1. Chapter 8 Oxidation and Reduction: Burn and Unburn

2. Oxidation and Reduction Reactions
Always occur together
Also known as redox reactions
reduction and oxidation
Occur in many places
Digestion of food
Batteries
Burning fossil fuels
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3. Three Views of Redox Reactions
1st view
Historically, reaction of oxygen with element or compound
Compound or element was oxidized
Reduction is the opposite
Loss of oxygen
Example: CH4 + O2  CO2 + 2 H2O
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4. Redox Practice Problems
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5. 2nd View of Redox Reactions
Oxidation is loss of H atoms
Reduction is gain of H atoms
Example: CH3OH  CH2O + H2
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6. 3rd View of Redox Reactions
Oxidation is loss of electrons
Reduction is gain of electrons
Example: Mg + Cl2  Mg Cl–
Mnemonic: OIL RIG
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7. Oxidation Numbers Just the charge on a simple ion
Increase in oxidation number – oxidation
Decrease in oxidation number – reduction
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8. Practice Using Oxidation Numbers
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9. Oxidizing and Reducing Agents
Oxidizing agent – element or compound that gets reduced
Causes oxidation of other substance
Reducing agent – element or compound that gets oxidized
Causes reduction of other substance
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10. Chapter 8

11. Electrochemistry
Oxidation–reduction reactions in which electrons are transferred from one substance to another can be used to produce electricity
Used in dry cells, storage batteries, and fuel cells
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12. Electrochemical Cell Anode – where oxidation occurs
Cathode – where reduction occurs
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13. Half-Reactions
Can break redox reactions into separate oxidation and reduction reactions
Oxidation: Zn(s)  Zn2+(aq) + 2 e–
Reduction: Cu2+(aq) + 2 e–  Cu(s)
Overall: Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq)
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14. Half-Reaction Practice Problems
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15. Chapter 8

16. Dry Cells Anode Cathode Found in common batteries
Zn(s)  Zn2+(aq) + 2 e–
Cathode
2 MnO2(s) + H2O + 2 e–  Mn2O3(s) + 2 OH–(aq)
Found in common batteries
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17. Lead Storage Batteries
Battery: series of electrochemical cells
Readily recharged
Durable but are heavy and contain H2SO4
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18. Other Batteries and Fuel Cells
Smaller, lighter batteries
Li–SO2, Li–FeS2
Other types of rechargeable batteries
Ni–Cad, Ni–metal hydride
Fuel Cells
Efficient to convert fuel to electricity
Require continuous supply of fuel
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19. Corrosion Costs U.S. ~$100 billion annually
In most air, Fe may be oxidized
2 Fe + O2 + 2 H2O  2 Fe(OH)2
Proceeds faster in presence of salt
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20. Other Types of Corrosion
Aluminum corrodes to produce Al2O3 on surface
Al2O3: very hard! so it prevents further corrosion of Al
Al2O3 corrodes in presence of Cl–
Why can you not use aluminum boats on the ocean?
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21. Silver tarnish occurs when Ag reacts with S2– Remove with polish
Takes a layer of Ag off item
Use aluminum
Make electrolytic cell
3 Ag+ + Al  3 Ag + Al3+
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22. Explosive Reactions
Chemical explosions typically result of oxidation–reduction reactions
Commonly involve N-containing compounds
Produce N2 gas
Example:
52 NH4NO3(s) + C17H36(l)  52 N2(g) + 17 CO2(g) H2O(g)
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23. Oxygen Abundant oxidizing agent Almost 2/3 of mass of humans is O
Found in nature as O2
~21% of Earth’s atmosphere
Oxygen reacts with many compounds
Useful: powers respiration, helps fossil fuels burn
Side problems: corrosion, food spoilage, and wood decay
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24. Ozone Another form of O O3 Powerful oxidizing agent
Destructive in lower atmosphere
Very useful in ozone layer in upper atmosphere
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25. Other Oxidizing Agents
Peroxide: H2O2
Converts to H2O in most reactions
3% solutions commonly available
Potassium dichromate: K2Cr2O7
Oxidizes ethanol
Used in old Breathalyzer test
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26. Change pigments to colorless products
Laundry bleach
5% NaOCl solution or Ca(OCl)2
Na2CO3 and H2O2
NaBO2 and H2O2
Change pigments to colorless products
Other stain removers may be solvents, reducing agents, or detergents
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27. Reducing Agents Production of metals Photography Antioxidants
SnO2 + C  Sn + CO2
Photography
Used in process to develop film
Antioxidants
Inhibit damage by O2 to cells
Some water soluble, some fat soluble
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28. Hydrogen H2 Not found free in nature Colorless gas Less dense than air
Highly flammable
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29. Used in many industrial processes Reactions may require a catalyst
N2 + 3 H2  2 NH3
Reactions may require a catalyst
Increases rate of reaction without being used up
Lowers activation energy
Minimum amount of energy needed to start reaction
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30. Redox Reactions in Living Things
Photosynthesis:
6 CO2 + 6 H2O + sunlight  C6H12O6 + 6 O2
Only reaction in nature that produces O2
Digestion
6 O2 + C6H12O6  6 H2O + 6 CO2 + energy
Other reactions that build or degrade molecules
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