Electrochemistry

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Chapter 11

• Electrochemistry

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Electromotive Force (???)
1 joule of work is produced or required when 1
coulomb of charge is transferred between two
points in the circuit that differ by a potential
of 1 volt.
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Galvanic Cells (Voltaic Cells )

• Galvanic cell - an electric cell that generates
  an electromotive force by an irreversible
  conversion of chemical to electrical energy
  cannot be recharged.
• The electron flow from the anode to the cathode
  is what creates electricity.
• In a galvanic cell, the cathode is positive while
  the anode is negative, while in a electrolytic
  cell, the cathode is negative while the anode is
  positive.

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Galvanic Cells
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Standard Reduction Potentials
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Standard Hydrogen Electrode

• 2H(aq)Zn(s) ?Zn2(aq)H2(g)
• Oxidation half-reaction
• Zn(s) ?Zn2(aq)2e-
• Standard hydrogen electrode
• 2H(aq)2e-?H2(g)
• The cathode consists of a platinum electrode in
  contact
• with 1 M H ions and bath by hydrogen gas at 1
  atm. We
• assign the reaction having a potential of exactly
  0 volts.

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Copper-Zinc Voltaic Cells
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The Cell Potentials

• E0cellE0(cathode)-E0 (anode)
• The value of E0 is not changed when a half
• reaction is multiplied by an integer.
• 2Fe32e- ?2Fe2 E0(cathode)0.77 V
• Cu ?Cu22e- -E0 (anode)-0.34 V
• Cu2Fe3 ?2Fe2Cu
• E0cellE0(cathode)-E0 (anode)0.43 V

oxidation
reduction
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Cell Diagrams

• For a copper-zinc voltaic cells
• Cu Zn ZnSO4(aq) CuSO4(aq) Cu
• 1. A vertical line indicates a phase boundary.
• 2. A dashed vertical line indicates the phase
  boundary between two miscible liquid.

Dashed line
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• PtL H2(g)HCl(aq)AgCl(s)Ag PtR
• Anode H2(g)2H2e-(PtL)
• Cathode AgCl(s)e-(PtR)AgCl- 2
• Overall 2AgCl(s) H2(g)2Ag 2H2Cl-
• CuLCd(s)CdCl2(0.1M)AgCl(s)Ag(s) CuR
• Anode CdCd22e-
• Cathode AgCle-AgCl- 2
• Overall Cd2AgCl2AgCd22Cl-

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Nernst Equation

• E0 standard reduction potential
• n moles of electrons
• F Faraday constant 96485 C/mol

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Thermodynamic-Free Energy

• The maximum cell potential is directly related to
  the free energy difference between the reactants
  and the products in the cell.

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Calculation of Equilibrium Constants for Redox
Reactions
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Reaction Quotient (Q)
The positive E (fRgtfL ) means that QltK0. As Q
increases toward K0, the cell emf decreases,
reaching zero when QK0
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The Equilibrium Constant of a Cu-Zn Cell

• ZnCu2(aq)Zn2(aq)Cu
• E00.34V-(-0.76V)1.10V

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Concentration Cells
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AgL Ag(0.1M) Ag(1M) AgR

• PtL Cl2(PL) HCl(aq) Cl2(PR) PtR

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Liquid Junction Potential

• Liquid junction the interface between two
  miscible electrolyte solutions.
• Liquid-junction potential A potential difference
  between two solutions of different compositions
  separated by a membrane type separator.
• The salt will diffuse from the higher
  concentration side to the lower concentration
  side.

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H
H
Cl-
Ag

- - - - - - - - - -

- - - - - - - - - -
HCl HCl
AgNO3 HNO3
a2 lt a1
a2 a1
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Liquid Junction Potential

• The diffusion rate of the cation and the anion of
  the salt will very seldom be exactly the same.
• Assume the cations move faster consequently, an
  excess positive charge will accumulate on the low
  concentration side, while an excess negative
  charge will accumulate on the high concentration
  side of the junction due to the slow moving
  anions.
• When the cell has a liquid junction, the observed
  cell emf includes the additional potential
  difference between the two electrolyte solutions.

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How to Solve the Liquid Junction Potential

• Liquid junction potentials are generally small,
  but they certainly cannot be neglected in
  accurate work.
• By connecting the two electrolyte solutions with
  a salt bridge, the junction potential can be
  minimized.
• A salt bridge consist of a gel made by adding
  agar to a concentrated aqueous KCl solution.

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A Cell Diagrams Containing a Salt Bridge

• For a copper-zinc voltaic cells
• Cu Zn ZnSO4(aq) CuSO4(aq) Cu

A salt bridge is symbolized by two vertical
dashed lines.
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Estimate the Liquid Junction Potential from EMF
Measurement

• Ag AgCl(s) LiCl(m) NaCl(m) AgCl(s) Ag
• m(LiCl)m(NaCl), E00
• Anode AgCl- (in LiCl(aq))AgCle-
• Cathode AgCle-AgCl- (in NaCl(aq))

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Estimate the Liquid Junction Potential from EMF
Measurement
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Applications of Electrochemistry

• pH meter
• ATP Synthase
• Potential for a resting nerve cell

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Determination of pH

• Pt H2(g) soln. X KCl(sat.) Hg2Cl2(s) Hg Pt

1/2H2(g)1/2Hg2Cl2Hg(l)H(aq,X)Cl-(aq) The
cell reaction and emf Ex
Junction potential between X and the saturated
KCl solution
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• If a second cell is set up to except that
  solution
• X is replaced by solution S, the emf Es of this
• cell will be

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Reference electrode saturated calomel electrode
(SCE)
Sensing electrode Ion Selective Electrode (ISE)
Pt Ag AgCl(s) HCl(aq) glass soln. X KCl(sat.)
Hg2Cl2(s) Hg Pt
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Determine the pH of a Solution by a pH Meter

• When the glass electrode is immersed in solution
  X, an equilibrium between H ions in solution and
  H ions in the glass surface is set up.
• This charge transfer between glass and solution
  produces a potential difference between the glass
  and solution.

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Ion Selective Electrodes (ISE) for PH Meter

• An ion selective electrode contains a glass,
  crystalline, or liquid membrane whose nature is
  such that the potential difference between the
  membrane and an electrolyte solution it is in
  contact with is determined by the activity of one
  particular ion.
• It is dependent on the concentration of an ionic
  species in the test solution and is used for
  electro-analysis.

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• Marcus theory for Electron transfer reactions
• Rudolph A. Marcus was awarded the 1992 Nobel
  Prize in chemistry

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Membrane Equilibrium
In a closed electrochemical system, the phase
equilibrium condition for two phases a and b
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Free-energy change during proton movement across
a concentration gradient

• The movement of protons from the cytoplasm into
  the matrix of the mitochondrion.

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Proton Pumping

• Proton pumping maintains a pH gradient of 1.4
  units, then DpH 1.4
• DG -2.303RT?pH
• - 2.303 (8.315 10-3
  kJ/mol)(298K)(1.4)
• - 7.99 kJ/mol
• Proton concentration gradient

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Free-energy change during solute movement across
a voltage gradient

• In mitochondria, electron transport drives proton
  pumping from the matrix into the intermembrane
  space.
• There is no compensating movement of other
  charged ions, so pumping creates both a
  concentration gradient and a voltage gradient.
• This voltage component makes the proton gradient
  an even more powerful energy source.

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Membrane Potential

• Dym yin yout0.14 V
• DG -nF Dym-(1)(96485)(0.14 ) - 13.5 kJ/mol

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Proton-motive force

• Proton-motive force (DP) is a Dy that combines
  the concentration and voltage effects of a proton
  gradient.
• DG-nFDP - 2.303 RT DpH nFDym
• (-7.99 kJ/mol)( - 13.5 kJ/mol)
• -21.5 kJ/mol

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ATP synthesis

• Mitochondrial proton gradient as a source of
  energy for ATP synthesis
• Estimated consumption of the proton gradient by
  ATP synthesis is about 3 moles protons per mole
  ATP.
• DG 50 kJ/mol for ATP synthesis
• DG 50 3(- 21.5) - 14.5 kJ/mol
• The synthesis of ATP is spontaneous under
  mitochondrial conditions.

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Potential for a resting nerve cell
Goldman-Hodgkin-Katz equation

• P permeability (???)
• D diffusion coefficient (????)
• t thickness of membrane (????)

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Resting Nerve Cell of a Squid

• Concentrations cell
• ?E(K)-95 mV
• ?E(Na)57 mV
• ?E(Cl-)-67 mV

P(K) /P(Cl-)2 P(K)/P(Na)25 The observed
potential for a resting squid nerve cell is
about -70 mV at 25oC.
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Resting Nerve Cell of a Squid

• The observed potential for a resting squid nerve
  cell is about -70 mV at 25oC.
• Hence Cl- is in electrochemical equilibrium, but
  K and Na are not.
• Na continuously flows spontaneously into the
  cell and K flows spontaneously out.
• Na-K pump

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Batteries

• Secondary batteries Voltaic cells whose
  electrochemical reactions can be reversed by a
  current of electrons running through the battery
  after the discharge of an electrical current.
• A secondary battery can be restored to nearly the
  same voltage after a power discharge.

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Lead Storage battery

• Anode reaction
• PbHSO4-? PbSO4H2e-
• Cathode reaction
• PbO2HSO4-3H2e- ? PbSO42H2O
• Cell reaction
• PbPbO2 2H2HSO4-? 2PbSO42H2O

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Dry Cell Battery

• Anode reaction
• Zn? Zn22e-
• Cathode reaction
• 2NH42MnO22e- ? Mn2O32NH3H2O
• Cell reaction
• 2MnO22NH4ClZn? Zn(NH3)2Cl2 Mn2O3
• H2O

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Alkaline Dry Cell

• Anode reaction
• Zn2OH- ? ZnOH2O2e-
• Cathode reaction
• MnO22H2O2e- ? Mn2O32OH-
• Cell reaction
• MnO2H2OZn? Mn2O3ZnO

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????????

• Anode reaction
• 2H24OH-?4H2O4e-
• Cathode reaction
• 4e-O22H2O ?4OH-
• Cell reaction
• 2H2O2 ? 2H2O

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?????????

• Polymer Electrolyte Membrane Fuel Cell (PEMFC)
• Proton Exchange Membrane Fuel cell (PEFC)

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Corrosion of Iron
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• Anodic Region
• Fe?Fe22e-
• Cathodic Region
• O22H2O4e- ?4OH-
• Overall Reaction
• 4Fe2(aq)O2(g)(42n) H2O(l)
• ?2Fe2O3?nH2O(s)8H(aq)

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Electrolysis

• Electrolytic Cell use electrical energy to
  produce chemical change
• The process of electrolysis involves forcing a
  current through a cell to produce a chemical
  change for which the cell potential is negative.

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standard galvanic cell standard
electrolytic cell
ZnCu2?Zn2Cu Zn2Cu?ZnCu2
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Electrolysis of Water

• Anode reaction 2H2O?O24H4e-
• Cathode reaction 4H2O4e-?2H24OH-
• 2H2O ?2H2O2 E0-2.06V

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Electrolysis of Mixture of Ions

• A solution in an electrolytic cell contains the
  ions Cu2, Ag and Zn2.
• The more positive the E0 value, the more the
  reaction has a tendency to proceed in the
  direction indicated.
• Ag gt Cu2 gt Zn2

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Electrolysis of NaCl/H2O System

• Anode reaction
• 2H2O?O24H4e- -E0-1.23 V
• 2Cl-?Cl22e- -E0-1.36 V
• The Cl- ion is first to be oxidized. A much
  higher
• potential than expected is required to oxidized
• water. The voltage required in excess of the
• excepted (overvoltage) is much greater for the
• production of O2 than for Cl2.

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electroplating
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Electrolysis of NaCl
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