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Electrochemical Cells
Separating Oxidation-Reduction to do work
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."
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
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!
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
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”
Electrochemical Cell
Robert Asato, Ph.D, Leeward Community College
Attach voltmeter here
How It Works
Oxidation: Zn → Zn2+
+ 2e-
Electrons are
produced at the
Zn/Zn2+
interface
This electrode is
called the anode
because
oxidation occurs
How It Works
Work
Electrons are
pushed through the
conductive wire
and through the
“work” device--a
motor, a light bulb
or a voltmeter
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
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
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
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+
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
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
Batteries
How do “Dry Cells” Work?
Inner half-cell:
e.g. Zn/Zn2+
connected to
central post
Electrons exit here
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
Cutaway of Dry Cell Battery

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Electrochemical cells

  • 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