john e. mcmurry robert c. fay c h e m i s t r y sixth edition chapter 17 electrochemistry © 2012...
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John E. McMurry • Robert C. Fay
C H E M I S T R YSixth Edition
Chapter 17Chapter 17ElectrochemistryElectrochemistry
© 2012 Pearson Education, Inc.
Galvanic CellsGalvanic Cells
Electrochemistry: The area of chemistry concerned with the interconversion of chemical and electrical energy
Galvanic (Voltaic) Cell: A spontaneous chemical reaction which generates an electric current
Electrolytic Cell: An electric current which drives a nonspontaneous reaction
Galvanic CellsGalvanic Cells
Cu(s)Cu2+(aq) + 2e
Reduction half-reaction:
Oxidation half-reaction:
Zn2+(aq) + Cu(s)Zn(s) + Cu2+(aq)
Zn2+(aq) + 2eZn(s)
Galvanic CellsGalvanic CellsZn2+(aq) + Cu(s)Zn(s) + Cu2+(aq)
Galvanic CellsGalvanic Cells
• Anode:• The electrode
where oxidation occurs.
• The electrode where electrons are produced.
• Is what anions migrate toward.
• Has a negative sign.
Galvanic CellsGalvanic Cells
• Cathode:• The electrode
where reduction occurs.
• The electrode where electrons are consumed.
• Is what cations migrate toward.
• Has a positive sign.
Galvanic CellsGalvanic Cells• Salt Bridge: a U-shaped tube that contains a gel
permeated with a solution of an inert electrolytes• Maintains electrical neutrality by a flow of ions• Anions flow through the salt bridge from the
cathode to anode compartment• Cations migrate through salt bridge from the
anode to cathode compartment
Galvanic CellsGalvanic CellsWhy do negative ions (anions) move toward the negative electrode (anode)?
Galvanic CellsGalvanic Cells
Zn2+(aq) + Cu(s)Zn(s) + Cu2+(aq)
Cu(s)Cu2+(aq) + 2e
Zn2+(aq) + 2eZn(s)
Overall cell reaction:
Anode half-reaction:
Cathode half-reaction:
No electrons should be appeared in the overall cell reaction
Shorthand Notation for Shorthand Notation for Galvanic CellsGalvanic Cells
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
Phase boundaryPhase boundary
Electron flow
Salt bridge
Cathode half-cellAnode half-cell
17.2 Shorthand Notation for 17.2 Shorthand Notation for Galvanic CellsGalvanic Cells
Cell involving gas◦ Additional vertical line due to presence of addition phase◦ List the gas immediately adjacent to the appropriate
electrode◦ Detailed notation includes ion concentrations and gas
pressure
E.g Cu(s) + Cl2(g) Cu2+(aq) + 2 Cl-(aq)
Cu(s)|Cu2+(aq)||Cl2(g)|Cl-(aq)|C(s)
ExampleExample Consider the reactions below
◦ Write the two half reactions◦ Identify the oxidation and reduction half◦ Identify the anode and cathode◦ Give short hand notation for a galvanic cell that employs
the overall reaction
Pb2+(aq) + Ni(s) Pb(s) + Ni2+(aq)
ExampleExample Given the following shorthand notation, sketch out the
galvanic cell
Pt(s)|Sn2+,Sn4+(aq)||Ag+(aq)|Ag(s)
Cell Potentials and Free-Energy Cell Potentials and Free-Energy Changes for Cell ReactionsChanges for Cell Reactions
Electromotive Force (emf): The force or electrical potential that pushes the negatively charged electrons away from the anode ( electrode) and pulls them toward the cathode (+ electrode).
It is also called the cell potential (E) or the cell voltage.
Standard Reduction Standard Reduction PotentialsPotentials
2H1+(aq) + Cu(s)H2(g) + Cu2+(aq)
Cu(s)Cu2+(aq) + 2e
2H+(aq) + 2eH2(g)
Overall cell reaction:
Anode half-reaction:
Cathode half-reaction:
The standard potential of a cell is the sum of the standard half-cell potentials for oxidation at the anode and reduction at the cathode:
E°cell = E°
ox + E°red
The measured potential for this cell: E°cell = 0.34 V
Standard Reduction Standard Reduction PotentialsPotentialsEo
cell is the standard cell potential when both products and reactants are at their standard states:◦Solutes at 1.0 M◦Gases at 1.0 atm◦Solids and liquids in pure form◦Temp = 25.0oC
Standard Reduction Standard Reduction PotentialsPotentialsSpotaniety of the reaction can be determined by
the positive Eocell value
The cell reaction is spontaneous when the half reaction with the more positive Eo value is cathode
Note: Eocell is an intensive property; the value is
independent of how much substance is used in the reaction
Ag+(aq) + e- Ag(s) Eored = 0.80
V2 Ag+(aq) + 2e- 2 Ag(s) Eo
red = 0.80V
Standard Reduction Standard Reduction PotentialsPotentials
The standard hydrogen electrode (S.H.E.) has been chosen to be the reference electrode.
Standard Reduction Standard Reduction PotentialsPotentials
2H+(aq, 1 M) + 2eH2(g, 1 atm)
H2(g, 1 atm)2H+(aq, 1 M) + 2e
E°red = 0 V
E°ox = 0 V
The standard hydrogen electrode (S.H.E.) has been chosen to be the reference electrode.
Standard Reduction Standard Reduction PotentialsPotentials
2H+(aq) + Cu(s)H2(g) + Cu2+(aq)
Cu(s)Cu2+(aq) + 2e
2H+(aq) + 2eH2(g)
Overall cell reaction:
Anode half-reaction:
Cathode half-reaction:
0.34 V = 0 V + E°red
E°cell = E°
ox + E°red
Cu(s)Cu2+(aq) + 2e
E° = 0.34 V
A standard reduction potential can be defined:
Standard Reduction Standard Reduction PotentialsPotentials
ExamplesExamples Of the two standard reduction half reactions below, write
the net equation and determine which would be the anode and which would be the cathode of a galvanic cell. Calculate Eo
cell
a. Cd2+(aq) + 2e- Cd(s) Eored = -0.40 V
Ag+(aq) + e- Ag(s) Eored = 0.80 V
b. Fe2+(aq) + 2e- Fe(s) Eored = -0.44 V
Al3+(aq) + 3e- Al(s) Eored = -1.66 V
Cell Potentials and Free-Energy Cell Potentials and Free-Energy Changes for Cell ReactionsChanges for Cell Reactions
1 J = 1 C x 1 V
volt (V)SI unit of electric potential
joule (J)SI unit of energy
coulomb (C)Electric charge
1 coulomb is the amount of charge transferred when a current of 1 ampere (A) flows for 1 second.
Cell Potentials and Free-Energy Cell Potentials and Free-Energy Changes for Cell ReactionsChanges for Cell Reactions
G° = nFE°
Cell potentialFree-energy change
Number of moles of electrons transferred in the reaction
faraday or Faraday constantThe electric charge on 1 mol of electrons and is equal to 96,500
C/mol e
G = nFE or
Cell Potentials and Free-Energy Cell Potentials and Free-Energy Changes for Cell ReactionsChanges for Cell Reactions
Calculate the standard free-energy change for this reaction at 25 °C. Is the reaction spontanous at this condition?
Zn2+(aq) + Cu(s)Zn(s) + Cu2+(aq)
The standard cell potential at 25 °C is 1.10 V for the reaction:
ExamplesExamples Calculate the cell potential at standard state (Eo
cell) for the following reaction. Then write the half reactions
I2(s) + 2 Br-(aq) 2I-(aq) + Br2(l) Go = 1.1 x 105J
Standard Cell Potentials and Standard Cell Potentials and Equilibrium ConstantsEquilibrium Constants
-nFE° = -RT ln K
G° = -RT ln KandUsing G° = -nFE°
log Kn
0.0592 VE° =
log KnF
2.303 RTln KnF
RT =E° =
in volts, at 25°C
The Nernst EquationThe Nernst EquationΔG = Δ G° + RT ln Q
Using:
Nernst Equation:
log Qn
0.0592 VE = E°
ln QnF
RTE = E°
log QnF
2.303RTE = E°
or
in volts, at 25 oC
Δ G = nFE and Δ G° = nFE°
The Nernst EquationThe Nernst Equation
What is the potential of a cell at 25 oC that has the following ion concentrations?
Cu2+(aq) + 2Fe2+(aq)Cu(s) + 2Fe3+(aq)
Consider a galvanic cell that uses the reaction:
[Fe2+] = 0.20 M[Fe3+] = 1.0 × 104 M [Cu2+] = 0.25 M
ExampleExample Calculate the concentration of cadmium ion in the galvanic
cell below
Cd(s)|Cd2+(aq)(?M)||Ni2+(aq)(0.100M)|Ni(s)
Standard Cell Potentials and Standard Cell Potentials and Equilibrium ConstantsEquilibrium Constants
Standard Cell Potentials and Standard Cell Potentials and Equilibrium constantsEquilibrium constants
From the Nernst Equation
log Qn
0.0592 VE = E° in volts, at 25 oC
At Equilibrium E = 0
log Kn
0.0592 VE° =
Standard Cell Potentials and Standard Cell Potentials and Equilibrium ConstantsEquilibrium Constants
Three methods to determine equilibrium constants:
3. K from electrochemical data:
K =[A]a[B]b
[C]c[D]d1. K from concentration data:
RT
-G°ln K =2. K from thermochemical data:
RT
nFE°ln K =ln KnF
RTE° = or
ExamplesExamples Calculate the equilibrium constant, Keq, for the reaction
below
Zn2+(aq) + 2e- Zn(s) Eored = -0.76 V
Sn2+(aq) + 2e- Sn(s) Eored = -0.14 V
ExampleExample What is the value of Eo for a redox reaction involving
the transfer of 2 mol electrons if its equilibrium constant is 1.8 x 10-5?