Chemistry 1020-Lecture 22-Notes
Alternate Energy Sources
Hydroelectric power (waterwheels, etc.) Indirect solar power
The reaction: 2 H2 + O2 --> 2 H2O
has an energy change of -572 kJ, or -286 kJ per mole of water.
Splitting water into hydrogen and oxygen will require input of the same amount of energy.
Photosynthesis accomplishes the reverse of this process. But a single direct event in which a photon strikes water and leads to breaking of the H-O bond is not feasible because to break the bond (bond energy of 459 kJ/mole) would require a photon of equivalent energy. This is
459 x 103 J/mole/6.02 x 1023 bonds/mole = 7.62 x 10-19 J/bond
Recall that E = hn = hc/l, so a photon of this energy would have a wavelength of:
6.63 x 10-34 J-s x 3.00 x 108 m/s /7.62 x 10-19 J/photon
2.61 x 10-7 m or 261 nm
Recall also that this is in the ultraviolet range, and most of the ultraviolet rays of the sun has been absorbed by ozone.
The actual splitting which occurs in photosynthesis uses light of longer wavelength and lower energy, but is a complex process requiring two photons for each H-O bond split . The molecule chlorophyll plays a role as an intermediate in this process.
Before describing this process, I need to describe something about the oxidation and reduction reactions involved in splitting water electrolytically. Recall that oxidation is the removal of electrons, and occurs at the anode. Reduction is the gaining of electrons, and occurs at the cathode.
Electrical energy is necessary to raise the electrons at the cathode to a high enough energy level (i.e., electrical potential) to reduce hydrogen ions to hydrogen gas. It must also lower the energy level (electrical potential) of the anode to a point where the anode will pull electrons from water to form oxygen. (See diagram below).
In photosynthesis, the energy level of the electrons are raised by means of photons of light being absorbed by the pigment chlorophyll, promoting them to a higher energy level so that they can be passed on to substances which ultimately could reduce hydrogen or some other difficult to reduce substance. The removal of that electron produces a very strong oxidant that is capable of removing electrons from water. The energy comes from light, however, rather than an electrical generator.
Your text describes a molecule that could act somewhat in the fashion of chlorophyll in plants. It is a derivative of Ruthenium that is capable of absorbing light and causing an electron to be promoted from the Ru metal to the bispyridine portion surrounding the metal. The electron so promoted is in a higher energy state and would be capable of reducing a water molecule to form hydrogen:
2 H2O + 2 e --> 2 OH- + H2
while the oxidized form of the complex should be capable of oxidizing water to form oxygen:
4 H2O --> 4 H+ + O2 + 4 e
Note, though, that each process must be a multi-step process with presumed intermediates. In photosynthesis, the process of oxidizing water takes place on a complex between manganese ions complexed with several water molecules so that one electron can be removed at a time.
The hope is to develop some kind of artificial substance that can behave as the chlorophyll and the manganese complex and harness several photons of light to carry out this process. We have yet to achieve that goal.
What if we could? Then we could consider the use of the hydrogen produced as a fuel. Hydrogen has many desirable properties: There are no polluting byproducts in its reaction with oxygen, and no greenhouse gas formed. The problem comes in the transport and storage, since hydrogen is a gas and is very flammable.
It is possible to react hydrogen with oxygen in an indirect way in what is called a fuel cell rather than in an explosive reaction. The fuel cell is in some sense just the reverse of the electrolytic cell described above, except that the electrodes are separated into different compartments, or half-cells, which are connected by a salt bridge. Compare the following with Figure 9.6 of your text:
One part of the fuel cell has the following reaction going on:
2 H2 + 4 OH- 4 H2O + 4 e
This is the anode, because hydrogen is being oxidized here. Notice in this case the anode is negative.
The second part has the reaction:
O2 + 2 H2O + 4 e 4 OH-
This is the cathode, because oxygen is being reduced here. Notice in this case the cathode is positive.
(We could write the following alternative reactions if they were carried out in acid solution rather than basic solution:
The electrons produced at the anode can be made to pass through an electrical circuit to the cathode providing electrical energy.
Discussion of batteries:
Batteries operate under the same principle. An oxidation reaction produces electrons with a high potential at the anode, and a reduction removes electrons from the cathode, giving it a positive charge, and the current moves from the negative to the positive electrode.
Zn + 2 OH- Zn(OH)2 + 2 e (oxidation: anode)
2 MnO2 + H2O + 2 e Mn2O3 + 2 OH- (reduction: cathode)
rechargeable lead storage battery:
Pb + SO42- PbSO4 + 2 e (oxidation: anode)
PbO2 + SO42- + 4 H+ + 2 e PbSO4 + 2 H2O (reduction: cathode)
Discussion of electric cars as pollution free. Disadvantages? Expense. Also pollution occurs where the electricity is generated.
Also disposal problem of the lead storage batteries.
Directly harnessing photons to generate electricity. Involves combining semiconductors in such a way that absorption of light can cause movement of electrons. Silicon does not require photons of very high energy to mobilize an electron.
Doping Si with Arsenic (containing 5 valence electrons) puts an extra electron in the crystal lattice, making it easier to move (called an n type semiconductor). Doping Si with Gallium (containing one less electron) produces a p type semiconductor, in which there is one less electron, or an electron hole, which also allows electrons to move more easily. Layering these one against the other (see Figure 9.11) allows charge separation when light is absorbed.
Efficiency is not yet very great. And such cells are very expensive in relation to the amount of current produced. But they have become very reliable in certain applications, such as generation of electricity on satellites and space probes.
Fusion-the Energy source in the sun
Recall our graph showing the mass defect:
The reaction occurring in the sun is the fusion of hydrogen nuclei to form helium, representing a much larger loss of mass per gram than the fission process. We did harness this energy in an uncontrolled way by adding small nuclei (lithium deuteride) to the material in a nuclear fission weapon. The heat from the fission explosion was sufficient to overcome the repulsive energies between the nuclei of the deuterium and lithium nuclei for fusion to occur, generating even more energy. This was the principle behind the hydrogen bomb. Of course we would like to be able to carry out the fusion process in a controlled way. The problem is getting enough thermal energy in a confined space to get the nuclei to fuse. Experiments have been done with use of magnetic fields to suspend the material, and laser light sources to pump enough energy into the system. There is hope that something practical will be developed, but no one knows how long it will take.
There was a brief period of excitement a few years ago when some scientists thought they had observed fusion of deuterium atoms in a room-temperature electrolytic cell. Repetition of the experiments by other scientists showed their conclusions tobe wrong, however.