Chapter 8 Oxidation and Reduction: Burn and Unburn

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

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

Redox Practice Problems Chapter 8

2nd View of Redox Reactions Oxidation is loss of H atoms Reduction is gain of H atoms Example: CH3OH  CH2O + H2 Chapter 8

3rd View of Redox Reactions Oxidation is loss of electrons Reduction is gain of electrons Example: Mg + Cl2  Mg2+ + 2 Cl– Mnemonic: OIL RIG Chapter 8

Oxidation Numbers Just the charge on a simple ion Increase in oxidation number – oxidation Decrease in oxidation number – reduction Chapter 8

Practice Using Oxidation Numbers Chapter 8

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

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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 Chapter 8

Electrochemical Cell Anode – where oxidation occurs Cathode – where reduction occurs Chapter 8

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

Half-Reaction Practice Problems Chapter 8

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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 Chapter 8

Lead Storage Batteries Battery: series of electrochemical cells Readily recharged Durable but are heavy and contain H2SO4 Chapter 8

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

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

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

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

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

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 Chapter 8

Ozone Another form of O O3 Powerful oxidizing agent Destructive in lower atmosphere Very useful in ozone layer in upper atmosphere Chapter 8

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

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 Chapter 8

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 Chapter 8

Hydrogen H2 Not found free in nature Colorless gas Less dense than air Highly flammable Chapter 8

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 Chapter 8

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 Chapter 8