Electrochemistry Part III: Reduction Potentials

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ElectrochemistryPart III Reduction Potentials

• Dr. C. Yau
• Fall 2009

Based on Brady Senese 5/eChap. 19 Sec 2 3
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Electrical Potential

• Every substance has the potential to gain
  electrons, or be reduced in oxidation state
• The relative ease of gaining electrons is termed
  the reduction potential, and is symbolized Ered
• If the matter being observed is in standard state
  then E is termed the standard reduction potential
  and is symbolized as E0red.
• (Often "red" is omitted and given as Eo)
• Units are V (volts)
• 1V1J/C where C is coulomb, a unit of charge.

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Standard Reduction Potentials (E0red)

• E0 are tabulated for nearly every known
  substance.
• A high value of E0 (E0 gt 0) means that the
  substance is easily reduced
• E0 is a relative number, arbitrarily determined.

• All substances are compared to H, which has a E0
  of 0.00 V.

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Cell Potentials

• The standard cell potential is calculated as
• E0cell E0cath-E0anode
• If the cell is non-standard
• EcellEcath-Eanode
• In spontaneous redox reactions, the cathode
  portion of the reaction has a higher reduction
  potential than that of the anode (EcathgtEanode)

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• Making Use of Standard Reduction Potentials Table
  (Table 19.1 p.779)
• Example 19.3 p.782
• What spontaneous reaction occurs if Cl2 and Br2
  are added to a solution that contains both Cl-
  and Br-? Assume standard states.
• What are the possibilities?
• Cl2 (g) 2e- 2Cl- (aq) Eo 1.36
• Br2 (g) 2e- 2Br- (aq) Eo 1.07
• What is Eo for Cl2 Br-?
• what is Eo for Br2 Cl-?

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Example 19.4 p.783

• Predict the reaction that will occur, at 25oC,
  when Ni and Fe are added to a solution that is
  1.00 M in both Ni2 and Fe2.
• Which reaction is at the cathode?
• Do Pract Exer 5, 6, 7 p.783

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Helpful Tips

• For a reaction to be spontaneous,
• the half-rxn with the larger (more positive)
  standard reduction potential Eo takes place as
  REDUCTION
• the other half-rxn will be reversed and occur as
  oxidation (change the sign of Eo)
• and ADD the two Eo together.
• This is the same as saying "to subtract one Eo
  from the other".

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Example 19.5 p.784

• A typical cell of a lead storage batter of the
  type used to start automobiles is constructed
  using electrodes made of lead and lead(IV) oxide
  PbO2 and with sulfuric acid as the electrolyte.
  The half-rxns their standard reduction
  potentials in this system are shown below. What
  is the cell rxn and what is the standard
  potential of the cell?
• PbSO4 (s) H(aq) 2 e- Pb(s) HSO4?
  (aq) Eo -0.36V
• PbO2(s)3H(aq)HSO4?(aq) 2e? PbSO4
  (s)2H2O (l)
• Eo 1.69V

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Example 19.6 p. 785

• At standard state, what would be the cell
  reaction and the standard cell potential of a
  galvanic cell employing the following half
  reactions?
• Al3 (aq) 3e- Al(s)
• Cu2 (aq) 2e- Cu(s)
• What is the overall net reaction?
• Note We do not multiply the Eo by two.
• Do Pract Exer 8, 9, 10 p.785

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Example 19.7 p. 787

• Determine whether the following rxns, at standard
  state, are spontaneous as written. If not, write
  the eqn for the rxn that is.
• Cu (s) 2H (aq) ?? Cu2 (aq) H2 (g)
• 3Cu(s) 2NO3?(aq) 8H(aq) ??
  3Cu2(aq) 2NO(g) 4 H2O(l)
• Do Pract Exer 11, 12 p.787

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Redox Trends in Periodic Table

• Be sure to review Sec 8.5 p.317
• Reactivities of metals and nonmetals are related
  to electronegativity.
• Metals react by losing electrons to form cations.
  Is this oxidation or reduction?
• Write a general eqn for metals reacting.
• The most reactive metal or active metal are
  ones that can lose electrons most readily (the
  most electronegative or least?)
• What is the general trend of ease of oxidation?
  General trend of ease of reduction?

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Redox Trends in Periodic Table

• Nonmetals react by gaining electrons to form
  anions. Is this oxidation or reduction?
• Write a general equation of a nonmetals reacting,
  using Group VIIA as an example.
• The most reactive nonmetals are ones that can
  gain electrons most readily (the most
  electronegative or least?)
• What is the general trend of ease of reduction?
  General trend of ease of oxidation?

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What do metals tend to do?

• Metals have tendencies to...
• be oxidized.
• be reduced.
• Metals tend to be
• oxidizing agents.
• reducing agents.

M ? M
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What do nonmetals tend to do?

• Nonmetals with high electronegativities have
  strong tendencies to...
• be oxidized.
• be reduced.
• Which two elements are the most powerful
  oxidizing agent?
• Ans. F2 followed by O2

X ? X-
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General Trends

• most
  active nonmetals
• (as oxidizing agents)
• easily reduced
• most active metals
• (as reducing agents)
• easily oxidized

Periodic Table
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Protection Against Corrosion

• What exactly is "corrosion" of a metal?
• We often talk about a metal being "oxidized."
• When an iron nail rusts, what exactly is it
  doing? What is rust?
• A simplified view of the rusting process
• Fe (s) Fe2O3 (s) oxidation or
  reduction?
• Why does iron rusts more quickly in contact with
  a metal such as Cu?
• It rusts more slowly in contact with other
  metalssuch as what?

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Facts about Rusting of Iron

• does not rust in dry air (moisture must be
  present)
• does not rust in air-free water (O2 must be
  present)
• loss of Fe and deposit of rust often occur at
  different places on the same object
• rusts more quickly at low pH
• rusts more quickly in contact with ionic
  solutions
• rusts more quickly in contact with a less active
  metal such as Cu
• rusts more slowly in contact with a more active
  metal such as Zn or Mg.

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• Fe acts as both anode cathode.
• Salt water acts as salt bridge.
• H is a catalyst speeds up process.

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Elimination of Corrosive Factors

• Keep dry
• Rinse off road salts (remove electrolytes)
• Protect with paint (keep oxygen, moisture, salts
  and acids out)
• What about contact with other metals?

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• Let's examine cell potentials of some metals
• Cu2 2e- ? Cu Eo 0.34 V
• Sn2 2e- ? Sn Eo -0.14 V
• Fe2 2e- ? Fe Eo - 0.44 V
• Zn2 2e- ? Zn Eo - 0.76 V
• Mg2 2e- ? Mg Eo - 2.37 V
• "Active metal" refers to metals that oxidizes
  easily, ones that do not reduce easily.
• Metals less active than Fe can oxidize it.
• Fe in contact with Cu, Fe is the anode, changing
  to Fe2.

less "active"
more "active"
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Cathodic Protection

• Cu2 2e- ? Cu Eo 0.34 V
• Sn2 2e- ? Sn Eo -0.14 V
• Fe2 2e- ? Fe Eo - 0.44 V
• Zn2 2e- ? Zn Eo - 0.76 V
• Mg2 2e- ? Mg Eo - 2.37 V
• If we place more active metal, such as Mg, in
  contact with Fe, then Fe is the cathode and no Fe
  is lost as Fe2.
• O2 is oxidizing the Mg instead of the Fe.
• e- from Mg (anode) travels thru Fe (cathode) to
  oxygen which is reduced to OH-

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• An active metal such as Mg or Al, is connected to
  underground Fe pipes to prevent corrosion. The
  active metal is "sacrificed" instead of the Fe.
  It has to be replaced from time to time.

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What about other metals?

• How do you protect other metals?
• Cu2 2e- ? Cu Eo 0.34 V
• Sn2 2e- ? Sn Eo -0.14 V
• Fe2 2e- ? Fe Eo - 0.44 V
• Zn2 2e- ? Zn Eo - 0.76 V
• Mg2 2e- ? Mg Eo - 2.37 V
• Just remember you would use a more active metal.
• What does "active" mean?
• Sacrificial metal is the more active metal.