Volume 1
1.10. OXYGEN

General Information • Ozone (O3). Allotropy • Applications of Oxygen • Physical Properties of O2 • Preparation of Oxygen • Chemical Properties of O2 • Exercises
1.10.1. General Information. As a chemical element, oxygen has the symbol O, atomic number 8 in the Periodic Table, and atomic weight (mass) of approximately 16 a.m.u. The valence of oxygen is 2. Oxygen is the most abundant element in the Earth's crust, approximately 50% by mass (Figure 1-31).
Figure 1-31. The mass-abundance of the most common elements in the Earth's crust (source).

As a simple substance, oxygen exists in the form of a diatomic molecule (O2) whose molecular mass is 32. The atmosphere (air) is approximately 21% oxygen admixed with nitrogen, N2 (78%), and roughly 1% of other gases, by volume.

1.10.2. Ozone (O3). Allotropy. There is another, less common form of oxygen as a simple substance, ozone. Ozone is a pale-blue gas. A molecule of ozone consists of three oxygen atoms, so the formula of ozone is O3. Ozone is much less stable and much more reactive than O2. Also, while oxygen O2 is odorless, ozone O3 has a characteristic smell and is toxic in high concentrations. Ozone is formed in the atmosphere from molecules of O2 during thunderstorms under the action of lightning (electric discharge). That is why it is often said that the air smells of ozone after a big thunderstorm. In the upper layers of the atmosphere, there is an ozone layer. This layer is of vital importance to life on our planet because it traps hazardous ultraviolet radiation from the sun.

Ozone (O3) is an allotropic modification (also known as allotropic form or just allotrope) of oxygen (O2). Allotropy (or allotropism) is the phenomenon of the existence of more than one simple substance for the same chemical element. Although allotropes are composed of atoms of the same type, they can and do exhibit distinct physical, chemical, and biological properties due to the difference in their structures. As mentioned above, oxygen O2 is nontoxic, odorless, and stable. In contrast, ozone O3 is toxic, has a characteristic smell, and much less stable.

Allotropes are known for many elements. One particularly well-known example is diamond and graphite, the two allotropes of carbon (Figure 1-32). Most people have seen diamond jewelry and used pencils, whose core, also known as pencil lead, is made of graphite. Diamond and graphite have totally different properties and appearance, despite the fact that both are composed entirely of carbon atoms. Diamond is clear and transparent, whereas graphite is black. Diamond is the hardest material known, whereas graphite is soft. Graphite conducts electricity, whereas diamond does not.
Figure 1-32. Allotropic forms of carbon (C), diamond (top) and graphite (bottom). Structure images source. Diamond picture source. Pencil photo source.

The difference between diamond and graphite stems from their different structures, the way carbon atoms are connected to one another and arranged in each of the two allotropes. These structures that are shown on the left in Figure 1-32 will be considered in more detail in Volume 3 of our course. For the time being, just remember the fact that graphite and diamond are different because their structures are different. There are many more allotropic forms of carbon. All of them have different structures and, consequently, possess distinct properties.

Another element that can produce allotropic forms is phosphorus (P). Several allotropes of phosphorus are known, the most common ones being white (sometimes called yellow) phosphorus and red phosphorus. Again, the difference in properties between the two is dramatic. Ivory-colored white phosphorus glows in the dark and is truly a diabolic substance that is both highly toxic and pyrophoric. A pyrophoric substance is a substance that ignites in air spontaneously, in the absence of any external ignition source. Chemical burns caused by white phosphorus are particularly painful and take a long time to heal. To avoid self-ignition, white phosphorus is conventionally stored under water. In sharp contrast, red phosphorus (which is indeed dark red in color) is not poisonous and neither self-ignites in air nor glows in the dark.
Digression. If you have read The Hound of the Baskervilles by Arthur Conan Doyle, you probably remember that the terrifying monstrous dog, as described in the novel, had a "luminous muzzle" and "blazing eyes". Genius fictional detective Sherlock Holmes and his friend Dr. Watson concluded that the glowing effect was due to the application of phosphorus. With all due respect, this conclusion does not hold water. The only allotrope of phosphorus that glows in the dark is white phosphorus, which is highly toxic and flammable. Applying white phosphorus onto a dog's skin and hair would cause severe burns and poisoning, and could even be lethal.
Allotropy is also characteristic of tin (Sn). At room temperature, tin is a silvery ductile metal. On cooling to 13 °C (about 56 °F) and below, however, this conventional form of tin transforms into a different allotrope, a brittle grey material that easily disintegrates to a powder. This allotropic transition is known as tin pest, tin blight, tin leprosy and tin plague. Watch the transition of the regular form of silvery tin metal to the brittle allotrope of tin here.
Digression. It has been proposed that Napoleon's catastrophic defeat in the 1812 French invasion of Russia may have been due to his army's uniform buttons. The buttons were made of tin, and the Russian winter of 1812 was brutally cold, as cold as -37 oC (-35 oF) at the beginning of December. The cold temperature was ideal for tin pest, which turned the solid tin metal buttons into a powder. That prompted the defeat. It is not easy to fight in the field when it is awfully cold and your pants are undone and keep falling down because the buttons are gone. It is sometimes said that the 1812 war tin buttons accident has been the greatest wardrobe malfunction in history. We do not know if the story of Napoleon's buttons is a true one or just an urban legend. What we do know though is that the allotropic transformation of tin at 13 oC and below is a scientifically established and well-documented fact.
Now here is a question for you. Atomic oxygen, O, is extremely short-lived because it dimerizes to give stable O2 molecules. Nonetheless, oxygen atoms have been detected experimentally. Do you think atomic oxygen can be identified as another allotropic form of oxygen? [My answer: Why not? The experimental demonstration of atomic oxygen shows that it can exist under certain conditions.]

1.10.3. Applications of Oxygen. Pure oxygen is broadly used for the smelting of iron ores into steel, metal cutting and welding, for rocket fuels, for making chemicals, and in medicine.

1.10.4. Physical Properties of O2. Oxygen is a colorless and odorless gas that is slightly more dense than air. At normal pressure (1 atmosphere), the boiling point of oxygen is -183 oC. Liquid oxygen is light blue in color. At -219 oC oxygen becomes a solid.

1.10.5. Preparation of Oxygen. In industry, pure oxygen is isolated from air by low-temperature (cryogenic) distillation or in a different process that is called vacuum swing adsorption. In the laboratory, oxygen is often obtained by heating potassium permanganate, KMnO4, according to the following equation.

2 KMnO4 = K2MnO4 + MnO2 + O2 (watch this video)

Small quantities of O2 can also be prepared by the decomposition of hydrogen peroxide in the presence of a catalyst, such as MnO2 or baker's yeast (see Section 1.8, including Experiment 5):

2 H2O2 = 2 H2O + O2

1.10.6. Chemical Properties of O2. Without oxygen in the atmosphere, there would be no life. The oxygen that we breathe is needed for many vital bioprocesses continuously occurring in our body. These bioprocesses are multistep chemical transformations, which, due to their complexity, are studied in advanced biochemistry and chemistry courses. In our introductory course, we will consider only very simple reactions of O2.

As demonstrated in this video, flammable substances burn in pure oxygen more rapidly and brightly than in air, which contains only about 20% O2. Among simple substances, hydrogen (H2), carbon (C), sulfur (S), phosphorus (P), magnesium (Mg), aluminum (Al), and even iron (Fe) react with oxygen to produce the corresponding oxides of these elements (Figure 1-33). Among the limited number of simple substances that do not react with O2 are some precious metals, such as gold and platinum.
Figure 1-33. Reactions of selected simple substances with oxygen.

Many complex substances (compounds) also readily react with oxygen. The burning of methane CH4, the main component of natural gas (Figure 1-34), is a great source of energy that is broadly used for heating purposes. Oxygen is as paramount for the combustion of fuels to produce energy as it is for respiration.
Figure 1-34. Combustion of methane.

When a substance has reacted with oxygen, we say that this substance has been oxidized. Reactions of various substances with O2 are conventionally accompanied with a considerable release of heat. Chemical transformations in which energy is released in the form of heat are called exothermic. The ones in which energy is consumed are called endothermic. All combustion reactions with O2 are exothermic.

Alas, it is not uncommon to hear some people say that oxygen is flammable and that oxygen can burn. These statements make no sense because oxygen on its own is not flammable. A flammable is a substance or a mixture of substances (such as natural gas and gasoline) that burn in the presence of oxygen (react with O2).

1.10.7. Exercises.

1. Balance the following chemical equations for reactions of various simple substances with O2. (Do not forget the tip: if the number of oxygen atoms on the right (product) is odd, use an appropriate coefficient to make it even. If you do not remember why, go back to subsection 1.8.6.)

(a) C + O2 = CO2

(b) Li + O2 = Li2O

(c) Ca + O2 = CaO

(d) B + O2 = B2O3

(e) Cu + O2 = CuO

2. Are there metals that do not react with O2? Answer

3. One of the chemical equations presented in Figure 1-33 is for a chemical reaction that had been widely used in flash photography for decades since the 1860s. Which one? Answer

4. Diamond and graphite are allotropes of carbon. Both of them burn in excess oxygen to produce carbon dioxide (CO2). Do you think CO2 produced from diamond is different in any way from CO2 formed from graphite? Answer

5. The combustion of ethane C2H6 in oxygen produces CO2 and H2O. Write a balanced chemical equation for this reaction. Answer

6. What color is liquid oxygen? Answer

7. Antimony (Sb) reacts with oxygen to give antimony (III) oxide. Write the formula of antimony (III) oxide and a balanced chemical equation for this reaction. Answer

8. Combustion reactions involving O2 (a) are always endothermic; (b) are always exothermic; (c) can be exothermic or endothermic. Answer