In the world there are a lot of different chemical compounds: about hundreds of millions. And all of them, as people, are individual. It is impossible to find two substances in which the chemical and physical properties would coincide with different compositions.
One of the most interesting inorganic substances that exist in this world are carbides. In this article we will discuss their structure, physical and chemical properties, application and analyze the subtleties of their preparation. But first, a little about the history of the discovery.
History
Metal carbides, the formulas of which we give below, are not natural compounds. This is due to the fact that their molecules tend to decompose upon interaction with water. Therefore, it is worth talking about the first attempts to synthesize carbides.
Beginning in 1849, there is mention of the synthesis of silicon carbide, however, some of these attempts remain unrecognized. Large-scale production began in 1893 by the American chemist Edward Acheson by the method, which was then named after him.
The history of the synthesis of calcium carbide also does not differ in a large amount of information. In 1862, it was received by the German chemist Friedrich Wöhler, heating fused zinc and calcium with coal.
Now let's move on to more interesting sections: chemical and physical properties. After all, it is in them that the whole essence of the use of this class of substances lies.
Physical properties
Absolutely all carbides are distinguished by their hardness. For example, one of the hardest substances on the Mohs scale is tungsten carbide (9 out of 10 possible points). In addition, these substances are very refractory: the melting point of some of them reaches two thousand degrees.
Most carbides are chemically inert and interact with a small amount of substances. They are not soluble in any solvents. However, dissolution can be considered interaction with water with the destruction of bonds and the formation of metal hydroxide and hydrocarbon.
We will discuss the latest reaction and many other interesting chemical transformations involving carbides in the next section.
Chemical properties
Almost all carbides interact with water. Some - easily and without heating (for example, calcium carbide), and some (for example, silicon carbide) - when heating water vapor to 1800 degrees. The reactivity in this case depends on the nature of the bond in the compound, which we will discuss later. In the reaction with water, various hydrocarbons are formed. This is because the hydrogen in the water combines with the carbon in the carbide. It is possible to understand which hydrocarbon will turn out (or both the limiting and unsaturated compounds can be obtained) based on the valency of the carbon contained in the starting material. For example, if we have calcium carbide, whose formula is CaC 2 , we see that it contains a C 2 2- ion. Therefore, two hydrogen ions with a charge + can be attached to it. Thus, we obtain the compound C 2 H 2 - acetylene. In the same way, from a compound such as aluminum carbide, the formula of which is Al 4 C 3 , we obtain CH 4 . Why not C 3 H 12 , you ask? After all, an ion has a charge of 12-. The fact is that the maximum number of hydrogen atoms is determined by the formula 2n + 2, where n is the number of carbon atoms. So, only a compound with the formula C 3 H 8 (propane) can exist, and that ion with a charge of 12- decays into three ions with a charge of 4-, which give methane molecules when combined with protons.
Of interest are carbide oxidation reactions. They can occur both when exposed to strong mixtures of oxidizing agents, and during ordinary combustion in an oxygen atmosphere. If everything is clear with oxygen: two oxides are obtained, then with other oxidizing agents it is more interesting. It all depends on the nature of the metal that is part of the carbide, as well as on the nature of the oxidizing agent. For example, silicon carbide, the formula of which is SiC, when reacted with a mixture of nitric and hydrofluoric acids forms hexafluorosilicic acid with the release of carbon dioxide. And when carrying out the same reaction, but with nitric acid alone, we obtain silicon oxide and carbon dioxide. Oxidizing agents also include halogens and chalcogenes. Any carbide interacts with them, the reaction formula depends only on its structure.
Metal carbides, the formulas of which we have considered, are far from the only representatives of this class of compounds. Now we will take a closer look at each commercially important compound of this class and then talk about their application in our lives.
What are carbides?
It turns out that carbide, whose formula, say, CaC 2, is significantly different in structure from SiC. And the difference is primarily in the nature of the bond between atoms. In the first case, we are dealing with a salt-like carbide. This class of compounds is named so because it behaves practically like a salt, that is, it is capable of dissociating into ions. Such an ionic bond is very weak, which makes it easy to carry out the hydrolysis reaction and many other transformations, including interactions between ions.
Another, probably more industrially important type of carbides are covalent carbides: such as, for example, SiC or WC. They are distinguished by high density and durability. As well as refractory and inert to diluted chemicals.
There are also metal-like carbides. Rather, they can be considered as alloys of metals with carbon. Among these, for example, cementite (iron carbide, the formula of which is different, but on average it is approximately the same: Fe 3 C) or cast iron, can be distinguished. They have a chemical activity intermediate in degree between ionic and covalent carbides.
Each of these subspecies of the class of chemical compounds that we are discussing has its own practical application. We will talk about how and where each of them is used in the next section.
The practical use of carbides
As we have already discussed, covalent carbides have the largest range of practical applications. These are abrasive and cutting materials, and composite materials used in various fields (for example, as one of the materials that make up the body armor), and auto parts, and electronic devices, and heating elements, and nuclear energy. And this is not a complete list of applications for these superhard carbides.
The narrowest applications are salt-forming carbides. Their reaction with water is used as a laboratory method for producing hydrocarbons. How this happens, we have already discussed above.
Along with covalent, metal-like carbides have the widest application in industry. As we have already said, such a metal-like type of the compounds we are discussing is steel, cast iron and other metal compounds interspersed with carbon. As a rule, the metal in such substances belongs to the class of d-metals. That is why he is inclined to form not covalent bonds, but as if introduced into the metal structure.
In our opinion, the practical applications of the above compounds are more than enough. Now take a look at the process of obtaining them.
Carbide production
The first two types of carbides that we examined, namely, covalent and salt-like, are most often obtained in one simple way: by reacting element oxide and coke at high temperature. In this case, part of the coke, consisting of carbon, combines with the atom of the element in the oxide composition, and forms carbide. The other part takes oxygen and forms carbon monoxide. This method is very energy-consuming, since it requires maintaining a high temperature (about 1600-2500 degrees) in the reaction zone.
Alternative reactions are used to make certain types of compounds. For example, decomposition of a compound that ultimately yields carbide. The reaction formula depends on the specific compound, so we will not discuss it.
Before completing our article, we will discuss some interesting carbides and talk about them in more detail.
Interesting connections
Sodium Carbide The formula of this compound is C 2 Na 2 . This can be thought of more as acetylenide (that is, the product of the replacement of hydrogen atoms in acetylene by sodium atoms) rather than carbide. The chemical formula does not fully reflect these subtleties, so they must be sought in the structure. This is a very active substance and, in any contact with water, very actively interacts with it with the formation of acetylene and alkali.
Magnesium Carbide Formula: MgC 2 . Interesting methods for producing this sufficiently active compound. One of them involves sintering of magnesium fluoride with calcium carbide at high temperature. As a result of this, two products are obtained: calcium fluoride and the carbide we need. The formula for this reaction is quite simple, and you can familiarize yourself with it in the specialized literature if you wish.
If you are not sure about the usefulness of the material presented in the article, then the next section is for you.
How can it be useful in life?
Well, firstly, knowledge of chemical compounds can never be superfluous. It is always better to be armed with knowledge than to remain without it. Secondly, the more you know about the existence of certain compounds, the better you understand the mechanism of their formation and the laws that allow them to exist.
Before proceeding to the end, I would like to give some recommendations on the study of this material.
How to study it?
Very simple. This is just a section of chemistry. And it should be studied in chemistry textbooks. Start with school information and move on to more in-depth, from university textbooks and reference books.
Conclusion
This topic is not as simple and boring as it seems at first glance. Chemistry can always be interesting if you find your goal in it.