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.