What is oxygen? Oxygen compounds

Oxygen (O) is a nonmetallic chemical element of group 16 (VIa) of the periodic table. It is a colorless, tasteless and odorless gas necessary for living organisms - animals that convert it to carbon dioxide, and plants that use CO ₂ as a carbon source and return O ₂ to the atmosphere. Oxygen forms compounds, reacting with almost any other element, and also displaces chemical elements from bonding with each other. In many cases, these processes are accompanied by the release of heat and light. The most important oxygen compound is water.

History of the discovery

In 1772 the Swedish chemist Karl Wilhelm Scheele first demonstrated what oxygen is, by obtaining it by heating potassium nitrate , mercury oxide , and many other substances. Regardless of him in 1774, the English chemist Joseph Priestley discovered this chemical element by thermal decomposition of mercury oxide and published his findings the same year, three years before the publication of Scheele. In 1775-1780 the French chemist Antoine Lavoisier interpreted the role of oxygen in breathing and burning, rejecting the theory of phlogiston, generally accepted at that time. He noted his propensity to form acids when combined with various substances and called the element oxygène, which in Greek means "giving birth to acid".

Prevalence

What is oxygen? Making up 46% of the mass of the earth's crust, it is the most common element of it. The amount of oxygen in the atmosphere is 21% by volume, and by weight in sea water it is 89%.

In rocks, the element combines with metals and nonmetals in the form of oxides, which are acidic (for example, sulfur, carbon, aluminum and phosphorus) or basic (calcium, magnesium and iron salts), and as saline compounds that can be thought of as formed from acidic And basic oxides, such as sulfates, carbonates, silicates, aluminates and phosphates. Although they are numerous, but these solids can not serve as sources of oxygen, since the breaking of the bond between the element and the metal atoms is too energy-consuming.

Features

If the temperature of oxygen is below -183 ° C, then it becomes a pale blue liquid, and at -218 ° C it is solid. Pure O ₂ is 1.1 times heavier than air.

During breathing, animals and some bacteria consume oxygen from the atmosphere and return carbon dioxide, while in the process of photosynthesis green plants absorb carbon dioxide in the presence of sunlight and release free oxygen. Almost all O ₂ in the atmosphere is produced by photosynthesis.

At 20 ° C, approximately 3 parts by volume of oxygen dissolve in 100 parts of fresh water, slightly less in seawater. This is necessary for the breathing of fish and other marine life.

Natural oxygen is a mixture of three stable isotopes: ¹⁶ O (99.759%), ¹⁷ O (0.037%) and ¹⁸ O (0.204%). Several artificially obtained radioactive isotopes are known. The longest-lived of them is ¹⁵ O (with a half-life of 124 s), which is used to study respiration in mammals.

Allotropes

A more clear idea of what oxygen is, you can get two of its allotropic forms, diatomic (O ₂ ) and triatomic (O ₃ , ozone). The properties of the diatomic form suggest that six electrons bind atoms and two remain unpaired, causing paramagnetism of oxygen. Three atoms in the molecule of ozone are not located on the same straight line.

Ozone can be obtained in accordance with the equation: 3O ₂ → 2O ₃ .

The process is endothermic (requires energy expenditure); The conversion of ozone back to diatomic oxygen is facilitated by the presence of transition metals or their oxides. Pure oxygen is converted to ozone under the effect of a glowing electrical discharge. The reaction also occurs when the ultraviolet is absorbed with a wavelength of about 250 nm. The emergence of this process in the upper layers of the atmosphere eliminates radiation that could damage life on the surface of the Earth. The smell of ozone is present in enclosed spaces with sparkling electrical equipment, such as generators. This gas is light blue. Its density is 1.658 times that of air, and it has a boiling point of -112 ° C at atmospheric pressure.

Ozone is a strong oxidizer capable of converting sulfur dioxide to trioxide, sulfide to sulfate, iodide to iodine (providing an analytical method for its evaluation), as well as many organic compounds in oxygen-containing derivatives such as aldehydes and acids. Ozone conversion of hydrocarbons from automobile exhaust to these acids and aldehydes is the cause of smog. In industry, ozone is used as a chemical reagent, disinfectant, for the treatment of sewage, water treatment and bleaching of tissues.

Methods of obtaining

The way in which oxygen is produced depends on how much gas is required. Laboratory methods are as follows:

1. Thermal decomposition of some salts, such as potassium chlorate or potassium nitrate:

• 2KClO ₃ → 2KCl + 3O ₂ .
• 2KNO ₃ → 2KNO ₂ + O ₂ .

The decomposition of potassium chlorate is catalyzed by transition metal oxides. For this, manganese dioxide (pyrolusite, MnO ₂ ) is often used. The catalyst reduces the temperature required to release oxygen from 400 to 250 ° C.

2. Decomposition of metal oxides under the effect of temperature:

• 2HgO → 2Hg + O2.
• 2Ag ₂ O → 4Ag + O ₂ .

Scheele and Priestley used the compound (oxide) of oxygen and mercury (II) to obtain this chemical element.

3. Thermal decomposition of metallic peroxides or hydrogen peroxide:

• 2BaO + O2 → 2BaO2.
• 2BaO ₂ → 2BaO + O ₂ .
• BaO ₂ + H ₂ SO ₄ → H ₂ O ₂ + BaSO ₄ .
• 2H ₂ O ₂ → 2H ₂ O + O _2.

The first industrial methods of separating oxygen from the atmosphere or for the production of hydrogen peroxide depended on the formation of barium peroxide from the oxide.

4. Electrolysis of water with small impurities of salts or acids, which provide the conductivity of electric current:

2H ₂ O → 2H ₂ + O ₂

Industrial production

If it is necessary to obtain large amounts of oxygen, fractional distillation of liquid air is used. Of the main components of air, it has the highest boiling point and, therefore, is less volatile than nitrogen and argon. In the process, the gas is cooled while expanding. The main steps of the operation are as follows:

• Air is filtered to remove solids;
• Moisture and carbon dioxide are removed by absorption in alkalis;
• Air is compressed, and the heat of compression is removed by conventional cooling procedures;
• Then he enters the coil in the chamber;
• Part of the compressed gas (at a pressure of about 200 atm) expands in the chamber, cooling the coil;
• The expanded gas returns to the compressor and passes several stages of subsequent expansion and compression, so that at -196 ° C air becomes liquid;
• The liquid is heated to distill the first light inert gases, then nitrogen, and liquid oxygen remains. Multiple fractionation produces a product that is fairly clean (99.5%) for most industrial purposes.

Use in industry

Metallurgy is the largest consumer of pure oxygen for the production of high-carbon steel: getting rid of carbon dioxide and other impurities of non-metals is faster and easier than using air.

Wastewater purification by oxygen is promising for more efficient treatment of liquid effluents than in other chemical processes. The burning of waste in closed systems using pure O _{2 is} becoming increasingly important.

The so-called rocket oxidizer is liquid oxygen. Pure O2 is used on submarines and in diving bells.

In the chemical industry, oxygen replaced conventional air in the production of substances such as acetylene, ethylene oxide and methanol. Medical applications include the use of gas in oxygen chambers, inhalers and children's incubators. An oxygen-enriched gaseous anesthetic provides support for life during general anesthesia. Without this chemical element, a number of industries using melting furnaces could not exist. That's what oxygen is.

Chemical properties and reactions

Large values of electronegativity and electron affinity of oxygen are typical for elements that exhibit nonmetallic properties. All oxygen compounds have a negative oxidation state. When two orbitals are filled with electrons, an O ² ion is formed. In peroxides (O ₂ ^2- ) it is assumed that each atom has a charge of -1. This property accept electrons by full or partial transmission and determines the oxidizing agent. When such an agent reacts with an electron donor substance, its own oxidation state decreases. The change (decrease) in the state of oxygen oxidation from zero to -2 is called recovery.

Under normal conditions, the element forms diatomic and triatomic compounds. In addition, there are extremely unstable tetrahalic molecules. In a diatomic form, two unpaired electrons are located on non-bonding orbitals. This is confirmed by the paramagnetic behavior of the gas.

The intense reactivity of ozone is sometimes explained by the assumption that one of the three atoms is in the "atomic" state. When reacted, this atom dissociates from O ₃ , leaving molecular oxygen.

The O2 molecule is weakly reactive at normal temperatures and ambient pressures. Atomic oxygen is much more active. The dissociation energy (O ₂ → 2O) is significant and amounts to 117.2 kcal per mole.

Connections

With non-metals such as hydrogen, carbon and sulfur, oxygen forms a large range of covalently bound compounds, among which nonmetallic oxides such as water (H ₂ O), sulfur dioxide (SO ₂ ) and carbon dioxide (CO ₂ ); Organic compounds such as alcohols, aldehydes and carboxylic acids; Common acids such as carbon (H ₂ CO ₃ ), sulfuric (H ₂ SO ₄ ) and nitric (HNO ₃ ); And the corresponding salts, such as sodium sulfate (Na ₂ SO ₄ ), sodium carbonate (Na ₂ CO ₃ ) and sodium nitrate (NaNO ₃ ). Oxygen is present as an O ² ion in the crystalline structure of solid metal oxides, such as the oxygen (oxide) compound and calcium CaO. Metallic superoxides (KO ₂ ) contain an O ₂ ^- ion, while metallic peroxides (BaO ₂ ) contain an O ₂ ^2- ion. Oxygen compounds generally have an oxidation state of -2.

Basic properties

Finally, we list the main properties of oxygen:

• Electron configuration: 1s ² 2s ² 2p ⁴ .
• Atomic number: 8.
• Atomic weight: 15,9994.
• Boiling point: -183.0 ° C.
• Melting point: -218.4 ° C.
• Density (if the oxygen pressure is 1 atm at 0 ° C): 1.429 g / l.
• Oxidation states: -1, -2, +2 (in compounds with fluorine).