John E. McMurry Robert C. Fay C H E M I S T R Y Sixth Edition Chapter 17 Electrochemistry © 2012 Pearson Education, Inc

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


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)


• Anode:• The electrode

where oxidation occurs.

• The electrode where electrons are produced.

• Is what anions migrate toward.

• Has a negative sign.


• 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)?



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)




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


The standard hydrogen electrode (S.H.E.) has been chosen to be the reference electrode.


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.


2H+(aq) + Cu(s)H2(g) + Cu2+(aq)

Cu(s)Cu2+(aq) + 2e

2H+(aq) + 2eH2(g)




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:


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


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.


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


Calculate the standard free-energy change for this reaction at 25 °C. Is the reaction spontanous at this condition?


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

From the Nernst Equation

log Qn

0.0592 VE = E° in volts, at 25 oC

At Equilibrium E = 0

log Kn

0.0592 VE° =


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?

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John E. McMurry Robert C. Fay C H E M I S T R Y Sixth Edition Chapter 17 Electrochemistry © 2012 Pearson Education, Inc