4. Degree of dissociation

Degree of dissociation (α) of electrolyte is the ratio:

α = .

α measures in percents (0…100%) or per unit (0…1). Degree of dissociation depends on:

• nature of chemical bonds in solute and solvent;

• concentration of solute;

• temperature of solution.

The main trends of these interdependences are:

1. Degree of dissociation is rising with decreasing of concentration of electrolyte;

2. Degree of dissociation is rising with rise in temperature.

Depending on degree of dissociation, all electrolytes are subdivided into three types: strong, medium and weak. This subdivision is relative, though.

Degree of dissociation of strong electrolytes is more than 30%, medium - from 3% till 30%, weak - less or equal than 3%. In Appendix 9 it is presented the division of main classes of inorganic substances as electrolytes.

5. Main classes of inorganic substances from viewpoint of theory of electrolytic dissociation

Traditional definition of an acid according Arrhenius’s theory: a chemical compound that contains Hydrogen and ionizes in aqueous solution to form Hydrogen ions:

HCl ↔ H⁺ + Cl^-;

HNO₃ ↔ H⁺ + NO₃^-.

An acid that can donate one proton (Hydrogen ion) per molecule is known as a monoprotic acid (like foregoing ones). Acids contained more than one proton per molecule are polyprotic and ionize in more than one stage. For example:

H2SO4 ↔ H+ + HSO4-;	H3PO4 ↔ H+ + H2PO4-;
HSO4-↔ H+ + SO42-;	H2PO4-↔ H+ + HPO42-;
	HPO42-↔ H+ + PO43-.

Traditional definition of a base according Arrhenius’s theory: this is a substance that contains hydroxide ions (OH^-) and dissociates to give hydroxide ions in aqueous solutions:

NaOH ↔ Na⁺ + OH^-;

KOH ↔ K⁺ + OH^-.

A base that can donate one hydroxyl OH^- per molecule is known as a monohydroxyl base (alkalis) (like foregoing ones). Bases contained more than one OH^--group per molecule are polyhydroxyl and ionize in more than one stages. For example:

Ca(OH)2 ↔ CaOH+ + OH-;

CaOH+ ↔ Ca2+ + OH-.

Neutral salts are dissociated with forming of metal cations (positive charged ions) and anions of acidic residues:

CaCl₂ ↔ Ca²⁺ + 2Cl^-;

Fe₂(SO₄)₃ ↔ 2 Fe³⁺ + 3SO₄^2-;

Ba(NO₂)₂ ↔ Ba²⁺ + 2NO₂^-.

Acidic and basic salts are ionized in some steps as salts and corresponding acids or bases. For example:

Acidic salt NH₄HSO₄:

1 step - like neutral salt:	NH4HSO4 ↔ NH4+ + HSO4-;
2 step - like acid:	HSO4-↔ H+ + SO42-.

Basic salt BaOHCl:

1 step - like neutral salt:	BaOHCl ↔ BaOH+ + Cl-;
2 step - like base:	BaOH+ ↔ Ba2+ + OH-.

All simple substances and oxides are weak electrolytes and do not dissociate in aqueous solutions.