History

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History
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Ellingham diagram

• Other possible reactions are
• (b) C(s) ½ O2(g) ? CO(g)
• (c) CO(g) ½ O2(g) ? CO2(g)

bronze Cu/Sn alloy brass Cu/Zn alloy
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Iron smelting
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Half-reactions

• 2H (aqu) 2e? ? H2(g) ?Gf? ? 0
• H 1 H2 pressure
  1 atm
• shorthand notation is H/H2 redox couple
• 1. ?G? ?nFE?
• n number of e? transferred
• F Faradays constant 96480 C / mol
• E? std. potential for a rxn or half-rxn
• E? gives ?G? and v.v. (thermodynamic data
  can be used to calc E?)
• note 1 kJ 1000 CV, so 1 eV ? 100 kJ/mol
  nE? ?G?

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Standard cell and potentials
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Half-reactions
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Nernst equation

• . Nernst equation
• E E? ? (0.059 / n) log Q
• Q reaction quotient, for aA ? bB cC
  Q Bb Cc / Aa
• Ex 2H2O ? O2(g) 4H(aqu) 4e?
  E? ?1.23V at pH0
• Ex What is the half-reaction potential to
  oxidize water at pH 2?
• E E?? (0.059/4)log H4 ?1.23V
  0.059(?pH) -1.23V 0.12V -1.11V
• Ex What is the water reduction potential at pH
  2?
• 2e? 2H(aqu) ? H2(g) E? 0 V
  at pH0
• E 0V ? (0.059/2) log 1/H2 0V ?
  0.059(?pH) ?0.12V

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Stability field for water
Note that E(O2/H2O) ? E(H2O/H2) 1.23V (pH
independent) E (V) 2e?
2H(aqu) ? H2(g) 0.00 - 0.059pH H2O
? ½O2(g) 2H 2e? -1.23 0.059pH H2O
? H2(g) ½ O2(g) -1.23V
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Kinetic factors

• Some redox reactions have slow kinetics, rates
  can be increased when overall Erxn gt 0.6V (high
  overpotential exists)
• Converse statement kinetically slow reactions
  may not occur at appreciable rates if Erxn lt 0.6
  V
• Examples of rapid reactions
• 1. Erxn gt 0.6V
• 2. outer-sphere mechanisms
• reaction does not make/break strong bonds
  or change coordination geometry
• Ex e? Fe(CN)63?(aqu) ?
  Fe(CN)64?(aqu) E? 0.38V
• hexacyanoferrate(III)
  hexacyanoferrate(II)
• ferricyanate ferrocyanate
• Ex e? Fe(?5?C5H5)2 ?
  Fe(?5?C5H5)2 E? 0.31V
• ferrocenium ferrocene

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Kinetic factors

• Examples of slow reactions
• 1. Erxn lt 0.6V
• 2. Reactions that make/break strong bonds
• Ex. reactions with H2, N2, O2 (water redox
  chemistry, N2 fixation)
• Reactions where n gt 1

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Kinetic factors

• surface passivation
• Ex Al anodization pH 7
• 2Al(s) 6OH?(aqu) ? Al2O3(s)
  3H2O 6e? E? 1.7V
• 1 ?m Al2O3 passive surface forms during
  reaction and acts as a barrier to OH- and O2
• Ex Si(m) in air forms a 30nm SiO2
  native oxide passivation layer

Gate 1.0 nm SiO2 on Si
http//nano.boisestate.edu/research-areas/gate-oxi
de-studies/
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Combining half-rxns

• Combining red red (or ox ox) half-reactions
• E? / V ?G? / kJ/mol
• 1. e? Mn3 ? Mn2 1.5 ?148
• 2. e? MnO2 4H ? Mn3 2H2O 0.95
  ?92
• 3. 2e? MnO2 4H ? Mn2 2H2O 1.23
  ?240
• E3 (n1E1 n2 E2) / n3 (1)(1.5)
  (1)(0.95) / 2 1.23V
• Combining red ox half-reactions
• 1. e? Mn3 ? Mn2 1.5V
• 2. 2H2O Mn3 ? e? MnO2 4H ?0.95V
• 2H2O 2Mn3 ? Mn2 MnO2 4H 0.55V
• this disproportionation is spontaneous in acidic
  soln, but slow

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Latimer Frost diagrams for Mn in acid
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Frost diagrams
prop to -?G?
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Frost diagrams
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Frost diagram for N
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pH effect

• Oxoacids are better oxidants in acidic solution
  than in basic solution
• 10e? 2HNO3 10H ? N2 6H2O E
  1.25V at pH0
• 10e? 2NO3- 6H2O ? N2 12OH?
  E 0.25V at pH14
• because they combine with H to lose oxo or
  hydroxy ligands

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Ligand effects
Note that e? Fe3(aqu) ?
Fe2(aqu) E? 0.77V But
e? Fe(CN)63?(aqu) ? Fe(CN)64?(aqu)
E? 0.38V gt cyano ligand stabilizes
Fe3 more than OH2 1.80V 0.80 AgO ?
Ag ? Ag(m) pH0 0.60 0.34 AgO ?
Ag2O ? Ag(m) pH14 1.69 Au ?
Au(m) pH0 0.60 Au(CN)2?
? Au(m) pH0
Zn(m)
Zn(CN)2(s) Au(s)
CN? poisoning ? inhibits cytochrome oxidase in
mitochondria
KOH
Zn(OH)42?(aqu) Au(s)
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Pourbaix diagram for Fe
e- Fe3 ? Fe2 E 0.77 V e-
Fe(OH)3 3H ? Fe2 3H2O E E0 -
3(0.059) pH e- Fe(OH)3 ? Fe(OH)2 OH- E
E0 - 0.059 pH
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Pourbaix diagram for Mn
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Example Group 13