1. Chapter 21 Thanks to D Scoggin Cabrillo College
Electrochemistry
Chapter 21
Thanks to D Scoggin
Cabrillo College

2. Electrochemistry and Redox
Oxidation-reduction: “Redox”
Electrochemistry:
study of the interchange between chemical change and electrical work
Electrochemical cells:
systems utilizing a redox reaction to produce or use electrical energy

3. Redox Review Redox reactions: electron transfer processes
Oxidation: loss of 1 or more e-
Reduction: gain of 1 or more e-
Oxidation numbers: imaginary charges
(Balancing redox reactions)

4. Oxidation Numbers (O.N.)
1. Pure element O.N. is zero
2. Monatomic ion O.N. is charge
3. Neutral compound: sum of O.N. is zero
Polyatomic ion: sum of O.N. is ion’s charge
*Negative O.N. generally assigned to more electronegative element

5. Oxidation Numbers (O.N.)
4. Hydrogen
assigned +1
(metal hydrides, -1)
5. Oxygen
assigned -2
(peroxides, -1; OF2, +2)
6. Fluorine
always -1

6. Oxidation-reduction Oxidation is loss of e-
O.N. increases (more positive)
Reduction is gain of e-
O.N. decreases (more negative)
Oxidation involves loss OIL
Reduction involves gain RIG

7. Redox Oxidation is loss of e- causes reduction “reducing agent”
Reduction is gain of e-
causes oxidation
“oxidizing agent”

9. Balancing Redox Reactions
1. Write separate equations (half-reactions) for oxidation and reduction
2. For each half-reaction
a. Balance elements involved in e- transfer
b. Balance number e- lost and gained
3. To balance e-
multiply each half-reaction by whole numbers

10. Balancing Redox Reactions: Acidic
4. Add half-reactions/cancel like terms (e-)
5. Acidic conditions:
Balance oxygen using H2O
Balance hydrogen using H+
Basic conditions:
Balance oxygen using OH-
Balance hydrogen using H2O
6. Check that all atoms and charges balance

11. Examples
Acidic conditions:
Basic conditions:

12. Types of cells Voltaic (galvanic) cells:
a spontaneous reaction generates electrical energy
Electrolytic cells:
absorb free energy from an electrical source to drive a nonspontaneous reaction

13. Common Components Electrodes:
conduct electricity between cell and surroundings
Electrolyte:
mixture of ions involved in reaction or carrying charge
Salt bridge:
completes circuit (provides charge balance)

14. Electrodes
Anode:
Oxidation occurs at the anode
Cathode:
Reduction occurs at the cathode
Active electrodes: participate in redox
Inactive: sites of ox. and red.

15. Voltaic (Galvanic) Cells
A device in which chemical energy is changed to electrical energy.
Uses a spontaneous reaction.

16. Oxidation
Reduction

20. Zn2+(aq) + Cu(s)  Cu2+(aq) + Zn(s)
Zn gives up electrons to Cu
“pushes harder” on e-
greater potential energy
greater “electrical potential”
Spontaneous reaction due to
relative difference in metals’ abilities to give e-
ability of e- to flow

21. Cell Potential
Cell Potential / Electromotive Force (EMF):
The “pull” or driving force on electrons
Measured voltage (potential difference)

22. Ecell = V

23. Cell Potential, E0cell
E0cell
cell potential under standard conditions
elements in standard states (298 K)
solutions: 1 M
gases: 1 atm

25. Standard Reduction Potentials
E0 values for reduction half-reactions with solutes at 1M and gases at 1 atm
Cu2+ + 2e  Cu
E0 = 0.34 V vs. SHE
SO42 + 4H+ + 2e  H2SO3 + H2O
E0 = 0.20 V vs. SHE

31. E0cell and DG0 E0cell > 0 DG0 < 0 Spontaneous
E0cell < 0 DG0 > 0 Not
E0cell = 0 DG0 = 0 Equilibrium

32. E0cell = E0cathode - E0anode
Calculating E0cell
E0cell = E0cathode - E0anode
Br2(aq)+2V3+ +2H2O(l)  2VO2+(aq)+ 4H+(aq)+ 2Br-(aq)
Given: E0cell = V
E0Br2 = V
What is E0V3+ and is the reaction spontaneous?

33. E0 values More positive: Stronger oxidizing agent
More readily accepts e-
More negative:
Stronger reducing agent
More readily gives e-
Stronger R.A. + O.A.  Weaker R.A. + O.A.

34. Free Energy and Cell Potential
n: number of moles of e-
F: Faraday’s constant
96485 C
mol of e-

35. DG0, E0, and K
At equilibrium: DG0 = 0 and K = Q
At 298 K:

37. Nernst Equation
Under nonstandard conditions

39. Concentration Cells
. . . a cell in which both compartments have the same components but at different concentrations

45. Batteries
A battery is a galvanic cell or, more commonly, a group of galvanic cells connected in series.

53. Fuel Cells Galvanic cells Reactants are continuously supplied.
2H2(g) + O2(g)  2H2O(l)
anode: 2H OH  4H2O + 4e
cathode: 4e + O H2O  4OH

56. Corrosion
Some metals, such as copper, gold, silver and platinum, are relatively difficult to oxidize.
These are often called noble metals.

60. Electrolysis
Forcing a current through a cell to produce a chemical change for which the cell potential is negative.

66. Stoichiometry
How much chemical change occurs with the flow of a given current for a specified time?
current and time  quantity of charge 
moles of electrons  moles of analyte 
grams of analyte