Download Applications of Redox Chemistry

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Electrochemical cells
Redox reactions involve the transfer of electrons between
two species. The flow of electrons is an electrical current.
Redox reactions can therefore be used to generate
electrical current. This is the basis of batteries and fuel cells.
An electrochemical cell is a
store of chemical energy in a
closed system.
All reactants and products are
contained within the casing of
the cell.
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Rechargeable and non-rechargeable cells
There are two types of cell: non-rechargeable and
rechargeable. They are also known as primary and
secondary cells.
Primary cells can only
Secondary cells can be
be used once.
recharged and reused.
Smaller cells contain fewer reactants and produce less
electrical energy. However, the reaction voltage doesn’t
change with cell size.
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How does a cell work?
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Primary cells
A primary cell can only be used once because it transfers
stored chemical energy into electrical energy by a nonreversible chemical reaction.
Primary cells are usually
cheaper to buy.
They are more reliable as
they do not discharge much
when they are not in use.
This makes them more useful
for applications such as
smoke detectors.
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Carbon–zinc
dry cell
Alkaline
dry cell
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Secondary cells
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Reactions in a zinc–carbon cell
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Primary or secondary cells?
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Batteries
A battery is more than one cell connected together
in a series arrangement.
When a conductor is connected across the two terminals, a
circuit is formed and electrons flow through it.
By connecting more than one cell together, the electromotive
force is increased. The battery voltage is the sum of the
voltages of each of the cells.
A battery is a closed system which contains the high energy
reactants and the low energy products in a sealed unit.
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Lead-acid battery
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Calculate the e.m.f
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Fuel cells
A fuel cell converts chemical energy into electrical
energy in a similar fashion to a battery. However, a fuel
cell is an open system – reactants flow in and products
flow out, rather than being stored in the cell.
Fuel cells are used in
spacecraft and in
hydrogen powered
cars.
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Hydrogen fuel cells
Hydrogen fuel cells have been used in spacecraft for many
years. New developments are incorporating them into cars to
replace the internal combustion engine.
In a hydrogen fuel cell, hydrogen and oxygen gases react to
form water.
At the anode:
2H2(g)
4H+(aq) + 4e–
At the cathode:
4H+(aq) + O2(g) + 4e–
2H2O(l)
Overall equation:
2H2(g) + O2(g)
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2H2O(l)
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How does a hydrogen fuel cell work?
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Label the fuel cell
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Production of hydrogen
Hydrogen is a very reactive element and must be produced
by a chemical reaction before it can be used in a fuel cell.
There are currently two main
methods of manufacturing hydrogen:

reaction of hydrocarbons with steam

electrolysis of acidified water.
Although a hydrogen fuel cell is not directly polluting,
processes used to produce hydrogen can be. Electricity for
electrolysis is often produced by fossil fuel power stations.
Carbon dioxide is produced from the reaction of
hydrocarbons with steam.
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Transporting and storing hydrogen
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Hydrogen-rich fuels
With the difficulty in transporting and storing hydrogen,
some fuel cell vehicles use hydrogen-rich fuels which are
converted to hydrogen by an onboard ‘reformer’.
The reformer uses
temperatures of
250–300°C to favour
reactions generating
hydrogen gas.
Hydrogen-rich fuels include methanol, natural gas and petrol.
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Direct methanol fuel cells
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Reactions in a direct methanol fuel cell
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Ethanol fuel cell
Ethanol is a less toxic and
more energy-dense alcohol
than methanol. Research into
developing an ethanol fuel
cell is ongoing.
Currently, platinum-based catalysts
are used to oxidize ethanol, but
don’t achieve complete oxidation. Cheaper, more efficient
catalysts capable of fully oxidizing ethanol are needed.
Ethanol can be made from renewable sources, such as the
fermentation of sugar. These energy resources are
considered carbon neutral – the amount of carbon dioxide
they release is equal to the amount absorbed during growth.
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Fuel cell vs. conventional vehicles
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Hydrogen economy
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Modern breath alcohol testers
Fuel cell technology has been applied to breath alcohol
testers. Ethanol in a person’s breath is oxidized into carbon
dioxide and water, producing a measurable electrical current.
electrical current
ethanol from
breath
oxygen
water
platinum
anode
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acid–electrolyte
membrane
platinum
cathode
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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