Last week, we were talking about batteries and I commented on the processes of oxidation and reduction.
Essentially, these two processes are complementary. Oxidation is the loss of electrons while reduction is gaining electrons. The electrons lost by one compound are picked up by another. Oxidation is always accompanied by reduction and vice versa.
Much of chemistry can be explained by this concept since chemical reactions essentially are about shifting electrons.
There is another concept associated with oxidation and reduction and that is oxidation number. This is related to the oxidation state of an atom which is determined by the number of missing or added electrons.
A simple example is the sodium atom. As a metal, sodium is in a zero oxidation state. It has exactly 11 protons in its nucleus and 11 electrons in the surrounding orbitals. As an ion, one of the electrons is lost and the sodium develops a positive charge as a result. The sodium ion is therefore Na+ and the oxidation number is +1.
Conversely, the chloride ions in sodium chloride are negatively charged and have an oxidation number of -1. One can think of the reaction of sodium metal with chlorine gas as a reduction/oxidation reaction in which there is an exchange of electrons resulting in a positively charged or oxidized sodium and a negatively charged or reduced chlorine.
This is the essence of redox chemistry which is often the bane of a chemistry student's existence.
However, simple redox reactions in which an electron is exchanged to create simple ions (i.e. Na + Cl -> Na+ + Cl-) are not the only use of oxidation numbers. And it is not always intuitively obvious how an oxidation number is obtained.
This is particularly true for organic compounds.
Consider a molecule of methane which is composed of a single carbon atom surrounded by four hydrogen atoms. There are a number of ways we could look at the oxidation number of the carbon atom but by convention, we assume each hydrogen atom has an oxidation number of +1 and as a result the carbon atom is -4.
Conversely, for carbon dioxide, we have a carbon atom attached to two oxygen atoms. Each oxygen atom is assigned a -2 oxidation number and the carbon is now +4. (Oxygen and two hydrogens combine to give water which has an overall charge of zero.)
In this sense, we can think of the combustion of methane as a redox reaction with the result that we convert a -4 carbon into a +4 carbon with the exchange of eight electrons. Overall, the reaction is balanced by the consumption of oxygen and the creation of water molecules.
That is, a single methane molecule will react with two molecules of oxygen to give a single carbon dioxide molecule and two molecules of water.
From a chemical point of view, there is no reason why this reaction can't simply be turned around and run the other direction. However, in our macroscopic world, this never happens spontaneously.
Carbon dioxide and water react but not to produce methane. They generate carbonic acid or hydrogen carbonate instead.
The reason behind this is the energy released in the combustion reaction. The energy loss results in the reaction going in one direction and one direction only.
The carbon atoms are converted from highly reduced to highly oxidized atoms by their reaction with oxygen.
The reverse reaction would require all of the energy lost during combustion to be put back into the carbon dioxide and water molecules in a way that would allow them to recreate methane and oxygen molecules.
It would be simplistic to say this is what plants do when they undergo photosynthesis. They do use sunlight to reverse the reaction but only part of the way. The result of photosynthesis is a sugar molecule in which the carbon atoms are partially reduced to a neutral oxidation number. Making methane from plant material requires a bit more energy.
All of this is a long and terribly chemical explanation for why we simply can't take the carbon dioxide generated during the combustion of fossil fuels and convert it back into more fossil fuels. The energetics are working against the process. Converting oxidized carbon back into reduced carbon is simply not a favourable reaction.
More to the point, it takes more energy to run the reaction backwards than is gained from running it forward as there are invariably losses along the way.
But it can be done using a variety of catalysts and reaction schemes. The trick is to find an alternative source of energy. Recent advances in synthetic leaves have scientists believing it might just be possible to build a system which would take water and carbon dioxide and convert them back into methane and oxygen.
More on that next week.
