OXYGEN: PROPERTIES, STRUCTURE, RISKS, USES - CHEMISTRY - 2024

The oxygen is a chemical element that is represented by the symbol O. is a highly reactive gas, which leads the group 16: chalcogens. This name is due to the fact that sulfur and oxygen are present in almost all minerals.

Its high electronegativity explains its great greed for electrons, which leads it to combine with a large number of elements; This is how a wide range of mineral oxides arises that enrich the earth's crust. Thus, the remaining oxygen composes and makes the atmosphere breathable.

Oxygen is often synonymous with air and water, but it is also found in rocks and minerals. Source: Pxhere.

Oxygen is the third most abundant element in the Universe, behind hydrogen and helium, and it is also the main constituent by mass of the Earth's crust. It has a percentage by volume of 20.8% of the Earth's atmosphere, and represents 89% of the mass of water.

It usually has two allotropic forms: diatomic oxygen (O ₂), which is the most common form in nature, and ozone (O ₃), found in the stratosphere. However, there are two others (O ₄ and O ₈) that exist in their liquid or solid phases, and under enormous pressure.

Oxygen is constantly produced through the process of photosynthesis, carried out by phytoplankton and land plants. Once produced, it is released so that living beings can use it, while a small portion of it dissolves in the seas, sustaining aquatic life.

It is therefore an essential element for living beings; not only because it is present in most of the compounds and molecules that form them, but also because it intervenes in all their metabolic processes.

Although its isolation is controversially attributed to Carl Scheele and Joseph Priestley in 1774, there are indications that oxygen was actually isolated for the first time in 1608, by Michael Sendivogius.

This gas is used in medical practice to improve the living conditions of patients with respiratory difficulties. Likewise, oxygen is used to allow people to fulfill their functions in environments where there is diminished or no access to atmospheric oxygen.

Commercially produced oxygen is used primarily in the metallurgical industry for the conversion of iron to steel.

History

Nitroarial spirit

In 1500, Leonardo da Vinci, based on the experiments of Philo of Byzantium carried out in the second century BC. C., concluded that a portion of the air was consumed during combustion and respiration.

In 1608, Cornelius Drebble showed that heating salpetre (silver nitrate, KNO ₃) produced a gas. This gas, as it would later be known, was oxygen; but Drebble couldn't identify it as a new item.

Then, in 1668, John Majow pointed out that a part of the air that he called "Spiritus nitroaerus" was responsible for fire, and that it was also consumed during respiration and the combustion of substances. Majow observed that substances did not burn in the absence of the nitroarial spirit.

Majow carried out the combustion of antimony, and observed an increase in the weight of antimony during its combustion. So Majow concluded that antimony combined with the nitroarial spirit.

Discovery

Although it did not receive the recognition of the scientific community, in life or after its death, it is probable that Michael Sandivogius (1604) is the true discoverer of oxygen.

Sandivogius was a Swedish alchemist, philosopher, and physician who produced the thermal decomposition of potassium nitrate. His experiments led him to the release of oxygen, which he called "cibus vitae": food of life.

Between 1771 and 1772, the Swedish chemist Carl W Scheele heated various compounds: potassium nitrate, manganese oxide, and mercury oxide. Scheele observed that a gas was released from them that increased combustion, and which he called "fire air."

Joseph Priestly's experiments

In 1774, the English chemist Joseph Priestly heated oxide of mercury by using a twelve-inch magnifying glass that concentrated sunlight. The mercury oxide released a gas that caused the candle to burn much faster than normal.

In addition, Priestly tested the biological effect of gas. To do this, he placed a mouse in a closed container that he expected to survive for fifteen minutes; however, in the presence of the gas, it survived an hour, longer than it estimated.

Priestly published his results in 1774; while Scheele did so in 1775. For this reason, the discovery of oxygen is often attributed to Priestly.

Oxygen in the air

Antoine Lavoisier, a French chemist (1777), discovered that air contains 20% oxygen and that when a substance burns, it is actually combining with oxygen.

Lavoisier concluded that the apparent weight gain experienced by the substances during their combustion was due to the weight loss that occurs in the air; since oxygen combined with these substances and therefore the masses of the reactants were conserved.

This allowed Lavoisier to establish the Law of Conservation of Matter. Lavoisier suggested the name of oxygen that came from the root acid "oxys" and "genes" formation. So oxygen means 'acid-forming'.

This name is wrong, since not all acids contain oxygen; for example, hydrogen halides (HF, HCl, HBr, and HI).

Dalton (1810) assigned water the chemical formula HO and therefore, the atomic weight of oxygen was 8. A group of chemists, including: Davy (1812) and Berzelius (1814) corrected Dalton's approach and concluded that the correct formula for water is H ₂ O and the atomic weight of oxygen is 16.

Physical and chemical properties

Appearance

Colorless, odorless and tasteless gas; while ozone has a pungent odor. Oxygen promotes combustion, but it is not itself a fuel.

Liquid oxygen. Source: Staff Sgt. Nika Glover, US Air Force

In its liquid form (top image) it is pale blue in color, and its crystals are also bluish; but they can acquire pink, orange, and even reddish tones (as will be explained in the section on their structure).

Atomic weight

15,999 u.

Atomic number (Z)

8.

Melting point

-218.79 ° C.

Boiling point

-182.962 ° C.

Density

Under normal conditions: 1,429 g / L. Oxygen is a gas that is denser than air. In addition, it is a poor conductor of heat and electricity. And at its (liquid) boiling point, the density is 1.141 g / mL.

Triple point

54.361 K and 0.1463 kPa (14.44 atm).

Critical point

154.581 K and 5.043 MPa (49770.54 atm).

Heat of fusion

0.444 kJ / mol.

Heat of vaporization

6.82 kJ / mol.

Molar caloric capacity

29.378 J / (mol · K).

Vapor pressure

At a temperature of 90 K it has a vapor pressure of 986.92 atm.

Oxidation states

-2, -1, +1, +2. The most important oxidation state is -2 (O ^2-).

Electronegativity

3.44 on the Pauling scale

Ionization energy

First: 1,313.9 kJ / mol.

Second: 3,388.3 kJ / mol.

Third: 5,300.5 kJ / mol.

Magnetic order

Paramagnetic.

Water solubility

The solubility of oxygen in water decreases as the temperature increases. For example: 14.6 mL of oxygen / L of water is dissolved at 0 ºC and 7.6 mL of oxygen / L of water at 20 ºC. The solubility of oxygen in drinking water is higher than in sea water.

In the condition of temperature 25 ºC and at a pressure of 101.3 kPa, drinking water can contain 6.04 mL of oxygen / L of water; while the water of sea water only 4.95 mL of oxygen / L of water.

Reactivity

Oxygen is a highly reactive gas that reacts directly with almost all elements at room temperature and high temperatures; except for metals with higher reduction potentials than copper.

It can also react with compounds, oxidizing the elements present in them. This is what happens when it reacts with glucose, for example, to produce water and carbon dioxide; or when wood or a hydrocarbon burns.

Oxygen can accept electrons by complete or partial transfer, which is why it is considered an oxidizing agent.

The most common oxidation number or state for oxygen is -2. With this oxidation number, it is found in water (H ₂ O), sulfur dioxide (SO ₂) and carbon dioxide (CO ₂).

Also, in organic compounds such as aldehydes, alcohols, carboxylic acids; common acids like H ₂ SO ₄, H ₂ CO ₃, HNO ₃; and its derived salts: Na ₂ SO ₄, Na ₂ CO ₃ or KNO ₃. In all of them, the existence of O ^2- could be assumed (which is not true for organic compounds).

Oxides

Oxygen is present as O ^2- in the crystal structures of metal oxides.

On the other hand, in metallic superoxides, such as potassium superoxide (KO ₂), oxygen is present as the O ₂ ^- ion. While in metal peroxides, to say barium peroxide (BaO ₂), oxygen appears as the ion O ₂ ^2- (Ba ²⁺ O ₂ ^2-).

Isotopes

Oxygen has three stable isotopes: ¹⁶ O, with 99.76% abundance; the ¹⁷ O, with 0.04%; and ¹⁸ O, with 0.20%. Note that ¹⁶ O is by far the most stable and abundant isotope.

Structure and electronic configuration

Oxygen molecule and its interactions

Diatomic oxygen molecule. Source: Claudio Pistilli

Oxygen in its ground state is an atom whose electronic configuration is:

2s ² 2p ⁴

According to the valence bond theory (TEV), two oxygen atoms are covalently bonded so that both separately complete their valence octet; in addition to being able to pair its two solitary electrons from the 2p orbitals.

In this way then, the diatomic oxygen molecule, O ₂ (upper image), appears, which has a double bond (O = O). Its energy stability is such that oxygen is never found as individual atoms in the gas phase but as molecules.

Because O ₂ is homonuclear, linear, and symmetric, it lacks a permanent dipole moment; therefore, their intermolecular interactions depend on their molecular mass and the London scattering forces. These forces are relatively weak for oxygen, which explains why it is a gas under Earth conditions.

However, when the temperature drops or the pressure increases, the O ₂ molecules are forced to coalesce; to the point that their interactions become significant and allow the formation of liquid or solid oxygen. To try to understand them molecularly, it is necessary not to lose sight of O ₂ as a structural unit.

Ozone

Oxygen can adopt other considerably stable molecular structures; that is, it is found in nature (or within the laboratory) in various allotropic forms. Ozone (bottom image), O ₃, for example, is the second best-known allotrope of oxygen.

Structure of the resonance hybrid represented by a sphere and rod model for the ozone molecule. Source: Ben Mills via Wikipedia.

Again, TEV sustains, explains and shows that in O _{3 there} must be resonance structures that stabilize the positive formal charge of oxygen in the center (red dotted lines); while the oxygens at the ends of the boomerang distribute a negative charge, making the total charge for ozone neutral.

In this way, the bonds are not single, but neither are double. Examples of resonance hybrids are very common in as many inorganic molecules or ions.

The O ₂ and O ₃, because their molecular structures are different, the same happens with their physical and chemical properties, liquid phases or crystals (even when both consist of oxygen atoms). They theorize that the large-scale synthesis of cyclic ozone is likely, the structure of which resembles that of a reddish, oxygenated triangle.

This is where the "normal allotropes" of oxygen end. However, there are two others to consider: O ₄ and O ₈, found or proposed in liquid and solid oxygen, respectively.

Liquid oxygen

Gaseous oxygen is colorless, but when the temperature drops to -183 ºC, it condenses into a pale blue liquid (similar to light blue). The interactions between O ₂ molecules is now such that even their electrons can absorb photons in the red region of the visible spectrum to reflect their characteristic blue color.

However, it has been theorized that in this liquid there are more than simple O ₂ molecules, but also an O ₄ molecule (lower image). It seems as if the ozone had been "stuck" by another oxygen atom that somehow intercedes for the positive formal charge just described.

Proposed model structure with spheres and rods for the tetraoxygen molecule. Source: Benjah-bmm27

The problem is that according to computational and molecular simulations, said structure for O ₄ is not exactly stable; however, they predict that they do exist as (O ₂) ₂ units, that is, two O ₂ molecules are so close that they form a kind of irregular framework (the O atoms are not aligned opposite each other).

Solid oxygen

Once the temperature drops to -218.79 ºC, oxygen crystallizes in a simple cubic structure (γ phase). As the temperature drops further, the cubic crystal undergoes transitions to the β (rhombohedral and -229.35 ° C) and α (monoclinic and -249.35 ° C) phases.

All these crystalline phases of solid oxygen occur at ambient pressure (1 atm). When the pressure increases to 9 GPa (~ 9000 atm), the δ phase appears, whose crystals are orange. If the pressure continues to increase to 10 GPa, the solid red oxygen or ε phase (again monoclinic) appears.

The ε phase is special because the pressure is so enormous that the O ₂ molecules not only arrange themselves as O ₄ units, but also O ₈:

Model structure with spheres and rods for the octa-oxygen molecule. Source: Benjah-bmm27

Note that this O ₈ consists of two O ₄ units where the irregular frame already explained can be seen. Likewise, it is valid to consider it as four O _2s aligned closely and in vertical positions. However, their stability under this pressure is such that O ₄ and O ₈ are two additional allotropes for oxygen.

And finally we have the ζ phase, metallic (at pressures greater than 96 GPa), in which the pressure causes the electrons to disperse in the crystal; just as it happens with metals.

Where to find and production

Minerals

Oxygen is the third element in the Universe by mass, behind hydrogen and helium. It is the most abundant element in the earth's crust, representing around 50% of its mass. It is mainly found in combination with silicon, in the form of silicon oxide (SiO ₂).

Oxygen is found as part of innumerable minerals, such as: quartz, talc, feldspars, hematite, cuprite, brucite, malachite, limonite, etc. Likewise, it is located as part of numerous compounds such as carbonates, phosphates, sulfates, nitrates, etc.

Air

Oxygen constitutes 20.8% of atmospheric air by volume. In the troposphere it is found primarily as a diatomic oxygen molecule. While in the stratosphere, a gaseous layer between 15 and 50 km from the earth's surface, it is found as ozone.

Ozone is produced by an electrical discharge on the O ₂ molecule. This allotrope of oxygen absorbs ultraviolet light from solar radiation, blocking its harmful action on human beings, which in extreme cases is associated with the appearance of melanomas.

Fresh and salt water

Oxygen is a major component of seawater and freshwater from lakes, rivers, and groundwater. Oxygen is part of the chemical formula of water, constituting 89% of it by mass.

On the other hand, although the solubility of oxygen in water is relatively low, the amount of oxygen dissolved in it is essential for aquatic life, which includes many species of animals and algae.

Living beings

The human being is made up of approximately 60% water and, at the same time, rich in oxygen. But in addition, oxygen is part of numerous compounds, such as phosphates, carbonates, carboxylic acids, ketones, etc., which are essential for life.

Oxygen is also present in polysaccharides, lipids, proteins, and nucleic acids; that is to say, the so-called biological macromolecules.

It is also part of harmful waste from human activity, for example: carbon monoxide and dioxide, as well as sulfur dioxide.

Biological production

Plants are responsible for enriching the air with oxygen in exchange for the carbon dioxide that we exhale. Source: Pexels.

Oxygen is produced during photosynthesis, a process by which marine phytoplankton and land plants use light energy to make carbon dioxide react with water, creating glucose and releasing oxygen.

It is estimated that more than 55% of the oxygen produced by photosynthesis is due to the action of marine phytoplankton. Therefore, it constitutes the main source of oxygen generation on Earth and is responsible for the maintenance of life on it.

Industrial production

Air liquefaction

The main method of producing oxygen in industrial form is that created in 1895, independently by Karl Paul Gottfried Von Linde and William Hamson. This method continues to be used today with some modifications.

The process begins with a compression of the air to condense the water vapor and thus eliminate it. Then, the air is sieved by being conducted by a mixture of zeolite and silica gel, for the elimination of carbon dioxide, heavy hydrocarbons and the rest of water.

Subsequently, the components of the liquid air are separated through a fractional distillation, achieving the separation of the gases present in it by their different boiling points. By this method it is possible to obtain oxygen with 99% purity.

Electrolysis of water

Oxygen is produced by electrolysis of highly purified water, and with an electrical conductivity that does not exceed 1 µS / cm. Water is separated by electrolysis into its components. Hydrogen as a cation moves towards the cathode (-); while oxygen moves towards the anode (+).

The electrodes have a special structure to collect the gases and subsequently produce their liquefaction.

Thermal decomposition

Thermal decomposition of compounds such as mercury oxide and salpetre (potassium nitrate) releases oxygen, which can be collected for use. Peroxides are also used for this purpose.

Biological role

Oxygen is produced by phytoplankton and land plants through photosynthesis. It crosses the lung wall and in the blood it is captured by hemoglobin, which transports it to different organs to later be used in cellular metabolism.

In this process, oxygen is used during the metabolism of carbohydrates, fatty acids and amino acids, to ultimately produce carbon dioxide and energy.

Respiration can be outlined as follows:

C ₆ H ₁₂ O ₆ + O ₂ => CO ₂ + H ₂ O + Energy

Glucose is metabolized in a set of sequential chemical processes, including glycolysis, the Krebs cycle, the electron transport chain, and oxidative phosphorylation. This series of events produces energy that accumulates as ATP (adenosine triphosphate).

ATP is used in various processes in cells including the transport of ions and other substances across the plasma membrane; the intestinal absorption of substances; the contraction of different muscle cells; the metabolism of different molecules, etc.

Polymorphonuclear leukocytes and macrophages are phagocytic cells that are capable of using oxygen to produce superoxide ion, hydrogen peroxide, and singlet oxygen, which are used to destroy microorganisms.

Risks

Breathing oxygen at high pressures can cause nausea, dizziness, muscle spasms, loss of vision, seizures, and loss of consciousness. In addition, breathing pure oxygen for a long period of time causes lung irritation, manifested by coughing and shortness of breath.

It can also be the cause of the formation of pulmonary edema: a very serious condition that limits respiratory function.

An atmosphere with a high concentration of oxygen can be dangerous, since it facilitates the development of fires and explosions.

Applications

Doctors

Oxygen is administered to patients who have respiratory failure; such is the case of patients with pneumonia, pulmonary edema or emphysema. They could not breathe ambient oxygen as they would be seriously affected.

Patients with heart failure with fluid accumulation in the alveoli also need to be supplied with oxygen; as well as patients who have suffered a severe cerebrovascular accident (CVA).

Occupational need

Firefighters who are fighting a fire in an environment with inadequate ventilation, require the use of masks and oxygen cylinders that allow them to carry out their functions, without putting their lives at risk.

The submarines are equipped with oxygen production equipment that allows sailors to stay in a closed environment and without access to atmospheric air.

Divers do their work submerged in water and thus isolated from atmospheric air. They breathe through oxygen pumped through tubes connected to their diving suit or the use of cylinders attached to the diver's body.

Astronauts carry out their activities in environments equipped with oxygen generators that allow survival during space travel and in a space station.

Industrial

More than 50% of the industrially produced oxygen is consumed in the transformation of iron into steel. The molten iron is injected with a jet of oxygen in order to remove the sulfur and carbon present; they react to produce the gases SO ₂ and CO ₂, respectively.

Acetylene is used in combination with oxygen to cut metal plates and also to produce their solder. Oxygen is also used in the production of glass, increasing the combustion in the firing of the glass to improve its transparency.

Atomic absorption spectrophotometry

The combination of acetylene and oxygen is used to burn samples of different origins in an atomic absorption spectrophotometer.

During the procedure, a beam of light from a lamp is impinged on the flame, which is specific for the element to be quantified. The flame absorbs the light from the lamp, allowing the element to be quantified.

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