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									CHEM 1405

Class Meeting 12




     CHEM1405      1
      Assignments and Reminders
• Reading Assignment
  – Chapter 5 by Thursday, Feb 23rd
• Homework Problems due TODAY Thursday Feb 23rd
  – Chapter 4 Problems 2, 4, 8, 9-12, 14, 16, 18, 20, 22, 26, 28


• Homework Problems due NEXT Thursday Mar 2nd
  – Chapter 4 problems 29-34
  – Chapter 5 problems 2, 4, 6, 8, 12, 14, 16, 18, 20

  Please use only one side of the page when submitting Homework

  Class website                    http://iws.ccccd.edu/jstankus/
                               CHEM1405                             2
                        Objectives
1. What are some important properties of oxygen?
2. What are some important properties of hydrogen?
3. List some simple ways of describing oxidation. What is its most
   fundamental definition?
4. List some simple ways of describing reduction. What is its most
   fundamental definition?
5. How do you identify an oxidation-reduction reaction? How do you use the
   definitions of oxidation and reduction to identify which substance is
   oxidized or reduced and which is the oxidizing agent or reducing agent?
6. What are some common oxidizing agents and their uses?
7. What are some common reducing agents and their uses?
8. What are some oxidation-reduction reactions that are important for living
   organisms?



                                CHEM1405                                3
Oxidation and Reduction Reactions
• Always occur together
           When one substance is oxidized,
           another is reduced
• Also known as redox reactions
  – reduction and oxidation
• Occur in many places
  – Digestion of food
  – Batteries
  – Burning fossil fuels

                      CHEM1405               4
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

                         CHEM1405           5
       Redox Practice Problems
1st view Gain or Loss of Oxygen Atoms
Note these are not complete chemical equations

Pb PbO2                        Lead gains Oxygen atoms
                                Lead is oxidized

SnO2 SnO                       Tin loses an oxygen atom
                                Tin is reduced


KClO3  KCl                     Potassium Chlorate loses oxygen
                                It is reduced


Cu2O  2 CuO
                                Copper gains oxygen
                                Copper is Oxidized
                          CHEM1405                         6
 2nd View of Redox Reactions

• Oxidation is loss of H atoms
• Reduction is gain of H atoms
• Example: CH3OH  CH2O + H2
     Methanol is oxidized to form formaldehyde




                  CHEM1405                       7
        Redox Practice Problems
• 2nd view Gain or Loss of Hydrogen Atoms
Note these are not complete chemical equations

C2H6O  C2H4O
           Compound Loses Hydrogen Atoms
           The compound is oxidized


C2H2  C2H6
           The Compound gains Hydrogen atoms
           The compound is reduced
                           CHEM1405              8
   3 rd   View of Redox Reactions
• Oxidation is loss of electrons
• Reduction is gain of electrons
• Example: Mg + Cl2  Mg2+ + 2 Cl–
  – Magnesium is oxidized
  – Chlorine is reduced
• Mnemonic: OIL RIG
  – Oxidation is loss of electrons
  – Reduction is gain of electrons


                       CHEM1405      9
        Redox Practice Problems
3rd view Oxidation is loss of electrons
         Reduction is gain of electrons
Note these are not complete chemical equations

Zn  Zn2+
  Zn loses 2 electrons to form Zn2+
                                Zinc is oxidized
Fe3+  Fe2+
Fe3+ gains an electron
            Iron(III) ion is reduced to the Iron(II) ion

                           CHEM1405                        10
        Redox Practice Problems
3rd view Oxidation is loss of electrons
         Reduction is gain of electrons
Note these are not complete chemical equations

S2-  S
S2- loses 2 electrons
                                  Sulfide ion is oxidized
AgNO3  Ag
 Remember AgNO3 is an ionic compound
Ag+ NO3-     so the silver ion gains an electron
                                         Silver is reduced
                           CHEM1405                     11
Summary of views of redox reactions




              CHEM1405           12
                     Oxidation Numbers
• Just the charge on a simple
  ion
• (rules for covalent compounds
  coming up next)
• Increase in oxidation
  number – oxidation
• Decrease in oxidation
  number – reduction
The oxidation number of an element is a means of designating the number of electrons
that its atoms lose, gain, or share in forming compounds
                                           CHEM1405                         13
        Oxidation Number Examples
Ionic compounds
    • NaCl is an ionic compound made up of
                  Na+ and Cl- ions
    Oxidation number of Na+ is +1
    Oxidation number of Cl- is -1

    • CaCl2
                 Ca2+ and Cl- ions
    Oxidation number of Ca2+ is +2
    Oxidation number of Cl- is -1
                       CHEM1405              14
          Oxidation Numbers
• Fairly straightforward for ionic compounds
• For covalent compounds (remember sharing
  of electrons) need some rules

  Following rules listed by priority
  Earlier rules take precedence




                       CHEM1405            15
               Oxidation numbers
1. The total of the oxidation numbers of all the atoms
  in a neutral molecule, an isolated atom, or a
  formula unit is 0.
Examples:
The oxidation number of the Fe atom is 0.
The sum of the oxidation numbers of all the atoms in
  each of the following molecules
               Cl2 , S8 , and C6H12O6 is 0,
The sum of the oxidation numbers of the ions in
  MgBr2 is 0
                         CHEM1405                 16
           Oxidation numbers
2. In their compounds, the Group 1A metals all
 have an oxidation number of +1, and the Group
 2A metals have an oxidation number of +2.
Examples:
The oxidation number
of Na in Na2SO4 is +1,
of Ca in Ca3(PO4)2 is +2


                   CHEM1405               17
                   Oxidation numbers

   3.In its compounds, hydrogen has an
     oxidation number of +1
   Examples:
   The oxidation number of H is +1
   in HCl, H2O, NH3 , and CH4

The principal exception to rule 3 is when H is bonded to an
element that is less electronegative than itself, as in metal hydrides.

                                CHEM1405                           18
             Oxidation numbers
4. In its compounds, oxygen has an oxidation
  number of -2.
Examples:
The oxidation number of O is -2
in CO, CH3OH, and C6H12O6


  The principal exception to rule 4 is when oxygen is
  bonded to itself, as in peroxides (for example, H2O2).

                         CHEM1405                          19
           Oxidation numbers
5. In their binary (two-element) compounds with
  metals, Group 7A elements have an oxidation number
  of -1, Group 6A elements have an oxidation number
  of -2, and Group 5A elements have an oxidation
  number of -3.

Examples:
The oxidation number
Of Br in CaBr2 is -1 because Br is in group 7A
Of S in Na2S is -2 because S is in group 6A
of N in Mg3N2 is -3 because N is in group 5A
                     CHEM1405                   20
      Oxidation Number Examples
I2
     Oxidation number of Iodine is 0 (rule 1)
Cr2O3
     Oxidation number of Oxygen is -2 (rule 4)
     Oxidation number of Chromium is +3 (rule 1)
AlCl3
     Oxidation number of chlorine is -1 (rule 5)
     Oxidation number of Aluminum is +3 (rule 1)


                         CHEM1405                  21
  Oxidation Number Examples
Na2SO4
  Oxidation number of Na is +1 (Rule 2)
  Oxidation number of O is -2 (rule 4)
  Oxidation number of S is +6 (rule 1)
CaH2
                               NEEDS WORK
  Oxidation number of Ca is +2 (Rule 2)
  Oxidation number of H is -1 (rule 3 exception)




                     CHEM1405                      22
 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



                     CHEM1405             23
 Identify the Oxidizing Agent and
          Reducing Agent
2 C + O2  2 CO
             Carbon gains oxygen and is oxidized
              therefore it must be Reducing agent
             Oxygen(O2) must be oxidizing agent
N2 + 3 H2  2 NH3
          Nitrogen gains hydrogen and is reduced
                       So it is the oxidizing agent
    Therefore Hydrogen (H2) is the reducing agent

                     CHEM1405                    24
 Identify the Oxidizing Agent and
          Reducing Agent
SnO + H2  Sn + H2O
                SnO loses oxygen and is reduced
              therefore it must be oxidizing agent
            Hydrogen(H2) must be reducing agent
Mg + Cl2    Mg2+ + 2 Cl-
       Magnesium loses electrons and is oxidized
                        So it is the reducing agent
    Therefore chlorine (Cl2) is the oxidizing agent

                     CHEM1405                    25
                  Oxygen
•   Abundant oxidizing agent
•   Almost 2/3 of mass of humans is O
•   Found in nature as O2
•   ~21% of Earth’s atmosphere




                     CHEM1405           26
                     Ozone

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



                       CHEM1405                27
      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




                      CHEM1405          28
• 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


                    CHEM1405              29
           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


                     CHEM1405              30
      Identifying Redox Reactions
In an oxidation-reduction reaction,

the oxidation number of one or more elements
  increases—an oxidation process—

and the oxidation number of one or more elements
  decreases—a reduction process.

A redox reaction must always have both an
  oxidation and a reduction.
                      CHEM1405                 31
                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)



                        CHEM1405                 32
Half-Reaction Practice Problems
Mg + Cl2  Mg2+ + 2 Cl-
Half reactions
         Mg  Mg2+ + 2e-
   Cl2 + 2e-  2 Cl-

Note that not only do the atoms balance
 but the electrons also balance

                  CHEM1405                33
            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




                     CHEM1405                  34
  Electrochemical
        Cell
• Anode – where
  oxidation occurs
• Cathode – where
  reduction occurs




                     CHEM1405   35
                        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




                              CHEM1405              36
           Lead Storage Batteries
• Battery: series of
  electrochemical cells
• Readily recharged
• Durable but are heavy
  and contain H2SO4




                          CHEM1405   37
  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



                       CHEM1405                38
                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




                     CHEM1405           39
      Other Types of Corrosion
• Aluminum corrodes to produce Al2O3 on
  surface
  – Al2O3: very hard! so it prevents further
    corrosion of Al
  – Anodization is the process to grow thick layers
    of Al2O3 on aluminum
• Al2O3 corrodes in presence of Cl–
  – Why can you not use aluminum boats on the
    ocean?

                       CHEM1405                       40
• 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+




                      CHEM1405               41
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

                    CHEM1405                    42
Applications of Redox Reactions
Thermite reaction
   Fe2O3 + 2 Al  Al2O3 + Fe

Highly Exothermic

Used for underwater welding


                    CHEM1405   43
             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)

                        CHEM1405                    44

								
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