Transcript CH08

Chapter 8: Energy from Electron Transfer
Electricity – the flow of electrons from a
negative electrode to a positive electrode
Direct current (DC) – electrons only flow in
one direction.
Alternating current (AC) – electrons
alternately flow in both directions.
Closed circuit – a conductive path is present
for flow of electrons.
Open circuit – no complete path for electrons
Voltage – electrical potential of each electron
higher voltage – more energy per electron
Current (amps) – number of electrons flowing
per second.
Voltage and amps combined give the power
for a particular circuit.
Power plants send out AC current.
Much less power loss over great distances
than with DC.
~7% loss is typical
Batteries produce DC current
Useful for portable power.
Computers, cell phones, … need DC
current
Energy is lost when converting between AC
and DC.
A battery is a system for the direct conversion of
chemical energy to electrical energy.
Batteries are found everywhere in today’s society
because they are convenient, transportable sources of
stored energy.
The “batteries”
shown here are more
correctly called
galvanic cells.
A series of galvanic cells that are wired together
constitutes a true battery – like the one in your car.
8.1
When an element forms a compound, or a
compound forms an element, electrons are
transferred:
Na + Cl → NaCl
Looking at each element independently:
Na → Na+1 + 1e-1
Cl + 1e-1 → Cl-1
Batteries are designed to direct the electron
flow through the wire, rather than directly
between the elements.
A Laboratory Galvanic Cell
Oxidation (at anode):
Zn(S)
Zn2+(aq)
Reduction (at cathode):
Cu2+(S)
Zn2+(aq)
8.1
A galvanic cell is a device that
converts the energy released in a
spontaneous chemical reaction
into electrical energy.
Chemical to
electrical energy
This is accomplished by the
transfer of electrons from one
substance to another.
Consider a nickel-cadmium (NiCad) battery:
Electrons are transferred from cadmium to
nickel.
8.1
Oxidation – when an atom loses an e-1
Na → Na+1 + 1e-1
Reduction – when an atom gains an e-1
Cl + 1e-1 → Cl-1
Both oxidation and reduction must occur for a
reaction, and to generate electricity.
Oxidation is loss of electrons: Cd  Cd2+ + 2 e–
Cd loses electrons
Reduction is gain of electrons: 2 Ni3+ + 2 e–  2 Ni2+
Ni3+ gains electrons
Oil Rig is a useful mnemonic device.
The transfer of electrons through an external circuit produces
electricity, the flow of electrons from one region to another that
is driven by a difference in potential energy.
8.1
The electron transfer process involves two
changes: the cadmium is oxidized, and the
nickel is reduced.
Each process is expressed as a half-reaction:
Oxidation half-reaction:
Reduction half-reaction:
Cd  Cd2+ + 2 e–
2 Ni3+ + 2 e–  2 Ni2+
Overall cell reaction:
2 Ni3+ + Cd  2 Ni2+ + Cd2+
The cadmium releases two electrons, resulting in a 2+ ion.
Two Ni3+ ions accept the two electrons and their respective charges go from 3+ to 2+.
The overall cell reaction does not have any electrons written in it – they must cancel.
8.1
To enable this transfer, electrodes (electrical conductors) are
placed in the cell as sites for chemical reactions.
Reduction occurs
at the cathode
Oxidation occurs
at the anode
The cathode receives the electrons.
The difference in electrochemical potential between the
two electrodes is the voltage (units are in volts).
8.1
Different elements have different attractions
for electrons, which results in different
voltages between the elements.
Various metals are selected for battery
electrodes depending on the desired voltage.
Alkaline AA batteries, for example, have a
voltage of about 1.5 V.
– Mercury batteries had the advantage of being very small
– Used in watches, cameras, hearing aids
– Toxicity and disposal concerns led to alternatives
8.2
Once all of the atoms in an electrode have
reacted, the battery is dead (no more
electrical chemical potential energy)
Some batteries may be rechargeable,
depending on the metals used – applying a
potential can reverse the reaction and restore
the original elements.
Many AA rechargeable batteries only have a
potential of 1.3 V, and will not work in some
devices that require the full 1.5 V.
Lead–Acid Storage Batteries
This is a true battery as it consists of a series of six cells.
Anode = Pb
Cathode = PbO2
Rechargeable:
discharging
Pb(s) + PbO2(s) + 2 H2SO4(aq)
H2O(l)
recharging
2 PbSO4(s) + 2
8.2
Hybrid vehicles use a combination of electric motors
and a gasoline generator for power.
The generator is used to charge up the batteries,
which power the motor.
With standard cars, when applying the brakes the
kinetic energy is lost as thermal energy.
With hybrids, brakes turn the kinetic into magnetic, and
then into electrical energy, which is used to recharge
the batteries. This is why hybrids are best in local
driving, with shorter trips and more stops/starts.
For hybrid vehicles, the energy source is still
gasoline.
For fully electric vehicles, the coal or nuclear
power plant supplies the energy.
Alternate power sources, such as fuel cells
based on H2 have also been proposed.
Fuel cells operate similarly to galvanic cells,
but need a constant supply of fuel.
This fuel, usually hydrogen, is highly reactive,
and requires energy to produce.
Solar energy has been proposed to split
water into hydrogen and oxygen for fuel cells.
Hydrogen can be adsorbed onto metal
substrates to reduce its potential hazards.
Honda FCX Powered by Fuel Cells
8.5
A fuel cell is a galvanic cell that produces electricity by
converting the chemical energy of a fuel directly into electricity
without burning the fuel.
Both fuel and oxidizer must constantly flow into the cell to
continue the chemical reaction.
8.5
Chemical Changes in a Fuel Cell
H2 + ½ O2
H2O
Anode reaction: (Oxidation half-reaction)
H2(g)  2 H+(aq) + 2 e–
Cathode reaction: (Reduction half-reaction)
½ O2(g) + 2 H+(aq) + 2 e–  H2O(l)
Overall (sum of the half-reactions)
H2(g) + ½ O2(g) + 2 H+(aq) + 2 e– 
Net equation:
H2(g) + ½ O2(g) 
2 H+(aq) + 2 e– + H2O(l)
H2O(l)
8.5
Comparing Combustion with Fuel Cell Technology
8.5
Methanol or Methane Gas Could be Converted to
Hydrogen Gas Using a Reformer
8.5
High Pressure Hydrogen Storage
Refueling a Honda FCX
Clarity with hydrogen gas
Hydrogen Storage in Metal Hydrides
Hydrogen is absorbed onto metal hydrides and
released with increased temperature or decreased
pressure.
8.6
If the “Hydrogen Economy” becomes reality,
where will we get the H2?
One potential source is fossil fuels:
165 kJ + CH4(g) + 2 H2O(g) → 4 H2(g) + CO2(g)
247 kJ + CO2(g) + CH4(g) → 2 H2(g) + CO(g)
But these reactions produce carbon dioxide and carbon monoxide.
Electrolysis of water could also be used:
249 kJ + H2O(g) → H2(g) + ½ O2(g)
8.6
Electrolysis of Water:
Hydrogen (and oxygen)
gas produced by the
electrolysis of water.
But this process
requires energy.
8.6
Use Energy from the Sun to Split Water:
• Using fossil fuels or electrolysis uses a lot of energy.
• Instead, use energy from the sun for this process.
8.6
Photovoltaic Cells
Energy of the future? An awesome source
of energy.
Our society has been using photovoltaic
cells (solar cells) at a minimum level, but
will we all begin picking up on this
natural form of energy as a way to
conserve our natural coal and petroleum
supply?
8.7
Other countries making use of solar energy
Solar Park Gut Erlasee in Bavaria. At peak
capacity, it can generate 12 MW.
It’s time to make important decisions and
advances in alternative energy technology
and new sources of renewable energy.
Harnessing the energy
of the sun for pumping water
8.7
How does a photovoltaic cell generate electricity?
• A Semiconductor can be used (usually made of crystalline silicon).
Semiconductors have a limited ability to conduct electricity.
• Two layers of semiconducting materials are placed in direct contact to
induce a voltage in the PV cell.
• n-type semiconductor – has an abundance of electrons
• p-type semiconductor – deficient in electrons
Arsenic-doped n-type
silicon semiconductor
Gallium-doped p-type
silicon semiconductor
8.7
Schematic diagram of a solar cell:
8.7
Other Renewable Energy Sources: Solar Thermal
Aerial view of the Solar
Millennium Andasol project in
Spain
Close-up of a portion of the
mirrored array
Other renewable energy sources include wind, water, and geothermal energy.
8.8