Redox processes are of great importance for animate and inanimate nature. For example, the combustion process can be attributed to OVR with the participation of atmospheric oxygen. In this redox reaction, it exhibits its non-metallic properties.
Also examples of OVR are digestive, respiratory processes, photosynthesis.
Classification
Depending on whether there is a change in the oxidation state of the elements of the starting material and the reaction product, it is customary to subdivide all chemical transformations into two groups:
- redox;
- without changing the degree of oxidation.
As examples of the second group are ionic processes that occur between solutions of substances.
Redox reactions are processes that are associated with a change in the oxidation state of the atoms that make up the starting compounds.
What is the degree of oxidation?
This is a conditional charge acquired by an atom in a molecule when an electron pair of chemical bonds shifts to a more electronegative atom.
For example, in a molecule of sodium fluoride (NaF) fluorine exhibits the maximum electronegativity, therefore its oxidation state is a negative value. Sodium in this molecule will be a positive ion. The sum of the oxidation states in the molecule is zero.
Definition Options
What ion is oxygen? Positive oxidation states are uncharacteristic for him, but this does not mean that this element does not show them in certain chemical interactions.
The very concept of the degree of oxidation has a formal character, it is not associated with the effective (actual) charge of the atom. It is convenient to use when classifying chemicals, as well as when recording ongoing processes.
Definition Rules
For non-metals, the lowest and highest oxidation states are distinguished. If eight is subtracted from the group number to determine the first indicator, the second value basically coincides with the group number in which the given chemical element is located. For example, the oxidation state of oxygen in compounds is usually -2. Such compounds are called oxides. For example, such substances include carbon dioxide (carbon dioxide), the formula of which is CO2.
Non-metals often show the maximum degree of oxidation in acids and salts. For example, in perchloric acid HClO 4, halogen has a valency of VII (+7).
Peroxides
The oxidation state of the oxygen atom in the compounds is usually -2, with the exception of peroxides. They consider oxygen compounds, which contain an incompletely reduced ion in the form of O 2 2- , O 4 2- , O 2 -.
Peroxide compounds are divided into two groups: simple and complex. Simple are those compounds in which the peroxide group combines with an atom or metal ion by an atomic or ionic chemical bond. Such substances are formed by alkali and alkaline earth metals (except lithium and beryllium). With an increase in the electronegativity of the metal within the subgroup, a transition from the ionic form of the bond to the covalent structure is observed.
In addition to peroxides of the species Me 2 O 2 , representatives of the first group (main subgroup) also have peroxides in the form of Me 2 O 3 and Me 2 O 4 .
If oxygen exhibits a positive oxidation state with fluorine, this indicator -1 in combination with metals (in peroxides).
Compounds where this group acts as ligands are considered complex peroxo compounds. Similar substances are formed by elements of the third group (main subgroup), as well as by subsequent groups.
Classification of complex peroxogroups
Five groups of such complex compounds are distinguished. The first is composed of peroxo acids, having the general form [Ep (O 2 2ā ) x L y ] zā . In this case, peroxide ions enter a complex ion or act as a monodentant (E ā O ā Oā), bridging (E ā O ā O ā E) ligand, forming a multicore complex.
If oxygen exhibits a positive oxidation state with fluorine, in combination with alkali and alkaline earth metals it is a typical non-metal (-1).
An example of such a substance is Caro acid (peroxomonomer acid) of the form H 2 SO 5 . The ligand peroxide group in such complexes acts as a bridging bond between non-metal atoms, for example, in peroxodisulfuric acid of the form H 2 S 2 O 8 , a white crystalline substance with a low melting point.
The second group of complexes is created by substances in which the peroxo group is part of a complex ion or molecule.
They are represented by the formula [ n n (O 2 ) x L y ] z .
The remaining three groups are peroxides in which there is crystallization water, for example, Na 2 O 2 Ć 8H 2 O, or crystallization hydrogen peroxide.
As typical properties of all peroxides, we distinguish their interaction with acid solutions and the release of active oxygen during thermal decomposition.
Chlorates, nitrates, permanganates, perchlorates can act as a source of oxygen.
Oxygen difluoride
When does oxygen show a positive oxidation state? In conjunction with the more electronegative fluorine ( oxygen fluoride ) OF 2. It is +2. This compound was first obtained by Paul Lebo at the beginning of the twentieth century, and was studied a little later by Ruff.
Oxygen exhibits a positive oxidation state in conjunction with fluorine. Its electronegativity is 4, so the electron density in the molecule is shifted toward the fluorine atom.
Oxygen Fluoride Properties
This compound is in a liquid state of aggregation, unlimitedly mixes with liquid oxygen, fluorine, ozone. Solubility in cold water is minimal.
How is the positive oxidation state explained? A large oil encyclopedia explains that it is possible to determine the highest + (positive) oxidation state by the group number in the periodic table. This value is determined by the largest number of electrons that a neutral atom can give off during complete oxidation.
Oxygen fluoride is produced by an alkaline method, which involves passing fluorine gas through an aqueous alkali solution.
In this chemical process, in addition to oxygen fluoride, ozone and hydrogen peroxide are also formed.
An alternative way to obtain oxygen fluoride is the electrolysis of a solution of hydrofluoric acid. In part, this compound also forms during combustion in a fluorine atmosphere of water.
The process proceeds according to a radical mechanism. Initially, free radicals are initiated, accompanied by the formation of an oxygen biradical. The next stage is the dominant process.
Oxygen difluoride exhibits bright oxidizing properties. By its strength it can be compared with free fluorine, and by the mechanism of the oxidation process - with ozone. The reaction needs a high activation energy, since atomic oxygen is formed in the first stage.
The thermal decomposition of this oxide, in which oxygen is characterized by a positive oxidation state, is a monomolecular reaction starting at temperatures from 200 Ā° C.
Distinctive features
When oxygen fluoride enters hot water, hydrolysis proceeds, the products of which will be ordinary molecular oxygen, as well as hydrogen fluoride.
The process is significantly accelerated in an alkaline environment. A mixture of water and oxygen difluoride vapor is explosive.
This compound intensively reacts with metallic mercury, and on noble metals (gold, platinum) forms only a thin fluoride film. This property explains the possibility of using these metals at ordinary temperature for contact with oxygen fluoride.
In the case of an increase in temperature, metal oxidation occurs. The most suitable metals for working with this fluorine compound are magnesium and aluminum.
Stainless steel and copper alloys slightly change their initial appearance under the influence of oxygen fluoride.
The high activation energy of the decomposition of this oxygen compound with fluorine allows it to be mixed safely with various hydrocarbons, carbon monoxide, explains the possibility of using oxygen fluoride as an excellent oxidizer of rocket fuel.
Conclusion
Chemists conducted a series of experiments that confirmed the feasibility of using this compound in gas-dynamic laser systems.
Questions related to the determination of the oxidation states of oxygen and other non-metals are included in the school chemistry course.
Such skills are important because they allow high school students to cope with the tasks offered in tests of a single state exam.