metals.
Start with Table J
Electrons travel from the more “Active metal”
toward the less active metal.
Metal above = oxidized
Ion on Metal below = reduced

41. Electrons flow
“Down Table J”
From metal
above to ion of
metal below e-

42. Parts of a Simple Battery (Voltaic Cell)
Made of Two “Half Cells” containing:
2 Metal Electrodes
2 Solutions of Ions
External Wire
Salt Bridge

43. Electrons need to flow in a “circuit” that is connected.
External Wire:
allows electrons to flow
between metal electrodes
Salt Bridge: allows ions
to flow between solutions

44. Zn/Zn+2//Cu+2/Cu What is Ox/Red? See Table J Metal above is oxidized
Ion of metal below reduced
Cu+2

45. flow in the external wire?
Which way do electrons
flow in the external wire?
See Table J
Electrons flow “Down” the
table from what is oxidized
towards what is reduced.
from Zn to Cu e-

46. Which electrode is negative? Which electrode is positive?
Electrons flow from negative to
positive electrode.
Negative electrode: Zn
Positive electrode: Cu e-

47. Which electrode is the anode and cathode? Anode: metal electrode
where oxidation occurs Zn
Cathode: metal electrode
where reduction occurs Cu

48. Remember AN OX RED CAT Anode is where oxidation happens
Cathode is where reduction happens

49. What are the Half Reactions? What is the Net Equation?
Ox:
Zn0 Zn+2 + 2e-
Red:
Cu+2 + 2e- Cu0
Net: (add ½ reactions)
Zn0 + Cu+2 Zn+2 + Cu0
Make sure final net equation is
balanced for electrons and atoms! e-

50. Which electrode gains/loses weight?
Look at half reactions!!
Which one forms solid metal?
Which one forms dissolved ions?
Ox:
Zn0 Zn+2 + 2e-
Red:
Cu+2 + 2e- Cu0
Zinc electrode loses mass
Copper electrode gains mass

51. “The negative ions complete the circuit”
Which way to do the ions in the salt
bridge “migrate” or move?
Remember:
“The negative ions complete the circuit”
(The ions actually end up moving towards
the solution of opposite charge that forms.)

52. Follow the ions

54. Means the reaction in the battery has
Dead Battery
Voltage = 0
Means the reaction in the battery has
reached EQUILIBRIUM.

55. You try it… Mg/Mg+2//Al+3/Al Draw and label Battery
What is oxidized/reduced?
What are the half reactions and net(balanced)?
What is the neg/pos electrode?
What is the anode/cathode?
Which way do e- flow in wire?
Which way do -/+ ions flow in salt bridge?
Which electrode gains/loses mass?

56. Finding Voltage of a Battery (Honors)
Use Voltage Table
Find your half reactions and record voltage
Note:
All ½ reactions shown are reductions.
For oxidation, reverse the sign of the voltage

57. Nerntz Equation (Honors)
Find voltage of a battery when the conc. of dissolved
ions is not 1 Molar (as on “standard voltage” table)
Ecell = E0 – log [product ion]x
n [reactant ion]y
n = total # of moles electrons being transferred
The concentration of dissolved ions can affect voltage.
Greater concentration of reactant ions (see net)
increases the overall voltage.

58. Electrolytic Cells & Electrolysis Reactions
Uses electricity to “split” or “lyse” a
compound into it’s neutral elements
An outside electrical source provides electrons
to force a non-spontaneous redox reaction to
occur.

59. Electrolysis Set Up Single Cell filled with electrolyte with +/- ions
Attach battery to two electrodes.
Electrodes are made of an inert substance
(like platinum or graphite) that conducts.
Electrodes don’t chemically change like in a battery,
they just provide current

60. Role of the Battery Pulls electrons off one electrode
Making it POSITIVE
Adds electrons onto one electrode
Making it NEGATIVE

61. Which Way do the Ions Move?
To electrode of
opposite charge

62. What is Oxidized/Reduced?
At neg. electrode
electrons are gained by ion
(reduction at CATHODE)
At positive electrode
electrons are lost by ion
(oxidation at ANODE)

63. Remember AN OX RED CAT Anode is where oxidation happens
Cathode is where reduction happens

64. Half Reactions & Net Equation
Rxn at Anode: (Ox)
Cl- Cl + 1e-
Or more correctly DIATOMIC!!!!!!
2Cl- Cl2 + 2e-
Rxn at Cathode: (Red)
Na+ + 1e- Na
(Multiply by 2 to balance electrons)
NET:
2Na+ + 2Cl Na + Cl2

65. Determining Voltage Needed
Use the Voltage Table to determine the total
voltage needed to run the Electrolytic cell.
Total voltage should be a NEGATIVE number

66. Electrolysis of Molten NaCl (l)

67. Electrolysis of PbCl2(l)
What is oxidized?
What is reduced?
What are the ox/red
half reactions?
What is the net
equation?
Negative Electrode
Positive Electrode

68. Electrolysis of PbCl2(l)
Oxidized: Cl- Reduced: Pb+2
Half Reactions
Ox: Cl-1 Cl + 1e-
2Cl-1 Cl2 + 2e-
Red: Pb+2 + 2e- Pb
Net: Pb+2 + 2Cl-1 Pb + Cl2

69. Electrolysis of NaCl(aq)

70. Electrolysis of NaCl (aq)

71. Electrolysis of Water At Positive Electrode: Ox: O-2 O + 2e-
but there is a diatomic!
2O-2 O2 + 4e-
At Negative Electrode
Red: H+1 + 1e- H
2H+1 + 2e- H2
Net: 2H2O 2H2 + O2

73. Lemon Battery Demo
Electrolysis of Copper Sulfate
Electrolysis of Water/ Silver Nitrate and Cu reaction
Electrolysis of Water (Animation)
Electrolysis of Water (Simple)

74. Electroplating Electrolysis reaction used to coat a
substance with a thin
layer of metal.
Often coating is a less
reactive metal that is
not easily oxidized or
corroded.

75. Electroplating Negative Electrode Red: Ag+ + 1e- Ag0
Is the OBJECT TO BE PLATED
so the positive metal ions would go towards it and be REDUCED.
It is the CATHODE
Red: Ag+ + 1e- Ag0

76. Electroplating Positive Electrode Ox: Ag0 Ag+ + 1e-
Made of plating metal
It dissolves into solution as metal strip gets OXIDIZED.
It is the ANODE
This replenishes the ions for plating.
Ox: Ag Ag+ + 1e-

77. Electroplating Problems (Honors)
Coulomb = measure of electrical charge
1 mole e- = 96,500 coulombs
# coulombs = # amps x seconds

78. Electroplating Problems (Honors)
Reduction:
Happens on object to be plated
Look at Reduction half reaction
Look at mole relationships
between electrons and metal atoms.
Ex: Ag+ + 1e- Ag0

79. Electroplating Problems (Honors)
You can now answer questions regarding the amount of a substance in moles or grams that can be electroplated over a certain amount of time.

80. Electroplating Problems (Honors)
If 10 amps are run through a CuSO4 solution for
5 minutes, calculate the grams of Cu that will plate
onto the spoon.
We Know:
1 mole e- = 96,500 coulombs
# coulombs = # amps x seconds
Red: Cu+2 + 2e- Cu0
1 mole Cu = 63.5 grams

81. # coulombs = 10 amps x 300 seconds = 3000 coulombs
So….Let’s start here
# coulombs = 10 amps x 300 seconds
= 3000 coulombs
3000 coul. x 1 mole e- x 1 mole Cu x 63.5g Cu = .987 grams
96,500 coul mole e mole Cu
Mole ratio from Reduction half reaction

82. You Try One How long will it take to deposit 20 grams of
silver from a solution of AgCl onto a copper
tray if a current of 5 amps is used?
Answer = 3, 574 sec
or 59.5 minutes or about 1 hour

83. You Try One How many amps are needed to deposit .504g.
of Iron in 40 minutes by passing a current
through a solution of Iron II Sulfate?
Answer: .72 amps