Electrochemistry

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Electrochemistry

• Electrochemical Process
• Chemical E -gt Electrical E
• OR
• Electrical E -gt Chemical E

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Re-dox Rxns

• LEO goes GER
• Half rxns show either oxidation or reduction
  alone
• e-s lost gained must be equal 1/2 rxns can be
  added to give the overall balanced equation

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Balancing Equations by the Half Rxn Method

• If the rxn is in acidic conditions, H and H2O
  can be added to either the R or P to balance H
  and O
• In basic conditions, OH- and H2O can be added
• MnO4-(aq) C2O42-(aq) -gt Mn2(aq) CO2 (aq)

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MnO4-(aq) C2O42-(aq) -gt Mn2(aq) CO2 (aq)

• Divide equation into 2 incomplete half rxns.
• Balance ea half rxn
• Balance the elements other than H O
• Balance O by adding H2O
• Balance H by adding H
• Balance the charge by adding e-s to the side w/
  the greater positive charge
• Multiply ea half rxn by an integer so e-s gained
  lost balance.
• Add 2 half rxns, cancel like species.
• Check to see it is balanced.

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Voltaic Cells

• The first battery
• Also called a Galvanic Cell
• Energy (e-s) produced in a spontaneous redox rxn
  are transferred through an external pathway so
  can be used to perform electrical work

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Voltaic Cells-cont

• Consists of 2 half cells
• Composed of a metal in a soln of its own ions
• A diff metal in a soln of its own ions makes up
  the other half cell
• An external circuit connects the metals

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Voltaic Cells-cont

• Anode electrode where oxidation occurs
• Cathode electrode where reduction occurs
• An Ox A Red Cat Studied Redox
• Anode Oxidation (LEO)
• Cathode Reduction (GER)
• E-s flow fr A -gt C
• Anode -gt Cathode
• Salt bridge- porous U-shaped tube of electrolyte
  th separates the half cells
• Needed to maintain neutrality by allowing
  migration of ions

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Cell EMF

• Std Reduction Potential Table
• -provides reduction voltages of half cells under
  std conditions
• - the more the voltage value of 2 half rxns is
  the one th is reduced (cathode)
• - the less () half rxn will be oxidized (anode)

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Cell EMF- cont

• Cell potential
• Ecell Ered (cathode) - Ered (anode)
• Intensive property, so changing coefficients WILL
  NOT change the Ecell
• The more positive the Ecell , the greater the
  driving force for reduction

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Oxidizing Reducing Agents

• The more positive the Ered value for a half-rxn,
  the greater the tendency for reduction, and so to
  oxidize another species
• Common oxidizing agents halogens, O2, and
  oxyanions such as MnO4-, Cr2O72-, and NO3-

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Oxidizing Reducing Agents-cont

• The smaller the Ered value of a half rxn, the
  more easily it is reversed for oxidation
• Common reducing agents H2, active metals such
  as the alkali alkali earth metals

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Oxidizing Reducing Agents-cont

• P.791, table 20.1 Appendix E
• Subs most readily reduced (good oxidizing agents)
  at top
• Subs most readily oxidized (good reducing agents)
  are at the bottom

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Spontaneity of Redox Rxns

• Use half-cell potentials to calc emf decide
  spontaneity
• E Ered (reduction process) - Ered (oxidation
  process)
• A positive E spontaneity

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Effect of Conc on EMF

• As a voltaic cell is discharged, the reactants
  are consumed products are generated, so the
  concs of the subs change
• The emf progressively drops, until E 0, the
  cell is dead
• Cell is at equilibrium
• Conc of P R dont change

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The Nernst Equation

• E E - 0.0592 V log Q ( T 298 K)
• n
• Q the rxn quotient (form of the Keq), w/ conc.
  at any given moment
• n the of e-s th are transferred

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Concentration Cells

• EMF depends upon conc
• Concentration cell The same metal can be both
  the anode cathode, so long as the conc used in
  the half cells is diff
• Works under nonstd conditions
• Anode cell w/ less conc
• Cathode cell w/ greater conc Calc the EMF w/
  the Nerst equation

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Electrolysis

• Passing an electric current through a sub,
  causing it to decompose into new subs.
• All ionic subs when molten or in soln can be
  electrolyzed
• Causes a nonspontaneous re-dox rxn to occur

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Electrolysis-cont

• Electrolytic Cell electrolysis apparatus
• Electrodes- 2 inert conducting rods
• Made of graphite or platinum
• Carry current into the electrolyte
• Cathode (-) electrode
• Joined to (-) battery terminal
• Attracts cations
• Anode () electrode
• Joined to () battery terminal
• Attracts anions

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Electrolysis -cont

• An Ox A Red Cat Studied Redox
• Anode Oxidation (LEO)
• Cathode Reduction (GER)
• E-s flow fr A -gt C
• Anode -gt Cathode

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Electrolysis of Molten Salts

• Can be prohibitive due to high melting pts
• 2NaCl(l) -gt 2Na(l) Cl2(g)
• Cathode 2Na(l) 2e- -gt 2Na (l)
• Anode 2Cl- (l)-gt Cl2(g) 2e-
• 2Na (l) 2Cl -(l) -gt 2Na (l) Cl2(g)
• Important industrial process for the production
  of active metals such as Na and Al

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Electrolysis of Aqueous Solns

• Complicated by the presence of water
• Water will either be
• Oxidized to produce O2(g) OR
• Reduced to produce H2(g)
• Rather than the ions of the salt

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Electrolysis of Aq Solns-cont

• Std Reduction Potential Table
• -provides reduction voltages of half cells under
  std conditions
• - the more the voltage value of 2 half rxns is
  the one th is reduced (cathode)
• - the less () half rxn will be oxidized (anode)

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Electrolysis of NaF(aq)

• Possible reactants are
• Na, F-, and H2O
• Possible reduction (cathode)
• Na or H2O
• (F- cant gain e-s)
• Possible oxidation (anode)
• F- or H2O

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Electrolysis of NaF(aq)

• Using Std red potential tables
• Na(aq) e- -gt Na(s) Ered -2.71 V
• 2H2O(l) 2e- -gt H2(g) 2OH-(aq) Ered -0.83
  V
• More () is favored for reduction
• H2(g) is produced at the cathode

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Electrolysis of NaF(aq)

• 2F-(aq) -gt F2(g) 2 e- Ered 2.87 V
• 4OH-(aq)-gt O2(g) 2H2O(l) 4e- Ered 0.40
  V
• Less () favored for oxidation
• O2(g) is produced at the anode

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Electrolysis of NaF(aq)

• Ecell Ecathode - Eanode
• Ecell -.83 - (.40) -1.23V
• a negative value is a nonspontaneous process

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Electrolysis of NaCl(aq)

• Produces H2(g), Cl2(g) and NaOH(aq)
• Cathode 2H2O(l) 2e- -gt H2(g)
  2OH-(aq)
• Anode 2Cl-(aq) -gt Cl2(g) 2e-
• 2H2O(l) 2Cl- -gt H2(g) 2OH-(aq)

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Quantitative Aspects of Electrolysis

• Na e- -gt Na
• 1 mole of Na will plate out 1 mole Na
• Cu2 2 e-s -gt Cu
• 2 moles of Cu2 -gt 1 moles Cu
• Al3 3e-s -gt Al
• 3 moles of Al3 --gt 1 moles of Al

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Quantitative Aspects of Electrolysis-cont

• In an electrolytic cell, the amt of sub reduced
  or oxidized is directly proportional to the
  number of e-s passed into the cell
• 1 F (faraday) 96,500C/mol e-
• Coulombs amps x secs