20.5
Student Notes
III. Standard Cell emfs and Standard Electrode
Potentials
A.
Cell emf –
1.
Cell emf is a measure of the
2.
Contribution of
the anode ( /oxidation potential)
and the cathode ( /reduction
potential)
3.
Ecell =
4.
Oxidation
potential =
So
we only need one, we use reduction potentials and can find oxidation
potentials; we call this electrode potentials, denoted
by the symbol E
5.
Ecell =
B.
Tabulating
Standard Electrode Potentials
1.
Standard emf - E°cell
-
2.
Standard
electrode potential - E° -
3.
Usually hydrogen
is used = 0.00 V
C.
Strengths of
Oxidizing and Reducing Agents
1.
Strongest
oxidizing agents are the oxidized species corresponding to half-reactions with
2.
Those reduction
half-reactions with lower (more negative) electrode potentials have a greater
tendency to go
Reduced species à oxidized species + n e-
3.
Strongest
reducing agents are the reduced species corresponding to half-reactions with
the
4.
Determining the
Relative Strengths of Oxidizing and Reducing Agents
a.
Order the
following oxidizing agents by increasing strength under standard-state
conditions: Cl2 (g), H2O2 (aq),
Fe3+ (aq).
b.
Order the
following reducing agents by increasing strength under standard-state
conditions: H2 (g), Al (s), Cu (s).
5.
The stronger oxidizing agent will be on the ______
side when the equation is written as a spontaneous reaction. (Alternatively,
the stronger reducing agent will be on the ______side of the spontaneous
reaction.)
6.
Determining
the direction of spontaneity from electrode potential
Consider
the reaction
Zn2+ (aq) + 2 Fe2+
(aq) à Zn (s) + 2 Fe3+ (aq)
Does
the reaction go spontaneously in the direction indicated, under standard
conditions?
D.
Calculating Cell emfs
1.
Calculating Cell emfs by Addition of Half-Reactions and Standard Potentials
a. Stronger
reducing agent on the _____ (more negative electrode potential)
b. Multiply
the half-reaction
c. Note: A
negative emf indicates that the cell reaction is
non-spontaneous as written,
2. Calculation
of Cell emfs using Standard Reduction Potentials
E°cell =
3. Calculating
the emf from Standard Potential
Calculate
the standard emf of the following voltaic cell at 25°
C using standard electrode potentials.
Al (s) / Al3+ (aq)
║ Fe2+ (aq) / Fe (s)
What is the cell reaction?