Water as a universal solvent.. Aqueous solutions.. Electrolytic dissociation.. Electrolyte.. Weak and strong electrolytes.. Carriers of electric charges in liquids.. Positive and negative ions.. Electrolysis.. Melts.. Nature of electric current in melts..
One of the conditions for the occurrence of an electric current is the presence of free charges capable of moving under the action of electric field. We talked about the nature of electric current in metals and.
In this lesson, we will try to figure out what particles carry electric charge in liquids and melts.
Water as a universal solvent
As we know, distilled water does not contain charge carriers and therefore does not conduct electric current, that is, it is a dielectric. However, the presence of any impurities already makes water a fairly good conductor.
Water has the phenomenal ability to dissolve almost everything in itself. chemical elements. When various substances (acids, alkalis, bases, salts, etc.) are dissolved in water, the solution becomes a conductor due to the breakdown of substance molecules into ions. This phenomenon is called electrolytic dissociation, and the solution itself is an electrolyte capable of conducting an electric current. All water basins on Earth are, to a greater or lesser extent, natural electrolytes.
The world ocean is a solution of ions of almost all elements of the periodic table.
Gastric juice, blood, lymph, all fluids in the human body are electrolytes. All animals and plants are also primarily composed of electrolytes.
According to the degree of dissociation, there are weak and strong electrolytes. Water is a weak electrolyte, and most inorganic acids are strong electrolytes. Electrolytes are also called conductors of the second kind.
Carriers of electric charges in a liquid
When dissolved in water (or other liquid) of various substances, they decompose into ions.
For example, common table salt NaCl (sodium chloride) in water separates into positive sodium ions (Na +) and negative chloride ions (Cl -). If the two poles in the resulting electrolyte are at different potentials, then the negative ions drift towards the positive pole while the positive ions drift towards the negative pole.
Thus, the electric current in a liquid consists of flows of positive and negative ions directed towards each other.
While absolutely pure water is an insulator, water containing even small impurities (natural or introduced from outside) of ionized matter is a conductor of electric current.
Electrolysis
Since the positive and negative ions of the solute drift in different directions under the influence of the electric field, the substance gradually separates into two parts.
This separation of matter into its constituent elements is called electrolysis.
Electrolytes are used in electrochemistry, in chemical current sources (galvanic cells and batteries), in electroplating production processes and other technologies based on the movement of electric charges in liquids under the influence of an electric field.
melts
The dissociation of a substance is possible without the participation of water. It is enough to melt the crystals of the chemical composition of the substance and get the melt. Melts of matter, like aqueous electrolytes, are conductors of the second kind, and therefore they can be called electrolytes. Electricity in melts has the same nature as the current in aqueous electrolytes - these are counter flows of positive and negative ions.
Using melts, in metallurgy, aluminum is obtained electrolytically from alumina. An electric current is passed through aluminum oxide and during electrolysis, pure aluminum accumulates at one of the electrodes (cathode). This is a very energy-intensive process, which, in terms of energy consumption, resembles the decomposition of water into hydrogen and oxygen using electric current.
In the aluminum electrolysis shop
« Physics - Grade 10 "
What are the carriers of electric current in a vacuum?
What is the nature of their movement?
Liquids, like solids, can be dielectrics, conductors, and semiconductors. Dielectrics include distilled water, conductors - solutions and melts of electrolytes: acids, alkalis and salts. Liquid semiconductors are molten selenium, sulfide melts, etc.
electrolytic dissociation.
When electrolytes are dissolved under the influence of the electric field of polar water molecules, electrolyte molecules decompose into ions.
The disintegration of molecules into ions under the influence of the electric field of polar water molecules is called electrolytic dissociation.
Degree of dissociation- the proportion of molecules in the dissolved substance that have decayed into ions.
The degree of dissociation depends on temperature, solution concentration and electrical properties solvent.
With increasing temperature, the degree of dissociation increases and, consequently, the concentration of positively and negatively charged ions increases.
Ions of different signs, when meeting, can again unite into neutral molecules.
Under constant conditions, a dynamic equilibrium is established in the solution, at which the number of molecules that decay into ions per second is equal to the number of pairs of ions that recombine into neutral molecules in the same time.
Ionic conduction.
Charge carriers in aqueous solutions or electrolyte melts are positively and negatively charged ions.
If a vessel with an electrolyte solution is included in an electrical circuit, then negative ions will begin to move towards the positive electrode - the anode, and positive - towards the negative - cathode. As a result, an electric current will flow through the circuit.
Conductivity aqueous solutions or melts of electrolytes, which is carried out by ions, is called ionic conductivity.
Electrolysis. With ionic conductivity, the passage of current is associated with the transfer of matter. On the electrodes, substances that make up electrolytes are released. At the anode, the negatively charged ions donate their extra electrons (in chemistry, this is called an oxidative reaction), and at the cathode, the positive ions gain the missing electrons (reduction reaction).
Liquids can also have electronic conductivity. Such conductivity is possessed, for example, by liquid metals.
The process of release of a substance at the electrode, associated with redox reactions, is called electrolysis.
What determines the mass of a substance released in a given time? Obviously, the mass m of the released substance is equal to the product of the mass m 0i of one ion by the number N i of ions that have reached the electrode during the time Δt:
m = m 0i N i . (16.3)
The ion mass m 0i is:
where M is the molar (or atomic) mass of the substance, and N A is the Avogadro constant, i.e. the number of ions in one mole.
The number of ions reaching the electrode is
where Δq = IΔt is the charge passed through the electrolyte during the time Δt; q 0i is the charge of the ion, which is determined by the valence n of the atom: q 0i \u003d ne (e is the elementary charge). During the dissociation of molecules, for example KBr, consisting of monovalent atoms (n = 1), K + and Br - ions appear. The dissociation of copper sulfate molecules leads to the appearance of doubly charged Cu 2+ and SO 2- 4 ions (n = 2). Substituting expressions (16.4) and (16.5) into formula (16.3) and taking into account that Δq = IΔt, a q 0i = ne, we obtain
Faraday's law.
Let us denote by k the coefficient of proportionality between the mass m of the substance and the charge Δq = IΔt passing through the electrolyte:
where F \u003d eN A \u003d 9.65 10 4 C / mol - Faraday constant.
The coefficient k depends on the nature of the substance (the values of M and n). According to formula (16.6) we have
m = kIΔt. (16.8)
Faraday's law of electrolysis:
The mass of the substance released on the electrode during the time Δt. during the passage of electric current, is proportional to the strength of the current and time.
This statement, obtained theoretically, was first established experimentally by Faraday.
The value k in formula (16.8) is called electrochemical equivalent given substance and expressed in kilograms per pendant(kg/C).
From formula (16.8) it can be seen that the coefficient k is numerically equal to the mass of the substance released on the electrodes during the transfer of a charge of 1 C by ions.
The electrochemical equivalent has a simple physical meaning. Since M / N A \u003d m 0i and en \u003d q 0i, then according to formula (16.7) k \u003d rn 0i / q 0i, i.e. k is the ratio of the ion mass to its charge.
By measuring the values of m and Δq, one can determine the electrochemical equivalents of various substances.
You can verify the validity of Faraday's law by experience. Let's assemble the installation shown in Figure (16.25). All three electrolytic baths are filled with the same electrolyte solution, but the currents passing through them are different. Let's denote the strength of the currents through I1, I2, I3. Then I 1 = I 2 + I 3 . By measuring the masses m 1 , m 2 , m 3 of the substances released on the electrodes in different baths, one can make sure that they are proportional to the corresponding currents I 1 , I 2 , I 3 .
Determination of the electron charge.
Formula (16.6) for the mass of the substance released on the electrode can be used to determine the electron charge. From this formula it follows that the electron charge modulus is equal to:
Knowing the mass m of the released substance during the passage of the charge IΔt, the molar mass M, the valency of n atoms and the Avogadro constant N A, one can find the value of the electron charge modulus. It turns out to be equal to e = 1.6 10 -19 C.
It was in this way that the value of the elementary electric charge was obtained for the first time in 1874.
Application of electrolysis. Electrolysis is widely used in engineering for various purposes. Electrolytically cover the surface of one metal with a thin layer of another ( nickel plating, chrome plating, gold plating and so on.). This durable coating protects the surface from corrosion. If good peeling of the electrolytic coating is ensured from the surface on which the metal is deposited (this is achieved, for example, by applying graphite to the surface), then a copy can be obtained from the relief surface.
The process of obtaining peelable coatings - electrotype- was developed by the Russian scientist B. S. Jacobi (1801-1874), who in 1836 applied this method to make hollow figures for St. Isaac's Cathedral in St. Petersburg.
Previously, in the printing industry, copies from a relief surface (stereotypes) were obtained from matrices (an imprint of a set on a plastic material), for which a thick layer of iron or another substance was deposited on the matrices. This made it possible to reproduce the set in the required number of copies.
Electrolysis removes impurities from metals. Thus, crude copper obtained from the ore is cast in the form of thick sheets, which are then placed in a bath as anodes. During electrolysis, the anode copper dissolves, impurities containing valuable and rare metals fall to the bottom, and pure copper settles on the cathode.
Aluminum is obtained from molten bauxite by electrolysis. It was this method of obtaining aluminum that made it cheap and, along with iron, the most common in technology and everyday life.
With the help of electrolysis, electronic circuit boards are obtained, which serve as the basis of all electronic products. A thin copper plate is glued onto the dielectric, on which a complex pattern of connecting wires is applied with a special paint. Then the plate is placed in an electrolyte, where the areas of the copper layer that are not covered with paint are etched. After that, the paint is washed off, and the details of the microcircuit appear on the board.
Liquids that are conductors include melts and electrolyte solutions, i.e. salts, acids and alkalis.
When electrolytes dissolve in water, their molecules break down into ions - electrolytic dissociation. The degree of dissociation, i.e. the fraction of molecules in a solute that have decomposed into ions depends on the temperature, the concentration of the solution, and the electrical properties of the solvent. With increasing temperature, the degree of dissociation increases and, consequently, the concentration of positively and negatively charged ions increases. Ions of different signs, when meeting, can again unite into neutral molecules. This process is called recombination. Under constant conditions, a dynamic equilibrium is established in the solution, at which the number of molecules that decay into ions per second is equal to the number of pairs of ions that recombine into neutral molecules in the same time.
Thus, free charge carriers in conductive liquids are positive and negative ions. If electrodes connected to a current source are placed in a liquid, then these ions will begin to move. One of the electrodes is connected to the negative pole of the current source - it is called the cathode - the other is connected to the positive - the anode. When connected to a current source, ions in an electrolyte solution, negative ions begin to move towards the positive electrode (anode), and positive ions, respectively, towards the negative (cathode). That is, an electric current is established. Such conductivity in liquids is called ionic, since ions are charge carriers.
When current passes through the electrolyte solution on the electrodes, a substance is released associated with redox reactions. At the anode, negatively charged ions donate their extra electrons (oxidation reaction), and at the cathode, positive ions accept the missing electrons (reduction reaction). This process is called electrolysis.
During electrolysis, a substance is released at the electrodes. The dependence of the mass of the released substance m on the strength of the current, the time of passage of the current and the substance itself was established by M. Faraday. This law can be obtained theoretically. So, the mass of the released substance is equal to the product of the mass of one ion m i by the number of ions N i that reached the electrode during the time Dt. The mass of the ion according to the formula for the amount of substance is m i \u003d M / N a, where M is molar mass substances, N a is the Avogadro constant. The number of ions that have reached the electrode is N i =Dq/q i, where Dq is the charge that passed through the electrolyte during the time Dt (Dq=I*Dt), q i is the charge of the ion, which is determined by the valency of the atom (q i = n*e, where n is the valency of the atom, e is the elementary charge). Substituting these formulas, we obtain that m=M/(neN a)*IDt. If we denote by k (proportionality factor) =M/(neN a), then we have m=kIDt. This is a mathematical notation of Faraday's first law, one of the laws of electrolysis. The mass of the substance released on the electrode during the time Dt during the passage of an electric current is proportional to the strength of the current and this time interval. The value of k is called the electrochemical equivalent of a given substance, which is numerically equal to the mass of the substance released on the electrodes during the transfer of a charge of 1 C by ions. [k]= 1 kg/C. k = M/(neN a) = 1/F*M/n , where F is Faraday's constant. F \u003d eN a \u003d 9.65 * 10 4 C / mol. The derived formula k=(1/F)*(M/n) is Faraday's second law.
Electrolysis is widely used in engineering for various purposes, for example, the surface of one metal is covered with a thin layer of another (nickel plating, chromium plating, copper plating, etc.). If good peeling of the electrolytic coating from the surface is ensured, a copy of the surface topography can be obtained. This process is called electroplating. Also, using electrolysis, metals are purified from impurities, for example, thick sheets of unrefined copper obtained from ore are placed in a bath as an anode. During electrolysis, copper dissolves, impurities fall to the bottom, and pure copper settles on the cathode. With the help of electrolysis, electronic circuit boards are also obtained. A thin complex pattern of connecting wires is glued onto the dielectric, then the plate is placed in the electrolyte, where the uncovered areas of the copper layer are etched away. After that, the paint is washed off and the details of the microcircuit appear on the board.
It is formed by the directed movement of free electrons and that in this case no changes in the substance from which the conductor is made do not occur.
Such conductors, in which the passage of an electric current is not accompanied by chemical changes in their substance, are called conductors of the first kind. These include all metals, coal and a number of other substances.
But there are also such conductors of electric current in nature, in which chemical phenomena occur during the passage of current. These conductors are called conductors of the second kind. These include mainly various solutions in water of acids, salts and alkalis.
If you pour water into a glass vessel and add a few drops of sulfuric acid (or some other acid or alkali) to it, and then take two metal plates and attach conductors to them by lowering these plates into the vessel, and connect a current source to the other ends of the conductors through a switch and an ammeter, then gas will be released from the solution, and it will continue continuously until the circuit is closed. acidified water is indeed a conductor. In addition, the plates will begin to be covered with gas bubbles. Then these bubbles will break away from the plates and come out.
When an electric current passes through the solution, chemical changes resulting in the release of gas.
Conductors of the second kind are called electrolytes, and the phenomenon that occurs in the electrolyte when an electric current passes through it is.
Metal plates dipped into the electrolyte are called electrodes; one of them, connected to the positive pole of the current source, is called an anode, and the other, connected to the negative pole, is called cathode.
What causes the passage of electric current in a liquid conductor? It turns out that in such solutions (electrolytes), acid molecules (alkalis, salts) under the action of a solvent (in this case, water) decompose into two components, and one particle of the molecule has a positive electrical charge, and the other negative.
The particles of a molecule that have an electric charge are called ions. When an acid, salt or alkali is dissolved in water, a large number of both positive and negative ions appear in the solution.
Now it should become clear why an electric current passed through the solution, because between the electrodes connected to the current source, it was created, in other words, one of them turned out to be positively charged and the other negatively. Under the influence of this potential difference, positive ions began to move towards the negative electrode - the cathode, and negative ions - towards the anode.
Thus, the chaotic movement of ions has become an ordered counter-movement of negative ions in one direction and positive ones in the other. This charge transfer process constitutes the flow of electric current through the electrolyte and occurs as long as there is a potential difference across the electrodes. With the disappearance of the potential difference, the current through the electrolyte stops, the orderly movement of ions is disturbed, and chaotic movement sets in again.
As an example, consider the phenomenon of electrolysis when an electric current is passed through a solution of copper sulphate CuSO4 with copper electrodes lowered into it.
The phenomenon of electrolysis when current passes through a solution of copper sulphate: C - vessel with electrolyte, B - current source, C - switch
There will also be a counter movement of ions to the electrodes. The positive ion will be the copper (Cu) ion, and the negative ion will be the acid residue (SO4) ion. Copper ions, upon contact with the cathode, will be discharged (attaching the missing electrons to themselves), i.e., they will turn into neutral molecules of pure copper, and deposited on the cathode in the form of the thinnest (molecular) layer.
Negative ions, having reached the anode, are also discharged (give away excess electrons). But at the same time they enter chemical reaction with anode copper, as a result of which a copper molecule Cu is added to the acidic residue SO4 and a molecule of copper sulfate CuS O4 is formed, which is returned back to the electrolyte.
Since this chemical process leaks long time, then copper is deposited on the cathode, which is released from the electrolyte. In this case, instead of the copper molecules that have gone to the cathode, the electrolyte receives new copper molecules due to the dissolution of the second electrode - the anode.
The same process occurs if zinc electrodes are taken instead of copper ones, and the electrolyte is a solution of zinc sulfate ZnSO4. Zinc will also be transferred from the anode to the cathode.
Thus, difference between electric current in metals and liquid conductors lies in the fact that in metals only free electrons, i.e., negative charges, are charge carriers, while in electrolytes it is carried by oppositely charged particles of matter - ions moving in opposite directions. Therefore they say that electrolytes have ionic conductivity.
The phenomenon of electrolysis was discovered in 1837 by B. S. Jacobi, who carried out numerous experiments on the study and improvement of chemical current sources. Jacobi found that one of the electrodes placed in a solution of copper sulphate, when an electric current passes through it, is covered with copper.
This phenomenon is called electroplating, finds now extremely large practical use. One example of this is the coating of metal objects with a thin layer of other metals, i.e. nickel plating, gilding, silver plating, etc.
gases (including air) normal conditions do not conduct electricity. For example, naked, being suspended parallel to each other, are isolated from one another by a layer of air.
However, under the influence of high temperature, a large potential difference, and other reasons, gases, like liquid conductors, ionize, i.e., particles of gas molecules appear in them in large numbers, which, being carriers of electricity, contribute to the passage of electric current through the gas.
But at the same time, the ionization of a gas differs from the ionization of a liquid conductor. If a molecule breaks up into two charged parts in a liquid, then in gases, under the action of ionization, electrons are always separated from each molecule and an ion remains in the form of a positively charged part of the molecule.
One has only to stop the ionization of the gas, as it ceases to be conductive, while the liquid always remains a conductor of electric current. Consequently, the conductivity of a gas is a temporary phenomenon, depending on the action of external causes.
However, there is another one called arc discharge or just an electric arc. The phenomenon of an electric arc was discovered at the beginning of the 19th century by the first Russian electrical engineer V. V. Petrov.
V. V. Petrov, doing numerous experiments, discovered that between two charcoal connected to a current source, a continuous electric discharge occurs through the air, accompanied by bright light. In his writings, V. V. Petrov wrote that in this case, "the dark peace can be quite brightly illuminated." So for the first time electric light was obtained, which was practically applied by another Russian electrical scientist Pavel Nikolaevich Yablochkov.
"Yablochkov's Candle", whose work is based on the use of an electric arc, made a real revolution in electrical engineering in those days.
The arc discharge is used as a source of light even today, for example, in searchlights and projectors. The high temperature of the arc discharge allows it to be used for . At present, arc furnaces powered by a very high current are used in a number of industries: for the smelting of steel, cast iron, ferroalloys, bronze, etc. And in 1882, N. N. Benardos first used an arc discharge for cutting and welding metal.
In gas-light tubes, fluorescent lamps, voltage stabilizers, to obtain electron and ion beams, the so-called glow gas discharge.
A spark discharge is used to measure large potential differences using a ball gap, the electrodes of which are two metal balls with a polished surface. The balls are moved apart, and a measured potential difference is applied to them. Then the balls are brought together until a spark jumps between them. Knowing the diameter of the balls, the distance between them, the pressure, temperature and humidity of the air, they find the potential difference between the balls according to special tables. This method can be used to measure, to within a few percent, potential differences of the order of tens of thousands of volts.
Liquids, like solid bodies, can be conductors, semiconductors and dielectrics. In this lesson, we will talk about liquid conductors. And not about liquids with electronic conductivity (molten metals), but about liquid conductors of the second kind (solutions and melts of salts, acids, bases). The type of conductivity of such conductors is ionic.
Definition. Conductors of the second kind are those conductors in which chemical processes occur when current flows.
For a better understanding of the process of current conduction in liquids, the following experiment can be presented: Two electrodes connected to a current source were placed in a bath of water, a light bulb can be taken as a current indicator in the circuit. If you close such a circuit, the lamp will not burn, which means there is no current, which means that there is a break in the circuit, and the water itself does not conduct current. But if you put a certain amount in the bathroom - table salt- and repeat the circuit, the light will turn on. This means that free charge carriers, in this case ions, began to move in the bath between the cathode and anode (Fig. 1).
Rice. 1. Scheme of experience
Conductivity of electrolytes
Where do the free charges come from in the second case? As mentioned in one of the previous lessons, some dielectrics are polar. Water has just the same polar molecules (Fig. 2).
Rice. 2. Polarity of the water molecule
When salt is added to water, water molecules are oriented in such a way that their negative poles are near sodium, positive - near chlorine. As a result of interactions between charges, water molecules break salt molecules into pairs of opposite ions. The sodium ion has a positive charge, the chlorine ion has a negative charge (Fig. 3). It is these ions that will move between the electrodes under the action of an electric field.
Rice. 3. Scheme of formation of free ions
When sodium ions approach the cathode, it receives its missing electrons, while chloride ions give up theirs when they reach the anode.
Electrolysis
Since the flow of current in liquids is associated with the transfer of matter, with such a current, the process of electrolysis takes place.
Definition. Electrolysis is a process associated with redox reactions in which a substance is released at the electrodes.
Substances that, as a result of such splitting, provide ionic conductivity are called electrolytes. This name was proposed by the English physicist Michael Faraday (Fig. 4).
Electrolysis makes it possible to obtain substances in a sufficiently pure form from solutions, therefore it is used to obtain rare materials, such as sodium, calcium ... in its pure form. This is what is known as electrolytic metallurgy.
Faraday's laws
In the first work on electrolysis in 1833, Faraday presented his two laws of electrolysis. In the first one, it was about the mass of the substance released on the electrodes:
Faraday's first law states that this mass is proportional to the charge passed through the electrolyte:
Here the role of the coefficient of proportionality is played by the quantity - the electrochemical equivalent. This is a tabular value that is unique for each electrolyte and is its main characteristic. Dimension of the electrochemical equivalent:
The physical meaning of the electrochemical equivalent is the mass released on the electrode when the amount of electricity in 1 C passes through the electrolyte.
If you recall the formulas from the topic of direct current:
Then we can represent Faraday's first law in the form:
Faraday's second law directly concerns the measurement of the electrochemical equivalent through other constants for a particular electrolyte:
Here: is the molar mass of the electrolyte; - elementary charge; - electrolyte valence; is Avogadro's number.
The value is called the chemical equivalent of the electrolyte. That is, in order to know the electrochemical equivalent, it is enough to know the chemical equivalent, the remaining components of the formula are world constants.
Based on Faraday's second law, the first law can be represented as:
Faraday proposed the terminology of these ions on the basis of the electrode to which they move. Positive ions are called cations because they move towards the negatively charged cathode, negative charges are called anions as they move towards the anode.
The above action of water to break a molecule into two ions is called electrolytic dissociation.
In addition to solutions, melts can also be conductors of the second kind. In this case, the presence of free ions is achieved by the fact that very active molecular movements and vibrations begin at a high temperature, as a result of which molecules break down into ions.
Practical application of electrolysis
The first practical application of electrolysis occurred in 1838 by the Russian scientist Jacobi. With the help of electrolysis, he received an impression of figures for St. Isaac's Cathedral. This application of electrolysis is called electroplating. Another area of application is electroplating - covering one metal with another (chrome plating, nickel plating, gilding, etc., Fig. 5)
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Homework
- What are electrolytes?
- What are the two fundamentally different types of liquids in which an electric current can flow?
- What are the possible mechanisms for the formation of free charge carriers?
- *Why is the mass released on the electrode proportional to the charge?
