Lecture 42 Electrochemistry VI

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Lecture 42 - Electrochemistry VI
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Review

• Electrolytic Cells
• Eocell lt 0
• Must apply a potential more negative than Eocell
  to make reaction proceed
• Overpotential

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Today

• Analytical Electrochemistry
• Corrosion

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Being Quantitative in Electrochemistry

• Total charge passed q (C)
• Current x time i x t (A ? s)
• (C/s ? s)
• (C)
• moles e- x F nF (mol e- ? C/mol e-)
• (C)
• Thus, nF it

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electroplating, for example
Battery
electron flow
Au3(aq)
Inert Electrode
(Fe) spoon
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at the spoon...

• Au3(aq) 3 e- Au(s)
• want 10 mm thick Au on spoon (40 cm2)
• find time required if current 0.1 A

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strategy

• 1. Find mass Au required
• 2. Find moles Au
• 3. Find moles e- required
• 4. Find t required

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• 1. mass of Au volume x density
• (10 x 10-6 m x 40 x 10-4 m2) x 17.0 x 103
  kg/m3
• 6.8 x 10-4 kg Au

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• 2. moles Au mass / AW

6.8 x 10-4 kg Ag

0.200 kg mol-1
3.4 x 10-3 mol Au
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• 3. moles e- required
• Au(aq) 3 e- Au(s)
• i.e. moles e- 3 ? moles Au(s) produced
• 3 ? 3.4 x 10-3 mol e-
• 1.02 ? 10-2 mol e- ( n)

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• 4. it nF
• thus,

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Analytical Chemistry

• e.g. measurement of pH
• pH -log10H(aq)

H(aq)
H(aq)
Pt(s)
H2(g, 1 atm)
Pt(s)
H2(g, 1 atm)
unknown concentration
1 M
reference cell
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• Oxidation
• H2(g, 1 atm) ? 2 H(aq, unknown) 2 e-
• Reduction
• 2 H(aq, 1M) 2 e- ? H2(g, 1 atm)

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• 2 H(aq, 1 M) ? 2 H(aq, unknown)

H(aq, unknown)2
Q
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0.0592
but, Ecell Eo -
log10(Q)
n
0.0592
0 -
log10(H(aq, unknown)2)
2
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0.0592
0 -
log10(H(aq, unknown)2)
2
- 0.0592 log10 H(aq, unknown)
but, pH - log10 H(aq, unknown)
Thus, Ecell 0.0592 x pH (Volts)
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It works for other ions also!

• NH4 wastewater, seawater
• HCO3- blood, groundwater
• F- drinking water, blood, urine
• Br- grain
• I- milk
• NO3- soil, fertilizer, drinking water
• K blood serum, soil, wine

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Corrosion

• costs billions each year
• is electrochemical
• can be slowed or prevented by chemical means

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Corrosion of Iron
Air
Water Droplet
O2
2 Fe2 3/2 O2 H2O ? Fe2O3H2O
O2
Fe2
H
e-
Fe ? Fe2 2 e-
e-
O2 4 H 4 e- ? 2 H2O
Fe metal
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Corrosion of Iron

• overall
• 2 Fe(s) O2(g) 4 H(aq) ?
• 2 Fe2(aq) 2 H2O(l) (Eo 1.68 V)
• and
• 2 Fe2(aq) 3/2 O2(g) H2O(l) ? Fe2O3½H2O(s)
  (rust)

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Preventing Corrosion

• Keep O2 and H2O away from Fe
• or
• Use Cathodic Protection

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Cathodic Protection

• 1. Passive - sacrificial anode used
• 2. Active - externally applied voltage

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Passive Cathodic Protection

• Fe ? Fe2 2 e- Eo 0.45 V
• Zn ? Zn2 2 e- Eo 0.76 V
• Zn2 2 OH- ? Zn(OH)2(s)

favored reaction
low solubility
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Passive Cathodic Protection
Zn2
Zn metal
e-
O2 4 H 4 e- ? 2 H2O
Fe metal
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Hot-Dip Galvanizing
Fe(s)
Zn(l)
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Protecting Underground Things
Passive Cathodic Protection
e-
Metal Pipe
Mg, for instance
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Active Cathodic Protection
e-

-
Metal Pipe
Inert Electrode
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Everything you ever wanted to know about
electrochemistry...

• Galvanic Electrolytic
• Anode Oxidation(-) Oxidation()
• Cathode Reduction() Reduction(-)
• Cu2(aq) 2 e- ? Cu(s) Eo 0.34 V
• Sn2(aq) 2 e- ? Sn(s) Eo -0.14 V
• Sn(s) ? Sn2(aq) 2 e- Eo 0.14 V

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Everything you ever wanted to know about
electrochemistry...

• Eocell Eoreduction Eooxidation
• DGo - nFEocell (Eocellgt0 means reaction
  proceeds)

Ecell Eocell -
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Everything you ever wanted to know about
electrochemistry...

• Primary batteries - irreversible
• Secondary batteries - rechargeable
• q it nF
• Electrolysis - reaction with higher positive
  potential proceeds
• Corrosion