In what cases does the dissociation constant make no sense?

Chemicals are a collection of atoms that are related 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 a single-nuclear system, if it is of different types of atoms, then it is non-nuclear. These systems are electrically neutral. As a result of external influences (temperature, light, radiation or molecules of a polar solvent with dipole polarization), chemical substances decay. Cations and anions, which are destroyed by molecules of a polar solvent (water) molecules of matter (electrolyte), are no longer electrically neutral. Any system tends to equilibrium. Using the example of weak electrolytes, it can be seen that the dissociation reactions are reversible. For strong electrolytes, this statement is not appropriate, since all molecules practically break up into ions. The propensity of the system to equilibrium is described by the equation of electrolytic dissociation KxAy ↔ x • K + + y • A- and shows the dissociation constant Kd = [K +] x · [A-] y / [KxAy].

It is seen from the above equation: the more undissociated molecules, the smaller the dissociation constant and vice versa. However, this does not apply to strong electrolytes, since it is established that with increasing their concentration, Kd does not increase, but decreases. This is due not to a decrease in the number of decaying molecules, but to an increase in the mutual attraction between the oppositely charged particles due to the reduction in the distance between them due to the increase in the concentration of the solution. Therefore, the ability of strong electrolytes to decay into ions is estimated by such an indicator as the apparent degree of dissociation, and Kd is not used, as it is meaningless. To solutions of weak electrolytes it does not make sense to apply the degree of dissociation, because with decreasing concentration the ratio of 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 a 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 particular substance KxAy.

Ordinary water (from natural sources or the one that flows from the tap) is not clean. The purest water contains hydronium ions [H3O + 1] and hydroxide ions [OH-1]. They are formed from two water molecules: H2O + H2O ↔ H3O + 1 + OH-1. This happens rarely, since water practically does not break up into ions, being a weak electrolyte. In a state of 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 hydronium ions, where water molecules predominate and only traces of ions are present. The dissociation constant of water is expressed by the equation: Kd = [H3O + 1] • [OH-1] / [H2O] • [H2O].

Dissociation of acid in solution means decomposition into protons H + and acid residue. The dissociation of polybasic acids proceeds in several stages (where only one hydrogen cation is split off), each stage is characterized by its value of the constant Kd. In the first stage, the hydrogen ion is split off more easily than in the subsequent stages, so the constant decreases from stage to stage. The acid dissociation constant Kd is an indicator of the strength of the acid: strong acids have a higher Kd value and vice versa. When the equilibrium of the process is reached, the decay rate and the rate of formation of the molecules are equal. For strong acids, the laws of chemical equilibrium can be used (only with allowance for the interionic interaction forces in solutions of strong electrolytes) for calculating Kg at a temperature of 25 ° C. For hydrochloric acid (HCl), Kd = 10000000, hydrobromic (HBr) Kd = 1000000000, hydroiodic (HJ) Kd = 100000000000, sulfuric (H2SO4) Kd = 1000, nitric (HNO3) Kd = 43.6, acetic (CH3COOH) Kd = 0.00002, cyanoboric (HCN) Kd = 0.0000000008. Knowing the properties of acids and comparing with the given values of Kd, it can be argued that the dissociation constant is the higher the stronger the acid.