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4/29/2012

Electrochemistry

ELECTROCHEMISTRY

• Study of chemical reactions which take place
in a solution at the interface of an electron
conductor (a metal or a semiconductor) and
an ionic conductor (the electrolyte)
• An electrochemical reaction is one which:
– A chemical reaction is driven by an external
applied voltage (electrolysis) [electrolysis; the
Greek word lysis means to break up]
– A voltage is created by a chemical reaction as in a
battery
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There are 2 types of electrochemical cell:

Electrodes

• 1
...
Electrolytic - a current causes a nonspontaneous chemical change - does work by
absorbing free energy from a source of electricity
to drive a non-spontaneous reaction such as
electroplating
...

• This causes ions of one of the electrodes to
preferentially enter the solution at one electrode,
and another ion to leave the solution at the other
electrode
...

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Electrolyte
• The anode is defined as the electrode where
oxidation occurs and the cathode is the electrode
where the reduction takes place
...

• Electrolytes commonly exist as solutions of acids,
bases or salts
...

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Volt & EMF

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Cell Potentials (cont
...
e
...

e-

• The force on the electrons causing them to
flow is referred to as the electromotive force
(EMF)
...

• An atom or ion that receives an electron from
another atom or ion has its oxidation state
decrease
...


• loss of electrons from an atom or molecule is
called oxidation, and the gain of electrons is
reduction
• Best remembered as “OIL RIG" (Oxidation Is
Loss, Reduction Is Gain)
• The atom or molecule which loses electrons is
known as the reducing agent, or reductant,
• The substance which accepts the electrons is
called the oxidizing agent, or oxidant
...

• Despite the name, an oxidation reaction does
not necessarily need to involve oxygen
...

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Broader definition of:
Oxidation and Reduction

Oxidation and Reduction
• In many organic compounds, the loss of
hydrogen implies oxidation of the molecule
from which it is lost (and the hydrogen is
reduced)
...


• Oxidation describes the loss of electrons /
hydrogen or gain of oxygen / increase in
oxidation state by a molecule, atom or ion
...

• This reaction can occur through the
application of an external voltage or through
the release of chemical energy
...


• Redox (shorthand for reduction-oxidation
reaction) describes all chemical reactions in
which atoms have their oxidation number
(oxidation state) changed
...

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Galvanic cell

Galvanic cell

• Electrochemical cell that converts chemical
energy into electrical energy, by using
spontaneous chemical reactions that take
place at the electrodes
...


• A simple galvanic cell will consist only of an
electrolyte and two different electrodes
...

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Galvanic Cell

A Half Cell

•
•
•
•

• metal = an electrode

Reduction occurs at the cathode
Oxidation occurs at the anode
CATions migrate towards the CAThode
ANions migrate towards the ANode

M
solution = an electrolyte

M+n(aq)

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Galvanic Cells

Salt Bridge

• Salt bridge/porous disk: allows for ion migration
such that the solutions will remain neutral
...

• The viscosity prevents mixing with the
electrolytes
...

• The chosen ions have similar mobility to
minimize junction potentials
...

• The standard electrode potential for the SHE is
zero, by definition
...


K+

Cu
NO3-

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Standard Potential Tables

Standard Potential Table

• if the measured potential is negative, the
electrode is more oxidizing than the SHE

• If the electrode has a positive potential with respect to the
SHE, then that means it is a strongly reducing electrode
F2 + 2e–
Co3+ + e–
Au+ + e–
Ce4+ + e–
Br2 + 2e–
Ag+ + e–
Cu2+ + 2e–
Sn4+ + 2e–
2H+ + 2e–

2F–
Co2+
Au
Ce3+
2Br–
Ag
Cu
Sn2+
H2

2H+ + 2e–
Pb2+ + 2e–
Sn2+ + 2e–
In3+ + 3e–
Fe2+ + 2e–
Zn2+ + 2e–
V2+ + 2e–
Cs+ + e–
Li+ + e–

+2
...
81
+1
...
61
+1
...
80
+0
...
15
0
...
0000
-0
...
14
-0
...
44
-0
...
19
-2
...
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Selected Standard Electrode Potentials
• F2(g) is the BEST oxidizing agent (and the most
easily reduced i
...
it is electronegative)
• Li+(aq) is the WORST oxidizing agent (and the
most difficult to reduce i
...
receive electrons )
• Li(s) is the BEST reducing agent (and the most
easily oxidized i
...
it is electropositive)
• F-(aq) is the WORST reducing agent (and the
most difficult to oxidize i
...
donate electrons)
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How do we obtain Standard Cell Potentials from
Standard Reduction Potentials?

• What would the Standard Cell Potential be for
a Cu|Cu2+ and Zn|Zn2+ cell?
• Step 1
...
R
...
equations:

• Step 2
...
Are the electrons balanced?
• Step 4
...
e
...

• What is the spontaneous cell reaction, anode,
cathode, E°?
• Step 1
...
Why NOT ??
• Compare

• Step 3
...

• A cell will always run spontaneously in the direction
that produces a positive cell potential

• The ENERGY release by (2) is certainly twice
that of (1), but remember that
Voltage = Joules/Coulombs
• (energy per coulomb of charge transferred)
• Equation (2) involves twice the energy, but it
also involves twice the coulombs - so the
VOLTAGE does not change
...

• The overall cell potential is then given by:

• The splitting of a substance or decomposing
by forcing a current through a cell to produce
a chemical change for which the cell potential
is negative
...
e
...

• The electrolyte is usually a solution of water
or other solvents in which ions are dissolved
...
23V
4H2O + 4e-  2H2 + 4OH- Eo=-0
...
06V
• Since it is the same reagent being used on both sides,
the two sides of the equation are added up
...

• The anode is a zinc container surrounded by a
thin layer of manganese dioxide
• A moist electrolyte paste of ammonium
chloride and zinc chloride is mixed with starch
...


• A primary cell is any kind of electrochemical
cell in which the electrochemical reaction of
interest is not reversible, so is used in
disposable batteries
...

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Zinc-carbon battery

Zinc-carbon battery

• A zinc-carbon dry cell or battery is packaged in a zinc
can that serves as both a container and anode
...
The electrolyte is a paste of zinc chloride and
ammonium chloride dissolved in water
...
Zinc-carbon batteries are
the least expensive primary batteries and thus a
popular choice by manufacturers when devices are
sold with batteries included
...

• The battery has an e
...
f
...
5 V
...
5 V
...

• The half-reactions are:
• Zn (s) + 2OH− (aq) → ZnO (s) + H2O (l) + 2e−
2MnO2 (s) + H2O (l) + 2e− →Mn2O3 (s) + 2OH− (aq)

• In an alkaline battery, the anode (negative
terminal) is made of zinc powder (which
allows more surface area for increased rate of
reaction therefore increased electron flow)
and the cathode (positive terminal) is
composed of manganese dioxide
...

Not usually available in "common" battery
sizes
Very common in laptop computers, moderate
to high-end digital cameras and camcorders,
and cellphones
...

Volatile: Chance of explosion if short circuited,
allowed to overheat, or not manufactured
with rigorous quality standards
...

• Each cell has a lead anode and a cathode
made from lead dioxide packed in a metal
plaque
...


• PbO2(s) + 4H+(aq) + SO42−(aq) + 2e− → PbSO4(s) + 2H2O(l) 0 = 1
...
35 V
• Pb(s) + PbO2(s) + 4H+(aq) + 2SO42−(aq) → 2PbSO4(s) + 2H2O(l)
Or
• Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
0 = 2
...

• lead-acid batteries are rechargeable
...

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Lead Acid Battery

Lithium rechargeable battery

• Moderately expensive
...

Moderate rate of self discharge
...

Environmental hazard due to Lead
...

• In this cell, the carbon in the anode can
reversibly form a lithium-carbon alloy
...

• It is also a light metal and therefore less mass
is required to generate 1 mole of electrons
...

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• Rechargeable, but not indefinitely
• Cd(s) + NiO2(s) + 2H2O(l) → Cd(OH)2(s) +
Ni(OH)2(s)
• Approximately 1
...


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Nickel-Cadmium (NiCad) Battery

Nickel-Cadmium (NiCad) Battery

• • Nickel–Cadmium Battery is Dry-Cell that is
rechargeable
...

• Cd(s) + 2OH–(aq) → Cd(OH)2(s) + 2e–
• Cathode/Reduction: Nickel (III) compound on
nickel metal
...
30V

• During discharge, Cd metal is oxidized at the
anode and nickel oxyhydroxide is reduced at the
cathode
...

• Recharging reverses this reaction
...

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Nickel-Cadmium (NiCad) Battery

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Nickel Metal Hydride Battery
• Nickel Metal Hydride battery is similar to NiCd
battery Dry-Cell that is rechargeable
...

• MH + OH–(aq) → M + H2O + e• Cathode/Reduction: Nickel (III) compound on
nickel metal
...
35V

• Inexpensive
...

Can withstand very high discharge rates with
virtually no loss of capacity
...

Environmental hazard due to Cadmium - use
now virtually prohibited in Europe

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Nickel Metal Hydride Battery

Nickel Metal Hydride Battery

• Hydrogen Stored at Anode as Hydride in Metal
Alloy
• Hydrogen Storage Metals: LaNi5, FeTi
• H2 Storage Capacity: LaNi5 0
...
Liquid H2 is
0
...

Not useable in higher drain devices
...

Newer chemistry has low self-discharge rate,
but also a ~25% lower energy density
...
Used in some cars
...

• Oxidation and reduction take place in the
anode and cathode separately
...

• Far more efficient than burning hydrogen,
boiling water, generating steam, running a
turbine to generate electricity
• Astronauts use the water produced in the cell
for drinking
...
The electrodes are
zinc and carbon, with an acidic paste
between them that serves as the electrolyte
...

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Summary
• Lithium-ion battery - With a very good
power-to-weight ratio, this is often found in
high-end laptop computers and cell
phones (rechargeable)
...

• Silver-zinc battery - This is used in
aeronautical applications because the
power-to-weight ratio is good
...

• Lead-acid battery - Automobiles (rechargeable)
...

• Nickel-metal hydride battery - This battery is
rapidly replacing nickel-cadmium (rechargeable)

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