Galvanic elements. Galvanic cells are primary chemical current sources (CHS), which use irreversible processes converting chemical energy into electrical energy. They are widely used as DC power supplies for small-sized and portable radio equipment.
In parallel connection elements, the battery capacity is equal to the sum of the capacities of the elements included in it, and with serial connection– the smallest capacity of the element included in it.
Capacity element a is the amount of electricity given off by the element during discharge and determined in ampere-hours.
Manganese-zinc elements and mercury-zinc elements are widely used.
Batteries. Batteries, like galvanic cells, are devices for directly converting chemical energy into electrical energy. Unlike galvanic cells, batteries are able to restore their functionality by delivering electrical energy to receivers by charging them from an external source of electrical energy. Therefore, a battery is a reusable device that can accumulate and store electrical energy for some time. It is a secondary chemical source of current. A reserve of chemical energy is created in it during charging from an external source. When charging a battery, the materials included in its composition are transformed into a state in which they can enter into a chemical reaction with each other, releasing electrical energy. Thus, batteries accumulate electrical energy when they are charged and consume it when discharged.
Batteries are characterized by the following main parameters.
EMF of the battery E, which depends on the composition of the active mass of the plates, on the temperature and concentration (density) of the electrolyte. The battery EMF is measured with a voltmeter with a high input resistance (more than 1000 Ohm/V). Since the EMF of a charged and partially discharged battery can be the same, it is impossible to judge the degree of discharge of the battery by the value of the EMF.
Battery voltage– potential difference between the positive and negative plates when the load is on. Voltage when charging U Z = E + I Z r 0, and when discharging U P = E - I P r 0,
where I З, I Р – charge and discharge currents in A; r 0 – internal resistance of the battery, Ohm (it is determined by the design of the electrodes, the density of the electrolyte, the degree of discharge of the battery, and the ambient temperature).
The nominal capacity of a battery is the amount of electricity in Ah that it can supply with a ten-hour discharge mode, constant current and electrolyte temperature of +25 o C. The current value of the 10-hour discharge mode is equal to the quotient of the nominal capacity (C 10) divided by 10 .
Batteries are capable self-discharge, i.e. reduce its capacity when the load circuit is open. The intensity of self-discharge depends on the ambient temperature, electrolyte composition and electrode material.
Depending on the composition of the electrolyte, batteries are either acidic or alkaline.
Acid batteries. The housing (made of hard rubber or plastic) houses positive and negative electrodes mounted in blocks. The active mass of the positive plate is lead dioxide (PbO 2), and the negative one is lead (Pb). The electrolyte is an aqueous solution of sulfuric acid. The nominal voltage of an acid battery is 2.0 V. When charging, the voltage is increased to 2.6 - 2.8 V. At the beginning of the discharge, the voltage quickly drops to 2.2 V. It should be remembered that it is impossible to discharge an acid battery below 1.8 V, since in this case a difficultly soluble substance is formed on the negative plates white coating(battery sulfation occurs). To protect the battery from sulfation, it is recommended to charge it every 30 days, regardless of the remaining capacity.
Disadvantages of acid batteries: they are difficult to maintain and have low durability, increased sensitivity to short circuits and overloads, they cannot be placed inside the electronic control unit (evaporations spoil the parts).
The industry produces SK type acid batteries with a nominal capacity from 36 to 5328 Ah, for example SK-148 (if this number 148 is multiplied by 36, you get a nominal capacity of 5328 Ah).
Alkaline batteries. They are easy to maintain, can be charged faster (4–7 hours instead of 10–12 hours for acid ones), and can be placed inside the REU without harm to them. The most commonly used alkaline batteries are nickel-cadmium (NC), nickel-iron (NI) and silver-zinc (SC). An aqueous solution of caustic potassium is used as an electrolyte.
For alkaline batteries, the emf is 1.5 V (in a discharged battery E = 1.3 V). The average density of the electrolyte in alkaline batteries is approximately constant during charging and discharging. Therefore, their condition is characterized mainly by the EMF value.
Alkaline batteries are produced by the factory without electrolyte. When preparing the electrolyte, special care must be taken, since mixing potassium hydroxide with water releases a large amount of heat. Solid alkali is broken into small pieces, while covering it with material so that the fragments do not get into the eyes or skin. The alkali is dipped into the water in pieces, continuously stirring the solution with a glass or steel rod.
Prerequisites for the emergence of galvanic cells. A little history. In 1786, the Italian professor of medicine, physiologist Luigi Aloisio Galvani discovered an interesting phenomenon: the muscles of the hind legs of a freshly opened frog corpse, suspended on copper hooks, contracted when the scientist touched them with a steel scalpel. Galvani immediately concluded that this was a manifestation of “animal electricity.”
After Galvani's death, his contemporary Alessandro Volta, being a chemist and physicist, would describe and publicly demonstrate a more realistic mechanism for the generation of electric current when different metals come into contact.
Volta, after a series of experiments, will come to the unequivocal conclusion that current appears in the circuit due to the presence in it of two conductors of different metals placed in a liquid, and this is not “animal electricity” at all, as Galvani thought. The twitching of the frog's legs was a consequence of the action of current generated by the contact of different metals (copper hooks and a steel scalpel).
Volta will show the same phenomena that Galvani demonstrated on a dead frog, but on a completely inanimate homemade electrometer, and will give in 1800 a precise explanation for the occurrence of current: “a conductor of the second class (liquid) is in the middle and is in contact with two conductors of the first class from two different metals... As a result, an electric current arises in one direction or another.”
In one of his first experiments, Volta dipped two plates - zinc and copper - into a jar of acid and connected them with wire. After this, the zinc plate began to dissolve, and gas bubbles appeared on the copper steel. Volta suggested and proved that an electric current flows through a wire.
This is how the “Volta element” was invented - the first galvanic cell. For convenience, Volta gave it the shape of a vertical cylinder (column), consisting of interconnected rings of zinc, copper and cloth, soaked in acid. A voltaic column half a meter high created a voltage that was sensitive to humans.
Since the research was started by Luigi Galvani, the name retained the memory of him in its name.
Galvanic cell is a chemical source of electric current based on the interaction of two metals and/or their oxides in an electrolyte, leading to the appearance of electric current in a closed circuit. Thus, in galvanic cells, chemical energy is converted into electrical energy.
Galvanic cells today
Galvanic cells today are called batteries. Three types of batteries are widely used: salt (dry), alkaline (they are also called alkaline, “alkaline” translated from English as “alkaline”) and lithium. The principle of their operation is the same as described by Volta in 1800: two metals, and an electric current arises in an external closed circuit.
The voltage of the battery depends both on the metals used and on the number of elements in the “battery”. Batteries, unlike accumulators, are not capable of restoring their properties, since they directly convert chemical energy, that is, the energy of the reagents that make up the battery (reducing agent and oxidizing agent), into electrical energy.
The reagents included in the battery are consumed during its operation, and the current gradually decreases, so the effect of the source ends after the reagents have reacted completely.
Alkaline and salt cells (batteries) are widely used to power a variety of electronic devices, radio equipment, toys, and lithium ones can most often be found in portable medical devices such as glucometers or in digital equipment such as cameras.
Manganese-zinc cells, which are called salt batteries, are “dry” galvanic cells that do not contain a liquid electrolyte solution.
The zinc electrode (+) is a glass-shaped cathode, and the anode is a powdered mixture of manganese dioxide and graphite. Current flows through the graphite rod. The electrolyte is a paste of ammonium chloride solution with the addition of starch or flour to thicken it so that nothing flows.
Typically, battery manufacturers do not indicate the exact composition of salt cells, however, salt batteries are the cheapest, they are usually used in devices where power consumption is extremely low: in watches, in remote controls remote control, in electronic thermometers, etc.
The concept of “nominal capacity” is rarely used to characterize zinc-manganese batteries, since their capacity greatly depends on operating modes and conditions. The main disadvantages of these elements are the significant rate of voltage decrease throughout the discharge and a significant decrease in the delivered capacity with increasing discharge current. The final discharge voltage is set depending on the load in the range of 0.7-1.0 V.
Not only the magnitude of the discharge current is important, but also the time schedule of the load. With intermittent discharge at high and medium currents, the performance of the batteries increases noticeably compared to continuous operation. However, at low discharge currents and months-long breaks in operation, their capacity may decrease as a result of self-discharge.
The graph above shows the discharge curves for an average salt battery for 4, 10, 20 and 40 hours for comparison with the alkaline one, which will be discussed later.
An alkaline battery is a manganese-zinc voltaic battery that uses manganese dioxide as the cathode, powdered zinc as the anode, and an alkali solution, usually in the form of potassium hydroxide paste, as the electrolyte.
These batteries have a number of advantages (in particular, significantly higher capacity, best job at low temperatures and at high load currents).
Alkaline batteries, compared to salt batteries, can provide more current for a longer period of time. A higher current becomes possible because zinc is used here not in the form of a glass, but in the form of a powder that has a larger area of contact with the electrolyte. Potassium hydroxide in the form of a paste is used as an electrolyte.
It is thanks to the ability of this type of galvanic cells to deliver significant current (up to 1 A) for a long time that alkaline batteries are most common today.
Electric toys, portable medical equipment, electronic devices, and cameras all use alkaline batteries. They last 1.5 times longer than salt ones if the discharge is low current. The graph shows the discharge curves at various currents for comparison with a salt battery (the graph was shown above) for 4, 10, 20 and 40 hours.
Lithium batteries
Another fairly common type of voltaic cell is lithium batteries - single non-rechargeable voltaic cells that use lithium or its compounds as the anode. Thanks to the use alkali metal they have a high potential difference.
The cathode and electrolyte of a lithium cell can be very different, so the term "lithium cell" combines a group of cells with the same anode material. For example, manganese dioxide, carbon monofluoride, pyrite, thionyl chloride, etc. can be used as a cathode.
Lithium batteries differ from other batteries in their long service life and high cost. Depending on the size chosen and the chemistries used, a lithium battery can produce voltages from 1.5 V (compatible with alkaline batteries) to 3.7 V.
These batteries have highest capacity per unit weight and long storage time. Lithium cells widely used in modern portable electronic equipment: to power clocks on computer motherboards, to power portable medical devices, wristwatch, calculators, photographic equipment, etc.
The graph above shows the discharge curves for two lithium batteries from two popular manufacturers. The initial current was 120 mA (per resistor of about 24 Ohms).
If missing electrical network, then to power electrical appliances, galvanic cells and batteries, otherwise called chemical current sources, are used. Let's consider the principle of their operation using the example of the first simplest element - the Volta element (Fig. 1). It consists of copper (Cu) and zinc (Zn) plates immersed in a solution of sulfuric acid (H2SO4). Due to the chemical reaction that occurs between zinc and sulfuric acid, excess electrons are produced on the zinc. Zinc is negatively charged and is the negative pole. The solution and the copper plate immersed in it become positively charged. As a result, an emf equal to approximately one volt is excited, which persists as long as the circuit is not closed.
If you close the circuit, a current will flow and hydrogen will begin to be intensely released inside the element, covering the surface of the plates with a layer of bubbles. This layer reduces the voltage at the poles of the element. This phenomenon is called polarization. The higher the current, the stronger the polarization and the faster the element voltage decreases.
Fig.1. The simplest Volta galvanic cell.
To eliminate polarization, substances capable of absorbing hydrogen and called depolarizers are introduced into the element. In order for the voltage at the poles to remain constant, the depolarizer must quickly absorb the hydrogen generated during operation of the element. By absorbing hydrogen, the depolarizer gradually becomes unusable. But usually before this, the electrolyte deteriorates and the zinc is corroded by the action of the electrolyte. At all electrical energy obtained in the cell due to the consumption of zinc, electrolyte and depolarizer; therefore, each element has a certain reserve of energy and can work only for a limited time.
The operation of galvanic cells is explained using theory electrolytic dissociation, according to which the molecules of a substance dissolved in water disintegrate (dissociate) into ions. This phenomenon is typical for all electrolytes, which are solutions of acids, alkalis and salts. In the Volta cell, a sulfuric acid molecule (H2SO4) in an aqueous solution breaks down into a negative acidic ion (SO4) and a positive hydrogen ion (H2), as shown in Fig. 2.
The chemical reaction between zinc and sulfuric acid is that the positive zinc ions go into solution, being attracted to the negative ions of the electrolyte. In this case, the zinc electrode itself becomes negatively charged. A potential difference arises between it and the electrolyte, and therefore electric field, which prevents the further transition of positive zinc ions into the solution. Therefore, a certain equilibrium is created with a certain potential difference between the zinc and the solution. For other metals and solutions, the value of the potential difference will be different.
To use the resulting potential difference, a second electrode made of a different metal is placed in the electrolyte. If the second electrode is zinc, then between it and the solutions there will be the same potential difference as that of the first electrode, but it will act oppositely, and the resulting potential difference between the electrodes will be zero. The cells usually have a negative electrode of zinc and a positive electrode of usually copper or carbon.
If you connect the electrodes of an element with a conductor, i.e., create a closed circuit, then under the influence of a potential difference, electrons will move from the zinc along the external circuit. Since they leave the zinc electrode, its negative potential begins to decrease and the electric field between it and the solution weakens. But then new positive zinc ions go into solution. This maintains a certain negative potential of the zinc electrode.
Fig.2. Ions in the electrolyte of the Volta cell.
When the cell operates, zinc is continuously dissolved in the electrolyte, which gradually turns into a solution of zinc sulfate (ZnSO4). Positive zinc ions, constantly passing into the electrolyte, attract negative ions of the acid residue. These ions in the electrolyte move in the direction from the copper plate to the zinc plate. But positive hydrogen ions are repelled by positive zinc ions and move in the opposite direction, that is, from zinc to copper. Thus, if in an external circuit the current represents the movement of electrons (as always in metal conductors), then in the electrolyte the current represents the movement of positive and negative ions in opposite directions. Hydrogen ions approach the copper plate and take electrons from it, turning into neutral atoms. As a result, a certain positive potential is maintained on the copper plate, despite the fact that electrons arrive at it from the external circuit. However, the copper plate is gradually covered with a layer of hydrogen. Between this layer and the electrolyte a potential difference arises, acting towards the main potential difference existing between the electrodes. The occurrence of such a counter-electromotive force is called polarization of the element. Due to polarization, the resulting potential difference decreases and the effect of the element deteriorates.
Galvanic cells are characterized by different parameters and, above all, electromotive force, internal resistance, maximum permissible discharge current and capacity.
The electromotive force is determined by the type of element, that is, the material of its electrodes, the substance of the electrolyte and the depolarizer. It is completely independent of the size of the element (the size of its electrodes), the amount of electrolyte and the amount of depolarizer.
The internal resistance of an element depends not only on its type, but also on its size, as well as how long the element has been in operation. The larger the element, the lower its internal resistance. As the element operates, the internal resistance increases. It increases especially sharply in depleted elements. The internal resistance of elements at the beginning of their operation usually ranges from a few ohms to tenths of an ohm. When an element is connected to a closed circuit, the voltage at its terminals is always slightly less than the EMF and decreases with increasing current, since the loss of part of the EMF at the internal resistance of the element increases. Sometimes the voltage for elements is indicated at the maximum discharge current at the beginning of the element’s operation (initial voltage).
Each element can be discharged with current to a certain value. Excessive current will cause accelerated polarization and the voltage will quickly become unacceptably low. A similar phenomenon, but to an even greater extent, occurs when the element is short-circuited. For most elements, the maximum permissible discharge current is a fraction of an ampere. The larger the element, the greater this current. Excessive current also leads to rapid depletion of the element.
The capacity of an element is the amount of electricity that it is capable of delivering when discharged with a current not exceeding the maximum permissible. Typically, the capacity of cells is measured in ampere-hours (ah), that is, the product of the discharge current in amperes and the number of operating hours of the cell. An element is considered discharged if its voltage has decreased by approximately 50% compared to its original value.
The operating time of a cell can be determined by dividing the capacity in ampere hours by the discharge current in amperes. In this case, the current should not exceed the maximum permissible value.
The cell capacity depends on the amount of zinc, electrolyte and depolarizer. The larger the size of the element, the greater the amount of substances included in its composition and the greater the capacity. In addition, the capacity depends on the discharge current, as well as on interruptions during discharge and their duration. The normal cell capacity corresponds to the maximum permissible discharge current during continuous discharge. If the current is less than the maximum and if the discharge occurs intermittently, then the capacity increases, and if the current exceeds the maximum, the capacity decreases, since part of the depolarizer does not participate in the reactions. Capacity also decreases with decreasing temperature. Therefore, the calculation of the operating time of an element based on its nominal capacity and discharge current is approximate.
2. MANGANESE – ZINC
AND OXIDE-MERCURY ELEMENTS.
Widespread We obtained manganese-zinc (MC) dry elements with a depolarizer of manganese dioxide.
A cup-type dry cell (Fig. 3) has a rectangular or cylindrical zinc vessel, which is a negative electrode. A positive electrode in the form of carbon is placed inside it.
sticks or plates, which are located in a bag filled with a mixture of manganese dioxide with coal or graphite powder. Carbon or graphite is added to reduce resistance. The carbon rod and the bag with the depolarizing mass are called an agglomerate. A paste composed of ammonia (NH4Cl), starch and some other substances is used as an electrolyte. For cup elements, the central terminal is the positive pole.
The operating voltage of a dry cell is slightly lower than its emf, equal to 1.5 V, and is approximately 1.3 or 1.4 V. With prolonged discharge, the voltage gradually decreases, since the depolarizer does not have time to absorb all the hydrogen released, and by the end discharge rate it reaches 0.7 V.
Fig.3. Dry element device.
Another design of a dry element, the so-called biscuits type, is shown in Fig. 4. In it, the positive electrode is a depolarizing mass (there is no carbon electrode). Biscuit elements have significantly best characteristics than cup ones.
Rice. 4. Construction of a dry biscuit element.
1 – depolarizer – positive electrode; 2 – zinc – negative electrode; 3 – paper;
4 – cardboard impregnated with electrolyte; 5 – polyvinyl chloride film.
In each element that has an electrolyte, even when the external circuit is open, a so-called self-discharge occurs, as a result of which the zinc electrode is corroded, and the electrolyte and depolarizer are depleted. Therefore, during storage, a dry cell gradually becomes unusable and its electrolyte dries out.
When the dry elements are completely discharged, their agglomerates are still operational and can be used to construct homemade self-filling elements. Such elements have an agglomerate and an electrode made of zinc sheet in a solution of ammonia, located in a glass or ceramic or plastic cup. In the absence of ammonia, you can use a solution of ordinary table salt with a small addition of sugar with slightly worse results. In addition to dry elements of the MC type, elements with manganese-air depolarization (MAD) are widely used. They are designed similarly to MC elements, but their positive electrode is made in such a way that outside atmospheric air enters the manganese dioxide through special channels. Air oxygen compensates for the loss of oxygen by manganese dioxide during depolarization. Therefore, depolarization can occur much longer and the cell capacity increases.
Physico-chemical processes in elements with manganese dioxide occur as follows. Ammonia, that is, ammonium chloride (NH4Cl), in an aqueous solution forms positive ammonium ions (NH4) and negative chlorine ions (Cl). Positive zinc ions go into solution and zinc acquires a negative potential. When the circuit is closed, when electrons in the external circuit move in the direction from zinc to coal, zinc dissolves all the time. Its ions pass into the electrolyte, due to which the negative potential of zinc is maintained. Zinc ions combine with chlorine ions to form a solution of zinc chloride (ZnCl2). At the same time, NH4 ions move towards the carbon electrode, take electrons from it and break down into ammonia (NH3) and hydrogen. This happens according to the equation
2NH4 = 2NH3 + H2.
The released hydrogen combines with a depolarizer, that is, manganese dioxide, forming manganese oxide and water:
H2 + MnO2 = MnO2 + H2O.
In recent years, dry sealed MC cells with an alkaline electrolyte (KOH) have been produced. They are cylindrical, disk and biscuit, their capacity is three to five times greater than that of cells with ammonia electrolyte. In addition, they allow several cycles of recharging with current with a return of 10% of the capacity. For such elements, the central electrode is zinc and is a minus, that is, the polarity of the terminals is opposite to the polarity of the terminals of conventional MC elements. Cells with alkaline electrolyte are used for long-term operation, for example, in electronic watches. In the designations of such elements, the letter A is placed in front.
All cells have an initial voltage of approximately 1.3 - 1.5 V, and a final voltage of 0.7 - 1 V. Dry cells or batteries should not be stored for longer than the period indicated on them before use; otherwise, continued functionality is not guaranteed. However, when stored for specified period there is a slight decrease in capacity, but not more than one third.
Recently, small-sized oxide-mercury (mercury-zinc) sealed elements with more high quality, rather than elements of the MC type. The structure of oxide-mercury elements is shown in Fig. 5. The element has a steel body consisting of two halves, separated from each other by a sealing insulating rubber gasket.
An active mass of mercury oxide (HgO) with graphite, which is a positive electrode, is pressed into one half of the body. The negative electrode is zinc powder pressed into the other half of the housing. Alkaline electrolyte (KOH) impregnates the porous spacer that separates the electrodes. These items are produced different sizes and different capacities (from tenths of an ampere hour to several ampere-hours)
hours). Their EMF is approximately 1.35 V. The shelf life of these elements is 2.5 years. Self-discharge does not exceed 1% per year. Compared to MC elements, mercury-
Rice. 5. Construction of a sealed mercury oxide element;
1 – steel body with a positive electrode; 2 – porous gasket; 3 – rubber sealing gasket; 4 – housing cover with a negative electrode.
but - zinc elements have a higher capacity, lower internal resistance, but a higher cost. They are widely used in electronic watches, pacemakers, photo exposure meters, and measuring instruments. The smallest elements have dimensions of only a few millimeters and a mass of tenths of a gram.
Important feature mercury oxide elements is voltage stability during discharge. Only at the very end of the discharge does the voltage drop sharply to zero.
3. CONNECTING ELEMENTS INTO BATTERIES.
It was said above that the EMF of an ordinary chemical element approximately equal to 1.5 V. To increase the EMF, a battery with a series connection of elements is used. In this case, the “+” of one element is connected to the “–” of the other, etc. The “minus” of the first and the “plus” of the last are the poles of the entire battery (Fig. 6.).
When connecting elements in series, the EMF increases as many times as the number of elements connected.
Fig.6. Serial and parallel connection of elements in a battery.
Less common is a parallel connection of cells, in which the positive poles of all the cells are connected together to form the positive pole of the battery, and the negative pole of the battery is obtained by connecting the negative poles of the cells (Fig. 6). When connecting elements in parallel, the battery's emf does not increase, but the capacity and maximum discharge current increase. Therefore, a parallel connection is used when it is necessary to obtain a higher discharge current and greater capacity than that of one element.
Much more often they resort to a mixed connection, in which both the EMF, the capacitance, and the maximum discharge current increase. In this case, several groups of elements are usually connected in parallel, and in each group as many elements as necessary are connected in series to obtain the required EMF.
Rice. 7. Mixed connection of elements into a battery.
The number of parallel groups is determined by the required maximum discharge current (Fig. 7). In general, it is advisable to construct batteries from series-connected cells with sufficient discharge current. And only in the case when it is necessary to obtain higher current or increased capacity, they resort to a mixed connection. The inclusion of additional elements according to the principle of mixed connection is also used to increase the voltage if the elements are heavily discharged.
When the battery is inactive, parallel groups of elements must be disconnected from each other, since due to even a slight difference in the EMF, one group can discharge to another.
Ministry of Education and Science of the Russian Federation
National Research Nuclear University "MEPhI"
Balakovo Engineering and Technology Institute
GALVANIC CELLS
Guidelines
on the course "Chemistry"
all forms of education
Balakovo 2014
Purpose of the work: to study the principle of operation of galvanic cells.
BASIC CONCEPTS
ELECTROCHEMICAL PROCESSES AT THE PHASE BOUNDARY
Atomic ions are located at the sites of metal crystal lattices. When a metal is immersed in a solution, a complex interaction of surface metal ions with polar solvent molecules begins. As a result, the metal is oxidized, and its hydrated (solvated) ions go into solution, leaving electrons in the metal:
Me + mH 2 O Me(H 2 O) 
+ ne -
The metal is charged negatively, and the solution is charged positively. Electrostatic attraction arises between those who have turned into liquid by hydrated cations and the metal surface and at the metal-solution interface a double electrical layer is formed, characterized by a certain potential difference - electrode potential.
Rice. 1 Electric double layer at the metal-solution interface
Along with this reaction, a reverse reaction occurs - the reduction of metal ions to atoms.
Me(H2O) 
+ ne 
Me + m H 2 O -
At a certain value of the electrode potential, equilibrium is established:
Me + mH 2 O 
Me(H2O) 
+ ne -
For simplicity, water is not included in the reaction equation:
Meh 
Me 2+ +ne -
The potential established under conditions of equilibrium of the electrode reaction is called the equilibrium electrode potential.
GALVANIC CELLS
Galvanic cells– chemical sources of electrical energy. They are systems consisting of two electrodes (conductors of the first kind) immersed in solutions of electrolytes (conductors of the second type).
Electrical energy in galvanic cells is obtained through the redox process, provided that the oxidation reaction is carried out separately on one electrode and the reduction reaction on the other. For example, when zinc is immersed in a copper sulfate solution, the zinc is oxidized and the copper is reduced
Zn + CuSO 4 = Cu + ZnSO 4
Zn 0 +Cu 2+ =Cu 0 +Zn 2+
It is possible to carry out this reaction so that the oxidation and reduction processes are spatially separated; then the transition of electrons from the reducing agent to the oxidizing agent will not occur directly, but through an electrical circuit. In Fig. Figure 2 shows a diagram of a Daniel-Jacobi galvanic cell; the electrodes are immersed in salt solutions and are in a state of electrical equilibrium with the solutions. Zinc, as a more active metal, sends more ions into the solution than copper, as a result of which the zinc electrode, due to the electrons remaining on it, is charged more negatively than the copper one. The solutions are separated by a partition that is permeable only to ions in an electric field. If the electrodes are connected to each other with a conductor (copper wire), then electrons from the zinc electrode, where there are more of them, will flow through the external circuit to the copper one. A continuous flow of electrons appears - an electric current. As a result of the departure of electrons from the zinc electrode, Zn begins to pass into solution in the form of ions, replenishing the loss of electrons and thereby trying to restore equilibrium.
The electrode at which oxidation occurs is called the anode. The electrode at which reduction occurs is called the cathode.
Anode (-) Cathode (+)
Rice. 2. Diagram of a galvanic cell
When a copper-zinc element operates, the following processes occur:
1) anodic – zinc oxidation process Zn 0 – 2e→Zn 2+;
2) cathodic – the process of reduction of copper ions Cu 2+ + 2e→Cu 0 ;
3) movement of electrons along the external circuit;
4) movement of ions in solution.
In the left glass there is a lack of SO 4 2- anions, and in the right glass there is an excess. Therefore, in the internal circuit of a working galvanic cell, the movement of SO 4 2- ions from the right glass to the left through the membrane is observed.
Summing up the electrode reactions, we get:
Zn + Cu 2+ = Cu + Zn 2+
Reactions take place at the electrodes:
Zn+SO 4 2- →Zn 2+ +SO 4 2- + 2e(anode)
Cu 2+ + 2e + SO 4 2- → Cu + SO 4 2- (cathode)
Zn + CuSO 4 → Cu + ZnSO 4 (total reaction)
Galvanic cell diagram: (-) Zn/ZnSO 4 | |CuSO 4 /Cu(+)
or in ionic form: (-) Zn/Zn 2+ | |Cu 2+ /Cu(+), where a vertical line denotes the interface between the metal and the solution, and two lines indicate the interface between two liquid phases - a porous partition (or a connecting tube filled with an electrolyte solution).
Maximum electrical work (W) when converting one mole of a substance:
W=nF 
E, (1)
where ∆E is the emf of the galvanic cell;
F - Faraday number equal to 96500 C;
n is the charge of the metal ion.
The electromotive force of a galvanic cell can be calculated as the potential difference between the electrodes that make up the galvanic cell:
EMF = E oxide. – E restore = E k – E a,
where EMF is electromotive force;
E oxid. – electrode potential of the less active metal;
E restore - electrode potential of the more active metal.
STANDARD ELECTRODE POTENTIALS OF METALS
It is impossible to directly determine the absolute values of the electrode potentials of metals, but the difference in electrode potentials can be determined. To do this, find the potential difference between the electrode being measured and the electrode whose potential is known. Most often, a hydrogen electrode is used as a reference electrode. Therefore, the EMF of a galvanic cell composed of the test and standard hydrogen electrode is measured, the electrode potential of which is taken equal to zero. The circuits of galvanic cells for measuring metal potential are as follows:
H 2, Pt|H + || Me n + |Me
Since the potential of the hydrogen electrode is conditionally equal to zero, the emf of the measured element will be equal to the electrode potential of the metal.
Standard electrode potential of the metal is called its electrode potential, which occurs when a metal is immersed in a solution of its own ion with a concentration (or activity) equal to 1 mol/l, under standard conditions, measured in comparison with a standard hydrogen electrode, the potential of which at 25 0 C is conventionally assumed to be zero. By arranging metals in a row as their standard electrode potentials (E°) increase, we obtain the so-called voltage series.
The more negative the potential of the Me/Me n+ system, the more active the metal.
Electrode potential of a metal immersed in a solution of its own salt at room temperature, depends on the concentration of ions of the same name and is determined by the Nernst formula:
,
(2)
where E 0 – normal (standard) potential, V;
R – universal gas constant equal to 8.31 J (mol. K);
F – Faraday number;
T - absolute temperature, K;
C is the concentration of metal ions in solution, mol/l.
Substituting the values of R, F, standard temperature T = 298 0 K and the conversion factor from natural logarithms (2.303) to decimal, we obtain a formula convenient for use:
(3)
CONCENTRATION GALVANIC CELLS
Galvanic cells can be composed of two electrodes of exactly the same nature, immersed in solutions of the same electrolyte, but of different concentrations. Such elements are called concentration elements, for example:
(-)Ag | AgNO 3 || AgNO 3 | Ag(+)
In concentration circuits for both electrodes, the values of n and E 0 are the same, therefore, to calculate the EMF of such an element, you can use
, (4)
where C 1 is the electrolyte concentration in a more dilute solution;
C 2 - electrolyte concentration in a more concentrated solution
POLARIZATION OF ELECTRODES
Equilibrium electrode potentials can be determined in the absence of current in the circuit. Polarization- change in electrode potential when an electric current passes.
E = E i - E p , (5)
where E is polarization;
E i is the potential of the electrode during the passage of electric current;
E p - equilibrium potential. Polarization can be cathodic E K (at the cathode) and anodic E A (at the anode).
Polarization can be: 1) electrochemical; 2) chemical.
OCCUPATIONAL SAFETY REQUIREMENTS
1. Experiments with unpleasant-smelling and toxic substances must be carried out in a fume hood.
2. When recognizing the gas being released by smell, you should direct the stream with hand movements from the vessel towards yourself.
3. When performing the experiment, you must ensure that the reagents do not get on your face, clothes, or a person standing next to you.
4. When heating liquids, especially acids and alkalis, hold the test tube with the opening away from you.
5. When diluting sulfuric acid, you should not add water to the acid; you should pour the acid carefully, in small portions into cold water, stirring the solution.
6. After finishing work, wash your hands thoroughly.
7. It is recommended to pour spent solutions of acids and alkalis into specially prepared containers.
8. All bottles with reagents must be closed with appropriate stoppers.
9. Reagents remaining after work should not be poured out or poured into reagent bottles (to avoid contamination).
Work order
Task 1
RESEARCH OF METALS ACTIVITY
Instruments and reagents: zinc, granulated; copper sulfate CuSO 4, 0.1 N solution; test tubes
Dip a piece of granulated zinc into a 0.1 N solution of copper sulfate. Leave it standing quietly on the tripod and watch what happens. Write an equation for the reaction. Conclude which metal can be taken as an anode and which as a cathode for the next experiment.
Task 2
GALVANIC CELL
Instruments and reagents: Zn, Cu – metals; zinc sulfate, ZnSO 4, 1 M solution; copper sulfate CuSO 4, 1 M solution; potassium chloride KCl, concentrated solution; galvanometer; glasses; U-shaped tube, cotton wool.
Pour up to ¾ of the volume of a 1 M metal salt solution, which is the anode, into one glass, and the same volume of a 1 M metal salt solution, which is the cathode, into the other glass. Fill the U-shaped tube with concentrated KCl solution. Cover the ends of the tube with thick pieces of cotton wool and lower them into both glasses so that they are immersed in the prepared solutions. Place a metal-anode plate in one glass, and a metal-cathode plate in another; mount a galvanic cell with a galvanometer. Close the circuit and mark the direction of the current using a galvanometer.
Draw a diagram of a galvanic cell.
Write electronic equations for the reactions occurring at the anode and cathode of this galvanic cell. Calculate the EMF.
Task 3
DETERMINING AN ANODE FROM A SPECIFIED SET OF PLATES
Instruments and reagents: Zn, Cu, Fe, Al – metals; zinc sulfate, ZnSO 4, 1 M solution; copper sulfate CuSO 4, 1 M solution; aluminum sulfate Al 2 (SO 4) 3 1 M solution; iron sulfateFeSO 4, 1 M solution; potassium chloride KCl, concentrated solution; glasses; U-shaped tube, cotton wool.
Make up galvanic pairs:
Zn/ZnSO 4 ||FeSO 4 /Fe
Zn/ZnSO 4 || CuSO4/Cu
Al/Al 2 (SO 4) 3 || ZnSO4/Zn
From the indicated set of plates and solutions of salts of these metals, assemble a galvanic cell in which zinc would be the cathode (task 2).
Write electronic equations for the reactions occurring at the anode and cathode of the assembled galvanic cell.
Write the redox reaction that underlies the operation of this galvanic cell. Calculate the EMF.
FORMULATION OF THE REPORT
The laboratory journal is filled out during laboratory classes as work is completed and contains:
date of completion of the work;
Name laboratory work and her number;
the name of the experiment and the purpose of its implementation;
observations, reaction equations, device diagram;
test questions and tasks on the topic.
CONTROL TASKS
1.Which of the following reactions are possible? Write reaction equations in molecular form and create electronic equations for them:
Zn(NO 3) 2 + Cu →
Zn(NO 3) 2 + Mg →
2. Draw up diagrams of galvanic cells to determine the normal electrode potentials of Al/Al 3+ , Cu/Cu 2+ paired with a normal hydrogen electrode.
3. Calculate the emf of the galvanic cell
Zn/ZnSO 4 (1M)| |CuSO 4 (2M)
What chemical processes occur during the operation of this element?
4. Chemically pure zinc almost does not react with hydrochloric acid. When lead nitrate is added to acid, partial evolution of hydrogen occurs. Explain these phenomena. Write down equations for the reactions that occur.
5. Copper is in contact with nickel and immersed in a dilute solution of sulfuric acid, what process occurs at the anode?
6. Draw up a diagram of a galvanic cell, which is based on a reaction proceeding according to the equation: Ni+Pb(NO 3) 2 =Ni(NO 3) 2 +Pb
7. A manganese electrode in a solution of its salt has a potential of 1.2313 V. Calculate the concentration of Mn 2+ ions in mol/l.
Time allotted for laboratory work
Literature
Main
1. Glinka. N.A. General chemistry: textbook. manual for universities. – M.: Integral – Press, 2005. – 728 p.
2. Korzhukov N. G. General and inorganic chemistry. – M.: MISIS;
INFRA-M, 2004. – 512 p.
Additional
3. Frolov V.V. Chemistry: textbook. allowance for colleges. – M.: Higher. school, 2002. –
4. Korovin N.V.. General chemistry: a textbook for engineering. direction and special universities – M.: Higher. school, 2002.–559 p.: ill..
4. Akhmatov N.S. General and inorganic chemistry: a textbook for universities. - 4th ed., corrected - M.: Higher. school, 2002. –743 p.
5. Glinka N.A. General chemistry assignments and exercises. – M.: Integral –Press, 2001. – 240 p.
6. Metelsky A.V. Chemistry in questions and answers: a reference book. – Mn.: Bel.En., 2003. – 544 p.
galvanic cells
Guidelines
to perform laboratory work
on the course "Chemistry"
for students of technical fields and specialties,
"General and inorganic chemistry"
for students of the direction "Chemical Technology"
all forms of education
Compiled by: Sinitsyna Irina Nikolaevna
Timoshina Nina Mikhailovna
Galvanic cells and batteries A galvanic element, or galvanic couple, is a device consisting of two metal plates (one of which can be replaced by coke plates), immersed in one or two different liquids, and serving as a source of galvanic current. A certain number of voltaic elements connected to each other in a known manner constitute a galvanic battery. The simplest element in terms of structure consists of two plates, immersed in a clay or glass glass, in which a liquid corresponding to the type of plate is poured; the plates should not have metallic contact in the liquid. D. elements are called primary if they are independent sources of current, and secondary, if they become effective only after a more or less prolonged exposure to sources of electricity that charge them. When considering the origin of voltaic elements, one must begin with the voltaic column, the ancestor of all subsequent galvanic batteries, or with the Voltaic cup battery. Voltage column. To compose it, Volta took pairs of dissimilar metal circles, folded or even soldered at the base, and cardboard or cloth circles moistened with water or a solution of caustic potassium. Initially, silver and copper mugs were used, and then usually zinc and copper. A pillar was made from them, as shown in the diagram. 1, namely: first, a copper plate is placed and a zinc plate is placed on it (or vice versa), on which a moistened cardboard circle is placed; this constituted one pair, on which was superimposed a second, again composed of copper, zinc and cardboard circles, superimposed on each other in the same order as in the first pair. Continuing to apply subsequent pairs in the same order, you can create a pillar; the pillar shown in the devil. 1, on the left, consists of 11 volt pairs. If a pole is installed on a plate of an insulating, i.e., non-conductive, substance, for example, glass, then, starting from the middle of it, one half of the column (the bottom in our drawing) will be charged with positive electricity, and the other (the top in the drawing) - negative. The intensity of electricity, imperceptible in the middle, increases as it approaches the ends, where it is greatest. Wires are soldered to the lowest and highest plates; bringing the free ends of the wires into contact gives rise to the movement of positive electricity from the lower end of the pole through the wire to the upper and the movement of negative electricity in the opposite direction; an electric, or galvanic, current is formed (see this word). Volta considered two plates of dissimilar metals to be a pair, and attributed to the liquid only the ability to conduct electricity (see Galvanism); but according to the view established later, the pair consists of two dissimilar plates and a liquid layer between them; therefore, the topmost and bottom plates of the pillar (Fig. 1 on the right) can be removed. Such a pillar will consist of 10 pairs, and then its lowermost plate will be copper, and its uppermost one will be zinc, and the direction of movement of electricity, or the direction of galvanic current, will remain the same: from the lower end of the pillar (now from zinc) to the upper (to copper). The copper end of the pole was called the positive pole, the zinc end was called the negative pole. Subsequently, in Faraday's terminology, the positive pole is called anode, negative - cathode. The Voltaic column can be laid horizontally in a trough, covered inside with an insulating layer of wax fused with harpius. Nowadays the voltaic pole is not used due to the great labor and time required to assemble and disassemble it; but in the past they used pillars made up of hundreds and thousands of pairs; Professor V. Petrov used it in St. Petersburg in 1801-2. During his experiments with a column, sometimes consisting of 4200 pairs (see Galvanism), Volta built his apparatus in another form, which is the form of later batteries. Volta's battery (corona di tazze) consisted of cups located around the circumference of a circle into which warm water or a salt solution was poured; in each cup there were two dissimilar metal plates, one opposite the other. Each plate is connected by wire to a dissimilar plate of the adjacent cup, so that from one cup to another along the entire circumference the plates constantly alternate: zinc, copper, then again zinc and copper, etc. In the place where the circle closes, in one cup there is zinc plate, in the other - copper; along the wire connecting these outer plates, current will flow from the copper plate (positive pole) to the zinc plate (negative pole). Volta considered this battery less convenient than a pole, but in fact it was the form of the battery that became widespread. In fact, the structure of the voltaic column was soon changed (Cruikshank): oblong wooden box, divided transversely by copper and zinc plates soldered together into small compartments into which liquid was poured, it was more convenient than an ordinary voltaic column. Even better was a box divided into compartments by wooden cross walls; copper and zinc plates were placed on both sides of each partition, being soldered together on top, where, in addition, an eyelet was left. A wooden stick passing through all the ears served to lift all the plates from the liquid or to immerse them. Elements with one liquid. Soon after, individual pairs or cells began to be made that could be combined into batteries in various ways. The electrical excitatory force of the elements depends on the metals and liquids and their components, and the internal resistance depends on the liquids and the size of the elements. To reduce the resistance and increase the current intensity, it is necessary to reduce the thickness of the liquid layer between dissimilar plates and increase the size of the immersed surface of the metals. This is done in Wollaston element(Wollaston - according to the more correct pronunciation Wulsten). The zinc is placed inside a bent copper plate, in which pieces of wood or cork are inserted to prevent the plates from touching; a wire, usually copper, is soldered to each of the plates; the ends of these wires are brought into contact with an object through which they want to pass a current flowing in the direction from copper to zinc along the outer conductors and from zinc to copper along internal parts element. In general, the current flows inside the liquid from a metal on which the liquid acts chemically more strongly, to another, on which it acts less strongly. In this cell, both surfaces of the zinc plate serve for the flow of electricity; This method of doubling the surface of one of the plates later came into use when arranging all elements with one liquid. The Wollaston element uses dilute sulfuric acid, which decomposes during the action of current (see Galvanic conductivity); the result of decomposition will be the oxidation of zinc and the formation of zinc sulfate, dissolving in water, and the release of hydrogen on the copper plate, which thereby comes into a polarized state (see Galvanic polarization and Galvanic conductivity), reducing the current strength. The variability of this polarized state is accompanied by variability in the current strength. Of many elements with one liquid we call media elements(Smee) and Grene, in the first - platinum or platinized silver among two zinc plates, all immersed in dilute sulfuric acid. The chemical action is the same as in Wollaston's element, and is polarized by hydrogen in platinum; but the current is less variable. The electrical excitation force is greater than in copper-zinc. Grenet's element consists of a zinc plate placed between two tiles cut from coke; the liquid for this element is prepared according to different recipes, but always from dichromopotassium salt, sulfuric acid and water. According to one recipe, for 2500 grams of water you need to take 340 grams of the named salt and 925 grams of sulfuric acid. The electrical excitation force is greater than in the Wollaston element. During the action of the Grenet element, zinc sulfate is formed, as in previous cases; but hydrogen, combining with the oxygen of chromic acid, forms water; chrome alum is formed in the liquid; polarization is reduced but not eliminated. For the Grenet element, a glass vessel with an expanded bottom, as shown in Fig. 7 table "Galvanic cells and batteries". So much liquid is poured in that the zinc plate Z, which is shorter than coke WITH, it was possible by pulling the rod attached to it T, remove from the liquid for the time when the element should remain inactive. Clamps V, V, connected - one with rod rim T, and therefore, with zinc, and the other with a rim of coal, are assigned to the ends of the conductor wires. Neither the records nor their frames have metallic contact with each other; the current flows along the connecting wires through external objects in the direction from coke to zinc. The carbon-zinc element can be used with a solution of table salt (in Switzerland, for telegraphs, calls) and then it is valid for 9-12 months. without care. Element of Lalande and Chaperone, improved by Edison, consists of a slab of zinc and another pressed from copper oxide. The liquid is a solution of caustic potassium. The chemical action is the oxidation of zinc, which then forms a compound with potassium; The separated hydrogen, oxidized by the oxygen of zinc oxide, becomes part of the resulting water, and copper is reduced. Internal resistance is low. The excitatory force is not determined with precision, but is less than that of the Daniel element. Elements with two liquids. Since the evolution of hydrogen on one of solids G. elements is a reason that reduces the strength of the current (actually electrically exciting) and makes it inconsistent, then placing the plate on which hydrogen is released in a liquid capable of donating oxygen to combine with hydrogen should make the current constant. Becquerel was the first to construct (1829) a copper-zinc element with two liquids for the named purpose, when the elements of Grenet and Lalande were not yet known. Later Daniel(1836) designed a similar element, but more convenient to use. To separate liquids, two vessels are needed: one glass or glazed clay vessel, containing a cylindrical, clay, slightly fired, and therefore porous, vessel into which one of the liquids is poured and one of the metals is placed; in the ring-shaped space between the two vessels another liquid is poured into which a plate of another metal is immersed. In the Daniel element, zinc is immersed in weak sulfuric acid, and copper is immersed in an aqueous solution of copper (blue) sulfate. Fig. 1 of the table depicts 3 Daniel elements connected into a battery; cylinders bent from zinc are placed in outer glass glasses, copper plates, also in the shape of a cylinder or bent like the letter S, are placed in inner clay cylinders. You can place it the other way around, i.e. copper in external vessels. The current flows from copper to zinc through external conductors and from zinc to copper through the liquid in the cell or battery itself, and both liquids decompose simultaneously: zinc sulfate is formed in a vessel with sulfuric acid, and hydrogen goes to the copper plate, at the same time copper sulfate (CuSO 4) decomposes into copper (Cu), which is deposited on the copper plate, and a separately non-existent compound (SO 4), which by a chemical process forms water with hydrogen before it has time to be released in the form of bubbles on the copper. Porous clay, easily wetted by both liquids, makes it possible for chemical processes to be transmitted from particle to particle through both liquids from one metal to another. After the action of the current, the duration of which depends on its strength (and this latter partly on external resistances), as well as on the amount of liquids contained in the vessels, all copper sulfate is consumed, as indicated by the discoloration of its solution; then the separation of hydrogen bubbles on copper begins, and at the same time the polarization of this metal. This element is called constant, which, however, must be understood relatively: firstly, even with saturated vitriol there is a weak polarization, but the main thing is that the internal resistance of the element first decreases and then increases. For this second and main reason, at the beginning of the action of the element, a gradual increase in current is noticed, the more significant, the less the current strength is weakened by external or internal resistances. After half an hour, an hour or more (the duration increases with the amount of liquid with zinc), the current begins to weaken more slowly than it increased, and after a few more hours it reaches its original strength, gradually weakening further. If a supply of this salt in undissolved form is placed in a vessel with a solution of copper sulfate, then this continues the existence of the current, as well as replacing the resulting solution of zinc sulfate with fresh dilute sulfuric acid. However, with a closed element, the liquid level with zinc gradually decreases, and with copper it increases - a circumstance that in itself weakens the current (from an increase in resistance for this reason) and, moreover, indicates a transition of liquid from one vessel to another (transfer of ions, see Galvanic conductivity, galvanic osmosis). Copper sulfate seeps into the vessel with zinc, from which the zinc releases copper purely chemically, causing it to precipitate partly on the zinc and partly on the walls of the clay vessel. For these reasons, there is a large waste of zinc and copper sulfate that is useless for current. However, Daniel's element is still one of the most constant. A clay glass, although wetted by liquid, presents great resistance to current; by using parchment instead of clay, the current can be significantly increased by reducing the resistance (Carré element); the parchment can be replaced by an animal bubble. Instead of diluted sulfuric acid, you can use a solution of table or sea salt for zinc; the excitatory force remains almost the same. Chemical effects have not been studied. Meidinger element. For frequent and continuous and, moreover, fairly constant, but weak current, the Meidinger element (Fig. 2 of the table), which is a modification of the Daniel element, can be used. The outer glass has an extension at the top, where a zinc cylinder is placed on the inner lip; At the bottom of the glass is placed another small one, in which is placed a cylinder rolled up from sheet copper, or a copper circle is placed at the bottom of the inner vessel, which is then filled with a solution of copper sulfate. After this, a solution of magnesium sulfate is carefully poured from above, which fills all the free space of the outer vessel and does not displace the vitriol solution, as it has a larger specific gravity. Nevertheless, through the diffusion of liquids, vitriol slowly reaches zinc, where it gives up its copper. To maintain the saturation of this solution, an overturned glass flask with pieces of copper sulfate and water is placed inside the element. Conductors go outward from the metals; their parts in the liquid have a gutta-percha shell. The absence of a clay jar in the element allows it to be used for a long time without changing its parts; but its internal resistance is high, it must be moved from place to place very carefully, and it contains a lot of copper sulfate, which is useless for current; in the flask of even a small element about 1/2 kilogram of vitriol is placed. It is very suitable for telegraphs, electric calls and other similar cases and can stand for months. Callot and Trouvé-Callot elements similar to Meidinger elements, but simpler than the latter. Kresten in St. Petersburg he also arranged a useful modification of the Meidinger element. Thomson element in the form of a dish or tray there is a modified Daniel's; porous flat membranes made of parchment paper separate one liquid from another, but you can do without membranes. Siemens element And Halske also belongs to the category of Daniel's. Element of Minotto. Copper circle at the bottom glass jar, on which crystals of copper sulfate are poured, and on top there is a thick layer of siliceous sand, on which a zinc circle is placed. Everything is filled with water. Lasts 1 1/2 to 2 years on telegraph lines. Instead of sand, you can take animal charcoal powder (Darsonval). Trouvé element. A copper circle on which is a column of circles made of pass-through paper, impregnated with copper sulfate on the bottom and zinc sulfate on the top. A small amount of water wetting the paper activates the element. The resistance is quite high, the action is long and constant. Grove element, platinum-zinc; platinum is immersed in strong nitric acid, zinc in weak sulfuric acid. The hydrogen released by the action of the current is oxidized by the oxygen of nitric acid (NHO 2), which turns into nitric anhydride (N 2 O 4), the released red-orange vapors of which are harmful to breathing and spoil all copper parts of the apparatus, which are therefore better made of lead. These elements can only be used in laboratories where there are fume hoods, and in an ordinary room they should be placed in a stove or fireplace; they have a high excitatory force and low internal resistance - all the conditions for a high current strength, which is the more constant the larger the volume of liquids contained in the element. Fig. 6 of the table shows such a flat-shaped element; outside it on the right there is a bent zinc plate connected to the platinum sheet of the element Z the second element, in the fold of which there is a flat clay vessel V for platinum. On the left is a platinum sheet clamped to the zinc element and belonging to the third element. With this form of elements, the internal resistance is very small, but the strong effect of the current does not last long due to the small amount of liquids. The current flows from the platinum through the outer conductors to the zinc, according to the general rule stated above. Bunsen element(1843), coal-zinc, completely replaces the previous one and is cheaper than it, since expensive platinum is replaced by coke tiles. The fluids are the same as in the Grove element, the electrical excitation force and resistance are approximately the same; the direction of the current is the same. A similar element is shown in Fig. 3 tables; charcoal tile marked with letter WITH, with a metal clamp with a + sign; this is the positive pole, or anode, of the element. From zinc cylinder Z with a clamp (negative pole, or cathode) there is a plate with another clamp, applied to the carbon slab of the second element in the case of a battery. Grove was the first to replace platinum in his element with coal, but his experiments were forgotten. Darsonval element, carbon-zinc; for coal, a mixture of nitric and hydrochloric acid, 1 volume each, with 2 volumes of water containing 1/20 sulfuric acid. Fora element.- Instead of a coke tile, a bottle made of graphite and clay is used; Nitric acid is poured there. This is apparently external change Bunsen element makes the use of nitric acid more complete. Sosnovsky element.- Zinc in a solution of sodium hydroxide or potassium hydroxide; coal in a liquid consisting of 1 volume of nitric acid, 1 volume of sulfuric acid, 1 volume of hydrochloric acid, 1 volume of water. Remarkable for its very high electrical excitatory power. Callan element.- Carbon of Bunsen elements is replaced by iron; the excitatory force remains the same as when using coal. Iron is not exposed to nitric acid, being in a passive state. Instead of iron, cast iron with some silicon content can be usefully used. Poggendorff element differs from the Bunsen element by replacing the nitric acid with a liquid similar to that used in the Grenet element. To 12 parts by weight of potassium dichromate, dissolved in 100 parts of water, add 25 parts of strong sulfuric acid. The excitatory force is the same as in the Bunsen element; but the internal resistance is greater. The oxygen in the said liquid given up for the oxidation of hydrogen is less than in nitric acid at the same volume. The absence of odor when using these elements, combined with other advantages, made it the most convenient to use. However, polarization has not been completely eliminated. Imshenetsky element, carbon-zinc. Graphite (carbon) plate in a solution of chromic acid, zinc in a solution of sodium sulfide salt. Great excitatory power, low internal resistance, almost complete utilization of zinc and very good use of chromic acid. Leclanche element, carbon-zinc; instead of an oxidizing liquid, it contains powder (large) of manganese peroxide at the coal slab, mixed with coke powder (Fig. 5 table) in an inner, liquid-permeable clay jar; A zinc stick is placed outside in one of the corners of the specially shaped flask. The liquid - an aqueous solution of ammonia - is poured from the outside and penetrates into the clay jar to the coal (coke), wetting the manganese peroxide; the top of the jar is usually filled with resin; holes are left for gases to escape. The excitatory force is average between the Daniel and Bunsen elements, the resistance is high. This element, left closed, gives a current of rapidly decreasing strength, but for telegraphs and home use it lasts for one to two years when adding liquid. When ammonia (NH 4 Cl) decomposes, chlorine is released into zinc, forming zinc chloride and ammonia with coal. Manganese peroxide, rich in oxygen, passes little by little into a compound of a lower oxidation state, but not in all parts of the mass filling the clay vessel. To make more complete use of manganese peroxide and reduce internal resistance, these elements are arranged without a clay jar, and tiles are pressed from manganese peroxide and coal, between which coke is placed, as shown in Fig. 4 tables. These types of elements can be made closed and easy to carry; glass is replaced by horn rubber. Geff also modified this element, replacing the ammonia solution with a solution of zinc chloride. Element of Marie-Devi, coal-zinc, contains, with coal, a dough-like mass of mercuric sulfate (Hg 2 SO 4), moistened with water, placed in a porous clay jar. Weak sulfuric acid or even water is poured onto the zinc, since the former will already be released from the mercury salt by the action of a current, during which hydrogen is oxidized, and with coal metallic mercury is released, so that after some time the element becomes zinc-mercury. The electrical excitatory force does not change from using pure mercury instead of coal; it is slightly larger than in the Leclanche element, the internal resistance is large. Suitable for telegraphs and in general for intermittent current action. These elements are also used for medical purposes, and they prefer to be charged with mercuric sulfate oxide (HgSO 4). The form of this element, convenient for medical and other purposes, is a tall cylinder of horn rubber, the upper half of which contains zinc and coal, and the lower half contains water and mercury sulfate. If the element is turned upside down, it acts, but in the first position it does not generate current. Warren Delarue element- zinc-silver. A narrow strip of silver protrudes from a cylinder of fused silver chloride (AgCl) placed in a tube of parchment paper; zinc has the shape of a thin rod. Both metals are placed in a glass tube sealed with a paraffin stopper. The liquid is a solution of ammonia (23 parts of salt per 1 liter of water). The electrical excitation force is almost the same (a little more) as in the Daniel element. Silver metal is deposited from silver chloride onto the silver strip of the element, and no polarization occurs. Batteries made from them were used for experiments on the passage of light in rarefied gases (V, Warren Delarue). Geff gave these elements a device that makes them convenient to carry; used for medical induction coils and for direct currents. Elements of Duchaumin, Partz, Figier. The first is zinc-carbon; zinc in a weak solution of table salt, coal - in a solution of ferric chloride. Unstable and little explored. Partz replaced zinc with iron; a solution of table salt has a density of 1.15, a solution of ferric chloride has a density of 1.26. Better than the previous one, although the electrical excitatory force is less. Figier uses one liquid in the iron-coal element, obtained by passing a stream of chlorine through a saturated solution of iron sulfate. Nyode element, carbon-zinc. The zinc is shaped like a cylinder surrounding a porous clay cylinder containing a coke slab covered with bleach. The element is sealed with a stopper filled with wax; a solution of table salt (24 parts per 100 parts water) is poured through the hole in it. The electrical excitatory force is large; with constant, somewhat prolonged action on external small resistance, it soon weakens, but after an hour or two of inactivity of the element it reaches its previous value. Dry elements. This name can be given to elements in which the presence of liquid is not apparent when it is sucked into the porous bodies of the element; it would be better to call them wet. These include the above-described copper-zinc Trouvé element and the Leclanche element, modified by Germain. This latter uses fiber extracted from coconuts; a mass is prepared from it that strongly absorbs liquid and gases, appears dry and only accepts pressure under pressure wet look. Easily portable and suitable for traveling telegraph and telephone stations. Gasner elements (carbon-zinc), which contain gypsum, probably impregnated with zinc chloride or ammonia (kept secret). The excitatory force is approximately the same as in the Leclanche element, some time after the onset of the latter’s action; internal resistance is less than that of Leclanche. In a dry Leclanche-Barbier cell, the space between the outer zinc cylinder and the inner hollow cylinder of agglomerate, which includes manganese peroxide, is filled with gypsum, a saturated solution of unknown composition. The first, rather lengthy tests of these elements were favorable for them. Gelatin-glycerin element Kuznetsova there is copper-zinc; consists of a cardboard box impregnated with paraffin with a bottom glued with tin inside and out. A layer of crushed copper sulfate is poured onto the tin, onto which a gelatin-glycerin mass containing sulfuric acid is poured. When this mass hardens, a layer of crushed amalgamated zinc is poured in, again filled with the same mass. These elements make up a battery like a voltaic column. Designed for calls, telegraphs and telephones. In general, the number of different dry elements is very significant; but in the majority, due to the secret composition of liquids and agglomerates, judgment about them is only possible in a practical, but not scientific way. Elements of large surface and low resistance. In cases where it is necessary to glow short, rather thick wires or plates, as, for example, during some surgical operations (see Galvanocaustics), elements with large metal surfaces immersed in liquid are used, which reduces the internal resistance and thereby increases the current. Wollaston's method of surface doubling is applied to the composition of surfaces from a large number of plates, as shown in Fig. 2, where y, y, y- plates of the same metal are placed in the spaces between the plates ts, ts, ts, ts other metal. All plates are parallel to each other and do not touch, but all of the same name are connected by external wires into one whole. This entire system is a uniform element of two plates, each with a surface area of six times that shown, with a thickness of the liquid layer between the plates equal to the distance between each two plates shown in the drawing. Already at the beginning of this century (1822), devices with large metal surface. These include the large Gare element, called the deflagrator. Long lengths of zinc and copper sheets, separated by flannel or wooden sticks, are rolled into a roller in which the sheets do not come into metallic contact with each other. This roller is immersed in a tub of liquid and produces a very high current when acting on very small external resistance. The surface of each sheet is about 50 square meters. feet (4 sq. meters). Nowadays, in general, they try to reduce the internal resistance of the elements, but give them a particularly large surface for some particular applications, for example, in surgery for cutting off painful growths with a hot wire or plate, for cauterization (see Galvanocaustics). Since conductors of low resistance are heated, current can be obtained precisely by reducing the internal resistance. Therefore, a large number of plates are placed in galvanocaustic elements, arranged similar to those shown in Fig. 2 texts. The device does not present any special features, but is adapted for convenient use; such, for example, are carbon-zinc cells or Chardin batteries with chrome liquid, used in Paris, Lyon, Montpellier and Brussels. Operators should be alerted to the need to use a very low-resistance current meter (ammeter, or ammeter) to ensure that the battery is in good condition before operation. Normal elements must retain their electrically exciting force or have a constant potential difference for as long as possible when they are kept open in order to serve as a normal unit of measure when comparing electrically exciting forces with each other. For this purpose, Rainier proposed a copper-zinc pair, in which the surface of copper is very large compared to zinc. The liquid is a solution of 200 parts of dry table salt in 1000 parts of water. Under this condition, the polarization of copper is very weak if this element is introduced into a circuit with high resistance and for a short time. Normal element Latimer Clark consists of zinc in a solution of zinc sulfate, mercury and mercury sulfide salt (Hg 2 SO 4). Normal element Fleming, copper-zinc, with solutions of copper sulfate and zinc sulfate of a certain, always constant density. Normal element London Post and Telegraph Office, copper-zinc, with a solution of zinc sulfate and crystals of copper sulfate with copper is very suitable. For the electrical excitatory force of the Fleming element, see the plate at the end of the article. Secondary elements, or batteries, originate from the secondary pillars of Ritter (see Galvanism), which remained without special attention for 50 years. A Ritter column, consisting of copper plates immersed in some liquid, became polarized after the action of a voltaic column on it, and after that it could itself generate a current, the direction of which was opposite to the primary current. In 1859, Plante constructed an element consisting of two lead sheets, coiled in a spiral like a Gare deflagrator, without mutual metallic contact, and immersed in weak sulfuric acid. By connecting one lead sheet to the anode (positive pole), and the other to the cathode of a battery of at least 2 Bunsen or Poggendorff cells connected in series, and thus passing a current flowing in the liquid from lead to lead, thereby causing the separation of oxygen on the lead plate , connected to the anode, and hydrogen on a sheet connected to the cathode. A layer of lead peroxide forms on the anode plate, while the cathode plate is completely cleared of oxides. Due to the heterogeneity of the plates, they form pairs with a large electrical excitatory force, giving a current in the direction opposite to the previous one. The great excitatory force developing in the secondary element and directed opposite to the excitatory force of the primary battery is the reason for the requirement that the latter exceed the first. Two Poggendorff elements connected in series have an exciting force of about 4 volts, but a Plante element only about 2 1/2. For charging 3 or 4 Plante cells connected in parallel (see. Galvanic batteries), in fact, the previous 2 Poggendorff elements would be sufficient, but their action would be very slow to oxidize such a large surface of lead; therefore, to simultaneously charge, for example, 12 Plante elements connected in parallel, you need the action of 3-4 Bunsen elements with an exciting force of 6-8 volts for several hours. Charged Plante cells connected in series develop an electrical excitation force of 24 volts and produce more incandescence, for example, than a charging battery, but the effect of the secondary battery will be shorter. The amount of electricity set in motion by the secondary battery is not more than the amount of electricity passed through it from the primary battery, but, being passed through the external conductors at a greater voltage or potential difference, is expended in a shorter time. Plante cells, after various practical improvements, were called batteries. In 1880, Faure came up with the idea of covering lead plates with a layer of red lead, i.e., ready-made lead oxide, which, under the action of the primary current, was further oxidized on one plate and deoxidized on the other. But the method of attaching red lead required technical improvements, which essentially consisted in the use of a lead grid, in which empty cells are filled with a test of red lead and litharge in weak sulfuric acid. The Fitz-Gerald battery uses lead oxide tiles without any metallic base; In general, there are a lot of battery systems and here is an image of only one of the best (Fig. 8 of the table). The Hagen lead grille is composed of two protrusions facing each other, which prevents pieces of lead oxide from falling out of the frame; specially depicted cuts along the lines ab And CD The main drawing explains the structure of this frame. One frame is filled with red lead, the other with litharge (the lowest oxidation state of lead). An odd number, usually five or seven, of plates are connected in the same way as explained in the devil. 2; in the first case 3, in the second 4 are covered with litharge. Of the Russian technicians, Yablochkov and Khotinsky benefited from the design of batteries. These secondary elements, which present one technical inconvenience - very heavy weight, received a variety of technical applications, by the way, to home electric lighting in cases where it is impossible to use direct current from dynamos for this purpose. Batteries charged in one place can be transported to another. They are now charged not with primary elements, but with dynamos, in compliance with some special rules (see Dynamos, Electric lighting). Composition of galvanic batteries. The battery is composed of elements in three ways: 1) series connection, 2) parallel connection, 3) combined from both previous ones. In fig. Table 1 shows a series connection of 3 Daniel elements: the zinc of the first pair, counting from the right, is connected by a copper tape to the copper of the second pair, the zinc of the second pair to the copper of the third. The free end of the copper of the first pair is the anode, or positive terminal of the battery; the free end of the third pair is the cathode, or negative terminal of the battery. To connect these same elements in parallel, all the zincs must be connected to each other with metal tapes and all copper sheets must be connected with tapes or wires into one whole separate from the zinc; the complex zinc surface will be the cathode, the complex copper surface will be the anode. The action of such a battery is the same as the action of a single cell, which would have a surface area three times larger than a single cell of the battery. Finally, the third connection method can be applied to at least 4 elements. By connecting them two in parallel, we get two complex anodes and the same two cathodes; By connecting the first complex anode with the second complex cathode, we obtain a battery of two elements with a double surface. Fuck it. 3 texts depict two different complex compounds of 8 elements, each represented by two concentric rings separated by black spaces. Without going into details, we note that according to appearance the method of composing these batteries differs from those just described. In (I) 4 elements are connected in series, but at one end the two outer zincs are connected by a metal strip KK, and on the opposite side, the two outer copper plates are connected by a plate AA, which is the anode, whereas QC -
the cathode of a complex battery, equivalent to 4 elements of double the surface connected in series. Drawing 3 (II) shows a battery equivalent to two elements of a quadruple surface connected in series. Cases when batteries are needed, composed in a certain way, are completely clarified by Ohm's formula (galvanic current), subject to the rule arising from it, that in order to obtain best action For any conductor, with a given number of galvanic cells, it is necessary to construct a battery from them in such a way that its internal resistance is equal to the resistance of the external conductor, or at least as close as possible to it. To this we must also add that with a series connection, the internal resistance increases in proportion to the number of connected pairs, and with a parallel connection, on the contrary, the resistance decreases in proportion to this number. Therefore, on telegraph lines, which present great resistance to galvanic current, batteries consist of elements connected in series; in surgical operations (galvanocaustics), a battery of parallel-connected elements is needed. Depicted in hell. 3 (I) the battery represents the best combination of 8 cells to act on an external resistance that is twice the internal resistance of a single cell. If the external resistance were four times less than in the first case, then the battery should be given the appearance of hell. 3 (II). This follows from calculations using Ohm's formula. [On elements and batteries, see the work of Niodet (in Russian translation by D. Golov - “Electrical elements” 1891); less detailed: "Die galvanischen Batterien", Hauck, 1883. Articles in the magazine "Electricity", 1891 and 1892] Comparison of galvanic cells among themselves. Notes related to this were partly given in the description of the elements. The merit of a galvanic cell is measured by the strength of the current it develops and the duration of its action, namely the product of the first quantity by the other. If we take the ampere as the unit of current (see Galvanic current), and the hour as the unit of time, then we can measure the performance of the galvanic cell in ampere-hours. For example, batteries, depending on their size, can provide from 40 to 90 ampere-hours. For methods of measuring the work delivered by electric current, equivalent to the work of the so-called steam horse for one hour, see Work, Energy of Electric Current.
