LEO the lion goes GER. If one substance is oxidized, anothe

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Outline 3/30/07

• Pick up Exam 2 from me
• Pick up CAPA 17 - outside
• Seminar today _at_ 4pm

• Today Start Chapter 19

• Redox reactions
• Balancing redox reactions

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Exam 2

• Hard exam?

Well done! 99, 99, 94, 92, 92, 89, 89, 88, 86,
86
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19.1 Recognizing Redox ReactionsDefinition Any
reaction which involves a change of oxidation
states for some of the species involved.
FeO(s) CO (g) ? Fe (s) CO2(g)

• Iron Oxide loses oxygen and is reduced.
• CO gains an oxygen and is oxidized.

What is actually happening atoms are gaining or
losing electrons
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Definitions!

• Reducing Agent (RA) agent that brings about
  reduction, RA is oxidized
• Oxidizing Agent (OA) agent that brings about
  oxidation, OA is reduced

RA loses electrons (is oxidized) OA gains
electrons (is reduced) LEO the lion goes GER
If one substance is oxidized, another substance
must be reduced. Thus the reactions are called
REDOX.
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Oxidation Numbers the electric charge an atom
has, or appears to have

• Treat polyatomic ions separately.
• Sum of all the oxidation numbers must equal the
  charge of the species.
• Hydrogen usually has an oxidation number of 1.
• The most electronegative atom in a species has a
  negative oxidation number equal to the number of
  electrons needed to complete its valence octet.

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Oxidation Number Hints

• 5. A pure element has an oxidation number of
  zero Mg(s), Cu(s), etc....
• For monatomic ions, the oxidation number is equal
  to the charge on the ion (Ca2 has a oxidation
  number of 2).
• 7. The oxidation number of H is 1 and O is
  2 in most compounds.
• Exceptions
• When H is bonded to a metal, it becomes a 1.
  For example, in CaH2, Ca2 and H-1).
• Oxygen can have a 1 charge only when it is
  called a peroxide (H2O2)the rest of the time it
  is -2 (except as O2 0).

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Example 1
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• SO4 2- Overall charge -2
• Assign oxidation number for most electronegative
  element first
• Oxidation number for Oxygen
• 6-8 -2
• Now find Oxidation number for S

• Oxidation number for sulfur 6

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Example 2

• Assign oxidation numbers to all atoms in these
  two reactions involving iron
• FeO(s) CO (g) ? Fe (l) CO2(g)
• FeCO3 (s) ? FeO(s) CO2(g)

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19.2 Balancing Redox Reactions

• You must balance electrons as well as atoms.
• The number of electrons transferred in a reaction
  may not be obvious.
• Oxidation and reduction always happens in pairs.
• If electrons are being lost by one species, then
  they must be gained by another.
• So, we separate the reaction into oxidation and
  reduction using half-reactions.

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How to balance redox rxns
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• Identify charge states is it a redox rxn?
• Separate into half reactions LEO-GER
• Balance each half-reaction for mass
• (adding H2O, H, OH-)
• Balance each half-reaction for charge e-
• Equate charge
• Add half-reactions together

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Example

• Separate the following unbalanced redox
  processes into half-reactions. All occur in
  aqueous solution
• Cr2O72-(aq) Fe2(aq) ? Cr2(aq) Fe3(aq)
• H2O2(aq) SO3- (aq) ? SO42-(aq)
• BrO2-(aq) ? BrO3-(aq) Br-(aq)