Ch' 20 OxidationReduction Reactions

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Ch. 20 Oxidation-Reduction Reactions
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Oxidation

• Originally meant the combination of an element
  with oxygen to produce oxides

2Fe (s) 3O2 (g) ? FeO2 (s)
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Oxidation

• Can involve burning gasoline or wood burns in
  air to produce CO2

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Other Oxidizing Agents

• Bleach (NaClO)
• Hydrogen Peroxide (H2O2)

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Reduction

• The opposite of oxidation- originally meant the
  loss of oxygen

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Redox Reactions

• Oxidation-reduction Reactions
• Oxidation is loss of electrons or gain of oxygen
• Reduction is gain of electrons or loss of oxygen

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Redox Reactions

• Mg S ? Mg 2 S 2-
• Magnesium Sulfur Magnesium
  Sulfur
• Atom Atom Ion
  Ion
• Mg? Mg 2 2 e- (Loses 2 e-) oxidized
• S ? S 2- (gains 2 e-) reduced

?
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• Reducing Agent the substance that loses
  electrons
• Oxidizing Agent The substance that gains
  electrons
• Mg S ? MgS

Oxidizing Agent
Oxidized
Reduced
Reducing Agent
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Covalent Compounds

• 2H2 (g) O2 (g) ? 2H2O (l)

Oxidized Gains Oxygen
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Rules for Covalent Products

• For carbon compounds the addition of oxygen OR
  the removal of hydrogen is always oxidation

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Corrosion

• Water in the environment, salts, and acids
  accelerate the rates of corrosion
• Salts and acids make the transfer of electrons
  easier

2 Fe (s) O2 (g) 2 H2O (l) ? Fe(OH)2 (s)
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Noble Metals

• Gold and platinum are called noble metals because
  they are resistant to corrosion

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Oxidation Numbers

• A positive or negative number assigned to an atom
  according to a set of rules
• Chemical book-keeping device

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Oxidation Numbers

• In binary ionic compounds, oxidation numbers
  equal ionic charges
• NaCl ? Na Cl- ? Na ox 1, Cl ox -1
• CaCl2 ? Ca 2 2Cl- ? Ca ox 2, Cl ox -1

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Oxidation Numbers

• As a general rule oxidation numbers are the
  charge if the electrons are assigned to the more
  electronegative element
• H2O
• Oxygen is more electronegative
• O Ox -2
• H Ox 1
• H2O

1 -2
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Rules for Assigning Oxidation Numbers

• 1. The oxidation number of a monatomic ion is
  equal to its charge (Fe 3 ? 3)

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Rules for Assigning Oxidation Numbers

• 2. The oxidation number for hydrogen in a
  compound is always 1 EXCEPT NaH where it is -1

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Rules for Assigning Oxidation Numbers

• 3. The oxidation number for oxygen in a compound
  is -2 EXCEPT in peroxides, such as H2O2 where it
  is -1

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Rules for Assigning Oxidation Numbers

• 4. The oxidation number of an element is always
  0 ( K ? 0, O2 ? 0)

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Rules for Assigning Oxidation Numbers

• For any neutral compound, the sum of the
  oxidation numbers must equal 0
• CaCl2 ? Ca 2 2Cl- ? 2 2- 0

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Rules for Assigning Oxidation Numbers

• For a polyatomic ion, the sum of the oxidation
  numbers must equal the ionic charge of the ion.
• NO3- ? O (-2) X 3 -6 ? N must 5
• SO42- ?S ?
• K2CrO4 ? Cr?
• K2Cr2O7 ? Cr ?

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Oxidation Number Changes in Chemical Reactions

• 1 5 -2 0 2 5 -2
  0
• 2AgNO3 (aq) Cu(s) ? Cu(NO3)2 (aq) 2Ag (s)
• Silver ions are reduced Ag ? Ag0
• Copper is oxidized Cu 0 ? Cu 2

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Examples

• Identify oxidation numbers for all atoms
• Identify which ions are being oxidized and which
  are reduced
• Identify the oxidizing and reducing agent
• 2H2 (g) O2 (g) ? 2H2O (l)

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Examples

• Identify oxidation numbers for all atoms
• Identify which ions are being oxidized and which
  are reduced
• Identify the oxidizing and reducing agent
• NH4NO2 (s) ? N2 (g) H2O (g)
• Hint consider each N in NH4NO2 separately

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Identifying Redox Reactions

• You can determine if a reaction is a redox
  reaction by using oxidation numbers to keep track
  of electrons
• N2 (g) O2 (g) ? 2NO (g)

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Balancing Redox Reactions using Oxidation Numbers

• Start with a skeleton equation (unbalanced)
• Fe2O3 (s) CO (g) ? Fe (s) CO2 (g)

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Balancing Redox Reactions using Oxidation Numbers

• Assign oxidation numbers to all atoms in the
  equation
• 3 -2 2 -2 0
  4 -2
• Fe2O3 (s) CO (g) ? Fe (s) CO2 (g)

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Balancing Redox Reactions using Oxidation Numbers

• Identify which atoms are oxidized and which are
  reduced
• 3 -2 2 -2 0
  4 -2
• Fe2O3 (s) CO (g) ? Fe (s) CO2 (g)
• Fe is reduced
• C is oxidized

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Balancing Redox Reactions using Oxidation Numbers

• Use bracketing to connect atoms that undergo
  oxidation and reduction
• 3 -2 2 -2 0
  4 -2
• Fe2O3 (s) CO (g) ? Fe (s) CO2 (g)

reduced -3
Oxidized 2
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Balancing Redox Reactions using Oxidation Numbers

• Make the total increase in oxidation number equal
  to the total decrease in oxidation number by
  using appropriate coefficients
• 3 -2 2 -2
  0 4 -2
• Fe2O3 (s) 3CO (g) ? 2Fe (s) 3CO2 (g)

-3 x 2 -6
2 x 3 6
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Balancing Redox Reactions using Oxidation Numbers

• Check to make sure the equation is balanced for
  charges AND atoms
• Fe2O3 (s) 3CO (g) ? 2Fe (s) 3CO2 (g)

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Balancing Redox Reactions using Half Reactions

• Half-reactions are equations that show either the
  reduction or the oxidation of a species in a
  redox reaction

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Balancing Redox Reactions using Half Reactions

• S (s) HNO3 (aq) ? SO2 (g) NO (g) H2O (l)
• 1. Write the unbalanced equation in ionic form
• S (s) H (aq) NO3- (aq) ? SO2 (g) NO (g)
  H2O (l)

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Balancing Redox Reactions using Half Reactions

• 2. Write separate half reactions for the
  oxidation and reduction processes
• S (s) H NO3- (aq) ? SO2 (g) NO (g) H2O
  (l)
• 0 4 -2
• Oxidation S (s) ? SO2 (g)
• 5 -2 2
  -2
• Reduction NO3- (aq) ? NO (g)

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Balancing Redox Reactions using Half Reactions

• Balance the atoms in the half reactions
• Oxidation S (s) 2H2O (l) ? SO2 (g) 4H (aq)
• Reduction4 H(aq) NO3- (aq)?NO (g)2H2O (l)

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Balancing Redox Reactions using Half Reactions

• Add sufficient electrons to one side of the
  reaction to balance the charges
• Oxidation
• S (s) 2H2O (l) ? SO2 (g) 4H (aq) 4 e-
• Reduction
• 4 H(aq) NO3- (aq) 3 e- ?NO (g)2H2O (l)

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Balancing Redox Reactions using Half Reactions

• 5. Multiply each half reaction by an appropriate
  number to make the number of electrons equal on
  both sides
• Oxidation
• (S (s) 2H2O (l) ? SO2 (g) 4H (aq) 4 e-)3
• 3S (s) 6H2O (l) ? 3SO2 (g) 12H (aq) 12 e-
• Reduction
• (4 H(aq) NO3- (aq) 3 e- ?NO (g)2H2O (l))4
• 16 H(aq) 4NO3- (aq) 12e- ?4NO (g)8 H2O (l)

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Balancing Redox Reactions using Half Reactions

• Add the half reactions to show an overall
  equation
• Oxidation
• 3S (s) 6H2O (l) ? 3SO2 (g) 12H (aq) 12 e-
• Reduction
• 16 H(aq) 4NO3- (aq) 12e- ?4NO (g)8 H2O (l)
• 3S (s) 6H2O (l) 16H(aq) 4NO3- (aq) 12e-
  ? 3SO2 (g) 12H(aq) 12 e- 4NO (g)8H2O (l)

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Balancing Redox Reactions using Half Reactions

• Subtract terms that are on both the left and the
  right side (spectator ions)
• 3S (s) 6H2O (l) 16H(aq) 4NO3- (aq) 12e-
  ? 3SO2 (g) 12H (aq) 12 e- 4NO (g)8H2O (l)
• 3S (s) 4H(aq) 4NO3- (aq) ? 3SO2 (g) (aq)
  4NO (g) 2H2O (l)

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Balancing Redox Reactions using Half Reactions

• 8. Add the ions and balance the equation
• 3S (s) 4H(aq) 4NO3- (aq) ? 3SO2 (g) (aq)
  4NO (g) 2H2O (l)
• 3S (s) 4HNO3 (aq) ? 3SO2 (g) (aq) 4NO (g)
  
• 
  2H2O (l)