Chapter 8: Energy from Electron Transfer
In his 2006 State of the Union address, President George
W. Bush proclaimed “. . . we are addicted to oil”
Are we doomed
as a country to
go on to the bitter
end in terms of
our addiction to
oil?
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
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
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
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
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
A Laboratory Galvanic Cell
Oxidation (at anode):
Zn (S)
Zn2+ (aq)
Reduction (at cathode):
Cu2+ (S)
Zn2+ (aq)
8.1
An Alkaline Cell
8.2
Mercury batteries had the advantage of being very small
Used in watches, cameras, hearing aids
Toxicity and disposal concerns led to alternatives
8.2
Lead-Acid Storage Batteries
This is a true battery as it consists of a series of six cells.
Anode = Pb
Cathode = PbO2
Rechargable:
discharging
Pb(s) + PbO2(s) + 2 H2SO4(aq)
re-charging
2 PbSO4(s) + 2 H2O(l)
8.3
Hybrid Vehicles
Combining the use of gasoline and battery technology
Many hybrids, unlike conventional
gasoline-powered cars, deliver
better mileage in city driving than
at highway speeds.
8.4
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- 
2 H+(aq) + 2 e- + H2O(l)
Net equation:
H2(g) + ½ O2(g) 
H2O(l)
8.5
Comparing Combustion with Fuel Cell Technology
Process
Reactants
Products
Comments
Traditional
Combustion
Fuel: gasoline,
hydrocarbons,
ethanol, wood, ect.
Oxidant:
O2 (from air)
H2O, heat, light,
sound;
CO2 and CO
(greenhouse gases)
Rapid process,
flame present,
lower efficiency,
best process for
producing heat
Fuel Cell
Technology
Fuel: H2
Oxidant:
O2 (from air)
H2O, electricity,
some heat
Slower process, no
flame, quiet and
more efficient, best
for generating
electricity
8.5
Fuel Cell Autos
Slim solid oxide fuel cell
8.6
If the “Hydrogen Economy” becomes reality,
where will we get the H2?
Hydrogen (and oxygen)
gas produced by the
electrolysis of water.
But this process
requires energy.
8.7
The Hydrogen Economy: New Potential Sources
of Hydrogen
A computer simulation of hydrogen
molecules (red) absorbed onto
both the interior and exterior of a
collection of carbon nanotubes
(blue).
Certain algae produce
H2
Via photosynthesis-very
promising technology.
Realistic future?
8.8
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?
Thin wafers of ultra pure Si
used for voltaic cells.
8.9
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
8.10