The instability constant of complex compounds

Probably, everyone who is familiar with school chemistry and is interested in it at least a little, knows about the existence of complex compounds. These are very interesting compounds with a wide range of applications. If you have not heard about this concept, then below we will explain everything to you. But let us begin with the history of the discovery of this rather unusual and interesting type of chemical compounds.

History

Complex salts were known even before the discovery of the theory and mechanisms that allow them to exist. They were named after the chemist who discovered this or that connection, and there were no systematic names for them. And, consequently, it was impossible to understand by the substance formula what properties it possesses.

This continued until 1893, while the Swiss chemist Alfred Werner did not offer his theory, for which 20 years later he received the Nobel Prize in Chemistry. It is interesting that he conducted his studies only by interpreting the various chemical reactions into which certain complex compounds entered. Studies were made before the discovery of the electron by Thompson in 1896, and after this event, decades later, the theory was augmented, in a much more modernized and complicated form, has survived to the present day and is actively used in science to describe phenomena occurring during chemical transformations With the participation of complexes.

So, before proceeding to a description of what a constant of instability is, let's look at the theory about which we spoke above.

The theory of complex compounds

Werner in his original version of the coordination theory formulated a number of postulates that formed the basis for it:

1. In any coordination (complex) compound, a central ion must be present. This is usually an atom of the d-element, more rarely some atoms of the p-elements, and only Li can act from the s-elements.
2. The central ion, together with the associated ligands (charged or neutral particles, for example water or anion of chlorine) forms the inner sphere of the wheel joint. She behaves in solution as one big ion.
3. The outer sphere consists of ions that are opposite in sign to the charge of the inner sphere. That is, for example, for a negatively charged sphere [CrCl ₆ ], the ^3- ion of the outer sphere can be metal ions: Fe ³⁺ , Ni ³⁺ , etc.

And now, if everything is clear with the theory, we can proceed to the chemical properties of complex compounds and their differences with conventional salts.

Chemical properties

In solution, complex compounds break up into ions, more precisely into internal and external spheres. We can say that they behave like strong electrolytes.

In addition, the inner sphere can also decay into ions, but in order for this to happen, it takes a lot of energy.

The outer sphere in complex compounds can be replaced by other ions. For example, if there was a chlorine ion in the outer sphere, and in the solution there is also an ion, which together with the inner sphere will give an insoluble compound, or in the solution there is a cation giving an insoluble compound with chlorine, there will be a replacement reaction of the outer sphere.

And now, before moving on to the definition of what constitutes instability, let's talk about a phenomenon that is directly related to this notion.

Electrolytic dissociation

You probably know this word from school. But still give a definition of this concept. Dissociation is the decomposition of molecules of a dissolved substance into ions in a solvent medium. This is due to the formation of sufficiently strong bonds of solvent molecules with the ions of the dissolved substance. For example, water has two oppositely charged ends, and some molecules are attracted by the negative end to the cations, and others are the positive end to the anions. So hydrates are formed - ions, surrounded by water molecules. Actually, this is the essence of electrolytic dissociation.

Now, in fact, let's return to the main topic of our article. What is the instability constant of complex compounds? Everything is simple enough, and in the next section we will analyze this concept in detail and in detail.

The instability constant of complex compounds

This indicator is in fact the direct opposite of the constant of stability of complexes. So with it we'll start.

If you have heard about the reaction equilibrium constant, then you will easily understand the material below. But if not, now we will briefly tell about this indicator. The equilibrium constant is defined as the ratio of the concentration of reaction products raised to the degree of their stoichiometric coefficients to the initial substances, in which the coefficients in the reaction equation are also taken into account. It shows in which direction the reaction will predominantly take place at a given concentration of the initial substances and products.

But how did we suddenly start talking about the equilibrium constant? In fact, the instability constant and the stability constant are, in fact, the equilibrium constants, respectively, of the reactions of destruction and the formation of the inner sphere of the complex. The relationship between them is very simple: _KH = 1 / K _ver .

To better understand the material, give an example. We take the complex anion [Ag (NO ₂ ) ₂ ] ^- and write down the equation of its decay reaction:

[Ag (NO ₂ ) ₂ ] ^- => Ag ⁺ + 2NO ₂ ^- .

The instability constant of the complex ion of this compound is 1.3 * 10 ^-3 . Hence, it is sufficiently stable, but still not to such an extent that it is considered very stable. The greater the stability of a complex ion in a solvent medium, the smaller the instability constant. Its formula can be expressed in terms of the concentrations of the initial and reacting substances: К _н = [Ag +] * [2NO ₂ ^- ] ² / [[Ag (NO ₂ ) ₂ ] ^- ].

Now that we have dealt with the basic concept, it is worthwhile to bring a little information about the various connections. The names of chemicals are written in the left column, and the constant of the instability of complex compounds is written in the right column.

Table

More detailed data on all known compounds are given in special tables in the manuals. In any case, the constant of instability of complex compounds, the table of which for several compounds is given above, is unlikely to seriously help you without using a directory.

Conclusion

After we found out how to calculate the instability constant, there remains only one question - about why this is all necessary.

The main purpose of this quantity is to determine the stability of a complex ion. So, we can predict the stability in the solution of a compound. This is very helpful in all areas, one way or another related to the use of complex substances. Enjoy your chemistry!