Have you ever wondered why some solutions conduct electricity and others do not? For example, everyone knows that it’s better not to take a bath while blowing a hair dryer. After all, water is a good conductor of electric current, and if a working hair dryer falls into water, then a short circuit cannot be avoided. In fact, water is not such a good conductor of current. There are solutions that conduct electricity much better. Such substances are called electrolytes. These include acids, alkalis and water-soluble salts.
Electrolytes - who are they?
The question arises: why do solutions of some substances pass electricity, but not others? It's all about charged particles - cations and anions. When dissolved in water, electrolytes decay into ions, which, under the action of an electric current, move in a given direction. Positive charged cations move toward the negative pole — the cathode, and negatively charged anions move toward the positive pole — the anode. The process of decomposition of a substance into ions during melting or dissolution in water is proudly called electrolytic dissociation.
This term was introduced by the Swedish scientist S. Arrhenius when he studied the properties of solutions to pass electricity. To do this, he closed the electric circuit through a solution of a substance and watched whether the light comes on while or not. If the incandescent light comes on, it means the solution conducts electricity, from which it follows that this substance is an electrolyte. If the bulb remains extinct, then the solution does not conduct electricity, therefore this substance is non-electrolyte. Non-electrolytes include solutions of sugar, alcohol, and glucose. But rasters of sodium chloride, sulfuric acid and sodium hydroxide conduct an excellent electric current, therefore, electrolytic dissociation proceeds in them.
How does dissociation proceed?
Subsequently, the theory of electrolytic dissociation was developed and supplemented by Russian scientists I.A. Kablukov and V.A. Kistyakovsky, applying the chemical theory of solutions to D.I. Mendeleev.
These scientists found that the electrolytic dissociation of acids, alkalis and salts occurs as a result of hydration of the electrolyte, that is, its interaction with water molecules. Ions, cations and anions formed as a result of this process will be hydrated, that is, bound to water molecules that surround them with a dense ring. Their properties are significantly different from unhydrated ions.
So, in a solution of strontium nitrate Sr (NO3) 2, as well as in solutions of cesium hydroxide CsOH, electrolytic dissociation occurs. Examples of this process can be expressed by the following reaction equations :
Sr (NO3) 2 = Sr2 + + 2NO3 -,
those. upon dissociation of one strontium nitrate molecule, one strontium cation and 2 nitrate anions are formed;
CsOH = Cs + + OH-,
those. upon dissociation of one cesium hydroxide molecule, one cesium cation and one hydroxide anion are formed.
Electrolytic acid dissociation occurs similarly. For iodic acid, this process can be expressed by the following equation:
HJ = H + + CJ-,
those. upon dissociation of one molecule of hydroiodic acid, one hydrogen cation and one iodine anion are formed.
The mechanism of dissociation.
The electrolytic dissociation of electrolyte substances proceeds in several stages. For substances with an ionic type of bond, such as NaCl, NaOH, this process includes three sequential processes:
Initially, water molecules having 2 opposite poles (positive and negative) and representing a dipole are oriented by the ions of the crystal. With a positive pole they attach to the negative ion of the crystal, and vice versa, with a negative pole - to the positive of the crystal ions;
then hydration of the crystal ions occurs with water dipoles,
and only after that the hydrated ions seem to diverge in different directions and begin to move randomly in the solution or melt until they are affected by an electric field.
For substances with a covalent polar bond, such as HCl and other acids, the dissociation process is similar, except that at the initial stage, the covalent bond becomes ionic due to the action of water dipoles. These are the main points of the theory of dissociation of substances.