Dissociation of salts, acids and alkalis. Theory and practical application

For physical chemistry and biochemistry, a general process is typical in which particles of matter - molecules, ions (positively charged particles called cations and negatively charged particles, called anions), the radicals decay into simpler particles. This process is called dissociation, which in translation from Latin "dissociation" means "separation". It is characterized by such an indicator as the "degree of dissociation", which shows the ratio of the number of dissociated particles to the total number of particles before decay, that is, the fraction of particles that have disintegrated. The process of decay of particles can occur as a result of certain effects on the substance, the nature of these effects determines the type of dissociation. Distinguish between thermal dissociation, photodissociation, dissociation under the influence of ionizing radiation, electrolytic dissociation. Dissociation is the opposite of association and recombination. This process is often confused with ionization.

Electrolytic dissociation is a kind of dissociation, proceeds under the influence of polar solvent molecules and is of a chemical nature. Substances that are dissociable in the solvent into ions and conduct an electric current are called electrolytes (acids, salts, bases). Substances that, when dissolved, do not dissociate into ions (alcohols, ethers, carbohydrates, etc.) are not electrolytes. The most important solvent of electrolytes is water. Water itself is characterized as a weak electrolyte. Polar solvents (for example, ethanol, ammonia and acetic acid) are also capable of dissolving electrolytes. Dissociation of acids, alkalis, as well as the dissociation of salts, occur in aqueous solutions. Salts are a class of chemical compounds whose molecules consist of positively charged particles (metal cations) and negatively charged particles (anions of acid residues). Acidic salts, in contrast to conventional salts, consist of two types of cations (metal and hydrogen) and anion of the acid residue. When dissolved in water, the salt molecules decompose into ions. Salt can be restored by evaporating water.

Distinguish between strong and weak electrolytes. In the classical theory of electrolytic dissociation, this process is considered to be reversible, but this applies only to weak electrolytes in dilute solutions. The electrolytic dissociation of acids, bases, salts is an irreversible process, since salts (practically all but some complex ones), acids and bases (those formed by alkaline and alkaline-earth metals) are strong electrolytes, and in weak solutions of their molecules completely (by 100 %) Dissociate into ions. Strong electrolytes: NaCl (sodium chloride), HNO3 (nitric acid), HClO3 (chloric acid), CaCl2 (calcium chloride), NaOH (sodium hydroxide). Weak electrolytes: NH4OH (ammonium hydroxide), H2CO3 (carbonic acid), CH3COOH (acetic acid) and most organic acids and bases. They dissociate partially when dissolving in water (usually this value ranges from 1 to 10%).

Therefore, it is true that in the solution of strong electrolytes only ions are contained, and in the solution of weak electrolytes, mostly unbroken molecules of matter. Dissociation of salts leads to the fact that the solution contains only ions of metal and acid residue (for example, sodium cation Na + and chlorine anion Cl-), and there are no salt molecules (NaCl). The dissociation of acid salts leads to the formation of a metal cation, a hydrogen cation and an anion of an acid residue in the solution. For example, the acid salt NaHCO3 (sodium hydrogen carbonate) dissociates into sodium cation (Na +), hydrogen cation (H-) and anion of acidic carbonic acid residue (CO3-).

If the solution (melt) of the electrolyte is placed in an electrolyzer (a vessel with a cathode and an anode), then when the voltage is applied, the directed movement of charged particles to the electrodes with the opposite charge begins: positive cations to the negatively charged cathode, and negative anions to the positively charged anode. This property of electrolytes, in particular, the dissociation of salts, is widely used in engineering. The method of electrolysis is the industrial production of aluminum, copper (by electrolytic refining). Electrolysis makes it possible to obtain the purest substances, this degree of purity can not be achieved by other methods (rectification, crystallization, etc.). With the help of electrolysis, the metals extracted from the ores are purified, since only the metal cation is deposited on the cathode, and the impurities remain in the solution or melt. Such a phenomenon as the dissociation of salts underlies the production of pure hydrogen and pure chlorine. In water , sodium chloride decomposes into ions: sodium cation and chlorine anion. The purest chlorine will be released at the anode, a hydrogen byproduct on the cathode, and another important byproduct, sodium hydroxide, will form in the solution.