We all breathe air, which is mainly composed of nitrogen and oxygen molecules with a minor addition of other elements. Thus, oxygen is one of the most important chemical elements. In addition, its molecules exist in a huge variety of chemical compounds used in everyday life. A hundred pages are not enough to describe all the properties of this element, so we restrict ourselves to the basic facts from history, as well as the basic characteristics of the element - the valency and oxidation state of oxygen, specific gravity, application, basic physical properties.
History of the discovery of a chemical element
The official opening date for the chemical element “oxygen” is August 1, 1774. It was on this day that the British chemist J. Priestley completed his experiment on the decomposition of mercury oxide in a hermetically sealed vessel. At the end of the experiment, the scientist received a gas that supported combustion. However, this discovery went unnoticed even by the scientists themselves. Mr. Priestley thought that he managed to isolate not a new element, but a component of air. Joseph Priestley shared his results with the famous French scientist and chemist Antoine Lavoisier, who was able to understand what the Englishman could not do. In 1775, Lavoisier was able to establish that the resulting "component of the air" is actually an independent chemical element, and called it oxygen, which in Greek means "forming acid". Lavoisier then believed that oxygen is found in all acids. Subsequently, the formulas of acids that did not contain oxygen atoms were derived, but the name stuck.
Oxygen - structural features of the molecule
This chemical element is a colorless gas, odorless and tasteless. The chemical formula is O 2 . Chemists call ordinary diatomic oxygen either “atmospheric oxygen” or “oxygen”.
The molecule of a substance consists of two bound oxygen atoms. There is also a molecule consisting of three atoms - O
3 . This substance is called ozone, in more detail it will be written below. A molecule with two atoms has an oxygen oxidation state of -2, since it has two unpaired ones capable of forming a covalent bond of an electron. The energy released during the decomposition (dissociation) of an oxygen molecule into atoms is 493.57 kJ / mol. This is quite significant.
The valency and oxidation state of oxygen
By valency of a chemical element is meant its ability to attach to itself a certain number of atoms of another chemical element. The valency of the oxygen atom is two. The valency of the oxygen molecule is also equal to two, since two atoms are connected to each other and have the ability to attach another atom of another compound to their structure, that is, form a covalent bond with it. For example, a water molecule of H 2 O was obtained as a result of the formation of a covalent bond between one oxygen atom and two hydrogen atoms.
Oxygen is found in many of the known chemical compounds. There is even a separate type of chemical compounds - oxides. These are substances obtained by combining virtually any chemical element with oxygen. The oxidation state of oxygen in oxides is -2. However, in some compounds this indicator may be different. This will be described in more detail below.
Physical properties of oxygen
Conventional diatomic oxygen is a gas that does not have color, odor, or taste. In the normal state, its density is 1.42897 kg / m 3 . The weight of 1 liter of substance is slightly less than 1.5 grams, that is, in its pure form, oxygen is heavier than air. When heated, the molecule dissociates into atoms.
As the temperature of the medium decreases to -189.2 ° C, oxygen changes its structure from gaseous to liquid. In this case, boiling occurs. With a decrease in temperature to -218.35 ° C, a change in the structure from liquid to crystalline is observed. At this temperature, oxygen has the form of bluish crystals.
At room temperature, oxygen is slightly soluble in water - 31 milliliters of oxygen per one liter of it. Solubility with other substances: 220 ml per 1 liter of ethanol, 231 ml per 1 liter of acetone.
Chemical properties of oxygen
A whole Talmud can be written about the chemical properties of oxygen. The most important property of oxygen is oxidation. This substance is a very strong oxidizing agent. Oxygen is able to interact with almost all known elements from the periodic table. As a result of this interaction, oxides are formed, as mentioned earlier. The oxidation states of oxygen in compounds with other elements are generally -2. An example of such compounds is water (H 2 O), carbon dioxide (CO 2 ), calcium oxide, lithium oxide, etc. But there is a certain category of oxides called peroxides or peroxides. Their feature is that in these compounds there is a “-O-O-” peroxide group. This group reduces the oxidizing properties of O 2 , so the oxidation state of oxygen in the peroxide is -1.
In combination with active alkali metals, oxygen forms superoxides or superperoxides. An example of such formations is:
- potassium superoxide (KO 2 );
- rubidium superoxide (RbO 2 ).
Their feature is that the oxidation state of oxygen in superoxides is -1/2.
In combination with the most active chemical element - fluorine, fluorides are obtained. They will be described below.
Highest degree of oxygen oxidation in compounds
Depending on what substance oxygen interacts with, there are seven degrees of oxygen oxidation:
- -2 - in oxides and organic compounds.
- -1 - in peroxides.
- -1/2 - in superoxides.
- -1/3 - in inorganic ozonides (true for triatomic oxygen - ozone).
- +1/2 - in the salts of the oxygen cation.
- +1 - in oxygen monofluoride.
- +2 - in oxygen difluoride.
As you can see, the highest degree of oxygen oxidation is achieved in oxides and organic compounds, and in fluorides it even has a positive degree. Not all types of interactions can be realized naturally. For the formation of certain compounds, special conditions are required, for example: high pressure, high temperature, exposure to rare compounds that are almost never found in nature. Consider the main compounds of oxygen with other chemical elements: oxides, peroxides and fluorides.
Classification of oxides by acid-base properties
There are four types of oxides:
- basic;
- acidic;
- neutral
- amphoteric.
The oxidation states of oxygen in the compounds of these species are -2.
- Basic oxides are compounds with metals with low oxidation states. Generally, when reacted with acids, the corresponding salt and water are obtained.
- Acid oxides are non-metal oxides with a high degree of oxidation. When water is added to them, acid is formed.
- Neutral oxides are compounds that do not react with either acids or bases.
- Amphoteric oxides are compounds with metals having a low value of electronegativity. They, depending on the circumstances, exhibit the properties of both acidic and basic oxides.
Peroxides, the degree of oxidation of oxygen in hydrogen peroxide and other compounds
Peroxides are oxygen compounds with alkali metals. They are obtained by burning these metals in oxygen. Peroxides of organic compounds are extremely explosive. They can also be obtained by absorption by oxygen oxides. Examples of peroxides:
- hydrogen peroxide (H 2 O 2 );
- barium peroxide (BaO 2 );
- sodium peroxide (Na 2 O 2 ).
All of them are united by the fact that they contain an oxygen group -OO-. As a result, the degree of oxidation of oxygen in peroxides is -1.
One of the best known compounds with the group —OO— is hydrogen peroxide. Under normal conditions, this compound is a pale blue liquid. By its chemical properties, hydrogen peroxide is closer to weak acid. Since the -OO- bond in the compound is weakly stable, even at room temperature the hydrogen peroxide solution can be decomposed into water and oxygen. It is the strongest oxidizing agent, however, when interacting with stronger oxidizing agents, hydrogen peroxide possesses the properties of the reducing agent. The degree of oxidation of oxygen in hydrogen peroxide, as in other peroxides, is -1.
Other types of peroxides are:
- superoxides (superoxides in which oxygen has an oxidation of -1/2);
- inorganic ozonides (extremely unstable compounds having ozone anion in their structure);
- organic ozonides (compounds with a -OOO- bond in their structure).
Fluorides, the oxidation state of oxygen in OF2
Fluorine is the most active element of all that is now known. Therefore, the interaction of oxygen with fluorine produces not oxides, but fluorides. They are named so because in this compound is not oxygen, but fluorine is an oxidizing agent. Fluorides cannot be obtained naturally. They are only synthesized by production by association of fluorine with an aqueous solution of KOH. Oxygen fluorides are divided into:
- oxygen difluoride (OF 2 );
- oxygen monofluoride (O 2 F 2 ).
Let us consider in more detail each of the compounds. Oxygen difluoride in its structure is a colorless gas with a pronounced unpleasant odor. When cooled, it condenses into a yellowish liquid. In a liquid state, it mixes poorly with water, but it is good with air, fluorine and ozone. In chemical properties, oxygen difluoride is a very strong oxidizing agent. The oxidation state of oxygen in OF2 is +1, that is, in this compound fluorine is an oxidizing agent, and oxygen is a reducing agent. OF 2 is very toxic, in terms of toxicity exceeds pure fluorine and approaches phosgene. The main use of this compound is as an oxidizing agent for rocket fuel, since oxygen difluoride is not explosive.
Oxygen monofluoride in the normal state is a yellowish solid. When melted, it forms a red liquid. It is a powerful oxidizing agent; when interacting with organic compounds it is extremely explosive. In this compound, oxygen shows oxidation states equal to +2, that is, in this fluorine compound, oxygen acts as a reducing agent, and fluorine as an oxidizing agent.
Ozone and its compounds
Ozone is a molecule that has three oxygen atoms bonded to each other. In normal condition, it is a blue gas. When cooled, it forms a deep blue liquid close to indigo. In the solid state, it is a dark blue crystal. Ozone has a pungent odor; in nature it can be felt in the air after a severe thunderstorm.
Ozone, like ordinary oxygen, is a very strong oxidizing agent. In chemical properties, it approaches strong acids. When exposed to oxides, ozone increases their oxidation state with the release of oxygen. But this reduces the degree of oxidation of oxygen. In ozone, the chemical bonds are not as strong as in O 2 ; therefore, under normal conditions, without the applied efforts, it can decompose into oxygen with the release of heat energy. With an increase in the temperature of exposure to the ozone molecule and with a decrease in pressure, the process of decomposition into diatomic oxygen with the release of heat accelerates. Moreover, if there is a large ozone content in space, then this process may be accompanied by an explosion.
Since ozone is a very strong oxidizing agent and a large amount of O 2 is released in almost all processes with its participation, ozone is an extremely toxic substance. However, in the upper atmosphere, the ozone layer acts as a reflector from the ultraviolet radiation of sunlight.
Organic and inorganic ozonides are created from ozone using laboratory tools. These are very unstable in their structure substances, therefore, their creation in natural conditions is impossible. Ozonides are stored only at low temperatures, since at ordinary temperatures they are extremely explosive and toxic.
The use of oxygen and its compounds in industry
Due to the fact that at one time, scientists found out the degree of oxidation of oxygen when interacting with other elements, it and its compounds were widely used in industry. Especially after turbo expanders were invented in the mid-twentieth century - units that can convert the potential energy of oxygen into mechanical energy.
Since oxygen is an extremely combustible substance, it is used in all industries where the use of fire and heat is necessary. When cutting and welding metals, compressed oxygen cylinders are also used to strengthen the gas-flame welding apparatus. Oxygen is widely used in the steel industry, where high temperature in blast furnaces is maintained using compressed O
2 . The maximum oxidation state of oxygen is -2. This characteristic of him is actively used for the manufacture of oxides with the aim of their further combustion and the release of thermal energy. Liquid oxygen, ozone and other compounds containing a large amount of O
2 are used as oxidizing agents for rocket fuel. Some organic compounds oxidized with oxygen are used as explosives.
In the chemical industry, oxygen is used as an oxidizing agent for hydrocarbons in acid-containing compounds, such as alcohols, acids, etc. In medicine, it is used under reduced pressure to treat patients with lung problems and to maintain the body's vital functions. In agriculture, small doses of pure oxygen are used to breed fish in ponds, to increase the proportion of cattle, etc.
Oxygen is a powerful oxidizing agent, without which existence is impossible
Much has been written above about what oxygen displays oxidation states when it reacts with various compounds and elements, what types of oxygen compounds exist, which types are life-threatening, and which are not. One thing may remain incomprehensible - how, for all its toxicity and high level of oxidation, is oxygen one of the elements without which life on Earth is impossible? The fact is that our planet is a very balanced organism that has adapted precisely to those substances that are contained in the atmospheric layer. She participates in the cycle, which is as follows: humans and all other animals consume oxygen and produce carbon dioxide, and plants in the vast majority consume carbon dioxide and produce oxygen. Everything in the world is interconnected, and the loss of one link in this chain can lead to the breaking of the entire chain. You should not forget about it and protect life on the planet as a whole, and not just its individual representatives.