Chemicals are a set of atoms that are connected to each other according to a certain law, more precisely, each of them is a system consisting of nuclei and electrons. If the system consists of one type of atom, then it can be called mononuclear, if it is composed of atoms of various types, then it is non-nuclear. These systems are electrically neutral. As a result of external influence (temperature, light, radiation, or molecules of a polar solvent with dipole polarization), the decomposition of chemicals occurs. Cations and anions into which molecules of a substance (electrolyte) decompose under the action of molecules of a polar solvent (water) are no longer electrically neutral. Any systems strive for equilibrium. Using weak electrolytes as an example, it can be seen that the dissociation reactions are reversible. For strong electrolytes, this statement is not suitable, since all molecules practically decay into ions. The tendency of the system to equilibrium is described by the electrolytic dissociation equation KxAy ↔ x • K + + y • A− and shows the dissociation constant Kd = [K +] x • [A−] y / [KxAy].
It can be seen from the above equation: the more undissociated molecules, the lower the dissociation constant and vice versa. However, this does not apply to strong electrolytes, since it was established that with an increase in their concentration, the CD does not increase, but decreases. This is explained not by a decrease in the number of decaying molecules, but by an increase in the forces of mutual attraction between oppositely charged particles due to a decrease in the distance between them due to an increase in the solution concentration. Therefore, the ability of strong electrolytes to decay into ions is estimated by such an indicator as the apparent degree of dissociation, and CD is not used, since it is meaningless. It does not make sense to apply the degree of dissociation to solutions of weak electrolytes, because with a decrease in the concentration, the ratio of the dissociated molecules to the total number increases before decay, but does not characterize the strength of the electrolyte. Their ability to dissociate into ions shows the dissociation constant, since it depends only on the temperature of the solution and the nature of the solvent, that is, Kd is a constant value for a specific substance KxAy.
Ordinary water (from natural sources or that which flows from a tap) is not clean. Pure water contains hydroxonium ions [H3O + 1] and hydroxide ions [OH-1]. They are formed from two water molecules: H2O + H2O ↔ H3O + 1 + OH-1. This rarely happens, since water practically does not decay into ions, being a weak electrolyte. In equilibrium, the concentrations of hydroxide ions and hydroxonium ions are: [H3O + 1] = [OH-1]. The process is reversible. Water usually exists as a mixture of molecules, hydroxide ions and hydroxonium ions, where water molecules predominate and only traces of ions are present. The dissociation constant of water is expressed using the equation: Cd = [H3O + 1] • [OH-1] / [H2O] • [H2O].
Dissociation of acid in solution means decay into H + protons and an acid residue. Dissociation of polybasic acids proceeds in several stages (where only one hydrogen cation is cleaved), each stage is characterized by its constant value Cd. In the first stage, the hydrogen ion is cleaved off more easily than in the subsequent stages; therefore, the constant decreases from stage to stage. The acid dissociation constant Kd is an indicator of acid strength : strong acids have a higher Kd value and vice versa. Upon reaching equilibrium of the process, the rate of decay and the rate of formation of molecules are equal. For strong acids, it is possible to apply (only taking into account the forces of interionic interaction in solutions of strong electrolytes) the laws of chemical equilibrium to calculate the Kd at a temperature of 25 ° C. For hydrochloric acid (HCl) Kd = 10000000, hydrobromic (HBr) Kd = 1000000000, iodic acid (HJ) Kd = 100000000000, sulfuric (H2SO4) Kd = 1000, nitric (HNO3) Kd = 43.6, acetic (CH3COOH) Kd = 0.00002, hydrogen cyanide (HCN) Cd = 0.0000000008. Knowing the properties of acids and comparing with the given values of Kd, it can be argued that the dissociation constant is higher, the stronger the acid.