term "dissociation" in Chemistry and Biochemistry refers to the process of disintegration of chemical compounds into ions and radicals.Dissociation - is the opposite phenomenon association or recombination, and is reversible.Quantification of dissociation is performed using a value as the degree of dissociation.It has lettering α and characterized in the dissociation reaction, occurring in homogeneous (homogeneous) systems according to the equation: CA ↔ R + A state of equilibrium.SC - a particle of the starting material, K and A - is the fine particles which broke as a result of dissociation of larger particles of the substance.From which it follows that the system will be dissociated and undissociated particles.Assuming that the n molecules disintegrated and not disintegrated N molecules, these values can be used to quantify the dissociation, which is calculated as a percentage: α = n • 100 / N or unit fractions: α = n / N.
That is, the degree of dissociation is the ratio of dissociated particles (molecules) of homogeneous (solution) the initial amount of particles (molecules) in the system (solution).If it is known that α = 5%, it means that only 5 out of 100 molecules of initial molecules are in the form of ions, and the remaining 95 molecules do not decompose.For each specific substance α is an individual, since it depends on the chemical nature of the molecule, as well as the temperature and the quantity of substance in a homogeneous system (in solution), i.e. on its concentration.Strong electrolytes, which include certain acids, bases and salts in the solution completely dissociate into ions, therefore not suitable for studying dissociation process.Therefore, to study weak electrolytes used whose molecules are dissociated into ions in the solution completely.
For reversible dissociation reaction of the dissociation constant (Kd), which characterizes the state of equilibrium is given by: Kd = [K] [A] / [SC].How constant and degree of dissociation are interconnected, it can be considered an example of a weak electrolyte.Based on the law of dilution built all logical reasoning: Kd = c • α2, where c - concentration of the solution (in this case = [CA]).It is known that in solution volume V 1 mol dm3 dissolved substance.In the initial state, the density of the molecules of the starting material can be expressed by: c = [SC] = 1 / V mol / dm3, and the ion concentration will be: [R] = [A] = 0 / V mol / dm3.Upon reaching equilibrium their values are changed: [SC] = (1 - α) / V mol / dm3 and [R] = [A] = α / V mol / dm3, whereas Kd = (α / V • α / V) /(1 - α) / V = α2 / (1 - α) • V.The case of little dissociating electrolytes, the dissociation degree (α) of which is close to zero, and the volume of the solution can be expressed in terms of known concentration: V = 1 / [Ka] = 1 / s.Then the equation can be transformed: Kd = α2 / (1 - α) • V = α2 / (1 - 0) • (1 / s) = α2 • s, and extracting the square root of the fraction cd / s, can be calculated dissociation degreeα.This law is valid if α is much smaller than 1.
For strong electrolytes is more suitable term apparent degree of dissociation.It is found, as the ratio of the apparent number of dissociated particles to real or isotonic formula determining factor (called factor of Van't Hoff and shows the true behavior of the substance in the solution): α = (i - 1) / (n - 1).Here i - Van't Hoff factor, and n - number of ions formed.For solutions, the molecules of which completely broke up into ions, α ≈ 1, and with a decrease in the concentration of α increasingly tends to 1. This is due to the theory of strong electrolytes, which states that the movement of cations and anions broken molecules strong electrolyte is difficult for several reasons.First, ions are surrounded by molecules of the polar solvent is an electrostatic interaction is called solvation.Second, oppositely charged cations and anions present in the solution, due to the forces of mutual attraction constitute associates or ion pairs.Associates behave as undissociated molecules.
