Excellent conductors of electric current - gold, copper, iron, aluminum, alloys. Along with them there is a large group of non-metal substances, 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? We will find out what relation electrolyte substances have to this common phenomenon.
What particles carry charges?
The world around is full of various conductors, as well as insulators. These properties of bodies and substances have been known since ancient times. The Greek mathematician Thales conducted an experiment with amber (in Greek - “electron”). Rubbing it on silk, the scientist observed the phenomenon of attraction of hair, wool fibers. Later it became known that amber is an insulator. There are no particles in this substance that could carry an electric charge. Good conductors are metals. They contain atoms, positive ions and free, infinitesimal negative particles - electrons. They provide the transfer of charges when they pass current. Strong electrolytes in dry form do not contain free particles. But upon dissolution and melting, the destruction of the crystal lattice occurs, as well as the polarization of the covalent bond.
Water, non-electrolytes and electrolytes. What is dissolution?
By donating or attaching electrons, the atoms of metallic and non-metallic elements turn into ions. A sufficiently strong bond exists between them in the crystal lattice. 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 upon interaction with water. In the 30s of the XIX century M. Faraday discovered that solutions of certain substances conduct current. The scientist introduced into science the following important concepts:
- ions (charged particles);
- electrolytes (conductors of the second kind);
- cathode;
- anode.
There are compounds - strong electrolytes, the crystal lattices of which are completely destroyed with the release of ions.
There are insoluble substances and those that are stored in molecular form, for example, sugar, formaldehyde. Such compounds are called non-electrolytes. They are not characterized by the formation of charged particles. Weak electrolytes (carbonic and acetic acid, ammonium hydroxide and a number of other substances) contain few ions.
Theory of Electrolytic Dissociation
In his works, the Swedish scientist S. Arrhenius (1859–1927) relied on the conclusions of Faraday. Subsequently, the provisions of his theory were clarified by Russian researchers I. Kablukov and V. Kistyakovsky. They found that when dissolved and melted, not all substances form ions, 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:
- Bases, acids and salts in solutions are in dissociated form.
- Strong electrolytes decompose reversibly into ions.
- The weak form few ions.
An indicator of the degree of dissociation of a substance (it is often expressed as a percentage) is the ratio of the number of molecules disintegrated into ions and the total number of particles in solution. Electrolytes are strong if the value of this indicator is more than 30%, while weak - less than 3%.
Electrolyte Properties
The theoretical conclusions of S. Arrhenius were supplemented by later studies of physicochemical processes in solutions and melts by Russian scientists. Got an explanation of the properties of bases and acids. The first group includes compounds in solutions of which only metal ions can be detected from cations, OH - particles are anions. Molecules of acids decompose into negative ions of the acid residue and hydrogen protons (H + ). The movement of ions in solution and melt is chaotic. Consider the results of an experiment for which it will be necessary to assemble a circuit, include carbon electrodes and an ordinary incandescent bulb in it. Let's check the conductivity of solutions of various substances: sodium chloride, acetic acid and sugar (the first two are electrolytes). What is an electric circuit? This is a current source and conductors interconnected. When the circuit is closed, the bulb will burn brighter in a solution of sodium chloride. The movement of ions becomes orderly. Anions go to the positive electrode, and cations - to the negative.
In this process, a small amount of charged particles is involved in acetic acid. Sugar is not an electrolyte, does not conduct current. An insulating layer will appear between the electrodes in this solution, the bulb will not light.
Chemical interactions between electrolytes
When draining solutions, one can observe how electrolytes behave. What are the ionic equations of such reactions? Consider the chemical interaction between barium chloride and sodium nitrate as an example:
2NaNO 3 + BaCl 2 + = 2NaCl + Ba (NO 3 ) 2 .
The electrolyte formulas are written in ionic form:
2Na + + 2NO 3– + Ba 2+ + 2Cl - = 2Na + + 2Cl - + Ba 2+ + 2NO 3– .
Substances taken for the reaction are strong electrolytes. In this case, the composition of the ions does not change. Chemical interaction between electrolyte solutions is possible in three cases:
1. If one of the products is insoluble.
Molecular equation: Na 2 SO 4 + BaCl 2 = BaSO 4 + 2NaCl.
We write the composition of electrolytes in the form of ions:
2Na + + SO 4 2– + Ba 2+ + 2Cl - = BaSO 4 (white precipitate) + 2Na + 2Cl - .
2. One of the resulting substances is gas.
3. Among the reaction products there is a weak electrolyte.
Water is one of the weakest electrolytes
Chemically pure water (distilled) does not conduct electric current. But in its composition there is a small amount of charged particles. These are H + protons and OH - anions. A negligible number of water molecules undergo dissociation. There is a quantity - the ionic product of water, which is constant at a temperature of 25 ° C. It allows you to know the concentration of H + and OH - . Hydrogen ions predominate in solutions of acids; hydroxide anions are predominant in alkalis. In neutral - the amount of H + and OH - coincides. The environment of solutions is also characterized by a 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 + and OH - ions, indicator substances change their color: litmus, phenolphthalein, methyl orange and others.
The properties of electrolyte solutions and melts are widely used in industry, engineering, agriculture and medicine. The scientific justification lies in the work of a number of prominent scientists who have explained the behavior of the particles of which salts, acids and bases are composed. In their solutions, numerous reactions of ion exchange proceed. 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).