Introduction to Electrochemistry Ch 20

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Introduction to Electrochemistry Ch 20
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Oxidation/Reduction

• Any reaction which involves a change in oxidation
  numbers transfer of electrons from one substance
  to another
• Oxidation and reduction always occur together
• Historically Oxidation was the addition of
  oxygen to a substance
• Oxidation Loss of electrons
• Reduction Gain of electrons
• LEO the lion says GER or OIL RIG
• Not all reactions are redox. If there is no
  change in ox numbers, they are not redox. Ex
  acid/base reactions

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Identifying Oxidation/Reduction

• Mg S ? Mg2 S-2 ? MgS
• Magnesium lost electrons Oxidized
• Sulfur gained electrons Reduced
• Mg ? Mg2 2e- Oxidation ½ reaction
• S 2e- ? S-2 Reduction ½ reaction
• Reducing Agent Substance that was oxidized
  (Magnesium)
• Oxidizing Agent Substance that was reduced
  (Sulfur)

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Practice with Covalent Compounds

• 2H2 O2 ? 2H2O
• Assign oxidation numbers to determine what is
  oxidized and what is reduced.
• Know the oxidation number rules on page 639!!!

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Practice

• Silver nitrate reacts with copper to produce
  copper (II) nitrate and silver.
• What is oxidized and what is reduced?
• Write the ionic equation
• Write the oxidation ½ reaction
• Write the reduction ½ reaction

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Balancing Redox Reactions

• Oxidation Change Method (Line Connect Method)
  pg 647-649
• Start with the skeleton equation
• Assign oxidation numbers to all atoms
• Identify which are oxidized and which are reduced
• Use bracketing lines to connect the changing
  atoms and write in the ox change
• Make the total e- lost equal to total gained
• Assign coefficients and check for balancing.
  Adjust water as needed

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Examples

• Hydrogen sulfide reacts with nitric acid to
  produce sulfuric acid, nitrogen dioxide, and water

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Balancing Redox Reactions

• Half-Reaction Method pg 650-651
• Start with the skeleton equation
• Write the complete ionic equation and assign
  oxidation numbers
• Write separate ½ reactions (ignore spectator
  ions)
• Balance the atoms in the ½ reactions

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Half-Reaction Method continued

• Balance other elements first, then balance
  Oxygen and Hydrogen
• If in acidic solution, use H2O and H
• If in basic solution, use OH- and H2O
• Balance charges by adding e-
• Make e- equal in both by multiplying each ½
  reactions

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Half-Reaction Method continued

• Add the balanced ½ reactions
• Add the spectator ions back in and balance the
  equation (you may simplify back into a formula
  equation from an ionic equation)

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Practice Problem

• Hydrogen sulfide reacts with nitric acid to
  produce sulfuric acid, nitrogen dioxide, an
  water.
• Some possibly helpful links
• http//www.chemguide.co.uk/inorganic/redox/equatio
  ns.html
• http//www.chemtutor.com/redox.htm

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Balancing Redox Equations
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Redox Reactions

• Environment acidic or basic is very important
• Particles available but sometimes are not given
  in the reaction

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acid hydrogen cation water base hydroxide
ion water
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Identify species undergoing

• 1. oxidation
• 2. reduction

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Split overall reaction into

• 1. Oxidation half-reaction
• 2. Reduction half-reaction

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Half-reaction in ACID Soln

• 1. Balance species changing oxidation
• 2. Balance oxygen by adding H2O

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Half-reaction in ACID Soln

• 3. Balance hydrogen by adding H
• 4. Balance charge by adding e-

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Half-reaction in ACID Soln
5. Add the two half- reactions eliminating
all electrons (least common multiple
concept)
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CHECK final equation for BALANCE of ATOMS and
CHARGE
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1. in acid AuCl4-(aq) AsH3(g) ? H3AsO3(aq)
Au(s) Cl-(aq)
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Half-reaction in BASIC Soln

• 1. Balance species changing oxidation
• 2. Balance oxygen by adding twice as many OH-

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Half-reaction in BASIC Soln

• 3. Balance hydrogen by adding H2O
• 4. Balance charge by adding e-

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Half-reaction in BASIC Soln
5. Add the two half- reactions eliminating
all electrons (least common multiple
concept)
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2. in base Am3(aq) S2O82-(aq) ?
AmO2(aq) SO42-(aq)
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OR Balance by acidic method then neutralize
the H by adding OH- and adjusting the H2O
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3. MnO4-(aq) H2C2O4(aq) ?
Mn2(aq) CO2(g)
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4. Bi(OH)3 SnO22- ?
Bi(s) SnO32-