What is the dissociation of water?

Particular case of dissociation (the process of decay of larger particles of matter - molecules of ions or radicals - into smaller particles) is electrolytic dissociation, in which neutral molecules of a substance called electrolyte dissolve into charged particles (as a result of the action of molecules of a polar solvent): cations And anions. This explains the ability of electrolyte solutions to conduct current.

It is accepted to divide all electrolytes into two groups: weak and strong. Water refers to weak electrolytes, the dissociation of water is characterized by a small amount of dissociated molecules, since they are fairly persistent and practically do not decompose into ions. Pure (without impurities) water conducts electricity weakly. This is due to the chemical nature of the molecule itself, when positively polarized hydrogen atoms are embedded in the electron shell of a relatively small oxygen atom, which is polarized negatively.

The strength and weakness of electrolytes is characterized by the degree of dissociation (denoted by α, often expressed in% of 0 to 100 or in fractions of a unit from 0 to 1) - the ability to break up into ions, that is, the ratio of the number of decayed particles to the number of particles before decay. Substances such as acids, salts and bases under the action of polar water molecules break up into ions completely. The dissociation of water is accompanied by the decomposition of H2O molecules into the proton H + and hydroxyl group OH-. If we represent the dissociation equation for the electrolyte in the form: M = K ++ A-, then the dissociation of water can be expressed by the equation: H2O + H ++ OH-, and the equation by which the degree of water dissociation is calculated can be represented in two forms Concentration of formed protons or concentration of formed hydroxyl groups): α = [H +] / [H2O] or α = [OH -] / [H2O]. Since the value of α is affected not only by the chemical nature of the substance, but also by the concentration of the solution or its temperature, it is customary to speak of the apparent (imaginary) degree of dissociation.

The tendency of molecules of weak electrolytes, including water, to decay into ions is characterized to a great extent by the dissociation constant (a particular case of the equilibrium constant), which is commonly referred to as Kd. To calculate this value, the law of acting masses is applied, which establishes the ratio between the masses of the obtained and the initial substances. The electrolytic dissociation of water is the decomposition of the original water molecules into hydrogen protons and the hydroxyl group, so the dissociation constant is expressed by the equation: Kd = [H +] • [OH -] / [H2O]. This value for water is constant and depends only on temperature, at a temperature equal to 25 ° C, Kd = 1.86 • 10-16.

Knowing the molar mass of water (18 grams / mole), and also neglecting the concentration of dissociated molecules and taking a mass of 1 dm3 of water per 1000 g, it is possible to calculate the concentration of undissociated molecules in 1 dm3 of water: [H2O] = 1000 / 18.0153 = 55.51 Mol / dm3. Then, from the equation of the dissociation constant, one can find the product of the concentrations of protons and hydroxyl groups: [H +] • [OH -] = 1.86 • 10-16 • 55.51 = 1 • 10-14. When the square root is extracted from the obtained value, the concentration of protons (hydrogen ions) is determined, which determines the acidity of the solution and is equal to the concentration of hydroxyl groups: [H +] = [OH -] = 1 • 10-7.

But in the nature of water, such purity does not exist because of the presence of dissolved gases in it or the contamination of water by other substances (in fact, water is a solution of various electrolytes); therefore, at 25oC, the concentration of hydrogen protons or the concentration of hydroxyl groups is different from 1-10-7. That is, the acidity of water is due to the flow not only of a process such as the dissociation of water. The hydrogen index is the negative logarithm of the concentration of hydrogen ions (pH), it is introduced to assess the acidity or alkalinity of water and aqueous solutions, since it is difficult to use numbers with negative degrees. For pure water, pH = 7, but since there is no pure water in nature, and the dissociation of water proceeds along with the dissolution of other dissolved electrolytes, the hydrogen index can be less or greater than 7, that is, for water, in practice, pH ≠ 7.