Lecture 41 Electrochemistry V

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Lecture 41 - Electrochemistry V
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Review

• Galvanic Cells
• Reaction is spontaneous
• Eocell gt 0
• The product is an electrical current
• Some can be reversed (recharged)

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Electrolytic Cells

• Reaction is not spontaneous
• Eocell lt 0
• Used for producing a chemical product

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Electrolysis of Water

• We want
• 2 H2O(l) ? 2 H2(g) O2(g)
• 2 H2O(l) ? O2(g) 4 H(aq) 4 e- Eo -1.23 V
• 2 H2O(l) 2 e- ? H2(g) 2 OH-(aq) Eo -0.83 V
• Eocell -2.06 V
• i.e. non-spontaneous!

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Electrolysis of Water
H2O(l) salt
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Electrolysis of Water
Note reversal of polarity!
battery
-

H2O(l) salt
anode
cathode
2 H2O 2 e- H2(g) 2 OH-
2 H2O O2(g) 4 H 4 e-
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Electrowinning of Metals
-

CuCl2(aq)
anode
cathode
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• at the cathode
• Cu2 Cu(s) ??
• or
• H2O H2(g) ??

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Rule of thumb

• The cathode reaction with the most positive
  reduction potential will occur preferentially

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• Cu2(aq) 2 e- ? Cu(s) Eo 0.15 V
• 2 H2O(l) 2 e- ? H2(g) 2 OH- Eo - 0.83 V

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at the anode Cl- Cl2(g) ?? or H2O O2(g) ??
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Same rule of thumb

• The anode reaction with the most positive
  oxidation potential will occur preferentially

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Same rule of thumb

• 2 Cl-(aq) ? Cl2(g) 2 e- Eo -1.36 V
• 2 H2O(l) ? O2(g) 4 H 4 e- Eo - 1.23 V
• Thus, we predict water gets oxidized at the anode.

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Overpotential

• Erequired gt Eocell for electrolysis
• In this case, Cl- is oxidized

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Electrowinning of Metals
-

CuSO4(aq)
anode
cathode
2 Cl- Cl2(g) 2 e-
Cu2(aq) 2 e- Cu(s)
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Overall Reaction

• Cu2(aq) 2 Cl-(aq) ? Cu(s) Cl2(g)
• Eocell -1.21 V

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Electrorefining of Metals
-

Impure Cu
Pure Cu
Cu
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Making Fluorine, F2(g)

• Electrolysis of HF 2 KF(l)
• oxidation 2 F- F2(g) 2 e-
• reduction 2 H 2 e- H2(g)

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Making Chlorine, Cl2(g)

• Electrolysis of brine (NaCl(aq))
• oxidation
• 2 Cl-(aq) Cl2(g) 2 e-
• reduction
• 2 H2O(l) 2 e- H2(g) 2 OH-(aq)

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Overall,

• 2 NaCl(aq) 2 H2O(l) Cl2(g) H2(g)
• 2 NaOH(aq)

cleaners, paper, etc
disinfection, plastics (PVC)
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Manufacture of Aluminum

• Why not just electrowin Al3(aq)?
• Al3(aq) 3 e- Al(s) Eo -1.66 V
• 2 H2O(l) 2 e- H2(g) 2 OH-(aq) Eo
  -0.83 V
• (reduction of water is favored)

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Manufacture of Aluminum

• Bauxite impure Al2O3(s)

m.p. 2045oC
Solution? 1. Lower the melting point 2. Perform
electrolysis
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Manufacture of Aluminum

• Al2O3(s) AlF3(s) 3 NaF(s)

Na3AlF6, cryolite
m.p. 900oC
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Manufacture of Aluminum

• reduction AlFxOy3-x-2y n e- Al(l) ?
• oxidation AlFxOy3-x-2y O2(g) n e- ?

sinks!
oxidizes C electrodes!