The degree of dissociation of weak and strong electrolytes

The term "dissociation" in chemistry and biochemistry refers to the process of disintegration of chemical compounds into ions and radicals. Dissociation is the opposite phenomenon of association or recombination, and it is reversible. The dissociation is quantified by a quantity such as the degree of dissociation. It has the letter designation α and characterizes in the dissociation reaction proceeding in homogeneous (homogeneous) systems according to the equation: KA ↔ K + A, state of equilibrium. KA is a particle of the initial substance, K and A are small particles, into which larger particles of matter have disintegrated as a result of dissociation. From which it follows that in the system there will be dissociated and undissociated particles. If we assume that n molecules decayed and not N molecules disintegrated, then these quantities can be used to quantify the dissociation, which is calculated as a percentage: α = n • 100 / N or in fractions of unity: α = n / N.

That is, the degree of dissociation is the ratio of dissociated particles (molecules) of a homogeneous system (solution) to the initial number of particles (molecules) in this system (solution). If it is known that α = 5%, then it means that only 5 molecules out of 100 starting molecules are in the form of ions, and the remaining 95 molecules do not decay. For each particular substance, α will be individual, since it depends on the chemical nature of the molecule, as well as on the temperature and the amount of matter in the homogeneous system (in solution), that is, on its concentration. Strong electrolytes, to which some acids, bases and salts belong, dissolve into ions in solution completely, for this reason they are not suitable for studying the process of dissociation. Therefore, weak electrolytes are used for the study, the molecules of which dissociate in the solution into ions not completely.

For the reversible dissociation reaction, the dissociation constant (Kd), characterizing the equilibrium state, is determined by the formula: Kd = [K] [A] / [KA]. How the constant and the degree of dissociation are related to each other can be considered by the example of a weak electrolyte. Based on the law of dilution Ostwald, all logical reasoning is built: Kd = c • α2, where c is the concentration of the solution (in this case, c = [KA]). It is known that 1 mole of substance is dissolved in the volume of the solution V dm3. In the initial state, the concentration of the molecules of the initial substance can be expressed: c = [KA] = 1 / V mol / dm3, and the ion concentrations will be: [K] = [A] = 0 / V mol / dm3. When equilibrium is reached, their values change: [KA] = (1 - α) / V mol / dm3 and [K] = [A] = α / V mol / dm3, then Kd = (α / V • α / V) / (1 - α) / V = α2 / (1 - α) • V. The case of few dissociating electrolytes is considered, the degree of dissociation (α) of which approaches zero, and the volume of the solution can be expressed through a known concentration: V = 1 / [KA] = 1 / s. Then the equation can be transformed: Kd = α2 / (1 - α) • V = α2 / (1 - 0) • (1 / с) = α2 • с, and by extracting the square root of the fraction Kd / c, we can calculate the degree of dissociation Α. This law is valid if α is much less than 1.

For strong electrolytes, the term "apparent degree of dissociation" is more suitable. It is found as the ratio of the apparent number of dissociated particles to the real one or from the formula for determining the isotonic coefficient (called the Van't Hoff coefficient and shows the true behavior of the substance in solution): α = (i-1) / (n-1). Here i is the isotonic coefficient, and n is the number of ions formed. For solutions whose molecules completely disintegrated into ions, α ≈ 1, and with increasing concentration α increasingly tends to 1. All this is explained by the theory of strong electrolytes, which argues that the motion of cations and anions of decomposed molecules of a strong electrolyte is hampered for several reasons. First: ions are surrounded by molecules of a polar solvent, this electrostatic interaction is called solvation. The second: unlike charged cations and anions that are in solution, due to the action of the forces of mutual attraction, form associates or ion pairs. Associates behave the same way as non-dissociated molecules.