Electricity in liquids is due to the movement of positive and negative ions. Unlike current in conductors where electrons move. Thus, if there are no ions in a liquid, then it is a dielectric, for example, distilled water. Since charge carriers are ions, that is, molecules and atoms of a substance, when an electric current passes through such a liquid, it will inevitably lead to a change chemical properties substances.
Where do positive and negative ions come from in a liquid? Let us say at once that charge carriers are not capable of forming in all liquids. Those in which they appear are called electrolytes. These include solutions of salts of acids and alkalis. When dissolving salt in water, for example, take table salt NaCl, it decomposes under the action of a solvent, that is, water into a positive ion Na called a cation and a negative ion Cl called an anion. The process of formation of ions is called electrolytic dissociation.
Let's conduct an experiment, for it we need a glass bulb, two metal electrodes, an ammeter and a direct current source. We fill the flask with a solution of common salt in water. Then we put two rectangular electrodes into this solution. We connect the electrodes to a direct current source through an ammeter.
Figure 1 - Flask with salt solution
When the current is turned on between the plates, an electric field will appear under the action of which salt ions will begin to move. Positive ions will rush to the cathode, and negative ions to the anode. At the same time, they will make a chaotic movement. But at the same time, under the action of the field, an ordered one will also be added to it.
Unlike conductors in which only electrons move, that is, one type of charge, two types of charges move in electrolytes. These are positive and negative ions. They move towards each other.
When the positive sodium ion reaches the cathode, it will gain the missing electron and become a sodium atom. A similar process will occur with the chlorine ion. Only when reaching the anode, the chlorine ion will give up an electron and turn into a chlorine atom. Thus, current is maintained in the external circuit due to the movement of electrons. And in the electrolyte, ions seem to carry electrons from one pole to another.
The electrical resistance of electrolytes depends on the amount of ions formed. In strong electrolytes, the level of dissociation is very high when dissolved. The weak are low. Also, the electrical resistance of the electrolyte is affected by temperature. With its increase, the viscosity of the liquid decreases and heavy and clumsy ions begin to move faster. Accordingly, the resistance decreases.
If the salt solution is replaced with a solution of copper sulfate. Then, when a current is passed through it, when the copper cation reaches the cathode and receives the missing electrons there, it will be restored to a copper atom. And if after that you remove the electrode, you can find copper deposits on it. This process is called electrolysis.
Liquids, like solids, can be conductors, semiconductors, and dielectrics. In this lesson, we will focus on 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, one can imagine next experience: 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 of salt in the bathroom 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 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 cleavage, 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 the molecules break down into ions.
Practical application of electrolysis
First practical use 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)
- Fatyf.narod.ru ().
- ChemiK ().
- Ens.tpu.ru ().
Homework
- What are electrolytes?
- What are the two fundamental different types 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?
Liquids, like solids, can be conductors, semiconductors, and dielectrics. In this lesson, we will focus on 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 of salt in the bathroom 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 the 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)
- Fatyf.narod.ru ().
- ChemiK ().
- Ens.tpu.ru ().
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?
Water as a universal solvent.. Aqueous solutions.. Electrolytic dissociation.. Electrolyte.. Weak and strong electrolytes.. Carriers 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 an 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 dissolved in water various substances(acids, alkalis, bases, salts, etc.), 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 refers to 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, ordinary 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 the direction of the electric field different sides, the substance is gradually divided 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 action of an electric field.
melts
The dissociation of a substance is possible without the participation of water. Enough to melt the crystals chemical composition substances and get a melt. Melts of matter, like aqueous electrolytes, are conductors of the second kind, and therefore they can be called electrolytes. The electric current 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 solid bodies, 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 electrical circuit, then the negative ions will begin to move towards the positive electrode - the anode, and the positive ones - towards the negative - the 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, negatively charged ions donate their extra electrons (in chemistry this is called oxidative reaction), and at the cathode, the positive ions get 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 valence 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 etc.). 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 layer is glued onto the dielectric copper plate, 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.
