Introduction to Oxidation-Reduction Reactions

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Introduction to Oxidation-Reduction Reactions

• Electron Transfer Reactions

2
Types of Chemical Reactions

• There are four types of chemical reactions
• Acid/Base
• Precipitation/Solubility
• Complex Formation/Complex Dissociation
• Oxidation/Reduction
• Any chemical reaction consists of one (or more)
  of these basic categories.

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Oxidation/Reduction Reactions

• Acid/Base reactions
• involve a donation /acceptance of protons
• Precipitation/ Solubility reactions
• involve a donation/ acceptance of negative charge
  
• what is being donated and accepted in a redox
  reaction?

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Oxidation/Reduction Reactions

• Electrons!
• Consider the reaction taking place in a
  disposable battery 2Zn 3MnO2 ? Mn3O4
  2ZnOHow can you tell that electrons are being
  donated and accepted? Which species is donating
  electron( s) and which is accepting electron (s)?

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REDOX REACTIONS

• Redox reactions are characterized by ELECTRON
  TRANSFER between an electron donor and electron
  acceptor.

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REDOX REACTIONS

• Transfer leads to
• increase in oxidation number of some element
  OXIDATION
• 2. decrease in oxidation number of some element
  REDUCTION

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Electron Transfer in Redox Reactions

• Oxidation
• Loss of electrons
• Gain in oxygen
• Reduction
• Gain of electrons
• Loss of oxygen
• LEO the lion goes Ger

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Example

• The reaction of a metal and non-metal
• All the electrons must be accounted for!

Mg
S
Mg 2
S2-

?

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Oxidation-Reduction

• Oxidation means an increase in oxidation state -
  lose electrons.
• Reduction means a decrease in oxidation state -
  gain electrons.
• The substance that is oxidized is called the
  reducing agent.
• The substance that is reduced is called the
  oxidizing agent.

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Assigning Oxidation States

• An Oxidation-reduction reaction involves the
  transfer of electrons.
• You should memorize these rules

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Rules for Oxidation States

• The charge the atom would have in a molecule (or
  an ionic compound) if electrons were completely
  transferred.
• The oxidation state of elements in their standard
  states is zero.
• Example Na, Be, K, Pb, H2, O2, P4 0

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Assigning Oxidation States

• Oxidation state for monatomic ions are the same
  as their charge.
• Example Li, Li 1 Fe3, Fe 3 O2-, O
  -2
• Oxygen is assigned an oxidation state of -2 in
  its covalent compounds except as a peroxide.

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Rules for Oxidation States

• The oxidation number of hydrogen is 1 except
  when it is bonded to metals in binary compounds.
  In these cases, its oxidation number is 1.
• Group IA metals are 1, IIA metals are 2 and
  fluorine is always 1.
• The sum of the oxidation numbers of all the atoms
  in a molecule or ion is equal to the charge on
  the molecule or ion.

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Practice in Oxidation States

• Assign the oxidation states to each element in
  the following.
• K2SO4
• NO3-
• H2SO4
• Fe2O3
• Fe3O4

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Identify the

• Oxidizing agent
• Reducing agent
• Substance oxidized
• Substance reduced
• On the worksheet

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Types of Oxidation-Reduction Reactions
Combination Reaction
0
0
4
-2
Decomposition Reaction
1
5
-2
1
-1
0
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Types of Oxidation-Reduction Reactions
Displacement Reaction a.k.a Single Replacement
0
1
2
0
Hydrogen Displacement
0
4
0
2
Metal Displacement
0
-1
-1
0
Halogen Displacement
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The Activity Series for Metals
Hydrogen Displacement Reaction
M is metal BC is acid or H2O B is H2
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Copper Demonstration

• Copper Pennies reacting with nitric acid.
• Can you figure out the equation?

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Types of Oxidation-Reduction Reactions
Disproportionation Reaction
Element is simultaneously oxidized and reduced.
1
-1
0
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Half-Reactions

• All redox reactions can be thought of as
  happening in two halves.
• One produces electrons - Oxidation half.
• The other requires electrons - Reduction half.

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Half-Reactions

• Write the half reactions for the following.
• Na Cl2 ? Na Cl-
• SO3- H MnO4- ? SO4- H2O Mn2

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Balancing Redox Equations

• In aqueous solutions the key is the number of
  electrons produced must be the same as those
  required.
• For reactions in acidic solution an 8 step
  procedure.

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Balancing Redox Equations

• Write separate half reactions
• For each half reaction balance all reactants
  except H and O
• Balance O using H2O

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Acidic Solution

• Balance H using H
• Balance charge using e-

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Acidic Solution

• Multiply equations to make electrons equal
• Add equations and cancel identical species
• Check that charges and elements are balanced.

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Practice

• Balance the following reactions
• Sn 2 (aq) 2Fe 3 ? Sn 4 (aq) 2Fe 2
• MnO4- (aq) C2O4-2 (aq) ? Mn2 (aq) CO2 (g)

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Practice

• The following reactions occur in aqueous
  solution. Balance them
• Cr(OH)3 OCl- OH- CrO4-2 Cl- H2O
• MnO4- Fe2 Mn2 Fe3

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Now for a tough one

• Fe(CN)6-4 MnO4- Mn2 Fe3 CO2 NO3-

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Basic Solution

• Do everything you would with acid, but add one
  more step.
• Add enough OH- to both sides to neutralize the H
  
• CrI3 Cl2 CrO4- IO4- Cl-
• CN- MnO4- ? CNO- MnO2

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Redox Titrations

• Same as any other titration.
• the permanganate ion is used often because it is
  its own indicator. MnO4- is purple, Mn2 is
  colorless. When reaction solution remains clear,
  MnO4- is gone.
• Chromate ion is also useful, but color change,
  orangish yellow to green, is harder to detect.