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John A. Schreifels

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Chapter 20 Electrochemistry Overview Half-Reactions Balancing oxidation reduction in acidic and basic solutions Voltaic cells Construction of voltaic cells ... – PowerPoint PPT presentation

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Title: John A. Schreifels


1
Chapter 20
  • Electrochemistry

2
Overview
  • Half-Reactions
  • Balancing oxidation reduction in acidic and
    basic solutions
  • Voltaic cells
  • Construction of voltaic cells
  • Notation for voltaic cells
  • Electromotive force (EMF)
  • Standard cell potentials
  • Equilibrium constants from EMFs
  • Concentration dependence of EMF
  • Electrolytic cells
  • Aqueous electrolysis
  • Stoichiometry of electrolysis

3
Electrochemistry
  • Electrochemistry - Field of Chemistry dealing
    with transfer of electrons from one species to
    another.
  • E.g. Zn in CuSO4(aq).
  • Electrochemical cell - combination of two half
    reactions to produce electricity from reaction.
  • E.g. Danielle cell Zn and Cu electrodes in
    salts of these ions.

4
Balancing Redox Reactions Oxidation Number Method
  • Determine oxidation for each atom- both sides
    of equation.
  • Determine change in oxidation state for each
    atom.
  • Left side make loss of electrons gain.
  • Balance on other side.
  • Insert coefficients for atoms that don't change
    oxidation state.
  • E.g. Balance
  • FeS(s)CaC2(s)CaO(s)?Fe(s)CO(g)CaS(s)
  • In acidic or basic solution balance as above,
    then balance charge with H or OH? on one side
    and water on other side.
  • Cancel waters that appear on both sides at end.
  • E.g. Balance which occurs in acidic solution

5
Balancing Half-Reaction Method
  • Write unbalanced half reactions for the oxidation
    and the reduction
  • Balance the number of elements except O and H for
    each.
  • Balance O's with H2O to the deficient side.
  • Balance H's with H to the hydrogen deficient
    side
  • Acidic add H
  • Basic add H2O to the deficient side and OH? to
    the other side.
  • Balance charge by adding e? to the side that
    needs it.
  • Multiply each half-reaction by integers to make
    electrons cancel.
  • Add the two half-reactions and simplify.
  • E.g. Balance
  • Acidic Zn(s) VO2(aq) ? Zn2(aq) V3(aq).
  • Basic Ag(s) HS?(aq) (aq) ?
    Ag2S(s) Cr(OH)3(s).

6
Galvanic (Voltaic) and Electrolytic Cells
  • Cell reaction Redox reaction involved in
    electrochemical cell.
  • Voltaic (galvanic) cell reaction is spontaneous
    and generates electrical current.
  • Electrolytic non-spontaneous reaction occurs
    due to passage of current from external power
    source. E.g. charging of batteries.

7
Galvanic Cell 2
  • anode electrode where oxidation occurs.
  • cathode electrode where reduction occurs.
  • salt bridge ionic solution connecting two
    half-cells (half-reactions) to prevent solutions
    from mixing.
  • E.g. Which is the anode and cathode in the cell
    to the right? Write the halfreactions.
  • Cd(s) 2Ag(aq) ? 2Ag(s) Cd2(aq)
  • Sign of electrodes (current flows from anode to
    cathode)
  • E.g. determine direction of electron flow for the
    reaction for a galvanic cell made from Ni(s) and
    Fe(s). The reaction is
  • 2Fe3(aq) 3Ni ? 2Fe(s) 3Ni2(aq)

8
Shorthand Notation for Galvanic Cells
  • Shorthand way of portraying electrodes in a
    voltaic electrochemical cell.
  • Redox couple-oxidized and reduced forms of same
    element when it is involved in electrochemical
    reaction. Shorthand Ox/Red
  • E.g. Cu2/Cu, Zn2/Zn.
  • 2 couples required for electrochemical reaction.
  • Shorthand rules
  • Anode reaction-left reduced form first.
  • Cathode-right oxidized form first.
  • Vertical line drawn between different phases
    including reaction of gases at metal electrode.
  • Double vertical drawn where salt bridge separates
    two half-reactions.
  • E.g. draw cell diagram for
  • Zn(s) Cu2(aq) ? Zn2(aq) Cu(s).
  • Fe3(aq) H2(g) ? Fe2(aq) 2H(aq).

9
Electrical Work
  • Earlier w ?P?V
  • In electrochemistry electrical pressure
    potential difference w E?q or charge times the
    electrical pressure.
  • Units Coulomb?Volts Joules (SI Units 1J
    1C?V)
  • Also want to relate to moles.
  • 1 mole e- 1 Faraday 1 F
  • F qe?N 1.602x10?19C?6.022x1023/mol
    9.65x104C/mol e-.

10
Cell Potentials for Cell Reactions Spontaneity
of Redox Reactions
  • ?G vs E
  • ? ?G ? ?E ? ?G ?nFE. Use this to calculate ?G
    for electrochemical reaction when cell voltage
    known.
  • E.g. Determine ?G for Zn/Cu cell if E ?1.100V

11
Standard Reduction Potentials
  • As with thermodynamic quantities, we list cell
    potentials at standard state 1M at 1 atm and
    usually 25?C.
  • Cell potential is the sum of half-cell potentials
    using Hess law.
  • Half-cell reactions for Daniell cell were
  • Potential at each electrode initially determined
    relative to SHE Standard hydrogen electrode.
  • 2H(aq)2e? ? H2(g) H 1M and PH21atm.
  • Other reaction run to determine if the SHE
    reaction proceeds spontaneously in direction
    written when connected to other half-cells.
  • E.g. the cell potential of copper at standard
    state conditions relative to SHE(acting as the
    anode) was 0.340 V determine the halfcell
    potential for Zn ? Zn2 if the potential for the
    Daniell cell (standard state conditions) was
    1.100 V.
  • Half-cell reactions reported as reductions.

12
Using Standard Reduction Potentials
  • Large negative value means oxidation strongly
    favored strong reducing agent.
  • Large positive value means reduction strongly
    favored strong oxidizing agent.
  • Relative values in table give an indication that
    one half-reaction favored over other. Summing
    half-cell reactions allow determination of
    standard cell potential.
  • Half-cell potential intensive property ?
    independent of amount of material ? we dont use
    stoichiometric coefficients for determining
    standard cell potentials.
  • E.g. determine the cell potential of
  • Br2(l) 2I?(aq) ? I2(l) 2Br?(aq)
  • E.g.2 determine the cell potential of
  • 2Ag(aq) Cu(s) ? 2Ag(s) Cu2(aq)
  • E.g.3 Determine cell potential
  • (aq) Fe(s) ? Fe2(aq) Mn2(aq) (balanced?).
  • when it is operated galvanically. Which is the
    oxidizing agent? reducing agent?
  • E.g. 4 Determine if the reaction below is
    spontaneous in the direction written.
  • Fe3(aq) Ag(s) ? ?

13
Spontaneity of Redox Reactions
  • ?G vs E
  • ? ?G ? ?E ? ?G ?nFE. Use this to calculate ?G
    for electrochemical reaction when cell voltage
    known.
  • E.g. Determine ?G for Zn/Cu cell if E 1.100V

14
  • Effect of Concentration on Cell EMF The Nernst
    Equation
  • Recall that ?G ?Go RTlnQ where
  • ?
  • or at 25C
  • which is called Nernst Equation.
  • E.g. Determine potential of Daniell cell at 25?C
    if Zn2 0.100 M and Cu2 0.00100 M.

15
Electrochemical Determination of pH
  • Electrodes can be used to determine acidity of
    solution by using hydrogen electrode with another
    one e.g. Hg2Cl2 half-cell.
  • E.g. determine the pH of a solution that
    develops a cell potential of 0.280 V (at 25C)
    given the cell below
  • Pt(s) H2(g) (1atm) H(? M) Pb2(1 M)
    Pb(s)
  • E.g. 2 determine the pH of a solution that
    develops a cell potential of ?0.200 V (at 25C)
    given the cell below (called a concentration
    cell).
  • Pt(s)H2(g)(1atm)H(1.00M)H(?M)H2(g)(1atm)Pt
    (s)

16
Standard Cell Potentials and Equilibrium Constants
  • Free energy and equilibrium constant for reaction
    studied can be determined from cell voltage.
  • Cell potential can be determined from ?G or from
    equilibrium constant.
  • Recall ?G ?nFE ?RTlnK.
  • E.g. Determine free energy and equilibrium
    constant for reaction below (unbalanced).
  • (aq) Fe(s) ? Fe2(aq) Mn2(aq)
  • E.g.2 Determine cell potential and equilibrium
    constant of Cl2/Br2 cell.
  • E.g.3 The following cell has a potential of
    0.578 V at 25C determine Ksp.
  • Ag(s)AgCl(s)Cl?(1.0 M)Ag(1.0 M)Ag(s).

17
Quantitative Aspects of Electrolysis
  • Current, i, measured units 1 ampere 1 coulomb
    per s (1 A 1 Cs?1).
  • Time of electrolysis, t (s), also measured.
  • Total charge, Q, calculated from the product
  • Q i?t
  • Charge on 1 mol of e?
  • mol of e? in an electrolysis obtained from
    balanced cell reaction
  • E.g. determine mol of electron involved in the
    electrolysis of the following
  • Ag(aq) e?? Ag(s)
  • 2Cl?(aq) 2e?? Cl2(g)
  • Amount deposited given by
  • E.g. Determine amount of Cu2 electrolyzed from
    solution at constant current of 6.00 A for period
    of 1.00 hour.

18
Electrical Work
  • Maximum electrical work ?G ?wmax ?nFE
  • n mol of electrons
  • F Faradays constant
  • E cell potential
  • Units Joules
  • Electrical Power 1 watt 1 J/s.
  • Energy often expressed as kilowatt hr
  • 1 kWhr 1000 W3600 s 3.6x106 J
  • E.g. determine the maximum work in kWhr required
    to produce 1.00 kg of Zn from Zn2 in a Daniell
    cell where the cell potential is ?1.100 V for the
    production of Zn metal.
  • Strategy
  • Determine n mol of Zn2 times 2.
  • Calculate work.
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