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

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

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)

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