Transcript Galvanic Cells

Galvanic Cells
From Chemistry to Electricity
Electrolytic Cells
From Chemistry to Electricity . . .
And back again!
Recall: the Galvanic Cell
Zn-2+
2e
Zn(s)
Zn2+
Cu2+
Zn2+
Cu2+
Cu(s)
Cu2+

Recall: the Galvanic Cell
Current →
Load
Conductor
Salt Bridge
Zn2+
Zn(s)
Anode
Zn2+
Cu2+
Electrolyte
Zn2+
Oxidation happens here
Cathode
Cu(s)
Cu(s)
Cu2+
Reduction happens here
Questions:
• Which way did the electrons go?
• Why?
• What happens when we use a different
pair of metals?

Voltage
• Voltage is also known as electromotive
force and potential difference
• It is a measure of how much energy
electrons have to get them moving
• It is related to the distance between two
metals are on the activity series
Lithium
Analogy
Potassium
Barium
This activity series is
the inverse of the
reduction table
Calcium
Sodium
e-
Magnesium
Aluminium
Zinc
e-
Iron
Nickel
Lead
(Hydrogen)
Copper
Silver
Gold
Potential Difference
Question:
• Can we push the electrons back up again?

The Electrolytic Cell
External Voltage
-
+
Cl-
Na+
Cathode
Na+
Cl-
Electrolyte: eg. NaCl
Anode

The Electrolytic Cell
External Voltage
-
+
Cl2(g)
Na(s)
Na(s)
Cathode
Electrolyte: eg. NaCl
Anode
 Similarities and Differences
Galvanic Cells
Electrolytic Cells
• Oxidation occurs at the
anode
• Reduction occurs at the
cathode
• Anode is negative
• Cathode is positive
• Does not require external
voltage source
• Changes chemical
reactions into electrical
energy
• Oxidation occurs at the
anode
• Reduction occurs at the
cathode
• Anode is positive
• Cathode is negative
• Requires external voltage
source
• Changes electrical
energy into chemical
reactions
Etymology
• Electrolysis comes from two Greek words:
electron and lysis (meaning to break.)
Therefore, the word means “breaking
apart using electrons
Water Reaction
2 H2O(l) → O2(g) + 4 H+(aq) + 4 e2 H2O(l) + 2 e- → H2(g) + 2 OH-(aq)
Water Reaction
Oxidation at the Anode: 2 H2O(l) → O2(g) + 4 H+(aq) + 4 e-
Reduction at the Cathode: 4 H2O(l) + 4 e- → 2 H2(g) + 4 OH-(aq)
6 H2O(l) → 2 H2(g) + O2(g) + 4 H+ + 4 OH-(aq)
The Electrolytic
Cell
External Voltage
H2(g)
-
+
O2(g)
Anode
Cathode
Electrolyte
Practice
• Where would the hydrolysis reaction be
useful?
• Draw an electrolytic cell for AgBr(l)?
• Draw a galvanic cell for the reaction
Au+3 (aq) + Ag(s)  Ag+1 (aq) + Au (s)
Review of Batteries
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•
•
•
•
Define the following terms:
Primary Cell
Secondary Cell
Power Density
Memory Effect
Nickel-Cadmium
Advantages
• Can be recharged 1000 times
or more
• Low cost/cycle
• Tough, stands up to abuse
Disadvantages
• Low energy density
• Memory effect
• Contains toxic metals
Popular Uses
• Two-way radios, power tools,
medical equipment
Lithium-Ion
Advantages
• High energy density
• No memory effect
Disadvantages
• More expensive than Ni-Cd
• Not fully mature, technology is
still evolving
Popular Uses
• Cell-phones, iPods, laptop
computers
Lead-Acid
Advantages
• Mature technology, well
understood
• Cheap and easy to
manufacture
• No memory effect
Disadvantages
• Very low energy density; most
applications require huge
batteries
• Limited number of full
discharge cycles
• Environmental concerns
Popular Uses
• Electric cars, golf carts,
scooters
Reusable Alkaline
Advantages
• Cheap to manufacture
• More economical than primary
alkaline cells
Disadvantages
• Limited current, cannot be
made on large scale
• Limited cycle life (about 10
cycles); fully discharging
shortens life
Popular Uses
• Personal CD players, radios,
flashlights
Fuel Cells
Behind the hype
The Limit of Batteries
• A battery is a fancy type of galvanic cell. It
changes chemicals into electricity.
• Eventually, all the chemical are reacted and the
battery goes dead.
• If the battery is a secondary cell, you can
recharge it, but this takes time and energy. Also,
there is a limited number of times you can do
this.
• Wouldn’t it be nice to be able to just open up a
battery, and pour in some more chemicals, like
refueling a car?
Fuel Cells: The Ultimate Battery
• A fuel cell is a type of
galvanic cell that
allows you to add
fresh chemicals
continuously. It will
continue to run as
long as you keep
adding fuel.
Fuel Cell Comparisons
Battery
Fuel Cell
Engine
Energy
Conversion
Chemical>
Electrical
Chemical>
Electrical
Chemical>
Mechanical>
Electrical
Fuel
Zinc or other
metal
H2, Methane,
Propane,
Methanol, etc.
Propane,
Methane,
Gasoline
Powered By
Electrochemical
Reaction
Electrochemical
reaction
Combustion
Reaction
Power Output
Low
Variable
High
Advantages of Fuel Cells
• Extremely versatile – can power everything from
cell phones to buses
• Can run on a variety of fuels
• More environmentally friendly than combustion
Disadvantages of Fuel Cells
• Technology is still
somewhat unreliable
• Some types still
produce greenhouse
gas emissions
• EXPENSIVE
The Promise of Hydrogen
• Many Fuel Cells are emission-free because they run on
hydrogen
O2(g)
H2(g)
22 He+-
Anode
Cathode
Electrolyte
The Promise of Hydrogen
• Many Fuel Cells are emission-free because they run on
hydrogen
H2(g)
H2O(l)
Anode
Cathode
Electrolyte
Overall Reaction: H2 + O2 → H2O
Question: Where does hydrogen come
from?
Types of Fuel Cells
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Proton Exchange Membrane (PEM)
Solid Oxide Fuel Cells (SOFC)
Alkaline Fuel Cells (AFC)
Direct Methanol Fuel Cells (DMFC)
There are many more, but we won’t get
into them here
Your Task
• You will work in groups of about 5
• Half of each group will argue “for” a
particular type of fuel cell the other half will
argue “against”
• Each half-group will prepare an extremely
short (60 s) presentation to convince the
audience of their stance
• The class will vote on who was most
convincing
What if there was a chemical
reaction that:
• Turned vehicles and buildings into dust
• Caused billions of dollars worth of damage
per year
• Was virtually unstoppable
• Had the potential to destroy an entire
planet’s atmosphere
Rust
The Silent Killer
Why does rust happen?
• Iron, like most metals, is a strong reducing
agent
• Earth’s atmosphere is 21% O2, which is a
powerful oxidizing agent
• Galvanic cells are easy to set up, and can
be as simple as a drop of water
The Rust Galvanic Cell
Oxidation: Fe(s)  Fe2+ + 2 e-
Reduction: O2(g) + 2H2O + 4e-  4 OH-
O2(g)
2 H2O(l)
Fe(s)
Particle
The Rust Galvanic Cell
Oxidation: Fe(s)  Fe2+ + 2 e-
Reduction: O2(g) + 2H2O + 4e-  4 OH-
O2(g)
2 H2O(l)
Fee2+
2
Fe(s)
Particle
The Rust Galvanic Cell
Oxidation: Fe(s)  Fe2+ + 2 e-
Reduction: O2(g) + 2H2O + 4e-  4 OH2 Fe(s) + O2(g) + 2H2O  2 Fe2+ + 4 OH-
Fe2+
Fe(s)
4 OH-
Particle
The Rust Galvanic Cell
2
Fe(s)
Fe2+
4 OH-
Particle
The Rust Galvanic Cell
2 Fe2+ + 4 OH-  2Fe(OH)2
2 Fe(OH)2
Fe(s)
Particle
The Rust Galvanic Cell
4 Fe(OH)2 + O2(g) + 2 H2O(l)  4 Fe(OH)3
O2(g)
2 H2O(l)
Fe(s)
4
Fe(OH)
4 e-
Particle
2
The Rust Galvanic Cell
4 Fe(OH)2 + O2(g) + 2 H2O(l)  4 Fe(OH)3
4 OH4 Fe(OH)2
Fe(s)
Particle
The Rust Galvanic Cell
4 Fe(OH)2 + O2(g) + 2 H2O(l)  4 Fe(OH)3
4 Fe(OH)3
Fe(s)
Particle
The Rust Galvanic Cell
4 Fe(OH)2 + O2(g) + 2 H2O(l)  4 Fe(OH)3
Fe(OH)3  Fe2O3·3 H2O
4 Fe2O3 · 3 H2O
Fe(s)
Questions
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How did the water become an electrolyte?
What was the anode?
What was the cathode?
Would this happen for other metals? Which
ones? How would it be different?
• Corrosion costs billions of dollars a year in
damage as boats, cars, trains, building, etc. all
gradually turn to dust. What can we do to
prevent rusting from causing so much damage?
Rust Prevention: Protective layer
• Adding a protective
layer of paint, plastic,
or glass prevents the
iron from coming in
contact with the
electrolyte
• What happens if the
protective layer
develops a scratch?
Rust Prevention: Galvanizing
• If you coat iron in a
thin layer of zinc, it is
called galvanization.
The layer both
protects the iron and
will act as the anode if
a scratch develops
• What happens when
all the zinc is
oxidized?
Fe2O3 + 3 Zn  3 ZnO + 2 Fe
Rust Prevention: Sacrificial Anode
• Some ships and gas
pipelines are protected by
putting a block of zinc,
aluminum, or magnesium
on them. The more
reactive metal is oxidized
and the iron stays intact.
• Who pays to replace the
sacrificial anode every
year?
Rust on Mars
• The surface of mars is
completely covered in
rust. Scientists think that
Mars might once have
had an atmosphere like
earth’s, but all of that
oxygen is now tied up in
Fe2O3.
• Question: why hasn’t this
happened on Earth?
Practice Questions
• In 2000, Transport Canada recalled thousands
of cars with corroded engine mounts in Nova
Scotia, New Brunswick, and PEI. Why was
corrosion such a problem in these provinces?
• A small scratch in a car door can quickly develop
into a major rust spot. Why does this happen?
• Does acid rain promote or prevent corrosion?
Explain?