Excellent conductors of electrical current - gold,copper, iron, aluminum, alloys. Along with them, there is a large group of nonmetallic substances, the melts and aqueous solutions of which also have the property of conductivity. These are strong bases, acids, some salts, collectively called "electrolytes." What is ionic conductivity? Let us find out what relation electrolytes have to this widespread phenomenon.
The world around is full of different conductors, and alsoinsulators. These properties of bodies and substances have been known since ancient times. Greek mathematician Thales conducted experience with amber (in Greek - "electron"). Having rubbed it on silk, the scientist observed the phenomenon of attraction of hair, fibers of wool. Later it became known that amber is an insulator. In this substance there are no particles that could carry an electric charge. Good conductors are metals. In their composition there are atoms, positive ions and free, infinitesimal negative particles - electrons. They provide the charge transfer when the current is passed. Strong electrolytes in dry form do not contain free particles. But during dissolution and melting, the crystal lattice breaks down, as well as the polarization of the covalent bond.
By giving or attaching electrons, atomsmetal and non-metallic elements are converted into ions. Between them in the crystal lattice there is a fairly strong connection. Dissolution or melting of ionic compounds, for example sodium chloride, leads to its destruction. In polar molecules there are neither bound nor free ions, they arise when interacting with water. In the 30s of the XIX century M. Faraday discovered that solutions of some substances conduct current. The scientist introduced into the science such important concepts:
There are compounds - strong electrolytes, the crystal lattices of which completely break down with the release of ions.
There are insoluble substances and those thatare preserved in a molecular form, for example, sugar, formaldehyde. Such compounds are called non-electrolytes. For them, the formation of charged particles is not characteristic. Weak electrolytes (coal and acetic acid, ammonium hydroxide and a number of other substances) contain few ions.
In his works the Swedish scientist S. Arrhenius (1859-1927) relied on the conclusions of Faraday. Later, the provisions of his theory were clarified by the Russian researchers I. Kablukov and V. Kistyakovsky. They found out that when dissolving and melting ions form not all substances, but only electrolytes. What is S. Arrhenius dissociation? This is the destruction of molecules, which leads to the appearance of charged particles in solutions and melts. The main theoretical provisions of S. Arrhenius:
The indicator of the degree of dissociation of a substance (itsoften expressed as a percentage) is the ratio of the number of molecules that have decayed into ions and the total number of particles in solution. Electrolytes are strong if the value of this indicator is more than 30%, in the case of weak ones - less than 3%.
Theoretical conclusions of S. Arrhenius was supplemented by later studies of physicochemical processes in solutions and melts conducted by Russian scientists. The properties of bases and acids were explained. The first include compounds in whose solutions from cations only metal ions can be detected, the anions are the particles OH-. The molecules of acids decompose into negative ions of the acid residue and hydrogen protons (H+). The motion of ions in solution and melt is chaotic. Consider the results of the experiment, for which you will need to assemble a chain, include carbon electrodes and an ordinary incandescent bulb. Let us check the conductivity of solutions of different substances: common salt, acetic acid and sugar (the first two are electrolytes). What is an electrical circuit? This is a source of current and conductors connected together. When the circuit is closed, the bulb will glow brighter in the salt solution. The motion of the ions acquires order. The anions are directed to the positive electrode, and the cations to the negative electrode.
In this process, acetic acid participatesa small amount of charged particles. Sugar is not an electrolyte, it does not conduct current. Between the electrodes in this solution there will be an insulating layer, the bulb will not burn.
When the solutions are drained, one can observe how they leadyourself electrolytes. What are the ionic equations of similar reactions? Consider, for example, the chemical interaction between barium chloride and sodium nitrate:
2NaNO3 + BaCl2 + = 2NaCl + Ba (NO3)2.
The formulas of electrolytes can be written in ionic form:
2Na+ + 2NO3- + Ba2+ + 2Cl- = 2Na+ + 2Cl- + Ba2+ + 2NO3-.
The substances taken for the reaction are strong electrolytes. In this case, the composition of the ions does not change. The chemical interaction between solutions of electrolytes is possible in three cases:
1. If one of the products is an insoluble substance.
Molecular equation: Na2SO4 + BaCl2 = BaSO24 + 2NaCl.
Let us write the composition of electrolytes in the form of ions:
2Na+ + SO42- + Ba2+ + 2Cl- = BaSO24 (white precipitate) + 2Na+ 2Cl-.
2. One of the formed substances is gas.
3. Among the reaction products there is a weak electrolyte.
Chemically pure water (distilled) does not conduct electric current. But in its composition there is a small amount of charged particles. These are protons H+ and anions OH-. A negligible number of water molecules undergo dissociation. There is a value - the ion product of water, which is constant at a temperature of 25 ° C. It allows one to know the concentrations of H+ and he-. Hydrogen ions predominate in solutions of acids, hydroxide anions are larger in alkalies. In neutral - the number H+ and he-. The solution medium also characterizes the hydrogen index (pH). The higher it is, the more hydroxide ions are present. The medium is neutral at a pH range close to 6-7. In the presence of H ions+ and he- change their color indicator substances: litmus, phenolphthalein, methylorange and others.
Properties of solutions and melts of electrolytesare widely used in industry, engineering, agriculture and medicine. The scientific justification is laid down in the works of a number of outstanding scientists who explained the behavior of particles, of which salts, acids and bases are composed. In their solutions, a variety of ion exchange reactions occur. They are used in many industrial processes, in electrochemistry, electroplating. Processes in living beings also occur between ions in solutions. Many non-metals and metals, toxic in the form of atoms and molecules, are indispensable in the form of charged particles (sodium, potassium, magnesium, chlorine, phosphorus and others).