Transcript Chapter 8 Oxidation and Reduction: Burn and Unburn

Chemistry for Changing Times 10th edition
Hill/Kolb
Chapter 8
Oxidation and Reduction:
Burn and Unburn
Daniel Fraser
University of Toledo, Toledo OH
©2003 Prentice Hall
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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Three Views of Redox
Reactions
st
1 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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Redox Practice Problems
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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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3rd View of Redox Reactions
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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
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Oxidation Numbers
• Just the charge on a
simple ion
• Increase in oxidation
number – oxidation
• Decrease in oxidation
number – reduction
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Practice Using Oxidation
Numbers
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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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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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Electrochemical Cell
• Anode – where
oxidation occurs
• Cathode – where
reduction occurs
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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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Half-Reaction Practice
Problems
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• Anode
Dry Cells
– 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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Lead Storage Batteries
• Battery: series of
electrochemical cells
• Readily recharged
• Durable but are heavy
and contain H2SO4
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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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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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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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• 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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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)
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Oxygen
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Abundant oxidizing agent
Almost 2/3 of mass of humans is O
Found in nature as O2
~21% of Earth’s atmosphere
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• Oxygen reacts with many compounds
– Useful: powers respiration, helps fossil fuels burn
– Side problems: corrosion, food spoilage, and wood
decay
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Ozone
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Another form of O
O3
Powerful oxidizing agent
Destructive in lower atmosphere
Very useful in ozone layer in upper
atmosphere
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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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• 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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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
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Hydrogen
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H2
Not found free in nature
Colorless gas
Less dense than air
Highly flammable
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• 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
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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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End of Chapter 8
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