particular case of dissociation (decay process larger particles of matter - molecular ions or radicals - to a smaller particle size) is an electrolytic dissociation, whereby the neutral molecule known electrolyte in solution (by the impact of the molecules of the polar solvent) is split into chargedparticle, cations and anions.This explains the ability of electrolyte solutions conduct electricity.
Adopted share all electrolytes into two groups: weak and strong.Water is a weak electrolyte, water dissociation characterized by a small amount of dissociated molecules, since they are sufficiently resistant and practically not dissociate into ions.Net (free of impurities), water conducts electricity poorly.This is due to the chemical nature of the molecule itself, when positively polarized hydrogen atoms are introduced in a relatively small electron shell of an atom of oxygen, which is negatively polarized.
strength and weakness of the electrolyte is characterized by a degree of dissociation (indicated by α, often this value is expressed in% from 0 to 100, or a fraction of a unit from 0 to 1) - the ability to split into ions, ie the ratio of the number of broken particles to the number of particles beforedecay.Substances such as acids, bases and salts thereof under the action of the polar water molecules dissociate into ions completely.The dissociation of water is accompanied by the collapse of the H2O molecules into a proton H + and OH- hydroxyl group.If the equation of dissociation of the electrolyte present in the form M = K ++ A-, then water dissociation can be expressed by the equation: OH N2O↔N ++, and the equation by which the calculated degree of water dissociation can be represented in two ways (bythe resulting concentration of protons or hydroxyl groups the resulting concentration): α = [H +] / [H2O] or α = [OH -] / [H2O].Since the value α to affect not only the chemical nature of the compound, but the solution concentration or the temperature, it is customary to speak about the apparent (imaginary) of the degree of dissociation.
Addiction molecules weak electrolytes, including water, dissociate into ions to a greater extent characterized by a dissociation constant (a special case of the equilibrium constant), which are referred to as CD.To calculate this value, the law of mass action, which sets the ratio between the masses and the derived starting materials.Electrolytic dissociation of water - is the collapse of the original water molecules into protons of hydrogen and hydroxyl group, so the dissociation constant is expressed by the equation: Kd = [H +] • [OH -] / [H2O].This quantity of water is a constant and depends only on the temperature, at a temperature of 25C, Kd = 1.86 • 10-16.
Knowing the molar mass of water (18 grams / mole), and neglecting the concentration of dissociated molecules and taking the mass of 1 dm3 of water per 1000 g, can 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 of the product can be found concentrations of protons and hydroxyl groups: [H +] • [OH -] = 1,86 • 10-16 • 55,51 = 1 • 10-14.When extracting the square root of the resulting value obtained concentration of protons (hydrogen ions), which determines the acidity of the solution and the concentration of hydroxyl groups equal to: [H +] = [OH -] = 1 • 10-7.
But naturally water such purity does not exist because of the presence therein of dissolved gases or water contamination by other substances (in fact water - this solution various electrolytes), however at 25 the concentration of hydrogen protons or concentrations of hydroxyl groups different from the value1 • 10-7.That is, the acidity of the water due to the flow of not only a process such as the dissociation of water.The pH indicator is the negative logarithm of the hydrogen ion concentration (pH), it is introduced to evaluate the acidity or alkalinity of the water and aqueous solutions, as the number of negative powers use difficult.Pure water pH 7, but as naturally pure water is not present, and the dissociation of water takes place along with the collapse of other dissolved electrolytes, the pH value may be less than or greater than 7, i.e. water practically ≠ pH 7.
