Chapter 8 Oxidation and Reduction Burn and Unburn

Chapter 8 Oxidation and Reduction: Burn and Unburn Chapter 8

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 2

• • 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: CH 4 + O 2 CO 2 + 2 H 2 O Chapter 8 3

Redox Practice Problems Chapter 8 4

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 8 5

3 rd View of Redox Reactions • • Oxidation is loss of electrons Reduction is gain of electrons Example: Mg + Cl 2 Mg 2+ + 2 Cl– Mnemonic: OIL RIG – Oxidation is loss of electrons – Reduction is gain of electrons Chapter 8 6

Oxidation Numbers • Just the charge on a simple ion • Increase in oxidation number – oxidation • Decrease in oxidation number – reduction Chapter 8 7

Practice Using Oxidation Numbers Chapter 8 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 9

Chapter 8 10

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 11

Electrochemical Cell • Anode – where oxidation occurs • Cathode – where reduction occurs Chapter 8 12

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 8 13

Half-Reaction Practice Problems Chapter 8 14

Chapter 8 15

• Anode Dry Cells – Zn(s) Zn 2+(aq) + 2 e– • Cathode – 2 Mn. O 2(s) + H 2 O + 2 e– Mn 2 O 3(s) + 2 OH–(aq) • Found in common batteries Chapter 8 16

Lead Storage Batteries • Battery: series of electrochemical cells • Readily recharged • Durable but are heavy and contain H 2 SO 4 Chapter 8 17

Other Batteries and Fuel Cells • Smaller, lighter batteries – Li–SO 2, Li–Fe. S 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 8 18

Corrosion • Costs U. S. ~$100 billion annually • In most air, Fe may be oxidized 2 Fe + O 2 + 2 H 2 O 2 Fe(OH)2 • Proceeds faster in presence of salt Chapter 8 19

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 8 20

• 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 8 21

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) + 122 H 2 O(g) Chapter 8 22

Oxygen • • • Abundant oxidizing agent Almost 2/3 of mass of humans is O Found in nature as O 2 ~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 23

Ozone • • • Another form of O O 3 Powerful oxidizing agent Destructive in lower atmosphere Very useful in ozone layer in upper atmosphere Chapter 8 24

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 8 25

• Laundry bleach – 5% Na. OCl solution or Ca(OCl)2 – Na 2 CO 3 and H 2 O 2 – Na. BO 2 and H 2 O 2 • Change pigments to colorless products • Other stain removers may be solvents, reducing agents, or detergents Chapter 8 26

Reducing Agents • Production of metals – Sn. O 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 8 27

Hydrogen • • • H 2 Not found free in nature Colorless gas Less dense than air Highly flammable Chapter 8 28

• Used in many industrial processes – N 2 + 3 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 8 29

Redox Reactions in Living Things • Photosynthesis: – 6 CO 2 + 6 H 2 O + sunlight C 6 H 12 O 6 + 6 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 8 30