Oxygen reactions essential, destructive
One of the most reactive substances in our environment is oxygen. It reacts with metals through oxidation and eventually leads to the formation of oxides. In the case of iron-based metals, this is visually obvious as the material will rust. Rust is simply a case of a chemical reaction between the metallic element and oxygen molecules to give various iron oxides.
These oxide compounds are not as structurally strong as the metal itself, which is why rusting can lead to the disintegration of an old car or a bed spring.
Other metals, such as aluminum, rapidly react with oxygen to form a passivating layer on their surface. This layer is atomically thin being only a few atoms deep but it serves as a shield and protects the underlying metal from further oxidation. This is why aluminum doesn’t rust away. But a vigorous reaction between aluminum and oxygen can be observed if the passivating layer is amalgamated with mercury.
Oxygen also reacts with nonmetallic elements such as sulfur and chlorine. Sulfur is one of the few elements which can be found in its elemental form and it readily burns to give sulphur dioxide, which is a noxious and poisonous gas. The various oxides of sulfur also lead to sulfate compounds and the generation of sulfuric acid. Chlorine reacts with oxygen giving both anionic and neutral compounds such as sodium hypochlorite – bleach – and chlorine dioxide.
Oxygen readily reacts with hydrogen to give dihydrogen monoxide, commonly called water.
This is one of the most important chemical compounds on the planet. Arguably, life would not exist without water. We certainly wouldn’t be a blue-green planet.
But of all the elements which react with oxygen perhaps the most important for our modern life is the reaction between carbon or carbon compounds and oxygen.
We even have a special name for these reactions. We call it combustion when it is overt, such as a fire or explosion. Within a cell, we call it metabolism.
Our entire body runs on the slow and continuous combustion of sugar compounds with oxygen.
While water is essential for supporting life, it is the slow reaction of glucose and oxygen to produce carbon dioxide and water which allows us to function.
Simple combustion involves the direct and sometimes rapid conversion of complex carbon compounds to carbon dioxide and water.
For example, the combustion of methane results in carbon dioxide, water and an awful lot of heat. In an excess of oxygen and under the right conditions, this reaction can even be explosive.
But if oxygen is deficient then complete combustion doesn’t occur. It takes two molecules of oxygen to react with every methane molecule. If only one molecule of oxygen is available then some partial produces are formed such formaldehyde or formic acid.
With larger simple hydrocarbons, such as propane, complete combustion to carbon dioxide requires five molecules of oxygen for every molecule of propane. If only four molecules of oxygen are present then carbon monoxide might be an end product. Even fewer oxygen molecules and a variety of carbon compounds, such as acetaldehyde, might be generated.
As carbon compounds get larger and more complex, combustion gets more and more complicated.
Partial oxidation becomes more prevalent. The reason wood smoke smells is due to the plethora of compounds generated as by-products.
The reason for this is many-fold. As combustion relies on a steady supply of oxygen, a fire can be smothered by removing access to oxygen. This can be accomplished by using a fire extinguisher, but it also naturally occurs within a combustion zone when the rate of oxidation is faster than the rate at which the oxygen can be replenished. Air must be drawn into any fire to keep it going but a rapid combustion or a large fire or a fire with a complex structure might not get enough air and the lack of oxygen results in smelly compounds.
This is the case for pretty much any campfire or wood stove.
The rate of reaction exceeds the capacity for the replenishment of oxygen. The result is the distinctive odours left on clothing after a night camping or the smell of smoke downstream from a house burning wood.
The compounds produced as byproducts of incomplete combustion tend to be hydrogen deficient compounds such as poly-aromatic hydrocarbons. These compounds have a significant impact on human health as they are potent carcinogens. A campfire might smell nice, but breathing in the smoke can come at a cost.
Ironically, it is the smell and the taste of these molecules which give cigarettes made from tobacco or marijuana their distinctive odours. And it is these compounds which lead to the cancer potential, not the nicotine or THC.
Oxygen reacts with all sorts of compounds in our environment, but not all of the results of such reactions are benign.