Stoichiometry is the understanding of the measure or relationship of chemical compounds to one another in chemical reactions.
It is the area of chemistry where students often stumble because it is related to the balanced chemical equation. For example, how many carbon dioxide molecules are generated during the combustion of, say, gasoline?
Well, it depends upon which chemical species you are talking about in gasoline since there are many different compounds present.
But for straight chain octane and all of its variants, the balanced equation tells you the answer is eight. Eight carbons in, eight carbons out. The mass of the universe is conserved - at least when it comes to chemical reactions.
Of course, things are a little more complicated.
For example, saying that "eight carbons in" equals "eight carbons out" is accurate but the "carbons out" are in the form of carbon dioxide while the "carbons in" are in the form of an alkane. The masses are not the same.
To handle this, chemists devised the concept of the "mole."
It is a huge number which often frightens people away from chemistry and science. It is 6.022 times ten raised to the 23rd power. That is, it's a six followed by 23 zeros.
However, all a mole does is allow us to convert atomic mass on the periodic table into grams.
It is just a conversion factor. The number of things in a mole isn't really important if we recognize it is just a conversion factor.
In this light, with oxygen weighing "15.9994" on the periodic table, it is fairly straightforward to recognize dioxygen - or "molecular oxygen" which consists of two oxygen atoms joined together - must weigh in at 31.9988.
One mole of oxygen molecules weighs 31.9988 grams.
Stoichiometry allows us to equate moles across a chemical reaction. Burning one mole of octane results in the generation of eight moles of carbon dioxide.
The stoichiometric relationship allows us to establish a molar relationship.
What this means for us is that combustion one mole or 114.23 grams of octane would result in the production of 352.06 grams of carbon dioxide. On this basis, and assuming that octane is representative of gasoline as a whole, one litre or 803 grams of gasoline results in the production of 2,474 grams of carbon dioxide. Roughly 2.5 kilograms of carbon dioxide is generated per litre of gasoline.
A more useful way of thinking about this, though, is the amount of energy available per mole of carbon dioxide emitted. How much energy can we get for each unit of carbon dioxide added to the atmosphere?
The maximum amount of energy that can be obtained from burning a mole of octane is 5,460 kilojoules. Compared to, say, methane at 802.5 kilojoules per mole, that would seem like a lot of energy.
However, octane generates eight times as much carbon dioxide as methane.
So, on a "per mole of carbon dioxide generated" basis, the answer for octane is 682.5 kilojoules per mole of carbon dioxide versus 802.5 kilojoules per mole of carbon dioxide for methane. In other words, burning gasoline generates 17.6 per cent more carbon dioxide compared to methane for the same amount of energy.
This is the basis for the argument methane or natural gas is a clean fuel but it has its disadvantages such as running a car's engine.
Gasoline makes a lot more sense as methane is not a suitable fuel.
But what about ethanol? There are lots of advocates suggesting that ethanol is a better fuel.
One mole of ethanol - made by fermenting glucose or sugar - will generate 1,367 kilojoules of heat.
However, it will also generate three moles of carbon dioxide (two from combustion and one from fermentation). The result is ethanol only delivers 455.7 kilojoules per mole of carbon dioxide released into the environment.
Put another way, ethanol is only 56 per cent as efficient as methane with regard to the emission of carbon dioxide.
Why, then, is the push on to use bio-fuels such as ethanol? Ethanol is made from sugar which can be obtained from starch and from cellulose which are made by plants absorbing carbon dioxide from the atmosphere and water from the ground. Photosynthesis, in theory, means carbon dioxide emissions from the combustion of ethanol will result in the generation of new plant material - of starch and cellulose. The system is renewable or recyclable.
The only problem with this notion is that the rate of growth of plants is not restricted by the amount of carbon dioxide in the air but by the availability of nutrients in the soil.
Fertilizers add a whole other carbon dioxide burden on the use of ethanol as a fuel.
When it comes to understanding the chemistry behind carbon dioxide emissions, stoichiometry tells us a lot.
