Oxygen (O) is a non-metallic chemical element of group 16 (VIa) of the periodic table. It is a colorless, tasteless and odorless gas required for living organisms - animals that turn it into 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 communication with each other. In many cases, these processes are accompanied by the release of heat and light. The most important oxygen compound is water.
Discovery story
In 1772, the Swedish chemist Karl Wilhelm Scheele first demonstrated what oxygen is, having obtained it by heating potassium nitrate , mercury oxide , as well as many other substances. Independently of it, in 1774, the English chemist Joseph Priestley discovered this chemical element by thermal decomposition of mercury oxide and published its findings in the same year, three years before Scheele's publication. In the years 1775-1780, the French chemist Antoine Lavoisier interpreted the role of oxygen in respiration and combustion, rejecting the phlogiston theory that was generally accepted at that time. He noted his tendency to form acids when combined with various substances and named the element oxygène, which in Greek means "giving birth to acid."
Prevalence
What is oxygen? Composing 46% of the mass of the earth's crust, it is its most common element. The amount of oxygen in the atmosphere is 21% by volume, and 89% by weight in seawater.
In rocks, the element is combined with metals and non-metals in the form of oxides, which are acidic (e.g., sulfur, carbon, aluminum and phosphorus) or basic (calcium, magnesium and iron salts), and as salt-like compounds that can be considered as formed from acidic and basic oxides such as sulfates, carbonates, silicates, aluminates and phosphates. Although they are numerous, but these solids cannot serve as sources of oxygen, since breaking the bond of an element with metal atoms is too energy-intensive.
Features
If the temperature of oxygen is below -183 ° C, then it becomes a pale blue liquid, and at -218 ° C - 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 as a result of photosynthesis.
At 20 ° C, about 3 volume parts 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 produced radioactive isotopes are known. The most long-lived of these is 15 O (with a half-life of 124 s), which is used to study respiration in mammals.
Allotropes
A clearer idea of what oxygen is allows one to obtain its two 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 ozone molecule are not located on one straight line.
Ozone can be obtained in accordance with the equation: 3O 2 → 2O 3 .
The process is endothermic (requires energy); the conversion of ozone back to diatomic oxygen is facilitated by the presence of transition metals or their oxides. Pure oxygen turns into ozone under the influence of a smoldering electric discharge. The reaction also occurs upon absorption of ultraviolet radiation with a wavelength of about 250 nm. The occurrence of this process in the upper atmosphere eliminates radiation, which could cause damage to life on the Earth's surface. The pungent smell of ozone is present in enclosed spaces with sparking electrical equipment such as generators. It is a light blue gas. 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 oxidizing agent capable of converting sulfur dioxide to trioxide, sulfide to sulfate, iodide to iodine (providing an analytical method for its assessment), as well as many organic compounds to oxygen-containing derivatives, such as aldehydes and acids. Ozone's conversion of hydrocarbons from automobile exhaust to these acids and aldehydes causes smog. In industry, ozone is used as a chemical reagent, disinfectant, for wastewater treatment, water treatment and bleaching of tissues.
Production Methods
The method for producing oxygen depends on how much gas is required. Laboratory methods are as follows:
1. Thermal decomposition of certain 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. Manganese dioxide (pyrolusite, MnO 2 ) is often used for this. The catalyst reduces the temperature necessary for oxygen evolution from 400 to 250 ° C.
2. The decomposition of metal oxides under the influence of temperature:
- 2HgO → 2Hg + O 2 .
- 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 metal peroxides or hydrogen peroxide:
- 2BaO + O 2 → 2BaO 2 .
- 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 for generating oxygen from the atmosphere or to produce hydrogen peroxide depended on the formation of barium peroxide from the oxide.
4. Electrolysis of water with small impurities of salts or acids that provide electrical current conductivity:
2H 2 O → 2H 2 + O 2
Industrial production
If necessary, to obtain large volumes 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 compared to nitrogen and argon. The process uses gas cooling as it expands. The main stages of the operation are as follows:
- air is filtered to remove particulate matter;
- moisture and carbon dioxide are removed by absorption in alkali;
- air is compressed and the heat of compression is removed by conventional cooling procedures;
- then it enters the coil located 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 is returned to the compressor and goes through several stages of subsequent expansion and compression, as a result of which the air becomes liquid at a temperature of -196 ° C;
- the liquid is heated to distill the first light inert gases, then nitrogen, and liquid oxygen remains. Multiple fractionation produces a product that is reasonably pure (99.5%) for most industrial purposes.
Industrial use
Metallurgy is the largest consumer of pure oxygen for the production of high-carbon steel: getting rid of carbon dioxide and other non-metal impurities is faster and easier than using air.
Wastewater treatment with oxygen is promising for a more efficient treatment of liquid effluents than in other chemical processes. Incineration in closed systems using pure O 2 is becoming increasingly important.
The so-called rocket oxidizer is liquid oxygen. Pure O 2 is used in submarines and in diving bells.
In the chemical industry, oxygen has 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 life support during general anesthesia. Without this chemical element, a number of industries using smelting furnaces could not exist. This is what oxygen is.
Chemical properties and reactions
Large values of electronegativity and electron affinity of oxygen are typical for elements that exhibit non-metallic 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 of accepting electrons by complete or partial transfer determines the oxidizing agent. When such an agent reacts with an electron donor substance, its own oxidation state decreases. Changing (decreasing) the state of oxygen oxidation from zero to -2 is called reduction.
Under normal conditions, the element forms diatomic and triatomic compounds. In addition, extremely unstable tetraatomic molecules exist. In a diatomic form, two unpaired electrons are located on non-binding 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 an “atomic” state. Entering into the reaction, this atom dissociates from O 3 , leaving molecular oxygen.
The O 2 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 wide range of covalently bonded compounds, including non-metal 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; general acids such as carbonic (H 2 CO 3 ), sulfuric (H 2 SO 4 ) and nitric (HNO 3 ); and corresponding salts, such as sodium sulfate (Na 2 SO 4 ), sodium carbonate (Na 2 CO 3 ) and sodium nitrate (NaNO 3 ). Oxygen is present in the form of an O 2– ion in the crystalline structure of solid metal oxides, such as a compound (oxide) of oxygen and calcium CaO. Metal superoxides (KO 2 ) contain an O 2 - ion, while metal 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 mass: 15,9994.
- Boiling point: -183.0 ° C.
- Melting point: -218.4 ° C.
- Density (if oxygen pressure is 1 atm at 0 ° C): 1.429 g / l.
- Oxidation states: -1, -2, +2 (in compounds with fluorine).