Electrochemistry

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Electrochemistry

• Fundamentals

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Oxidation Reduction

• Oxidation of iron (II) to iron (III) by
  permanganate
• Fe2 MnO4- ? Fe3 Mn2
• Oxidation losing electrons
• Reduction gaining electrons

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Electricity - Units

• Charge (q)
• 1 atomic charge 1.602 X 10 -19 Coulombs
• qm X F, F Faraday Constant 96490 Coul/mol
• Current (i) Amps Coul/sec
• iq/t
• Electrical Potential Volts Joules/Coul
• V or E IR
• ?G -nFE

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Electrochemical Cells
Electrolytic Cells require energy Galvanic
Cells produce energy
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Cell Components Cell Notation
Anode Oxidation Zn (s) ? Zn2 2 e-
Cathode Reduction Cu2 2 e- ? Cu (s)
Cell Notation or Line Notation Zn (s)?Zn(NO3)2 ,
(aq, 1.0M) ?? Cu((NO3)2 , (aq, 1.0M),? Cu (s)
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Standard Electrode Potentials

• The electrode potential of a half cell reaction
  when all reactants products exist at unit
  activity
• Written as standard reduction potentials
• Cu2 e- ? Cu
• E 0.153 Volts

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Electro-chemical Series
Strongly oxidizing
Strongly reducing
http//mysite.freeserve.com/chemsheets2/A2509.doc,
9 February 2004
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Concentration Effects The Nernst Equation

• Electrode potentials measure reaction driving
  force
• This driving force is concentration dependent
• The driving force is quantified by the Nernst
  Equation

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The Nernst Equation
E standard electrode potential R gas
constant (8.314 J/mol-K) T temperature
(Kelvins) n mols of electrons F Faraday
constant (96,485 Coul/mol)
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Old Mans Nernst Equation
aA bB ? cC dD
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Problem

• A palladium wire in a 0.025 F hydrochloric acid
  solution which is saturated with hydrogen gas at
  a pressure of 0.50 atm and which is connected
  through a potassium chloride salt bridge to a
  cell consisting of a zinc electrode immersed in a
  0.050 M zinc nitrate solution.
• Use line notation to sketch cell
• Find E, Ecell determine if electrolytic or
  galvanic

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Problem

• A metallic silver electrode in 0.015 F silver
  nitrate is connected through a salt bridge to a
  0.028 F nickel chloride solution into which a
  nickel electrode is immersed.
• Select a material for the salt bridge
• Use line notation to sketch cell
• Find E, Ecell determine if electrolytic or
  galvanic

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Problem

• A platinum wire immersed in a 1F sulfuric acid
  solution containing 0.10 M cerium (IV) and 0.05 M
  cerium (III). This solution is in contact via a
  semipermeable membrane to a 10 F sodium hydroxide
  solution containing 0.05 M permanganate ion and
  0.001 M manganate ion containing a gold
  electrode.
• Use line notation to sketch cell
• Find E, Ecell determine if electrolytic or
  galvanic

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Standard Electrode Potentials Equilibrium
Constants

• Galvanic cells produce current because the net
  cell reaction is not at equilibrium

anytime
at equilibrium
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Problem

• Determine E for the reduction
• AgCl (s) ? Ag (s) Cl- (aq)
• Ksp (AgCl) 1.8 X 10-10

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Qualitative Effects

• PtFe3 (aq), Fe2 (aq)H (aq), H2 (g)Pt

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Qualitative Effects II

• PtCr3(aq), Cr2(aq)AgCl (s), Cl- (aq)Ag

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Cell Current

• Electrodes Conductor
• Electrons serve as charge carriers
• In the bulk solution
• Charge movement involves migration of anions
  cations
• Ions from salt bridge neutralize changes in
  charge of anode cathode cells
• At the electrode surface
• Oxidation reduction provide mechanism for ionic
  conduction in solution to couple with electron
  conduction of electrode

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Structure of Solution Potential
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Solution Structure

• The Double Layer

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Faradaic versus NonFaradaic Currents

• Faradaic Current
• Direct transfer of electrons at electrode surface
  by oxidation or reduction
• Solution Current
• Non Faradaic Current
• Current converted to energy by solution
  Friction
• Found in AC electrochemical systems
• Capacitive current increases with electrode area
  and frequency
• Electrode charging

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Ohmic Potential or IR Drop

• Electrical potential required to overcome the
  resistance of ions to movement
• Follows Ohms law, V IR
• Always subtracted from overall cell potential
• Ecell Ecathode Eanode EIR

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Cell Polarization

• Cell polarization create nonlinear relationships
  in current voltage curves
• e.g.
• Ideal polarized electrode
• Ideal non-polarized electrode

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