The study of the interchange of
chemical and electrical energy.
Review of Terms-REDOX
oxidation-reduction (redox) reaction:
involves a transfer of electrons from the
reducing agent to the oxidizing agent.
How “beaters” rust-video
Review of Terms-OXIDATION
LEO: oxidation: loss of electrons
- OXIDATION occurs at the
ANODE of electrochemical cell
-Anions/negative ion move TO the anode
Review of Terms-REDUCTION
GER: reduction: gain of electrons
- REDUCTION occurs at the
CATHODE of electrochemical cell.
-Cations/positive ions move TO the
ANODE and CATHODE-video
The overall reaction is split into two half-reactions,
one involving oxidation and one reduction.
8H+ + MnO4 + 5Fe2+ Mn2+ + 5Fe3+ + 4H2O
Reduction: 8H+ + MnO4 + 5e Mn2+ + 4H2O
Oxidation: 5Fe2+ 5Fe3+ + 5e
. . . forcing a current through a cell to
produce a chemical change for which
the cell potential is negative.
… use chemical energy to make non-
spontaneous reaction take place
Electrolysis of Water
2H2O (l) 2H2 (g) + O2 (g)
2H2O (l) O2 (g) + 4H+ + 4e -
2H2O (l) + 4e - 2H2 (g) + 4OH -
-A device in which chemical energy is
changed to electrical energy.
-A spontaneous chemical reaction
occurs to generate electrical energy.
Cell Potential or Electromotive
Force (emf): The “pull” or driving
force on the electrons.
Standard Reduction Potentials-
Table 18.1 Page 481
The E values corresponding to reduction
half-reactions with all solutes at 1M and all
gases at 1 atm.
Cu2+ + 2e Cu E = 0.34 V vs. SHE
SO42 + 4H+ + 2e H2SO3 + H2O
E = 0.20 V vs. SHE
Calculation of E
ETotal = Ered + Eox
Reduction: Cu2+ + 2e Cu E red = +0.339 V
Oxidation: Zn Zn2+ + 2e E ox = +0.762 V
ETotal= +0.339 V + +0.762 V = +1.101 volts
Strength of Oxidizing and
The more positive Ered , the stronger the
The more positive Eox , the stronger the
of Redox Reactions
If ETotal= +, then the reaction is spontaneous.
ETotal= -,then the reaction is non-spontaneous.
Therefore, cell-Zn/Zn2+||Cu2+/Cu, with
ETotal= +1.101 volts is spontaneous.
A battery is a galvanic cell or,
more commonly, a group of
galvanic cells connected in series.
. . . galvanic cells for which the reactants are
2H2(g) + O2(g) 2H2O(l)
anode: 2H2 + 4OH 4H2O + 4e
cathode: 4e + O2 + 2H2O 4OH
Some metals, such as
copper, gold, silver
and platinum, are
relatively difficult to
oxidize. These are
often called noble
Stoichiometry of Electrolysis
- How much chemical change occurs with
the flow of a given current for a specified
current and time quantity of charge
moles of electrons moles of analyte
grams of analyte
emf and Work
work ( J )
emf potential difference (V)
ch arg e (C)
Free Energy and Cell
G = nFE
n = number of moles of electrons
F = Faraday = 96,485 coulombs per mole of
Calculation of Equilibrium
Constants for Redox Reactions
At equilibrium, Ecell = 0 and Q = K.
log( K ) at 25 C
. . . a cell in which both compartments
have the same components but at
The Nernst Equation
We can calculate the potential of a cell in
which some or all of the components are not
in their standard states.
E E logQ