What is called electric current in liquids? Electric current in liquids: its origin, quantitative and qualitative characteristics. Cathode and anode. Cations and anions
>>Physics: Electricity in liquids
Liquids, like solids, can be dielectrics, conductors and semiconductors. Dielectrics include distilled water, conductors include solutions and melts of electrolytes: acids, alkalis and salts. Liquid semiconductors are molten selenium, molten sulfides, etc.
Electrolytic dissociation. When dissolving electrolytes under the influence electric field Polar water molecules disintegrate electrolyte molecules into ions. This process is called electrolytic dissociation.
Degree of dissociation, i.e., the proportion of molecules in the dissolved substance that have broken up into ions depends on the temperature, 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.
When ions of different signs meet, they can again combine into neutral molecules - recombine. Under constant conditions, a dynamic equilibrium is established in the solution, in which the number of molecules that disintegrate into ions per second is equal to the number of pairs of ions that, at the same time, recombine into neutral molecules.
Ionic conductivity. Charge carriers in aqueous solutions or molten electrolytes are positively and negatively charged ions.
If a vessel with an electrolyte solution is turned on electrical circuit, then negative ions will begin to move towards the positive electrode - the anode, and positive ions - towards the negative - cathode. As a result, an electric current will be established. Since charge transfer in aqueous solutions or electrolyte melts is carried out by ions, such conductivity is called ionic.
Liquids can also have electronic conductivity. Liquid metals, for example, have such conductivity.
Electrolysis. In ionic conduction, the passage of current is associated with the transfer of matter. At the electrodes, the substances that make up the electrolytes are released. At the anode, negatively charged ions give up their extra electrons (in chemistry this is called an oxidation reaction), and at the cathode positive ions receive the missing electrons (reduction reaction). The process of release of a substance at the electrode associated with redox reactions is called electrolysis.
Application of electrolysis. Electrolysis is widely used in technology for various purposes. Electrolytically coat the surface of one metal with a thin layer of another ( nickel plating, chrome plating, copper plating and so on.). This durable coating protects the surface from corrosion.
If you ensure good peeling of the electrolytic coating from the surface on which the metal is deposited (this is achieved, for example, by applying graphite to the surface), then you can get a copy from the relief surface.
In the printing industry, such copies (stereotypes) are obtained from matrices (a type imprint on a plastic material), for which a thick layer of iron or other substance is deposited on the matrices. This allows you to reproduce the set in the required number of copies. If previously the circulation of a book was limited to the number of prints that could be obtained from one set (the set is gradually erased during printing), now the use of stereotypes allows one to significantly increase the circulation. True, at present, using electrolysis, stereotypes are obtained only for high-quality printed books.
The process of obtaining peelable coatings - electrotype- was developed by the Russian scientist B. S. Jacobi (1801-1874), who in 1836 used this method to make hollow figures for St. Isaac's Cathedral in St. Petersburg.
Using electrolysis, metals are purified from impurities. Thus, the crude copper obtained from the ore is cast into the form of thick sheets, which are then placed in a bath as anodes. During electrolysis, the copper of the anode dissolves, impurities containing valuable and rare metals fall to the bottom, and pure copper settles on the cathode.
Using electrolysis, aluminum is obtained from molten bauxite. It was this method of producing aluminum that made it cheap and, along with iron, the most common in technology and everyday life.
Using electrolysis, electronic circuit boards are obtained, which serve as the basis for all electronic products. A thin copper plate is glued onto the dielectric, onto which a complex pattern of connecting wires is painted with 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 this, the paint is washed off and the details of the microcircuit appear on the board.
In solutions and melts of electrolytes, free electrical charges appear due to the disintegration of neutral molecules into ions. The movement of ions in the field means the transfer of matter. This process is widely used in practice (electrolysis).
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1. What is called electrolytic dissociation?
2. Why does a substance transfer occur when current passes through an electrolyte solution, but does not transfer a substance when passing through a metal conductor?
3. What are the similarities and differences between the intrinsic conductivity of semiconductors and electrolyte solutions?
G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade
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Liquids, like solids, can be conductors, semiconductors and dielectrics. This lesson will focus on conductive fluids. 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 type are conductors in which chemical processes occur when current flows.
To better understand the process of current conduction in liquids, we can imagine the following experiment: Two electrodes connected to a current source were placed in a bath of water; a light bulb can be taken in the circuit as a current indicator. If you close such a circuit, the lamp will not light, 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 place a certain amount of table salt in the bathroom and repeat the circuit, the light bulb will light up. This means that free charge carriers began to move in the bath between the cathode and anode, in in this case ions (Fig. 1).
Rice. 1. Experiment scheme
Conductivity of electrolytes
Where do free charges come from in the second case? As was said in one of previous lessons, some dielectrics are polar. Water has polar molecules (Fig. 2).
Rice. 2. Polarity of the water molecule
When salt is added to water, the water molecules are oriented in such a way that their negative poles are near sodium, positive poles are near chlorine. As a result of interactions between charges, water molecules break salt molecules into pairs of unlike 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 influence of an electric field.
Rice. 3. Scheme of formation of free ions
When sodium ions approach the cathode, it receives its missing electrons, and chlorine ions give up theirs when they reach the anode.
Electrolysis
Since the flow of current in liquids is associated with the transfer of matter, at 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 from solutions in a fairly pure form, so it is used to obtain rare materials such as sodium, calcium... in their pure form. This is what is called electrolytic metallurgy.
Faraday's laws
In the first work on electrolysis in 1833, Faraday presented his two laws of electrolysis. The first dealt with the mass of the substance released on the electrodes:
Faraday's first law states that this mass is proportional to the charge passing through the electrolyte:
Here the role of the proportionality coefficient is played by the quantity - the electrochemical equivalent. This is a tabular value that is unique for each electrolyte and is its main characteristic. Electrochemical equivalent dimension:
The physical meaning of the electrochemical equivalent is the mass released on the electrode when an amount of electricity of 1 C passes through the electrolyte.
If you remember the formulas from the topic about direct current:
Then we can represent Faraday’s first law as:
Faraday's second law directly concerns the measurement of the electrochemical equivalent through other constants for a specific electrolyte:
Here: - molar mass of the electrolyte; - elementary charge; - electrolyte valence; - Avogadro's number.
The quantity 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 a terminology for these ions based on 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 described 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 type. In this case, the presence of free ions is achieved by the fact that at high temperatures very active molecular movements and vibrations begin, as a result of which the molecules are broken down into ions.
Practical application of electrolysis
First practical use electrolysis occurred in 1838 by the Russian scientist Jacobi. Using electrolysis, he obtained an impression of the figures for St. Isaac's Cathedral. This application of electrolysis is called galvanoplasty. Another area of application is electroplating - coating 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 liquids in which electric current can flow?
- What could be the mechanisms for the formation of free charge carriers?
- *Why is the mass deposited on the electrode proportional to the charge?
Everyone is familiar with the definition of electric current. It is represented as the directed movement of charged particles. Such movement in different environments has fundamental differences. As a basic example of this phenomenon, one can imagine the flow and propagation of electric current in liquids. Such phenomena are characterized by various properties and are seriously different from the ordered movement of charged particles, which occurs under normal conditions not under the influence of various liquids.
Figure 1. Electric current in liquids. Author24 - online exchange of student work
Formation of electric current in liquids
Despite the fact that the process of conducting electric current is carried out through metal devices (conductors), the current in liquids depends on the movement of charged ions that have acquired or lost similar atoms and molecules for some specific reason. An indicator of such movement is a change in the properties of a certain substance where ions pass. Thus, it is necessary to rely on the basic definition of electric current in order to form a specific concept of the formation of current in various liquids. It has been determined that the decomposition of negatively charged ions promotes movement into the region of a current source with positive values. Positively charged ions in such processes will move in the opposite direction - towards the negative current source.
Liquid conductors are divided into three main types:
- semiconductors;
- dielectrics;
- conductors.
Definition 1
Electrolytic dissociation is the process of decomposition of molecules of a certain solution into negative and positive charged ions.
It can be established that an electric current in liquids can occur after a change in composition and chemical properties liquids used. This completely contradicts the theory of the propagation of electric current by other means when using a conventional metal conductor.
Faraday's experiments and electrolysis
The flow of electric current in liquids is a product of the process of movement of charged ions. Problems associated with the occurrence and propagation of electric current in liquids became the reason for the study of the famous scientist Michael Faraday. With the help of numerous practical research was able to find evidence that the mass of substance released during electrolysis depends on the amount of time and electricity. In this case, the time during which the experiments were carried out matters.
The scientist was also able to find out that in the process of electrolysis, when releasing a certain amount of a substance, the same amount is required electric charges. This quantity was accurately established and recorded in constant value, which is called the Faraday number.
In liquids, electric current has different propagation conditions. It interacts with water molecules. They significantly impede all movement of ions, which was not observed in experiments using a conventional metal conductor. It follows from this that the generation of current at electrolytic reactions won't be that big. However, as the temperature of the solution increases, the conductivity gradually increases. This means that the voltage of the electric current is increasing. Also, during the electrolysis process, it was noticed that the probability of a certain molecule breaking up into negative or positive ion charges increases due to large number molecules of the substance or solvent used. When the solution is saturated with ions above a certain norm, the reverse process occurs. The conductivity of the solution begins to decrease again.
Currently, the electrolysis process has found its application in many fields and areas of science and production. Industrial enterprises it is used in the production or processing of metal. Electrochemical reactions are involved in:
- electrolysis of salts;
- electroplating;
- surface polishing;
- other redox processes.
Electric current in vacuum and liquids
The propagation of electric current in liquids and other media is quite difficult process, which has own characteristics, features and properties. The fact is that in such media there are completely no charges in bodies, which is why they are usually called dielectrics. The main goal of the research was to create conditions under which atoms and molecules could begin to move and the process of generating electric current began. For this, it is customary to use special mechanisms or devices. The main element of such modular devices are conductors in the form of metal plates.
To determine the main current parameters it is necessary to use well-known theories and formulas. The most common is Ohm's law. It acts as a universal ampere characteristic, where the principle of dependence of current on voltage is implemented. Recall that voltage is measured in units of Amperes.
To conduct experiments with water and salt, it is necessary to prepare a vessel with salt water. This will give a practical and visual understanding of the processes that occur during the formation of electric current in liquids. The installation must also contain rectangular electrodes and power supplies. For full-scale preparation for experiments, you need to have an ampere installation. It will help conduct energy from the power supply to the electrodes.
They will act as guides metal plates. They are dipped into the liquid being used, and then the voltage is connected. The movement of particles begins immediately. It happens in a chaotic manner. Whenever magnetic field Between the conductors, all the processes of particle movement are ordered.
The ions begin to change charges and combine. Thus, cathodes become anodes, and anodes become cathodes. There are also several other important factors to consider in this process:
- level of dissociation;
- temperature;
- electrical resistance;
- use of alternating or direct current.
At the end of the experiment, a layer of salt forms on the plates.
Electric current in gases
Charge carriers: electrons, positive ions, negative ions.
Charge carriers appear in the gas as a result of ionization: due to irradiation of the gas, or collisions of heated gas particles with each other.
Electron impact ionization.
A_(fields)=eEl
e=1.6\cdot 10^(19)Cl ;
E - field direction;
l is the mean free path between two successive collisions of an electron with gas atoms.
A_(fields)=eEl\geq W - ionization condition
W is the ionization energy, i.e. energy required to remove an electron from an atom
The number of electrons increases in geometric progression, as a result, an electron avalanche occurs, and consequently a discharge in the gas.
Electric current in liquid
Liquids, just like solids, can be dielectrics, conductors and semiconductors. Dielectrics include distilled water, conductors include solutions of electrolytes: acids, alkalis, salts and molten metals. Liquid semiconductors are molten selenium and sulfide melts.
Electrolytic dissociation
When electrolytes dissolve under the influence of the electric field of polar water molecules, the electrolyte molecules disintegrate into ions. For example, CuSO_(4)\rightarrow Cu^(2+)+SO^(2-)_(4).
Along with dissociation, the reverse process occurs - recombination , i.e. combining ions of opposite signs into neutral molecules.
The carriers of electricity in electrolyte solutions are ions. This conductivity is called ionic .
Electrolysis
If electrodes are placed in a bath with an electrolyte solution and current is applied, then negative ions will move to the positive electrode, and positive ions to the negative.
At the anode (positive electrode), negatively charged ions give up extra electrons ( oxidation reaction), and at the cathode (negative electrode), positive ions receive the missing electrons (reduction reaction).
Definition. The process of releasing substances on electrodes associated with redox reactions is called electrolysis.
Faraday's laws
I. The mass of the substance that is released on the electrode is directly proportional to the charge flowing through the electrolyte:
m=kq
k is the electrochemical equivalent of the substance.
q=I\Delta t , then
m=kI\Delta t
k=\frac(1)(F)\frac(\mu)(n)
\frac(\mu)(n) - chemical equivalent of the substance;
\mu - molar mass;
n - valence
Electrochemical equivalents of substances are proportional to chemical ones.
F - Faraday's constant;
Liquids that are conductors include melts and solutions of electrolytes, i.e. salts, acids and alkalis.
When electrolytes are dissolved in water, their molecules disintegrate into ions - electrolytic dissociation. The degree of dissociation, i.e. The proportion of molecules in the dissolved substance that have broken up 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. When ions of different signs meet, they can again unite into neutral molecules. This process is called recombination. Under constant conditions, a dynamic equilibrium is established in the solution, in which the number of molecules that disintegrate into ions per second is equal to the number of pairs of ions that, at the same time, recombine into neutral molecules.
Thus, free charge carriers in conducting liquids are positive and negative ions. If electrodes connected to a current source are placed in the liquid, 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 pole - the anode. When connected to a current source, ions in the electrolyte solution begin to move negative ions towards the positive electrode (anode), and positive ions, respectively, towards the negative electrode (cathode). That is, an electric current will be established. Such conductivity in liquids is called ionic, since the charge carriers are ions.
When current passes through an electrolyte solution, a substance is released at the electrodes associated with redox reactions. At the anode, negatively charged ions give up their extra electrons (oxidation reaction), and at the cathode, positive ions take up 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 current strength, 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 =M/N a, where M is the molar mass of the substance, N a is Avogadro’s constant. The number of ions reaching the electrode is equal to N i =Dq/qi, where Dq is the charge that passed through the electrolyte during the time Dt (Dq=I*Dt), qi is the charge of the ion, which is determined by the valency of the atom (q i = n*e, where n – valency of the atom, e – elementary charge). When substituting these formulas, we obtain that m=M/(neN a)*IDt. If we denote by k (proportionality coefficient) =M/(neN a), then we have m=kIDt. This is a mathematical representation 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 period of time. The value k is called the electrochemical equivalent of a given substance, which is numerically equal to mass a substance released on the electrodes when transferred by ions of a charge equal to 1 C. [k]= 1 kg/Cl. k = M/(neN a) = 1/F*M/n, where F is Faraday’s constant. F=eN a =9.65*10 4 C/mol. The derived formula k=(1/F)*(M/n) is Faraday's second law.
Electrolysis is widely used in technology for various purposes, for example, covering the surface of one metal with a thin layer of another (nickel plating, chrome plating, copper plating, etc.). If you ensure good peeling of the electrolytic coating from the surface, you can obtain a copy of the surface topography. This process is called electroplating. Electrolysis is also used to purify metals from impurities, for example, thick sheets of crude copper obtained from ore are placed in a bath as an anode. During the electrolysis process, copper dissolves, impurities fall to the bottom, and pure copper settles on the cathode. Electronic circuit boards are also produced using electrolysis. A thin complex pattern of connecting wires is glued onto the dielectric, then the plate is placed in an electrolyte, where the areas of the copper layer that are not covered with paint are etched. After this, the paint is washed off and the details of the microcircuit appear on the board.