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Chapter 8 Oxidation and Reduction:
Burn and Unburn
Chemistry for Changing Times 10th editionHill/Kolb
Daniel Fraser
University of Toledo, Toledo OH
©2003 Prentice Hall
Chapter 8 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
Chapter 8 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
Chapter 8 5
2nd View of Redox Reactions• Oxidation is loss of H atoms
• Reduction is gain of H atoms
• Example: CH3OH CH2O + H2
Chapter 8 6
3rd View of Redox Reactions
• Oxidation is loss of electrons
• Reduction is gain of electrons
• Example: Mg + Cl2 Mg2+ + 2 Cl–
• Mnemonic: OIL RIG– Oxidation is loss of electrons– Reduction is gain of electrons
Chapter 8 7
Oxidation Numbers
• Just the charge on a simple ion
• Increase in oxidation number – oxidation
• Decrease in oxidation number – reduction
Chapter 8 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
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
Chapter 8 12
Electrochemical Cell
• Anode – where oxidation occurs
• Cathode – where reduction occurs
Chapter 8 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)
Chapter 8 16
Dry Cells• Anode
– Zn(s) Zn2+(aq) + 2 e–
• Cathode– 2 MnO2(s) + H2O + 2 e– Mn2O3(s) + 2 OH–(aq)
• Found in common batteries
Chapter 8 17
Lead Storage Batteries• Battery: series of
electrochemical cells
• Readily recharged
• Durable but are heavy and contain H2SO4
Chapter 8 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
Chapter 8 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
Chapter 8 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?
Chapter 8 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+
Chapter 8 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)
+ 122 H2O(g)
Chapter 8 23
Oxygen
• Abundant oxidizing agent
• Almost 2/3 of mass of humans is O
• Found in nature as O2
• ~21% of Earth’s atmosphere
Chapter 8 24
• Oxygen reacts with many compounds– Useful: powers respiration, helps fossil fuels burn– Side problems: corrosion, food spoilage, and wood
decay
Chapter 8 25
Ozone
• Another form of O
• O3
• Powerful oxidizing agent
• Destructive in lower atmosphere
• Very useful in ozone layer in upper atmosphere
Chapter 8 26
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
Chapter 8 27
• 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
Chapter 8 28
Reducing Agents
• Production of metals– SnO2 + C Sn + CO2
• Photography– Used in process to develop film
• Antioxidants– Inhibit damage by O2 to cells
– Some water soluble, some fat soluble
Chapter 8 29
Hydrogen
• H2
• Not found free in nature
• Colorless gas
• Less dense than air
• Highly flammable
Chapter 8 30
• Used in many industrial processes – 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
Chapter 8 31
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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