Galvanic cells - structure, principle of operation, types and main characteristics. Galvanic cells

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): an oblong wooden box, divided across by copper and zinc plates soldered together, into small compartments into which liquid was poured, 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, separate pairs or elements began to be made that could be connected into batteries in various ways, the benefits of which were especially clearly revealed after Ohm expressed the formula for the strength of the current depending on the electroexcitatory (or electromotive) force of the elements and on the resistances encountered by the current as in external conductors and inside elements (see Galvanic current). 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 through the internal parts of the 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 lower part is used, as shown in Fig. 7 table "Galvanic cells and batteries". So much liquid is poured so 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 release of hydrogen on one of the solid bodies of hydrogen elements is a reason that reduces the strength of the current (actually electrically exciting) and makes it unstable, then placing the plate on which the hydrogen is released in a liquid capable of donating oxygen to combine with hydrogen should make current is 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, which contains 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 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 in water solution 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 there is another small one, in which a cylinder rolled up from sheet copper is placed, or a copper circle is placed on the bottom internal vessel, then filled with a solution of copper sulfate. After this, a solution of magnesium sulfate is carefully poured on top, which fills the entire free space of the outer vessel and does not displace the vitriol solution, as it has a higher 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 you to use it 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. A copper circle is at the bottom of a 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 great excitatory power and low internal resistance - all the conditions for great strength current, which is 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 bar, 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 nitric acid with a liquid similar to that used in the Grenet element. For 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 force, 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, in 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 silver strip 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 made 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 in the form of a cylinder surrounding a porous clay cylinder containing coke tiles filled 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 takes on a wet appearance under pressure. 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 practical, but not scientific.

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 a large metal surface were installed. 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. To charge 3 or 4 Plante elements 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 exciting 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, due to the action of the primary current, was further oxidized on one plate and deoxidized on the other. But the method of attaching the red lead required technical improvements, which essentially consisted in the use of a lead grid, in which the empty cells are filled with a dough 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 - a very large weight, have received various technical applications, among other things, for home electric lighting in cases where it is impossible to use the direct current of 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 in 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 - 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 the best effect on any conductor with a given number of galvanic elements, it is necessary to compose a battery from them in such a way that inside it the resistance was 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 the 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 between 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 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.

1. Galvanic cell

A galvanic cell is a chemical source of electric current, named after Luigi Galvani. The principle of operation of a galvanic cell is based on the interaction of two metals through an electrolyte, leading to the generation of electric current in a closed circuit. The emf of a galvanic cell depends on the material of the electrodes and the composition of the electrolyte. These are primary CITs, which, due to the irreversibility of the reactions occurring in them, cannot be recharged.

Galvanic cells are disposable sources of electrical energy. The reagents (oxidizing agent and reducing agent) are included directly in the composition of the galvanic cell and are consumed during its operation. A galvanic cell is characterized by emf, voltage, power, capacity and energy transferred to the external circuit, as well as storability and environmental safety.

The EMF is determined by the nature of the processes occurring in the galvanic element. The voltage of a galvanic cell U is always less than its EMF due to the polarization of the electrodes and resistance losses:

U = Eе – I(r1–r2) – ΔE,

where Ee is the EMF of the element; I – current strength in the operating mode of the element; r1 and r2 – resistance of conductors of the first and second kind inside the galvanic cell; ΔE is the polarization of a galvanic cell, consisting of the polarizations of its electrodes (anode and cathode). Polarization increases with increasing current density (i), determined by the formula i = I/S, where S is the cross-sectional area of ​​the electrode, and increasing system resistance.

During the operation of a galvanic cell, its EMF and, accordingly, voltage gradually decrease due to a decrease in the concentration of reagents and an increase in the concentration of products of redox processes on the electrodes (remember the Nernst equation). However, the slower the voltage decreases during the discharge of a galvanic cell, the greater the possibilities for its use in practice. The capacitance of an element is the total amount of electricity Q that a galvanic cell is capable of delivering during operation (during discharge). The capacity is determined by the mass of reagents stored in the galvanic cell and the degree of their conversion. With an increase in the discharge current and a decrease in the operating temperature of the element, especially below 00C, the degree of conversion of reagents and the capacity of the element decrease.

The energy of a galvanic cell is equal to the product of its capacitance and voltage: ΔН = Q.U. The greatest energy elements with a high EMF value, low mass and high degree transformation of reagents.

Storability is the length of the storage period of an element during which its characteristics remain within the specified parameters. As the temperature of storage and operation of an element increases, its shelf life decreases.

Composition of a galvanic cell: reducing agents (anodes) in portable galvanic cells, as a rule, are zinc Zn, lithium Li, magnesium Mg; oxidizers (cathodes) - oxides of manganese MnO2, copper CuO, silver Ag2O, sulfur SO2, as well as salts CuCl2, PbCl2, FeS and oxygen O2.

The most widespread production in the world remains the production of manganese-zinc elements Mn-Zn, widely used to power radio equipment, communication devices, tape recorders, flashlights, etc. The design of such a galvanic cell is shown in the figure.

The current-generating reactions in this element are:

At the anode (–): Zn – 2ē → Zn2+ (in practice, the zinc shell of the element body gradually dissolves);

At the cathode (+): 2MnO2 + 2NH4+ + 2ē → Mn2O3 + 2NH3 + H2O.

The following processes also take place in the electrolytic space:

At the anode Zn2+ + 2NH3 →2+;

At the cathode Mn2O3 + H2O → or 2.

In molecular form, the chemical side of the operation of a galvanic cell can be represented by the total reaction:

Zn + 2MnO2 + 2NH4Cl → Cl2 + 2.

Galvanic cell diagram:

(–) Zn|Zn(NH3)2]2+|||MnO2 (C) (+).

The EMF of such a system is E = 1.25 ÷ 1.50V.

Galvanic cells with a similar composition of reagents in an alkaline electrolyte (KOH) have better output characteristics, but they are not applicable in portable devices due to environmental hazards. Ag-Zn silver-zinc cells have even more advantageous characteristics, but they are extremely expensive and therefore not cost-effective. Currently, more than 40 are known various types portable galvanic cells, called “dry batteries” in everyday life.

2. Electric batteries

Electric batteries (secondary HIT) are rechargeable galvanic cells that can be recharged using an external current source (charger).

Batteries are devices in which, under the influence of an external current source, chemical energy is accumulated in the system (the process of charging the battery), and then during operation of the device (discharging), the chemical energy is again converted into electrical energy. Thus, when charging, the battery acts as an electrolyzer, and when discharging, it acts as a galvanic cell.

In a simplified form, a battery consists of two electrodes (anode and cathode) and an ionic conductor between them - an electrolyte. Oxidation reactions occur at the anode, both during discharge and during charging, and reduction reactions occur at the cathode.

Until recently, acid lead and alkaline nickel-cadmium and nickel-iron batteries remained the most common in Russia, and even in Transnistria.

The electrodes in it are lead grids, one of which is filled in the pores with lead oxide IV powder - PbO2. The electrodes are connected to the electrolyte through a porous separator. The entire battery is placed in a tank made of ebonite or polypropylene.

When such a device operates, the following electrode processes occur in it:

A). Discharge or operation of the battery as a source of electrical energy.

At the anode: (–) Pb – 2ē → Pb2+;

at the cathode: (+) PbO2 + 4H+ + 2ē → Pb2+ + 2H2O.

The lead cations formed on the electrodes interact with the anions of the electrolyte to release a white precipitate of lead sulfate

Pb2+ + SO42– = ↓PbSO4.

The total current-generating reaction of the battery discharge process:

Pb + PbO2 + 2H2SO4 = 2PbSO4↓ + 2H2O,

and the circuit of a working battery as a galvanic cell has the form (–) Pb|PbSO4||PbO2 (+).

The voltage at the terminals of a working battery reaches 2.0÷2.5V. During operation of the device, the electrolyte is consumed, and sediment accumulates in the system. When the concentration of active hydrogen ions [H+] becomes critical for the reaction at the cathode, the battery stops working.

B). Charging or restoring the chemical potential of a battery for its subsequent conversion into electrical energy. To do this, the battery is connected to an external current source in such a way that the negative pole is supplied to the “anode” terminal, and the positive pole is supplied to the “cathode” terminal. In this case, reverse processes occur on the electrodes under the influence of external voltage, restoring them to their original state.

Metallic lead restores the electrode surface (–): PbSO4 + 2ē → Pb + SO42;

The resulting lead oxide IV fills the pores of the lead lattice (+): PbSO4 + 2H2O – 2ē → ↓PbO2 + 4H+ + SO42.

Total reduction reaction: 2PbSO4 + 2H2O = Pb + PbO2 + 2H2SO4.

You can determine when the battery charging process is complete by the appearance of gas bubbles above its terminals (“boiling”). This is due to the occurrence of side processes of reduction of hydrogen cations and oxidation of water with increasing voltage during electrolyte reduction:

2Н+ + 2ē → Н2; 2H2O – 4ē → O2 + 2H2.

The battery efficiency reaches 80% and the operating voltage maintains its value for a long time.

The emf of the battery can be calculated using the equation:

RT α4(H+) α2(SO42–)

EE = EE0 + –––– ℓn –––––––––––––– (solid phases in Comp.

2F α2(H2O) are taken into account).

It should be noted that concentrated sulfuric acid (ω(H2SO4) > 30%) cannot be used in the battery, because at the same time, its electrical conductivity decreases and the solubility of metallic lead increases. Lead-acid batteries are widely used in all types of motor vehicles, telephone and power stations. However, due to the high toxicity of lead and its products, lead batteries require hermetically sealed packaging and full automation of their operation processes.

A) In alkaline batteries, the positive electrode is made of a nickel grid impregnated with gel-like nickel hydroxide II Ni(OH)2; and negative - from cadmium or iron. The ionic conductor is a 20% solution of potassium hydroxide KOH. The total current-forming and generating reactions in such batteries have the form:

2NiOOH + Cd + 2H2O ◄====== 2Ni(OH)2 + Cd(OH)2; EE0 = 1.45V.

2NiOOH + Fe + 2H2O ◄====== 2Ni(OH)2 + Fe(OH)2; EE0 = 1.48V.

The advantages of these batteries include their long service life (up to 10 years) and high mechanical strength, while the disadvantages are low efficiency and operating voltage. Alkaline batteries are used to power electric cars, loaders, mining electric locomotives, communications and electronic equipment, and radios. Let us also remember that cadmium is a highly toxic metal, which requires compliance with safety rules when disposing of used devices.

EMF and current. It must be remembered that the battery must contain elements with the same characteristics. Work plan Draw equivalent circuits: Rheostat connection circuits Potentiometer connection circuits Galvanic elements connection circuits. Conclusion From the constructed circuits and conditions, each circuit has its own EMF value; on each circuit it is determined differently. Answers to...

Developments of electroplating technology in the 19th – 20th centuries. remains largely open. It seems that it can be solved based on the reconstruction of the process of creating galvanic production; tracing which areas of science and technology, their specific achievements, it owes its formation; consideration of the socio-economic prerequisites for the emergence and development of electroplating technology. ...

The current is lower than in galvanostegy; in iron galvanoplastic baths it does not exceed 10-30 a/m2, while during iron plating (electroplating) the current density reaches 2000-4000 a/m2. Galvanic coatings must have a fine-crystalline structure and uniform thickness in different areas of the coated products - protrusions and recesses. This requirement is especially important in electroplating...

A galvanic cell is a source of electrical energy; its operating principle is based on chemical reactions. Most modern batteries and accumulators fall within the definition and fall into this category. Physically, a galvanic cell consists of conducting electrodes immersed in one or two liquids (electrolytes).

general information

Galvanic cells are divided into primary and secondary in accordance with their ability to produce electricity. Both types are considered sources and serve different purposes. The former generate current during a chemical reaction, the latter function exclusively after charging. Below we will discuss both varieties. Based on the amount of liquids, two groups of galvanic cells are distinguished:

The inconstancy of power sources with a single liquid was noticed by Ohm, revealing the unsuitability of Wollaston's galvanic cell for experiments in the study of electricity. The dynamics of the process are such that at the initial moment of time the current is high and initially increases, then within a few hours it drops to the average value. Modern batteries are capricious.

History of the discovery of chemical electricity

It is a little known fact that in 1752 galvanic electricity was mentioned by Johann Georg. The publication A Study of the Origin of Pleasant and Unpleasant Sensations, published by the Berlin Academy of Sciences, even gave the phenomenon a completely correct interpretation. Experiment: silver and lead plates were connected at one end, and the opposite ones were applied to the tongue from different sides. The taste of iron sulfate is observed on the receptors. Readers have already guessed that the described method of checking batteries was often used in the USSR.

Explanation of the phenomenon: apparently, there are some metal particles that irritate the receptors of the tongue. Particles are emitted from one plate upon contact. Moreover, one metal dissolves. Actually, the principle of operation of a galvanic cell is evident, where the zinc plate gradually disappears, giving off the energy of chemical bonds to the electric current. The explanation was made half a century before Alessandro Volta's official report to the Royal Society of London on the discovery of the first power source. But, as often happens with discoveries, for example, electromagnetic interaction, the experience went unnoticed by the general scientific community and was not properly studied.

Let us add that this turned out to be due to the recent abolition of prosecution for witchcraft: few people decided, after the sad experience of the “witches,” to study incomprehensible phenomena. The situation was different with Luigi Galvani, who had been working at the department of anatomy in Bologna since 1775. His specialties were considered irritants nervous system, but the luminary left a significant mark not in the field of physiology. Beccaria's student was actively involved in electricity. In the second half of 1780, as follows from the scientist’s memoirs (1791, De Viribus Electricitatis in Motu Muscylary: Commentarii Bononiensi, volume 7, p. 363), the frog was once again dissected (the experiments continued for many years).

It is noteworthy that the assistant noticed an unusual phenomenon, exactly as with the deflection of a compass needle by a wire carrying an electric current: the discovery was made only indirectly related to scientific research People. The observation concerned twitching of the frog's lower limbs. During the experiment, the assistant touched the internal femoral nerve of the animal being dissected, and the legs twitched. There was an electrostatic generator on the table nearby, and a spark flashed across the device. Luigi Galvani immediately set about repeating the experiment. What succeeded? And again the car sparked.

A parallel connection with electricity was formed, and Galvani wanted to know whether a thunderstorm would act in a similar way on a frog. It turned out that natural disasters do not have a noticeable impact. Frogs attached with copper hooks to spinal cord to the iron fence, twitched regardless of weather conditions. The experiments could not be carried out with 100% repeatability; the atmosphere had no effect. As a result, Galvani found a host of pairs made of different metals, which, when in contact with each other and the nerve, caused the frog's legs to twitch. Today the phenomenon is explained by varying degrees of electronegativity of materials. For example, it is known that aluminum plates cannot be riveted with copper; metals form a galvanic couple with pronounced properties.

Galvani rightly noted that a closed electrical circuit is formed, and suggested that the frog contains animal electricity, discharged like a Leyden jar. Alessandro Volta did not accept the explanation. After carefully studying the description of the experiments, Volta put forward the explanation that a current arises when two metals combine, directly or through the electrolyte of the body of a biological being. The cause of the current lies in the materials, and the frog serves as a simple indicator of the phenomenon. Volta quote from a letter addressed to the editor of a scientific journal:

Conductors of the first kind ( solids) and the second kind (liquids) upon contact in some combination give rise to an impulse of electricity; today it is impossible to explain the reasons for the occurrence of the phenomenon. The current flows in a closed circuit and disappears if the integrity of the circuit is broken.

Voltaic pole

Giovanni Fabroni contributed to the series of discoveries, reporting that when two plates of a galvanic pair are placed in water, one begins to collapse. Therefore, the phenomenon is related to chemical processes. Meanwhile, Volta invented the first power source, which for a long time served for the study of electricity. The scientist constantly looked for ways to enhance the action of galvanic couples, but did not find them. During the experiments, the design of a voltaic column was created:

1. Zinc and copper mugs were taken in pairs in close contact with each other.
2. The resulting pairs were separated by wet cardboard circles and placed on top of each other.

It’s easy to guess that it turned out to be a series connection of current sources, which, when summed up, enhanced the effect (potential difference). The new device caused a shock that was noticeable to the human hand when touched. Similar to Muschenbroek's experiments with the Leyden jar. However, it took time to replicate the effect. It became obvious that the energy source is of chemical origin and is gradually being renewed. But getting used to the concept of new electricity was not easy. The voltaic column behaved like a charged Leyden jar, but...

Volta organizes an additional experiment. He supplies each of the circles with an insulating handle, brings them into contact for a while, then opens them and conducts an examination with an electroscope. By that time, Coulomb’s law had already become known; it turned out that zinc was charged positively, and copper – negatively. The first material gave electrons to the second. For this reason, the zinc plate of the voltaic column is gradually destroyed. A commission was appointed to study the work, to which Alessandro’s arguments were presented. Even then, through inference, the researcher established that the tension of individual couples adds up.

Volta explained that without wet circles placed between the metals, the structure behaves like two plates: copper and zinc. No amplification occurs. Volta found the first row of electronegativity: zinc, lead, tin, iron, copper, silver. And if we exclude the intermediate metals between the extreme ones, the “driving force” does not change. Volta established that electricity exists as long as the plates are in contact: the force is not visible, but is easily felt, therefore it is true. On March 20, 1800, the scientist wrote to the President of the Royal Society of London, Sir Joseph Banks, to whom Michael Faraday also addressed for the first time.

English researchers quickly discovered: if top plate(copper) drop water, gas is released at the indicated point in the contact area. They did the experiment from both sides: the wires of a suitable circuit were enclosed in flasks with water. The gas was examined. It turned out that the gas is flammable and is released only from one side. The wire on the opposite side has noticeably oxidized. It has been established that the first is hydrogen, and the second phenomenon occurs due to excess oxygen. It was established (May 2, 1800) that the observed process was the decomposition of water under the influence of electric current.

William Cruikshank immediately showed that a similar thing could be done with solutions of metal salts, and Wollaston finally proved the identity of the voltaic column with static electricity. As the scientist put it: the effect is weaker, but has a longer duration. Martin Van Marum and Christian Heinrich Pfaff charged a Leyden jar from the element. And Professor Humphrey Davy found that pure water cannot serve as an electrolyte in this case. On the contrary, the more the liquid is able to oxidize zinc, the better the voltaic column acts, which was quite consistent with Fabroni’s observations.

Acid greatly improves performance by accelerating the process of generating electricity. In the end, Davy created a coherent theory of the voltaic column. He explained that metals initially have a certain charge, which, when the contacts are closed, causes the action of the element. If the electrolyte is able to oxidize the surface of the electron donor, the layer of depleted atoms is gradually removed, revealing new layers capable of producing electricity.

In 1803, Ritter assembled a column of alternating circles of silver and wet cloth, the prototype of the first battery. Ritter charged it from a voltaic column and observed the discharge process. Correct interpretation The phenomenon was given by Alessandro Volta. And only in 1825, Auguste de la Rive proved that the transfer of electricity in a solution is carried out by ions of the substance, observing the formation of zinc oxide in a chamber with clean water, separated from an adjacent membrane. The statement helped Berzelius create a physical model in which the electrolyte atom was imagined to be composed of two oppositely charged poles (ions) capable of dissociating. The result was a harmonious picture of the transfer of electricity over a distance.

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 loss 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.

The 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

Exercise 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 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. ON THE. 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

In the first experiments of scientists, two metal plates were lowered into a container with acid: copper and zinc. The plates were connected with a conductor, after which gas bubbles appeared on the copper plate, and the zinc plate began to dissolve. It has been proven that an electric current passes through a conductor. This research was started by the Italian scientist Galvani, and from him the name galvanic cells was given.

After this, the scientist Volta developed a cylindrical form of this element in the form of a vertical column, including a set of rings of copper, zinc and cloth, connected to each other, and impregnated with acid. The half-meter-high vertical element developed by Volt produced a voltage that a person could feel.

Galvanic cells are sources of electrical energy that produce electric current by the chemical reaction of two metals in an electrolyte. Chemical energy in galvanic cells is converted into electrical current.

Principle of operation

The action of galvanic cells is based on the fact that two different metals in an electrolyte medium interact with each other, resulting in the formation of an electric current in an external circuit.

Such chemical elements today they are called batteries. The voltage of the battery depends on the types of metals used and the number of elements contained in it. The entire battery device is located in a metal cylinder. The electrodes are metal meshes coated with a reducing agent and an oxidizing agent.

Batteries cannot restore lost properties, since they directly convert the chemical energy of an oxidizing agent and a reducing agent into electrical energy. During the operation of the battery, chemical reagents are gradually consumed, and the electric current decreases.

The negative terminal of the battery is made of zinc or lithium, it loses electrons and is a reducing agent. Another positive terminal plays the role of an oxidizing agent; it is made from magnesium oxide or metal salts. Electrolyte composition in normal conditions does not pass electric current through itself. When the electrical circuit is closed, the electrolyte begins to disintegrate into ions, which causes its appearance. electrical conductivity. The electrolyte most often consists of a solution of acid or sodium and potassium salts.

Types and features of the device

Batteries are widely used to power various electronic devices, instruments, digital equipment and are divided into three types:

1. Alkaline.
2. Saline.
3. Lithium.

Salt galvanic cells

These batteries are manganese-zinc batteries, and are the most used at present.

The advantages of salt batteries are:

• Acceptable electrical parameters for many applications.
• Ease of use.
• Low price due to low production costs.
• Simple manufacturing technology.
• Cheap and available raw materials.

For a long time, this type of battery has been the most popular due to the ratio of quality and price. However, in recent years, manufacturing plants have been reducing the production of galvanic salt cells, and even refusing to produce them, as the requirements for power supplies are being increased by manufacturers of electronic equipment.

The disadvantages of salt batteries are:

• Short shelf life, no more than 2 years.
• A sharp drop in properties with decreasing temperature.
• A sharp decrease in capacity when the operating current increases to the operating values ​​of modern consumers.
• Rapid voltage reduction during operation.

Galvanic salt cells may leak at the end of their discharge, which is associated with leakage of electrolyte due to an increase in the volume of the positive electrode, which squeezes out the electrolyte. The active mass of the positive electrode consists of manganese dioxide and electrolyte. Soot and graphite added to the active mixture increase the electrical conductivity of the active mixture. Their share is from 8 to 20% depending on the brand of battery. To increase the operating life of the oxidizer, the active mixture is saturated with electrolyte.

The negative electrode is made of purified zinc, which is resistant to corrosion. It contains a small proportion of cadmium or lead, which are corrosion inhibitors. Previously, ammonium chloride was used as an electrolyte in batteries. It participates in the reaction of current formation and creates the permeability of ions. But this electrolyte did not show good results, and it was replaced by zinc chloride with impurities of calcium chloride. Manganese-acid elements work longer and show better results at lower temperatures.

In salt galvanic cells, the negative pole is the zinc housing 7. The positive electrode 6 is made of an active pressed mass impregnated with electrolyte. In the center of this mass there is a carbon rod 5, treated with paraffin to retain moisture in the electrolyte. The upper part of the rod is covered with a metal cap. Separator 4 contains a thick electrolyte. Gases generated during battery operation enter gas chamber 1. The top of the battery is covered with a gasket 3. The entire galvanic cell is enclosed in a case 2 made of cardboard or foil.

Alkaline batteries

Alkaline batteries appeared in the middle of the last century. In them, manganese dioxide acts as an oxidizing agent, and zinc powder acts as a reducing agent. This makes it possible to increase the surface area. To protect against corrosion, amalgamation was previously used. But after the ban on mercury, purified zinc powders are used with the addition of other metals and corrosion inhibitors.

The active substance in the anode of an alkaline (alkaline) battery is purified zinc in powder form with the addition of aluminum, indium or lead. The cathode active mixture includes manganese dioxide, acetylene black or graphite. The electrolyte of alkaline batteries consists of sodium hydroxide or potassium hydroxide with the addition of zinc oxide.

The powder anode can significantly increase the use of the active mixture, in contrast to salt batteries. Alkaline batteries have a significantly higher capacity than salt batteries, with equal overall dimensions. They performed well in cold weather.

A special feature of the design of alkaline elements is zinc powder, so instead of a zinc glass, a steel case is used for the positive terminal. The active mixture of the positive electrode is located near inner wall steel case. An alkaline battery has the ability to accommodate more active mixture of the positive electrode, unlike a salt battery.

A cellophane separator moistened with electrolyte is inserted into the active mixture. A brass negative electrode runs through the center of the battery. The remaining volume between the separator and the negative current lead is filled with anode paste in the form of powdered zinc impregnated with a thick electrolyte. Typically, an alkali saturated with special zinc compounds is used as an electrolyte. This makes it possible to prevent the consumption of alkali at the beginning of the element’s operation and reduce corrosion. The weight of alkaline batteries is higher than salt ones due to the steel case and the higher density of the active mixture.

In many basic parameters, alkaline galvanic cells are superior to salt cells. Therefore, the production of alkaline batteries is currently increasing.

Lithium batteries

Lithium galvanic cells are used in various modern devices. They are available in various sizes and types.

There are lithium batteries and those that differ greatly from each other. Batteries contain a solid organic electrolyte, unlike other types of cells. Lithium cells are used in places where medium and low discharge currents and stable operating voltage are required. A lithium battery can be recharged a certain number of times, but batteries are not designed for this and are only used once. They must not be opened or recharged.

Basic requirements for production

• Reliable sealing of the case. Leakage of electrolyte and penetration of other substances from the external environment must not be allowed. Violation of the tightness leads to their fire, since lithium is a highly active element. Galvanic cells with broken seals are not suitable for use.
• Manufacturing must take place in sealed rooms with an argon atmosphere and humidity control.

The shape of lithium batteries can be cylindrical, disk or prismatic. The dimensions are practically no different from other types of batteries.

Area of ​​use

Lithium galvanic cells have a longer service life compared to other elements. The scope is very wide:

• Space industry.
• Aviation production.
• Defense industry.
• Kids toys.
• Medical equipment.
• Computers.
• Photo and video cameras.

Advantages

• Wide operating temperature range.
• Compact size and weight.
• Long-term operation.
• Stable parameters in various conditions.
• Large capacity.