US - USING SUNLIGHT
Topic Overview
Redox Chemistry
Modern Methods 
Hydrogen as a Fuel
Solar Cells
The Ideal Gas Law
Hydrogen as a Fuel
When burnt, hydrogen gives out a lot of energy. Since photosynthesis produces oxygen from water, could sunlight be used to produce oxygen and hydrogen from water? The O-H bond energy is +463 kJ mol-1, which is 7.69 x 10-19J to break one bond, so there is not enough energy available from sunlight directly.
Electron transfer processes for breaking down water are:
1/2O2 + 2H+ + 2e- E
= +0.81V2H+ + 2e-
H2 E = -0.42VUsing the electrode potential values it can be seen that the decomposition of water will not work.
Plants are able to do this because chlorophyll plays a key part by using the energy absorbed from sunlight to create new substances which can oxidise waterin the first stages of photosynthesis. We are looking for a substance which is a good absorber of sunlight, is able to oxidise water and in its excitedstate can reduce hydrogen ions. Some of the most successful research so far on decomposing water in this way is based on complexes of ruthenium.
Hydrogen can be obtained by electrolysis of water, but a lot of electrical energy is needed - is it worth it? There are two major advantages:
- hydrogen can be stored,
- it can be used in the internal combustion engine, hence saving fossil fuels and not producing pollution.
Fuel cells have been designed which use hydrogen and oxygen to produce electrical energy.
2H+ + 2e- E = -0.42V1/2O2 + 2H+ + 2e-
H2O E =+0.81VFuels cells have been used in space craft as a source of electrical energy.
Solar Cells
Solar cells generate electricity directly from sunlight. They depend on the photovoltaic effect - when sunlight shines on some materials asmall current is generated. Solar cells are made from semiconductor materials. These become better conductors as their temperature riseswhich is the opposite to metals. Silicon is the most widely used as it is extremely abundant on Earth, and thus cheap to produce. It is acrystalline element with each silicon covalently bonded to four others in a tetrahedral arrangement. The conductivity of silicon also increases whenexposed to light.
Electrons in atoms are in definite, well-separated energy levels. However, in solids the levels group into energy bands. In semiconductors thebands are either completely filled or completely empty. In order to move, an electron would have to be excited into a higher, unfilled band. Whenphotons with enough energy are absorbed, electrons can move into the lowest unfilled band, leaving vacancies in the lower band. Electrons move around inthe lowest unfilled band, until eventually they will fall back into vacant sites in the lower band, giving out energy, unless prevented.
In a solar cell this is done by creating an electric field at a junction between two types of silicon semiconductor, called n-type and p-typesemiconductors. In n-type silicon, a few of the silicon atoms have been replaced by phosphorus atoms which have an extra electron which must occupy thelowest unfilled band and better conductivity. In p-type silicon, a few of the silicon atoms have been replaced by boron atoms which have one electronfewer. The highest occupied band is not completely filled, hence conductivity is increased. The process is called doping.
In a solar cell the two types are put together. At the junction, there are electron vacancies in the highest occupied band in the p-type material, andmobile electrons in the lowest unfilled band of the n-type material.
Electrons move across the junction from the n-type material into the p-type material. This creates a separation of charge on each side of the junction,which prevents further movement of electrons.
In a solar cell, a thin wafer of silicon is used and two layers are created in it. One part per thousand of phosphorus is added to a very thin toplayer, and one part per million of boron to the thicker bottom layer.
If sunlight falls on the n-type material, electrons are excited to the lowest unfilled band. They are repelled away from the junction and move awaytowards the upper surface. This leaves behind vacancies in the highest occupied band in the n-type material. Electrons move across the junctionfrom the highest occupied band of the p-type material. If the upper and lower surfaces of the wafer are connected to an external circuit, electrons flowfrom top surface to bottom surface to restore balance of charge.
A typical silicon cell has an area of 100 cm2 and can deliver 3A at 0.5 V in full sunlight. Hence large panels are needed in order togenerate significant amounts of power. Cells are connected in series to form modules giving out 12V d.c. The modules can be connected together toform larger systems.
They are non-polluting, give no noise, have no moving parts, require minimal maintenance and can be used anywhere. The limiting factor is the cost ofproduction.
The Ideal Gas Law
Gases are easy to compress because there is plenty of empty space between the particles. Boyle found that if the pressure is doubled, the volume ishalved. This law can be written:
P V = constant
Charles found that the volume of a gas is affected by temperature. He found that the volume is directly proportional to the absolute temperature at aconstant pressure. This relationship can be written as:
V / T = constant
Combining Boyle's Law and Charles' Law gives:
P V / T = constant
Thus, for n moles of gas this can be written:
P V / T = nR
or
P V = nRT
where R is the universal gas constant, which experimentation has proved to be 8.31 JK-1mol-1
The Ideal Gas Equation is only approximately obeyed by real gases, but it is useful to determine the volume and pressure of gases underdifferent conditions. However, the gas law follows within most temperatures accessable within our laboratory, and only deviates near0K and at higher temperatures with certain real gases. It has been found that 1 mole of a gas at 273K and 101,325Pa occupies 22.4dm3.
