Chapter 8 Oxidation and Reduction:

Burn and Unburn

Chemistry for Changing Times 10th editionHill/Kolb

Daniel Fraser

University of Toledo, Toledo OH

©2003 Prentice Hall

Chapter 8 2

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 3

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 4

Redox Practice Problems

Chapter 8 5

2nd View of Redox Reactions• Oxidation is loss of H atoms

• Reduction is gain of H atoms

• Example: CH3OH CH2O + H2

Chapter 8 6

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

Chapter 8 7

Oxidation Numbers

• Just the charge on a simple ion

• Increase in oxidation number – oxidation

• Decrease in oxidation number – reduction

Chapter 8 8

Practice Using Oxidation Numbers

Chapter 8 9

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 10

Chapter 8 11

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 12

Electrochemical Cell

• Anode – where oxidation occurs

• Cathode – where reduction occurs

Chapter 8 13

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 14

Half-Reaction Practice Problems

Chapter 8 15

Chapter 8 16

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

Chapter 8 17

Lead Storage Batteries• Battery: series of

electrochemical cells

• Readily recharged

• Durable but are heavy and contain H2SO4

Chapter 8 18

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 19

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 20

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 21

• 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 22

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 23

Oxygen

• Abundant oxidizing agent

• Almost 2/3 of mass of humans is O

• Found in nature as O2

• ~21% of Earth’s atmosphere

Chapter 8 24

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

decay

Chapter 8 25

Ozone

• Another form of O

• O3

• Powerful oxidizing agent

• Destructive in lower atmosphere

• Very useful in ozone layer in upper atmosphere

Chapter 8 26

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 27

• 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 28

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

Chapter 8 29

Hydrogen

• H2

• Not found free in nature

• Colorless gas

• Less dense than air

• Highly flammable

Chapter 8 30

• 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

Chapter 8 31

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 32

End of Chapter 8