Oxygen

The special position of oxygen in chemistry

Swedish chemist J. Berzelius deservedly called oxygen "the axis around which chemistry revolves". The unique position of this element is largely due to the properties of its most important compound — water (H₂O).

Since most chemical reactions take place precisely in aqueous solutions, oxides stand out as a completely distinct class of inorganic compounds. Interacting with water as a medium, they are capable of forming acids and bases. If hydrogen fluoride (HF), for instance, acted as the primary solvent on the planet, the chemical paradigm would change: fluorides would become the key class of substances, while metal oxides would be considered merely a special case of binary salts. Thus, the global significance of oxygen is directly linked to the physicochemical properties of water, its liquid state of aggregation, and its phenomenal dissolving capacity.

Biological significance ("Element of Life")

Oxygen is the absolute leader in abundance within the biosphere. It occurs in nature in the form of water (oceans and cellular environment), minerals (compounds with Si, C, S, P, and metals), as well as in free form (O₂) in the atmosphere, where it enters through photosynthesis.

In living organisms, the element performs two critically important functions:

• Structural: Water makes up over 60% of the mass of living beings, forming the internal environment of cells. In addition, oxygen atoms are embedded in the framework of key biomolecules — proteins, fats, and carbohydrates.
• Energy: Molecular dioxygen (O₂) is necessary for cells as a potent oxidizing agent. It oxidizes glucose and fat breakdown products, releasing energy. Without a continuous supply of this gas, the life of most organisms stops within minutes. The only exceptions are anaerobic microorganisms whose metabolism proceeds without molecular oxygen.

Interestingly, under normal conditions, the chemical activity of oxygen is low. This kinetic limitation saves living organisms from instantaneous spontaneous oxidation (combustion) to CO₂ and H₂O. All biochemical oxidation processes in cells are triggered exclusively with the participation of protein catalysts — enzymes.

Allotropic modifications: dioxygen and ozone

The element is capable of forming two simple substances that differ drastically in their properties and biological effects.

Dioxygen (O₂) This is a colorless, odorless gas that condenses into a blue liquid upon deep cooling (below −182.95 °C). Classical theory assumed a double bond between the atoms, but the actual physical properties of the gas proved its paramagnetism. In the modern model, the molecule has a triple bond that is significantly weakened by the presence of two unpaired electrons in antibonding orbitals. Energetically, such a structure is close to the strength of a double bond.

Ozone (O₃) Trioxygen is a blue gas with a pungent specific odor, liquefying into a dark blue liquid at −111.35 °C. The molecular geometry is an isosceles triangle with an obtuse angle of 117°. Ozone is unique in that it is the only case of a polar molecule among all simple substances. The central atom at the apex of the obtuse angle bears a positive charge (+δ) and exhibits trivalency, which is unpredictable from the perspective of classical theory. The chemical bond in ozone is delocalized (π-bond), continuously redistributing between the terminal atoms as resonance structures. Unlike life-sustaining dioxygen, ozone is toxic, although its powerful oxidizing properties are actively used in ozonizers for purifying drinking water.

Preparation and regeneration

In industry, molecular oxygen is obtained by rectification (fractional distillation) of liquid air and stored in gas cylinders, traditionally painted blue.

In laboratory practice, displacing oxygen from complex compounds is extremely difficult (possible only with fluorine, which is too dangerous). Therefore, thermal decomposition of oxygen-rich compounds is used:

1. Nitrates: $2KNO_3 \rightarrow 2KNO_2 + O_2$
2. Potassium chlorate (chlorate of potash) with a catalyst: $2KClO_3 \xrightarrow{t, MnO_2} 2KCl + 3O_2$
3. Hydrogen peroxide: $2H_2O_2 \xrightarrow{kat} 2H_2O + O_2$

Of particular practical importance is air regeneration in confined spaces (submarines, spacecraft, gas masks). For this purpose, potassium superoxide (KO₂) is used, which can simultaneously absorb carbon dioxide and release fresh oxygen via the reaction: $4KO_2 + 2CO_2 = 3O_2 + 2K_2CO_3$.

Reactivity

Oxygen is a universal oxidizing agent for all elements of the Periodic Table, except for fluorine. It reacts with most substances whose atoms are fundamentally capable of forming bonds with oxygen.

However, the gas will not react with compounds in which the elements have already reached their highest oxidation states (for example, carbon dioxide CO₂ or sulfuric acid H₂SO₄). Direct interaction is also impossible if the element does not form stable oxygen bonds (chlorine in NaCl) or if oxygen is physically unable to displace a stronger oxidizing agent (fluorine in CaF₂). Reactions with other substances (metals, organic compounds, ammonia) proceed with the formation of the corresponding oxides. Rapid oxidation accompanied by a powerful release of energy is called combustion, the rate of which is directly proportional to the concentration of oxygen.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Electron balance and the mechanism of reactions with potassium superoxide.
• Features of the redox properties of oxygen upon interaction with active metals.
• Resonance structures and $\pi$-bond delocalization in the ozone molecule.
• Application of the heavy isotope ¹⁸O in scientific research (tracer technique).
• Specifics of the thermal decomposition of potassium permanganate to produce O₂.

Other topics of section "Chapter 10. Group VIA Elements"

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