john e. mcmurry robert c. fay c h e m i s t r y chapter 17 electrochemistry
TRANSCRIPT
John E. McMurry • Robert C. Fay
C H E M I S T R Y
Chapter 17Chapter 17ElectrochemistryElectrochemistry
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
Electrochemical Cell Electrochemical Cell ComponentsComponents Two conductors (anode and cathode) Electrolytes solution: solution that each electrode is
emerse in it External circuit: provide a pathway for electron to
move from one electrode to another Salt Bridge: provide neutrality
Galvanic CellsGalvanic CellsZn2+(aq) + Cu(s)Zn(s) + Cu2+(aq)
External circuit
Galvanic CellsGalvanic Cells
• Anode:• The electrode
where oxidation occurs.
• The electrode where electrons are produced.
• Is what anions migrate toward.
• Has a negative sign.
Anode (-) Cathode (+)
Galvanic CellsGalvanic Cells
• Cathode:• The electrode
where reduction occurs.
• The electrode where electrons are consumed.
• Is what cations migrate toward.
• Has a positive sign.
Anode (-) Cathode (+)
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)?
Shorthand Notation for Shorthand Notation for Galvanic Cells or Voltaic CellGalvanic Cells or Voltaic Cell
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
Phase boundaryPhase boundary
Electron flow
Salt bridge
Cathode half-cellAnode half-cell
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)
Mg(s) + Ni+2(aq) Mg2+(ag) + Ni(s)
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 Given the following shorthand notation, sketch out the
galvanic cell
Pt(s)|Sn2+,Sn4+(aq)||Ag+(aq)|Ag(s)
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
Standard Reduction Standard Reduction PotentialsPotentials
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.
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
Eocell 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
E°cell = E°
ox + E°red
Standard Reduction Standard Reduction PotentialsPotentials
Spotaniety of the reaction can be determined by the positive Eo
cell 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
V
2 Ag+(aq) + 2e- 2 Ag(s) Eored = 0.80V
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
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) ΔGo = 1.1 x 105J
Standard Cell Potentials and Standard Cell Potentials and Equilibrium constantsEquilibrium constants
Nernst Equation: describe the relationship between Ecell and the concentration of species involved in the cell reaction
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
The Nernst EquationThe Nernst Equation What is the cell potential at 25oC for the following short
hand redox reaction?
Ni(s)/Ni2+ (1.0M)||Sn2+ (1.0 x 10-4M)/Sn(s)
Given
Ni2+(aq) + 2e- Ni(s) Eored = -0.23 V
Sn2+(aq) + 2e- Sn(s) Eored = -0.14 V
ExampleExample Calculate the concentration of cadmium ion in the galvanic
cell below
Cd(s)|Cd2+(aq)(?M)||Ni2+(aq)(0.100M)|Ni(s)
Ecell = 0.30V
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?
Electrolysis and Electrolytic Electrolysis and Electrolytic CellsCells Anode: where oxidation takes place
◦ Anions are oxidized at this electrode◦ labeled positive to reflect anions attraction to
anode Cathode: where reduction takes places
◦ Cations are reduced at this electrode◦ Labeled negative to reflect the cations attraction to
cathode
Electrolysis and Electrolytic Electrolysis and Electrolytic CellsCells
Electrolysis: The process of using an electric current to bring about chemical change.
Electrolysis and Electrolytic Electrolysis and Electrolytic CellsCells• Electrolysis: The process of
using an electric current to bring about chemical change.
• Process occurring in galvanic cell and electrolytic cells are the reverse of each other
• In an electrolytic cell, two inert electrodes are dipped into an aqueous solution
Electrolysis of Molten Electrolysis of Molten SaltsSalts Write the half-reactions for the electrolysis of the
following molten compounds KCl
KOH