Chapter 8 Oxidation and Reduction: Burn and Unburn Chemistry for Changing Times 10 th edition Hill/Kolb Daniel Fraser University of Toledo, Toledo OH ©2003 Prentice Hall

Chapter 82 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 83 Three Views of Redox Reactions 1 st view Historically, reaction of oxygen with element or compound –Compound or element was oxidized Reduction is the opposite –Loss of oxygen Example: CH 4 + O 2  CO H 2 O

Chapter 84 Redox Practice Problems

Chapter 85 2 nd View of Redox Reactions Oxidation is loss of H atoms Reduction is gain of H atoms Example: CH 3 OH  CH 2 O + H 2

Chapter 86 3 rd View of Redox Reactions Oxidation is loss of electrons Reduction is gain of electrons Example: Mg + Cl 2  Mg Cl – Mnemonic: OIL RIG –Oxidation is loss of electrons –Reduction is gain of electrons

Chapter 87 Oxidation Numbers Just the charge on a simple ion Increase in oxidation number – oxidation Decrease in oxidation number – reduction

Chapter 88 Practice Using Oxidation Numbers

Chapter 89 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 810

Chapter 811 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 812 Electrochemical Cell Anode – where oxidation occurs Cathode – where reduction occurs

Chapter 813 Half-Reactions Can break redox reactions into separate oxidation and reduction reactions Oxidation: Zn(s)  Zn 2+ (aq) + 2 e – Reduction: Cu 2+ (aq) + 2 e –  Cu(s) Overall: Zn(s) + Cu 2+ (aq)  Cu(s) + Zn 2+ (aq)

Chapter 814 Half-Reaction Practice Problems

Chapter 815

Chapter 816 Dry Cells Anode –Zn(s)  Zn 2+ (aq) + 2 e – Cathode –2 MnO 2 (s) + H 2 O + 2 e –  Mn 2 O 3 (s) + 2 OH – (aq) Found in common batteries

Chapter 817 Lead Storage Batteries Battery: series of electrochemical cells Readily recharged Durable but are heavy and contain H 2 SO 4

Chapter 818 Other Batteries and Fuel Cells Smaller, lighter batteries –Li–SO 2, Li–FeS 2 Other types of rechargeable batteries –Ni–Cad, Ni–metal hydride Fuel Cells –Efficient to convert fuel to electricity –Require continuous supply of fuel

Chapter 819 Corrosion Costs U.S. ~$100 billion annually In most air, Fe may be oxidized 2 Fe + O H 2 O  2 Fe(OH) 2 Proceeds faster in presence of salt

Chapter 820 Other Types of Corrosion Aluminum corrodes to produce Al 2 O 3 on surface –Al 2 O 3 : very hard! so it prevents further corrosion of Al Al 2 O 3 corrodes in presence of Cl – –Why can you not use aluminum boats on the ocean?

Chapter 821 Silver tarnish occurs when Ag reacts with S 2– Remove with polish –Takes a layer of Ag off item Use aluminum –Make electrolytic cell –3 Ag + + Al  3 Ag + Al 3+

Chapter 822 Explosive Reactions Chemical explosions typically result of oxidation–reduction reactions Commonly involve N-containing compounds –Produce N 2 gas Example: 52 NH 4 NO 3 (s) + C 17 H 36 (l)  52 N 2 (g) + 17 CO 2 (g) H 2 O(g)

Chapter 823 Oxygen Abundant oxidizing agent Almost 2/3 of mass of humans is O Found in nature as O 2 ~21% of Earth’s atmosphere

Chapter 824 Oxygen reacts with many compounds –Useful: powers respiration, helps fossil fuels burn –Side problems: corrosion, food spoilage, and wood decay

Chapter 825 Ozone Another form of O O 3 Powerful oxidizing agent Destructive in lower atmosphere Very useful in ozone layer in upper atmosphere

Chapter 826 Other Oxidizing Agents Peroxide: H 2 O 2 –Converts to H 2 O in most reactions –3% solutions commonly available Potassium dichromate: K 2 Cr 2 O 7 –Oxidizes ethanol –Used in old Breathalyzer test

Chapter 827 Laundry bleach –5% NaOCl solution or Ca(OCl) 2 –Na 2 CO 3 and H 2 O 2 –NaBO 2 and H 2 O 2 Change pigments to colorless products Other stain removers may be solvents, reducing agents, or detergents

Chapter 828 Reducing Agents Production of metals –SnO 2 + C  Sn + CO 2 Photography –Used in process to develop film Antioxidants –Inhibit damage by O 2 to cells –Some water soluble, some fat soluble

Chapter 829 Hydrogen H 2 Not found free in nature Colorless gas Less dense than air Highly flammable

Chapter 830 Used in many industrial processes –N H 2  2 NH 3 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 831 Redox Reactions in Living Things Photosynthesis: –6 CO H 2 O + sunlight  C 6 H 12 O O 2 –Only reaction in nature that produces O 2 Digestion –6 O 2 + C 6 H 12 O 6  6 H 2 O + 6 CO 2 + energy Other reactions that build or degrade molecules

Chapter 832 End of Chapter 8