Chapter 8 Oxidation and Reduction

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Chapter 8 Oxidation and Reduction

• Outline
• 1. Introduction
• 2. Basic concept
• 3. Electrochemistry
• 4. Examples of redox reactions
• 5. Examples of common oxidizing and reducing
  agents

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1. Introduction to Oxidation and Reduction
Reactions

• Also known as redox reactions
• reduction and oxidation
• Occur in many places. e.g
• Digestion of food
• Tree consuming CO2 and making O2
• Burning fossil fuels
• Rust
• Batteries

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2. Basic concepts
Three Views of Redox Reactions

• 1st view
• Historically, reaction of oxygen with element or
  compound.
• Oxidation gains oxygen atoms
• Reduction loses oxygen atoms.
• Example
• In 2H2 O2 ? 2 H2O
• H2, gains O, is oxidized
• In 2 PbO2 ?2 PbO O2
• PbO2 , loses O, it is reduced

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Redox Practice Problems
In each of the following reactions, is the
reactant undergoing oxidation or reduction?
(These are not complete chemical reactions.) a.
Pb ? PbO2 b. SnO2 ? SnO c. KClO3 ? KCl d. Cu2O ?
2 CuO
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2nd View of Redox Reactions

• Oxidation loses H atoms
• Reduction gains H atoms
• Example
• In CO 2 H2 ? CH3OH
• CO, gains H, is reduced
• In CH3OH ? CH2O H2
• CH3OH, loses H, is oxidized.

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Redox Practice Problems
In each of the following reactions, is the
reactant undergoing oxidation or reduction?
(These are not complete chemical reactions.) a.
C6H6O ? C2H4O b. C2H2 ? C2H6
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3rd View of Redox Reactions

• Oxidation loses electrons
• Reduction gains electrons
• Example

2 Na(s) Cl2(g) ? 2 NaCl(s) Na ? Na 1 e
Na, loses e-, it is oxidized Cl2 2 e ? 2 Cl
Cl2, gains e-, is reduced
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Redox Practice Problems
In each of the following reactions, is the
reactant undergoing oxidation or reduction?
(These are not complete chemical reactions.) a.
Zn ? Zn2 b. Fe3 ? Fe2
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Summary Oxidation and Reduction

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Exercise. Identify the Element Being Oxidized and
the Element Being Reduced.

• 2 C(s) O2(g) ? 2 CO(g)
• Mg(s) Cl2(g) ? MgCl2(s)
• Mg(s) Fe2(aq) ? Mg2(aq) Fe(s)

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Oxidizing and Reducing Agents

• Oxidizing agent agent that gets reduced
• Causes oxidation of other substance
• Reducing agent agent that gets oxidized
• Causes reduction of other substance

Example
2 Na(s) Cl2(g) ? 2 NaCl(s) Na is oxidized,
Cl is reduced. Na is the reducing agent, Cl2 is
the oxidizing agent.
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Exercise. Identify the Oxidizing and Reducing
Agents.

• 2 C(s) O2(g) ? 2 CO(g)
• Mg(s) Cl2(g) ? MgCl2(s)
• Mg(s) Fe2(aq) ? Mg2(aq) Fe(s)

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Oxidation State and oxidation number

• For reactions that are not metal nonmetal, or
  do not involve O2, we need a method for
  determining how the electrons are transferred.
• Chemists assign a number to each element in a
  reaction called an oxidation state that allows
  them to determine the electron flow in the
  reaction.
• Although they look like them, oxidation states
  are not ion charges!
• Oxidation states are imaginary charges assigned
  based on a set of rules.
• Ion charges are real, measurable charges.

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Rules for Assigning Oxidation States

• Rules are in order of priority.
• Free elements have an oxidation state 0.
• Na(s) 0 and Cl2(g) 0 in 2 Na(s) Cl2(g)?2
  NaCl(s).
• Monoatomic ions have an oxidation state equal to
  their charge.
• Na 1 and Cl -1 in NaCl(s).
• a. The sum of the oxidation states of all the
  atoms or ions in a neutral compound is 0.
• Na 1 and Cl -1 in NaCl, and (1) (-1) 0.

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Rules for Assigning Oxidation States, Continued

• b. The sum of the oxidation states of all the
  atoms in a polyatomic ion equals the charge on
  the ion.
• N 5 and O -2 in NO3, (5) 3(-2) -1.
• a. Group 1A metals have an oxidation state of 1
  in all their compounds.
• Na 1 in NaCl.
• b. Group 2A metals have an oxidation state of
  2 in all their compounds.
• Mg 2 in MgCl2.

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Rules for Assigning Oxidation States, Continued

• In their compounds, nonmetals usually have
  oxidation states according to the table below.
• Nonmetals higher on the table take priority.

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Exercise. Assign an Oxidation State to Each
Element in the Following

• F2
• Mg2
• KCl
• SO2
• PO43-
• BaO2

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Oxidation Numbers and redox reactions

• Increase in oxidation number oxidation
• Decrease in oxidation number reduction

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Exercise. Assign Oxidation States and Identify
the Oxidizing and Reducing Agents in Each of the
Following

• 3 H2S 2 NO3 2 H 3 S 2 NO 4 H2O
• MnO2 4 HBr MnBr2 Br2 2 H2O

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3. 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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In one pot reaction, electric current
flowsdirectly between atoms
Principles of electrochemistry
Zn Cu2 ? Zn2 Cu
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Electric Current Flowing Indirectly Between Atoms
Zn Cu2 ? Zn2 Cu
Current will be produced if the Cu2 and Zn are
separated and connected with a wire.
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Electrochemical Cell
Cathode
Anode
Anode where oxidation occurs
Zn(s) ? Zn2(aq) 2 e
Cathode where reduction occurs
Cu2(aq) 2 e ? Cu(s)
Overall
Zn Cu2 ? Zn2 Cu
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Electrodes
Cathode

• Anode
• Electrode where oxidation occurs.
• Anions attracted to it.
• Connected to positive end of battery in
  electrolytic cell.
• Loses weight in electrolytic cell.
• Cathode
• Electrode where reduction occurs.
• Cations attracted to it.
• Connected to negative end of battery in
  electrolytic cell.
• Gains weight in electrolytic cell.
• Electrode where plating takes place in
  electroplating.

Anode
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Half-Reactions

• A Redox reaction can be split 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)

One can use half reactions to balance reactions
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Half-Reaction Practice Problems
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(No Transcript)
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Exercise. Balance the Following EquationCu
I2 ? Cu2 I
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Batteries and redox chemistry
Battery a device converting chemical energy into
electrical energy
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Examples of batteries
1. Dry Cells

• Electrolyte NH4Cl/graphite
• Anode Zn
• Zn(s) ? Zn2(aq) 2 e
• Cathode carbon
• 2 MnO2(s) H2O 2 e ? Mn2O3(s) 2 OH(aq)
• Overall reaction
• 2 MnO2(s) 2 NH4(aq) 2 H2O(l) 2 e- 2
  NH4OH(aq) 2 Mn(O)OH(s)
• Cell voltage 1.5 v.

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2. Alkaline Dry Cell

• Same as dry cell, except electrolyte is alkaline
  KOH paste.
• Anode Zn.
• Zn(s) 2OH(aq) ? ZnO(s) H2O(l) 2e
• Cathode carbon.
• 2MnO2(s) H2O(l) 2e ? Mn2O3(s) 2OH(aq)
• Cell voltage 1.54 v.
• Longer shelf life than acidic dry cells and
  rechargeable little corrosion of zinc.
• Overall reaction?

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3. Lead Storage Battery

• Electrolyte 6 M H2SO4.
• Anode Pb.
• Pb(s) SO42-(aq) PbSO4(s) 2 e-
• Cathode Pb coated with PbO2.
• PbO2 is reduced.
• PbO2(s) 4 H(aq) SO42-(aq) 2 e- PbSO4(s)
  2 H2O(l)
• Rechargeable, heavy, containing corrosive H2SO4
• Overall reaction?

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4. The Silver-Zinc Cell A Button Battery
Zn(s),ZnO(s)KOH(satd)Ag2O(s),Ag(s)
Overall reaction
Zn(s) Ag2O(s) ? ZnO(s) 2 Ag(s) Ecell
1.8 V
Half reactions?
watches, camera
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Zn(s),ZnO(s)KOH(satd)Ag2O(s),Ag(s)
Half reactions?

• Zn(s) Ag2O(s) ? ZnO(s) 2 Ag(s)

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Fuel Cells

• Like batteries in which reactants are constantly
  being added.
• So it never runs down!
• Anode and cathode both Pt-coated metal.
• Electrolyte is OH solution.
• Anode reaction 2 H2 4 OH ? 4 H2O(l) 4 e-.
• Cathode reaction O2 4 H2O 4 e- ? 4 OH.

Overall reaction
2 H2 4 OH ? 4 H2O(l) 4 e-.

O2 4 H2O 4 e- ? 4 OH.
2 H2 O2 ? 4 H2O
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Dead Battery
Why battery can not used forever?
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4. Examples of redox reactions
In Photosynthesis by plants Carbon dioxide
water sunlight sugar oxygen
The only reaction in nature that produces O2
6 CO2 6 H2O energy ? C6H12O6 6 O2

• In our digestion of food
• 6 O2 C6H12O6 ? 6 H2O 6 CO2 energy

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• Reactions of metals with oxygen and rust

Many metals react with oxygen in air to give
oxides or hydroxides. e.g.
Rusting of iron 2 Fe O2 2 H2O ? 4
Fe(OH)2 4 Fe(OH)2 O2 2 H2O ? 4 Fe(OH)3
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Preventing Corrosion

• The spontaneous oxidation of a metal by chemicals
  in the environment is called Corrosion.
• Since many materials we use are active metals,
  corrosion can be a very big problem.
• One way to reduce or slow corrosion is to coat
  the metal surface to keep it from contacting
  corrosive chemicals in the environment.
• Paint.
• Some metals, like Al, form an oxide that strongly
  attaches to the metal surface, preventing the
  rest from corroding.

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Reactions in combustion

• Cooking with natural gas

CH4 O2 ? CO2 H2O

• Traveling with a car, burning gasoline (a mixture
  of hydrocarbons, mostly octane)

2 C8H18 25 O2 ? 16 CO2 18 H2O
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Explosive Reactions

• Chemical explosions typically result of
  oxidationreduction reactions.
• Redox reactions that occur rapidly with the
  production of gases (often nitrogen) are often
  explosive.
• Example
• 52 NH4NO3(s) C17H36(l) ? 52 N2(g) 17 CO2(g)
  122 H2O(g)
• (ANFO Ammonium Nitrate/Fuel Oil)
• 2 C7H5N3O6 (TNT) ? 3 N2 (g) 5 H2O (g) 7 CO
  (g) 7 C

Buildings topple by explosives in Yunong Village
in south China's Shenzhen on Sunday May 22, 2005.
The demolition, dubbed 'China's No. 1 blast' by
Hong Kong media, reportedly covered an area of
51,000 square meters (57,000 square feet) the
largest ever in China.
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Industrial applications

• Metallurgy Reducing Agents
• Coke (C) is used as a reducing agent in smelting
  of metals such as tin
• SnO2 C ? Sn CO2
• Aluminum is used to reduce chromium oxide to
  chromium metal
• Cr2O3 2 Al ? Al2O 2 Cr
• H2 is used to produce fertilizer
• N2 3 H2 ? 2 NH3

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5. Common oxidizing agents
1) Oxygen, O2

• Abundant oxidizing agent
• 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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• 2) Hydrogen Peroxide H2O2
• Converts to H2O in most reactions
• 3 solutions commonly available
• A common oxidizing agent used as a disinfectant
  or to bleach hair.
• 3) Potassium dichromate K2Cr2O7
• Oxidizes ethanol
• Used in old Breath analyzer test

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Disinfectant and antiseptics

• Disinfectants ?? are antimicrobial agents that
  are applied to non-living objects to destroy
  microorganisms, the process of which is known as
  disinfection.
• Antiseptics (???) are antimicrobial substances
  that are applied to living tissue/skin to reduce
  the possibility of infection, sepsis, or
  putrefaction.

An antiseptic solution of Povidone-iodine applied
to an abrasion
Disinfection of a floor using a mop
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• 4) Laundry bleach
• Change pigments to colorless products
• 5 NaOCl solution or Ca(OCl)2, most common
• Na2CO3 and H2O2
• NaBO2 and H2O2

• 5) Chlorine
• used as a disinfectant in the treatment of
  drinking water, wastewater, swimming pool