1. Chapter 17 Electrochemistry
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
4/14/ :02:19 PM
Chapter 17 Electrochemistry
Copyright © 2011 Pearson Prentice Hall, Inc.

2. Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry
Galvanic Cells
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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
Copyright © 2011 Pearson Prentice Hall, Inc.

3. Electrochemical Cell Components
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

4. Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry
Galvanic Cells
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Zn2+(aq) + Cu(s)
Zn(s) + Cu2+(aq)
External circuit
Be careful about assumptions. Don’t always place the same electrode on the same side. For instance, in the past, some textbooks would commonly placed the anode on the left side which led some students to believe that the anode always belongs on the left.
Students often are confused as the purpose and need of a salt bridge.
Copyright © 2011 Pearson Prentice Hall, Inc.

5. Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry
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Galvanic Cells
Anode:
The electrode where oxidation occurs.
The electrode where electrons are produced.
Is what anions migrate toward.
Has a negative sign.
Anode (-)
Cathode (+)
The wire convention is used for the sign at the anode and cathode.
“Anode” and “oxidation” both begin with vowels.
“Cathode” and “reduction” both begin with consonants.
Copyright © 2011 Pearson Prentice Hall, Inc.

6. Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry
4/14/ :02:19 PM
Galvanic Cells
Cathode:
The electrode where reduction occurs.
The electrode where electrons are consumed.
Is what cations migrate toward.
Has a positive sign.
Anode (-)
Cathode (+)
The wire convention is used for the sign at the anode and cathode.
“Anode” and “oxidation” both begin with vowels.
“Cathode” and “reduction” both begin with consonants.
Copyright © 2011 Pearson Prentice Hall, Inc.

7. Chapter 17: Electrochemistry
4/14/2017
Galvanic 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
“Anode” and “oxidation” both begin with vowels.
“Cathode” and “reduction” both begin with consonants.
Copyright © 2008 Pearson Prentice Hall, Inc.

8. Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry
4/14/ :02:19 PM
Galvanic Cells
Why do negative ions (anions) move toward the negative electrode (anode)?
Copyright © 2011 Pearson Prentice Hall, Inc.

9. Shorthand Notation for Galvanic Cells or Voltaic Cell
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
4/14/ :02:19 PM
Shorthand Notation for Galvanic Cells or Voltaic Cell
Salt bridge
Anode half-cell
Cathode half-cell
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
Electron flow
Phase boundary
Phase boundary
Copyright © 2011 Pearson Prentice Hall, Inc.

10. Example 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)

11. 17.2 Shorthand Notation for Galvanic 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) Cl-(aq)
Cu(s)|Cu2+(aq)||Cl2(g)|Cl-(aq)|C(s)

12. Example
Given the following shorthand notation, sketch out the galvanic cell
Pt(s)|Sn2+,Sn4+(aq)||Ag+(aq)|Ag(s)

13. Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry
Galvanic Cells
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Anode half-reaction:
Zn(s)
Zn2+(aq) + 2e
Cathode half-reaction:
Cu2+(aq) + 2e
Cu(s)
Overall cell reaction:
Zn(s) + Cu2+(aq)
Zn2+(aq) + Cu(s)
No electrons should be appeared in the overall cell reaction
Copyright © 2011 Pearson Prentice Hall, Inc.

14. Standard Reduction Potentials
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
4/14/ :02:19 PM
Standard Reduction Potentials
Potentials can only be measured in pairs which is why we need to establish a reference point for our half-cell potentials. Much like what was done for standard enthalpies and standard free energies.
Copyright © 2011 Pearson Prentice Hall, Inc.

15. Cell Potentials and Free-Energy Changes for Cell Reactions
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
4/14/ :02:19 PM
Cell Potentials and Free-Energy Changes 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.
The standard hydrogen electrode (S.H.E.) has been chosen to be the reference electrode.
H2(g, 1 atm)
2H+(aq, 1 M) + 2e
E°ox = 0 V
E°red = 0 V
2H+(aq, 1 M) + 2e
H2(g, 1 atm)
Copyright © 2011 Pearson Prentice Hall, Inc.

16. Standard Reduction Potentials
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

17. Standard Reduction Potentials
Spotaniety 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 V
2 Ag+(aq) + 2e-  2 Ag(s) Eored = 0.80V

18. Standard Reduction Potentials
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
Standard Reduction Potentials
4/14/ :02:19 PM
Anode half-reaction:
H2(g)
2H+(aq) + 2e
Cathode half-reaction:
Cu2+(aq) + 2e
Cu(s)
Overall cell reaction:
H2(g) + Cu2+(aq)
2H+(aq) + Cu(s)
E°cell = E°ox + E°red
0.34 V = 0 V + E°red
A standard reduction potential can be defined:
Recall that the 0.34 V was measured for the cell with the half-cells as described above. The oxidation half-cell potential is defined to be 0 V since it is the S.H.E.
Note also that the “ox” and “red” subscripts for oxidation and reduction have been dropped. While this can be very confusing to students, it is common usage.
Cu(s)
Cu2+(aq) + 2e
E° = 0.34 V
Copyright © 2011 Pearson Prentice Hall, Inc.

19. Standard Reduction Potentials
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
4/14/ :02:19 PM
Standard Reduction Potentials
Copyright © 2011 Pearson Prentice Hall, Inc.

20. Examples
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 Eocell
Cd2+(aq) + 2e-  Cd(s) Eored = V
Ag+(aq) + e-  Ag(s) Eored = 0.80 V
Fe2+(aq) + 2e-  Fe(s) Eored = V
Al3+(aq) + 3e-  Al(s) Eored = V

21. Cell Potentials and Free-Energy Changes for Cell Reactions
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
4/14/ :02:19 PM
Cell Potentials and Free-Energy Changes for Cell Reactions
faraday or Faraday constant
The electric charge on 1 mol of electrons and is equal to 96,500 C/mol e
DG = nFE or
DG° = nFE°
Free-energy change
The “°” refers to standard free-energy change and standard cell potential.
Cell potential
Number of moles of electrons transferred in the reaction
Copyright © 2011 Pearson Prentice Hall, Inc.

22. Cell Potentials and Free-Energy Changes for Cell Reactions
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
4/14/ :02:19 PM
Cell Potentials and Free-Energy Changes for Cell Reactions
The standard cell potential at 25 °C is 1.10 V for the reaction:
Zn2+(aq) + Cu(s)
Zn(s) + Cu2+(aq)
Calculate the standard free-energy change for this reaction at 25 °C. Is the reaction spontanous at this condition?
Worked example 17.3 p689.
The sign for a spontaneous reaction for free-energy: -
The sign for a spontaneous reaction for cell potential: +
Copyright © 2011 Pearson Prentice Hall, Inc.

23. Examples
Calculate the cell potential at standard state (Eocell) for the following reaction. Then write the half reactions
I2(s) Br-(aq)  2I-(aq) + Br2(l) ΔGo = 1.1 x 105J

24. Standard Cell Potentials and Equilibrium constants
Nernst Equation: describe the relationship between Ecell and the concentration
of species involved in the cell reaction V
E = E° 
log Q
in volts, at 25 oC n
At Equilibrium E = 0
log K n V
E° =

25. Standard Cell Potentials and Equilibrium Constants
Chapter 17: Electrochemistry
Standard Cell Potentials and Equilibrium Constants
4/14/2017
Copyright © 2008 Pearson Prentice Hall, Inc.

26. Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry
The Nernst Equation
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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 = V
Sn2+(aq) + 2e-  Sn(s) Eored = V
Problem p696.
Copyright © 2011 Pearson Prentice Hall, Inc.

27. Example
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

28. Examples
Calculate the equilibrium constant, Keq, for the reaction below
Zn2+(aq) + 2e-  Zn(s) Eored = V
Sn2+(aq) + 2e-  Sn(s) Eored = V

29. Example
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?

30. Electrolysis and Electrolytic Cells
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

31. Electrolysis and Electrolytic Cells
Chemistry: McMurry and Fay, 6th Edition
Chapter 17: Electrochemistry
Electrolysis and Electrolytic Cells
4/14/ :02:19 PM
Electrolysis: The process of using an electric current to bring about chemical change.
Copyright © 2011 Pearson Prentice Hall, Inc.

32. Electrolysis and Electrolytic Cells
Chapter 17: Electrochemistry
4/14/2017
Electrolysis and Electrolytic Cells
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
Copyright © 2008 Pearson Prentice Hall, Inc.

33. Electrolysis of Molten Salts
Write the half-reactions for the electrolysis of the following molten compounds
KCl
KOH