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Electrochemistry

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Galvanic cell - an electric cell that generates an electromotive ... A salt bridge consist of a gel made by adding agar to a concentrated aqueous KCl solution. ... – PowerPoint PPT presentation

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Title: Electrochemistry


1
Chapter 11
  • Electrochemistry

2
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.
3
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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8
Standard Reduction Potentials
9
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.

10
Copper-Zinc Voltaic Cells
11
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 ?2Fe2Cu2
  • E0cellE0(cathode)-E0 (anode)0.43 V

oxidation
reduction
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13
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.
  • 3. Cell emf EfR-fL

Dashed line
14
Cell Diagrams
  • A positive emf for a cell diagram means that the
  • cell reaction corresponding to this diagram will
  • occur spontaneously when the cell is connected
  • to a load.
  • Oxidation at the left electrode sends electrons
  • flowing out of this electrode to the right
    electrode,
  • and electrons flow spontaneously from low to
  • high f therefore fRgtfL and Egt0.

15
  • 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-

16
Nernst Equation
  • E0 standard reduction potential
  • n moles of electrons
  • F Faraday constant 96485 C/mol

17
Thermodynamic-Free Energy
  • The maximum cell potential is directly related to
    the free energy difference between the reactants
    and the products in the cell.

18
Calculation of Equilibrium Constants for Redox
Reactions
19
Reaction Quotient (Q)
The positive E (fRgtfL ) means that QltK0. As Q
increases toward K0, the cell emf decreases,
reaching zero when QK0
20
The Equilibrium Constant of a Cu-Zn Cell
  • ZnCu2(aq)Zn2(aq)Cu
  • E00.34V-(-0.76V)1.10V

21
Concentration Cells
22
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.

25
H
H
Cl-
Ag

- - - - - - - - - -

- - - - - - - - - -
HCl HCl
AgNO3 HNO3
a2 lt a1
a2 a1
26
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.

27
Liquid Junction Potential
  • When the cell has a liquid junction, the observed
    cell emf includes the additional potential
    difference between the two electrolyte solutions.

28
The Liquid Junction Potential for a Cu-Zn Cell
  • The Cu2 ions diffuse into the ZnSO4 solution
    faster than the Zn2 ions diffuse into the CuSO4
    solution.
  • This produces a small excess of positive charge
    on ZnSO4 side of boundary and a small excess of
    negative charge on CuSO4 side
  • The negative charge builds up until a steady
    state is reached with a potential difference
  • f(ZnSO4)-f(CuSO4)EJ

29
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.

30
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.
31
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))

32
Estimate the Liquid Junction Potential from EMF
Measurement
33
Applications of Electrochemistry
  • pH meter
  • ATP synthase
  • Potential for a resting nerve cell

34
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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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.

39
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
40
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.

41
Membrane Equilibrium
In a closed electrochemical system, the phase
equilibrium condition for two phases a and b
42
ATP Synthase
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45
Free-energy change during proton movement across
a concentration gradient
  • The movement of protons from the cytoplasm into
    the matrix of the mitochondrion.

46
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

47
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.

48
Membrane Potential
  • Dym yin yout0.14 V
  • DG -nF Dym-(1)(96485)(0.14 ) - 13.5 kJ/mol

49
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

50
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.

51
Potential for a resting nerve cell
Goldman-Hodgkin-Katz equation
  • P permeability (???)
  • D diffusion coefficient (????)
  • t thickness of membrane (????)

52
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.
53
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

54
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

59
Alkaline Dry Cell
  • Anode reaction
  • Zn2OH- ? ZnOH2O2e-
  • Cathode reaction
  • MnO22H2O2e- ? Mn2O32OH-
  • Cell reaction
  • MnO2H2OZn? Mn2O3ZnO

60
????????
  • Anode reaction
  • 2H24OH-?4H2O4e-
  • Cathode reaction
  • 4e-O22H2O ?4OH-
  • Cell reaction
  • 2H2O2 ? 2H2O

61
?????????
  • Polymer Electrolyte Membrane Fuel Cell (PEMFC)
  • Proton Exchange Membrane Fuel cell (PEFC)

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Corrosion of Iron
64
  • Anodic Region
  • Fe?Fe22e-
  • Cathodic Region
  • O22H2O4e- ?4OH-
  • Overall Reaction
  • 4Fe2(aq)O2(g)(42n) H2O(l)
  • ?2Fe2O3?nH2O(s)8H(aq)

65
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.

66
standard galvanic cell standard
electrolytic cell
ZnCu2?Zn2Cu Zn2Cu?ZnCu2
67
Electrolysis of Water
  • Anode reaction 2H2O?O24H4e-
  • Cathode reaction 4H2O4e-?2H24OH-
  • 2H2O ?2H2O2 E0-2.06V

68
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

69
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
72
Electrolysis of NaCl
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