Chapter 8 Oxidation and Reduction: Burn and Unburn

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Chapter 8 Oxidation and Reduction Burn and
Unburn
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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

• 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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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

• 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

• Oxidationreduction 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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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

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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
• LiSO2, LiFeS2
• Other types of rechargeable batteries
• NiCad, Nimetal 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
  oxidationreduction 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

• Abundant oxidizing agent
• Almost 2/3 of mass of humans is O
• Found in nature as O2
• 21 of Earths 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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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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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

• 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