Yesterday we talked in miserable detail about why oxygen is unique, and today we will finish it by discussing two important topics: photosynthesis and ozone.
Photosynthesis obviously is important because it is the source of all of the oxygen in the atmosphere. Ozone is important for two reasons, in addition to its interesting chemistry. in the upper atmosphere ozone literally saves our lives, and at ground level kills us. More after the fold.
Photosynthesis occurs when green (usually, but some algae have other pigments) plants absorb carbon dioxide and water, and then utilize solar energy to combine them into carbohydrates. For simplicity, we will make the assumption that the sole product is dextrose, but is reality many high polymers of dextrose are created. The overall reaction is:
6CO2 + 6H20 → C6H12O6 + 6O2 ΔH = +2818 kJ mol-1
This means that to form one mol (about 180 grams, about a third of a pound) of glucose requires the input of 2818 kJ (kilo Joules) of energy, about 673 Calories for a reference to food energy units.
There is a problem, because chlorophyll absorbs mostly red light (remember green is the complementary color of red, so that is why leaves look green), and red photons have only about 170 kJ mol-1 of energy. What we are really doing is "unburning" sugar, so it makes sense that it requires energy. However, you can see that it takes a multiple photon event to produce a sugar molecule. How do plants do that?
Photosynthesis is not a simple process. Without the gory details, the energy from many photon reactions (the so-called "light" reactions) are stored in intermediate compounds in the chloroplast (the part of the plant’s cell that contains the hardware for sugar synthesis) and is released later into the rest of the mechanism (the so-called "dark" reactions) that actually cause the reaction with carbon dioxide. Wikipedia has a pretty good writeup on it, but there are some technical issues with it, and not all of the chemistry has been worked out even yet, although we are getting close.
Chlorophyll is pretty good at collecting photons, about 90% efficient at the optimum wavelength, but only in a rather limited bandwidth. There are several forms of chlorophyll, each with a slightly different absorption maximum, and other pigments harvest different wavelengths and channel the energy into the mechanism. That is why fall leaves tend to turn colors, as the chlorophyll decomposes and the other, more stable, pigments last for a while. Yellow pigments absorb in the blue (high energy photons) and red ones in the green (higher energy than chlorophyll absorbs, but not as high as blue photons). Nature is pretty wonderful.
The important part for us is that the overall reaction releases oxygen into the atmosphere and fixes carbon. Until the advent of green plants, life as we know it was not possible. Interestingly, oxygen is toxic to forms of life that predate green plants, and these are known as anaerobic bacteria. Some of them are pretty nasty, like the germs that cause botulism and tetanus.
Now to ozone. Ozone is an endothermic material, meaning it contains more energy than its elements. Therefore, it is unstable and pure, liquid or solid ozone is apt to detonate, forming oxygen. Ozone is composed of three oxygen atoms, in a bent configuration. For you wonks out there, we are looking at a molecule with a similar geometry as water, and it is polar due to a single lone pair of electrons on the central atom.
Ozone is formed in the upper atmosphere by direct irradiation of oxygen by sunlight, which provides the energy to make the reaction proceed. Oxygen is only slightly opaque to the ultraviolet, but ozone is strongly opaque to it, being dissociated back to an oxygen molecule and an oxygen atom, which recombine (oxygen atoms are lonely) to regenerate the ozone. The ozone layer is several kilometers thick, but because of the altitude and low pressure, would be only a few millimeters thick at sea level.
The problem with chlorofluorocarbons (the "Freons") is that they interfere with the regeneration of ozone after the dissociation just mentioned. Freons are very stable, and eventually diffuse to the ozone layer, where they act as "chain reaction inhibitors" by intercepting the oxygen atoms before they can recombine with oxygen molecules (remember that I said that oxygen atoms are lonely). The energetics of forming oxygen-chlorine adducts are better than forming ozone, so the oxygen atoms are scavenged and become unavailable for further ozone formation. Any chlorinated material will do this, but the Freons are just about the worst because they are so stable that they survive the long journey into the upper atmosphere.
Why is ozone bad at ground level? Ozone is a very powerful oxidizing agent (I will go into that in a little bit) and combines with essentially anything. It is produced by sunlight acting on unburnt hydrocarbons mainly from automobile exhaust by a complex chain reaction involving free radicals, and then combines with everything else. It also oxidizes the tissues in your lungs, so that is the problem.
By the way, ozone has some good uses. It is being utilized to sterilize water, and leaves fewer toxic products than chlorine does, but the equipment to make it is expensive and energy intensive (remember, pure ozone is explosive, so there is no way to transport it). It is also beginning to be used to bleach paper and flour, again with fewer toxic byproducts than are produced by using chlorine.
Last question: Why is ozone so reactive, when you told me last night that oxygen is particularly unreactive? I thought that you would never ask! If you recall, last night we discussed the triplet nature of the oxygen molecule (two unpaired electrons) that makes its reaction with normal, singlet matter (no unpaired electrons) "spin forbidden", thus lowering the rate of reaction of oxygen with normal matter by a factor of over a billion. Well, ozone is free of this problem, because all of its electrons are paired, making it a singlet, and singlet-singlet reactions do not have to take spin into account. Ozone behaves like oxygen on steroids, reacting instantly with any material that can be oxidized, just about. Now you know the rest of the story. I will stick around for a while to field questions and comments, because you all know that I am not a "hit and run" diarist. Warmest regards, Doc.