Description of and mathematics of Galvanic and Voltaic cells suitable for regular and AP high school chemistry.

1. Electrochemical Cells
Separating Oxidation-Reduction to do work

2. History
We attribute the discovery of electrochemical cells to Luigi
Galvani (1737-1798), a citizen of the Papal States
"Therefore having noticed that frog preparations which hung
by copper hooks from the iron railings surrounding a balcony
of our house contracted not only during thunder storms but
also in fine weather, I decided to determine whether or not
these contractions were due to the action of atmospheric
electricity....Finally....I began to scrape and press the hook
fastened to the back bone against the iron railing to see
whether by such a procedure contractions might be excited,
and whether instead of an alteration in the condition of the
atmospheric electricity some other changes might be
effective. I then noticed frequent contractions, none of which
depended on the variations of the weather."

3. What He Did
Traditional metals used: Zn & Cu
▪Galvani discovered that if two metals were immersed in a
saline solution and connected by a wire or touched to a muscle,
an electrical current made the muscle twitch
•
We know now that a small amount of each metal dissolved to
make the corresponding Zn and Cu ions, forming an oxidation-
reduction reaction.

Zn0
→ Zn2+
+ 2e-

Cu2+
+ 2e-
→ Cu0
–Electrons transferred from Zn to Cu2+
through the muscle, which twitched
from the stimulus

4. Alternatively
Typically, we see an electron-transfer reaction,
Zn0
+ Cu2+
→ Zn2+
+ Cu0
▪The reaction is the same: Zn oxidized & Cu2+
reduced (cf.
P. 677)
▸But in the case above, the Zn comes directly into contact
with the copper ions in solution
▸And electrons pass directly from the metal to the ion
▸In Galvani’s reaction, he separated the reaction into two
half-reactions using salt water and the animal muscle.
–So the electrons had to do some work first!

5. Activity Series
Metals & Nonmetals
▪Recall from the STAAR guide
▸Elements above will react with the ions of
elements below
▸Which means they will give electrons to
the ions of elements below them
▸Thus Ca will react with H+
▸And Cu with Ag+
▸Can we quantify this?
Schoolisland.com

6. Alessandro Volta
Toward a quantifying of electric potential
▪The Voltaic cell
▸Alternate copper, water-soaked
cardboard, and zinc
▸The more “cells” of
copper/cardboard/zinc, the higher the
electric potential or driving force
▸We call this driving force “voltage”

7. Electrochemical Cell
Robert Asato, Ph.D, Leeward Community College
Attach voltmeter here

8. How It Works
Oxidation: Zn → Zn2+
+ 2e-
Electrons are
produced at the
Zn/Zn2+
interface
This electrode is
called the anode
because
oxidation occurs

9. How It Works
Work
Electrons are
pushed through the
conductive wire
and through the
“work” device--a
motor, a light bulb
or a voltmeter

10. How It Works
Reduction: Cu2+
+ 2e-
→ Cu
Electrons at the
cathode collide
with Cu2+
ions
attracted to this
locus of negative
charge, and reduce
them to metallic Cu

11. How It Works
Ion migration--the salt bridge
Sulfate (SO4
2-
)
migrates through the
salt bridge and Zn2+
in the other direction
to keep charges
balanced

12. Experimental Results
Each half-cell, measured against a standard half-cell, gives a
predictable voltage (standard 298K, 1 M solutions)
▪Standard: H2 gas/H+
(aq) 0.00 V
▸Fe2+
/Fe -0.44 V
▸PbSO4/Pb -0.36 V
▸Pb2+
/Pb -0.13 V
▸Cu2+
/Cu +0.34 V
▸Ag+
/Ag +0.80 V
▸Br2/Br-
+1.07 V
▸PbO2/Pb2+
+1.46 V

13. Your Turn
Given this table, calculate the standard voltage of the
following full cells
▪Fe2+
/Fe // Ag+
/Ag
▸1.24 V
▪Pb2+
/Pb // Br2/Br-
▸1.20 V
▪Cu2+
/Cu // PbO2/Pb2+
▸1.12 V
▪Thus we can predict the “force” with which electrons are
driven from one half-cell to the other
PotentialCell
-0.44 VFe2+/Fe
-0.36 VPbSO4/Pb
-0.13 VPb2+/Pb
0.00 VH2 gas/H+ (aq)
+0.34 VCu2+/Cu
+0.80 VAg+/Ag
+1.07 VBr2/Br-
+1.46 VPbO2/Pb2+

14. How to Write It
The shorthand diagram
▪Fe2+
/Fe // Ag+
/Ag
PotentialCell
-0.44 VFe2+/Fe
-0.36 VPbSO4/Pb
-0.13 VPb2+/Pb
0.00 VH2 gas/H+ (aq)
+0.34 VCu2+/Cu
+0.80 VAg+/Ag
+1.07 VBr2/Br-
+1.46 VPbO2/Pb2+
Oxidation
reaction first
with product
on left and
oxidized
species on
right

15. How to Write It
The shorthand diagram
▪Fe2+
/Fe // Ag+
/Ag
PotentialCell
-0.44 VFe2+/Fe
-0.36 VPbSO4/Pb
-0.13 VPb2+/Pb
0.00 VH2 gas/H+ (aq)
+0.34 VCu2+/Cu
+0.80 VAg+/Ag
+1.07 VBr2/Br-
+1.46 VPbO2/Pb2+
Reduction
reaction
second with
reduced
species on
the left and
product on
the right

16. Batteries
How do “Dry Cells” Work?
Inner half-cell:
e.g. Zn/Zn2+
connected to
central post
Electrons exit here

17. Batteries
How do “Dry Cells” Work?
Outer half-cell:
e.g. Cu/Cu2+
connected to
outer post
Electrons enter here
Electrolyte
inside case is a
solid-like gel

18. Cutaway of Dry Cell Battery