The emergence of e. d.s. in a galvanic cell. The simplest Volta copper-zinc galvanic cell (Fig. 156) consists of two plates (electrodes): zinc 2 (cathode) and copper 1 (anode), immersed in electrolyte 3, which is an aqueous solution of sulfuric acid H 2 S0 4. When sulfuric acid is dissolved in water, a process of electrolytic dissociation occurs, i.e., some of the acid molecules disintegrate into positive hydrogen ions H 2 + and negative ions of the acid residue S0 4 -. At the same time, the zinc electrode is dissolved in sulfuric acid. When this electrode is dissolved, positive zinc ions Zn+ go into solution and combine with negative ions SO 4 - an acidic residue, forming neutral molecules of zinc sulfate ZnS04. In this case, the remaining free electrons will accumulate on the zinc electrode, as a result of which this electrode acquires a negative charge. A positive charge is formed in the electrolyte due to the neutralization of some of the negative ions S0 4. Thus, in the boundary layer between the zinc electrode and the electrolyte, a certain potential difference arises and an electric field is created, which prevents the further transition of positive zinc ions into the electrolyte; in this case, the dissolution of the zinc electrode stops. The copper electrode practically does not dissolve in the electrolyte and acquires the same positive potential as the electrolyte. Potential difference of copper? Cu and zinc? Zn of electrodes with an open external circuit is e. d.s. E of the galvanic cell under consideration.
The emf created by a galvanic cell depends on the chemical properties of the electrolyte and the metals from which the electrodes are made. Usually such combinations of metals and electrolyte are selected at which e.g. d.s. the largest, but in almost all used elements it does not exceed 1.1 -1.5 V.
When any electrical energy receiver is connected to the electrodes of a galvanic cell (see Fig. 156), current I will begin to flow through the external circuit from the copper electrode (positive pole of the element) to the zinc electrode (negative pole). In the electrolyte at this time, the movement of positive zinc ions Zn + and hydrogen H 2 + from the zinc plate to the copper and negative ions of the acid residue S0 4 will begin - from the copper plate to the zinc plate. As a result, the balance of electrical charges between the electrodes and the electrolyte will be disrupted, as a result of which positive zinc ions will again begin to flow into the electrolyte from the cathode, maintaining a negative charge on this electrode; New positive ions will be deposited on the copper electrode. Thus, between the anode and cathode there will always be a potential difference necessary for the passage of current through the electrical circuit.
Polarization. The considered Volta galvanic cell cannot operate for a long time due to the harmful phenomenon of polarization that occurs in it. The essence of this phenomenon is as follows. Positive hydrogen ions H 2 + directed to copper electrode 1 interact with the free electrons present on it and turn into neutral hydrogen atoms. These atoms cover the surface of the copper electrode with a continuous layer 4, which impairs the performance of the galvanic cell for two reasons. Firstly, additional emission occurs between the hydrogen layer and the electrolyte. d.s. (emf of polarization) directed against the main emf. d.s. element, therefore its resulting e. d.s. E decreases. Secondly, a layer of hydrogen separates the copper electrode from the electrolyte and prevents new positive ions from approaching it. In this case, the internal resistance of the galvanic element increases sharply.
To combat polarization in all galvanic cells, special substances are placed around the positive electrode - depolarizers, which easily react chemically with hydrogen. They absorb hydrogen ions approaching the positive electrode, preventing them from depositing on this electrode.
The industry produces galvanic cells of various types (with different electrodes and electrolytes), having different designs. The most common are carbon-zinc cells, in which the carbon and zinc electrodes are immersed in an aqueous solution of ammonium chloride (ammonia) or table salt, and manganese peroxide is used as a depolarizer.
Dry elements. A type of galvanic cell is a dry cell (Fig. 157), used in batteries of pocket flashlights, radios, etc. In this cell, the liquid electrolyte is replaced by a dough-like mass consisting of a solution of ammonia mixed with sawdust and starch, and the zinc electrode is made in in the form of a cylindrical box used as a vessel in which the electrolyte and carbon electrode are placed. To remove gases generated during operation of the element, a gas outlet tube is provided in it.
Capacity. The ability of chemical current sources to deliver electrical energy is characterized by their capacity. Capacity refers to the amount of electricity stored in galvanic cells or batteries. Capacity is measured in ampere hours. The rated capacity of a chemical current source is equal to the product of the rated (calculated) discharge current (in amperes) given by the chemical current source when a load is connected to it, and the time (in hours) until its e. d.s. will not reach the minimum acceptable value. During long-term operation, the amount of electricity that a galvanic cell can produce decreases, since the active chemical substances present in it, which ensure the occurrence of electricity, are gradually consumed. d.s; at the same time e decreases. d.s. element and its capacity and its internal resistance increases.
A galvanic cell has a nominal capacity only if a relatively short time has passed since its manufacture. The capacity of a galvanic cell gradually decreases, even if it does not produce electrical energy (after 10-12 months of storage, the capacity of dry cells decreases by 20-30%). This is explained by the fact that chemical reactions in such elements occur continuously and the active chemical substances stored in them are constantly consumed.
A decrease in the capacity of chemical current sources over time is called self-discharge. The capacity of a galvanic cell also decreases when it is discharged with a high current.
An example of a chemical galvanic cell is the Jacobi-Daniel element (Fig. 6). It consists of a copper electrode (a copper plate immersed in a CuSO 4 solution) and a zinc electrode (a zinc plate immersed in a ZnSO 4 solution). EDL appears on the surface of the zinc plate and equilibrium is established
Zn ⇄ Zn 2+ + 2ē
In this case, the electrode potential of zinc arises, and the electrode circuit will have the form Zn|ZnSO 4 or Zn|Zn 2+.
Similarly, EDS also appears on the copper plate and equilibrium is established
Cu ⇄ Cu 2+ + 2ē
Therefore, the electrode potential of copper arises, and the electrode circuit will have the form Cu|CuSO 4 or Cu|Cu 2+.
At the Zn electrode (electrochemically more active), the oxidation process occurs: Zn – 2ē → Zn 2+. At the Cu electrode (electrochemically less active), the reduction process occurs: Cu 2+ + 2ē → Cu.
Rice. 6 Scheme of a copper-zinc galvanic cell
The overall equation for the electrochemical reaction is:
Zn + Cu 2+ → Zn 2+ + Cu
or Zn + CuSO 4 → ZnSO 4 + Cu
Since the circuit of a chemical galvanic cell is written according to the “right plus” rule, the circuit of the Jacobi–Daniel element will have the form
The double line in the diagram indicates electrolytic contact between the electrodes, usually carried out through a salt bridge.
In a manganese-zinc galvanic cell (Fig. 7), as in a copper-zinc cell, the anode is a zinc electrode. The positive electrode is pressed from a mixture of manganese dioxide with graphite and acetylene black in the form of a column of “agglomerate”, in the middle of which a carbon rod - a current conductor - is placed.
Rice. 7 Diagram of a dry manganese-zinc cell
1 – anode (zinc cup), 2 – cathode (a mixture of manganese dioxide with graphite), 3 – graphite conductor with a metal cap,
4 - electrolyte
The electrolyte containing ammonium chloride used in manganese-zinc cells has a slightly acidic reaction due to the hydrolysis of NH 4 CI. In an acidic electrolyte, a current-generating process occurs on the positive electrode:
МnO 2 + 4Н + + 2ē → Мn 2+ + 2Н 2 O
In an electrolyte with a pH of 7-8, there are too few hydrogen ions and the reaction begins to occur with the participation of water:
MnO 2 + H 2 O + ē → MnOOH + OH -
MnOOH is an incomplete hydroxide of manganese (III) - manganite.
As hydrogen ions are consumed in the current-generating process, the electrolyte changes from acidic to neutral or even alkaline. It is not possible to maintain the acid reaction in a salt electrolyte when discharging elements. It is impossible to add acid to the salt electrolyte, as this will cause severe self-discharge and corrosion of the zinc electrode. As manganite accumulates on the electrode, it can partially react with zinc ions formed during the discharge of the zinc electrode. In this case, a sparingly soluble compound is obtained - hetaerolite, and the solution is acidified:
2MnOOH + Zn 2+ → ZnO∙Mn 2 O 3 + 2H +
The formation of hetaerolyte prevents the electrolyte from becoming too alkalized when the cell is discharged.
In addition to electrolysis, another option for the occurrence of a redox reaction is possible. In this case, electrons from the reducing agent to the oxidizing agent pass through a metal conductor through an external electrical circuit. As a result, an electric current arises in the external circuit, and such a device is called galvanic element. Galvanic cells are chemical current sources- devices for direct conversion of chemical energy into electrical energy, bypassing its other forms.
Galvanic cells based on various metals and their compounds have found wide practical application as chemical current sources.
In a galvanic cell, chemical energy is converted into electrical energy. The simplest galvanic cell consists of two vessels with solutions of CuSO 4 and ZnSO 4, into which copper and zinc plates are immersed, respectively. The vessels are connected to each other by a tube called a salt bridge, filled with an electrolyte solution (for example, KCl). Such a system is called copper-zinc galvanic cell.
Schematically, the processes occurring in a copper-zinc galvanic cell or, in other words, the diagram of a galvanic cell are presented in the figure below.
Galvanic cell diagram
The zinc oxidation process occurs at the anode:
Zn - 2e - = Zn 2+.
As a result of this, zinc atoms are converted into ions, which go into solution, and the zinc anode dissolves and its mass decreases. Note that the anode in a galvanic cell is the negative electrode (due to the electrons obtained from the zinc atoms) as opposed to the electrolysis process where it is connected to the positive terminal of the external battery.
Electrons from zinc atoms move through an external electrical circuit (metal conductor) to the cathode, where the process of reduction of copper ions from a solution of its salt takes place:
Cu 2+ + 2е – = Cu.
As a result, copper atoms are formed, which are deposited on the surface of the cathode, and its mass increases. The cathode in a galvanic cell is a positively charged electrode.
The overall equation for the reaction occurring in a copper-zinc galvanic cell can be represented as follows:
Zn + Cu 2+ = Zn 2+ + Cu.
In fact, the reaction of replacing copper with zinc in its salt occurs. The same reaction can be carried out in another way - immerse a zinc plate in a CuSO 4 solution. In this case, the same products are formed - copper and zinc ions. But the difference between the reaction in a copper-zinc galvanic cell is that the processes of electron loss and gain are spatially separated. The processes of electron release (oxidation) and addition (reduction) of electrons do not occur in direct contact of the Zn atom with the Cu 2+ ion, but in different places in the system - respectively, at the anode and cathode, which are connected by a metal conductor. With this method of carrying out this reaction, electrons move from the anode to the cathode along an external circuit, which is a metal conductor. A directed and ordered flow of charged particles (in this case electrons) is electricity. An electric current arises in the external circuit of the galvanic cell. You need to enable JavaScript to vote
In modern conditions, the most common chemical current sources are galvanic cells. Despite their individual shortcomings, they are widely used in electronics, and constant work is being done to improve them. The operating principle of a galvanic cell is quite simple. Copper and zinc plates are immersed in an aqueous solution of sulfuric acid, which then act as positive and negative poles.
Operating principle of a galvanic cell
When the poles are connected using a conductor, a simple electrical circuit appears. The flow of current inside the element will occur from a negative charge to a positive one, that is, from the zinc plate to the copper one. The movement of charged particles along the external circuit will be in the opposite direction.
When exposed to electric current, the movement of sulfuric acid residues, as well as hydrogen ions, will occur in different directions. At the same time, hydrogen transfers a charge to the copper plate, and the remaining acid transfers to the zinc plate. Thus, voltage will be maintained at the terminals. At the same time, hydrogen bubbles settle on the copper plate, weakening the overall effect of the element and creating additional voltage. This voltage is known as the electromotive force of polarization. To avoid this phenomenon, a substance is introduced into the composition that can absorb hydrogen atoms and perform a depolarization function.
Galvanic cells: advantages and disadvantages
A variety of materials are used to make modern galvanic cells. The most common are materials based on carbon-zinc elements used for finger-type ones.
Their main positive quality is considered to be relatively low cost. However, such elements have low power and a short shelf life. The best option is to use alkaline elements. Here, the electrolyte is not coal, but an alkali solution. During discharge, no gas is released, which ensures complete sealing. Alkaline elements have a longer shelf life.
The general principle of operation of a galvanic cell is exactly the same for all types. For example, elements based on mercury oxide are structurally similar to alkaline ones. They are characterized by increased resistance to high temperatures, high mechanical strength and stable voltage values. The disadvantage is the toxicity of mercury, which requires careful handling of waste elements.
In order to draw up a diagram of a galvanic cell, it is necessary to understand the principle of its operation and structural features.
Consumers rarely pay attention to batteries and rechargeable batteries, although these are the most popular power sources.
Chemical current sources
What is a galvanic cell? Its circuit is based on an electrolyte. The device includes a small container containing the electrolyte, which is adsorbed by the separator material. In addition, the diagram of two galvanic cells assumes the presence of What is the name of such a galvanic cell? The scheme linking two metals together assumes the presence of an oxidation-reduction reaction.
The simplest galvanic cell
It involves the presence of two plates or rods made of different metals, which are immersed in a solution of a strong electrolyte. During the operation of this galvanic cell, an oxidation process occurs at the anode, associated with the release of electrons.
At the cathode - reduction, accompanied by the acceptance of negative particles. Electrons are transferred through the external circuit to the oxidizing agent from the reducing agent.
Example of a galvanic cell
In order to draw up electronic circuits of galvanic cells, it is necessary to know the value of their standard electrode potential. Let us analyze a variant of a copper-zinc galvanic cell that operates on the basis of the energy released during the interaction of copper sulfate with zinc.
This galvanic cell, the diagram of which will be given below, is called a Jacobi-Daniel element. It includes which is immersed in a solution of copper sulfate (copper electrode), and it also consists of a zinc plate located in a solution of its sulfate (zinc electrode). The solutions come into contact with each other, but in order to prevent them from mixing, the element uses a partition made of porous material.
Operating principle
How does a galvanic cell operate, the circuit of which is Zn ½ ZnSO4 ½½ CuSO4 ½ Cu? During its operation, when the electrical circuit is closed, the process of oxidation of metallic zinc occurs.
On its surface of contact with the salt solution, the transformation of atoms into Zn2+ cations is observed. The process is accompanied by the release of “free” electrons, which move along the external circuit.
The reaction occurring at the zinc electrode can be represented as follows:
The reduction of metal cations is carried out on a copper electrode. Negative particles that enter here from the zinc electrode combine with copper cations, precipitating them in the form of metal. This process looks like this:
If we add up the two reactions discussed above, we obtain a summary equation that describes the operation of a zinc-copper galvanic cell.
The zinc electrode serves as the anode, and copper serves as the cathode. Modern galvanic cells and batteries require the use of a single electrolyte solution, which expands the scope of their application and makes their operation more comfortable and convenient.
Types of galvanic cells
The most common are carbon-zinc elements. They use a passive carbon current collector in contact with the anode, which is manganese oxide (4). The electrolyte is ammonium chloride, used in paste form.
It does not spread, which is why the galvanic cell itself is called dry. Its feature is the ability to “recover” during operation, which has a positive effect on the duration of their operational period. Such galvanic cells have low cost, but low power. When the temperature decreases, they reduce their efficiency, and when it increases, the electrolyte gradually dries out.
Alkaline cells require the use of an alkali solution, so they have quite a few areas of application.
In lithium cells, the active metal acts as an anode, which has a positive effect on the service life. Lithium is negative; therefore, with small dimensions, such elements have a maximum rated voltage. Among the disadvantages of such systems is the high price. Opening lithium power sources is explosive.
Conclusion
The operating principle of any galvanic cell is based on redox processes occurring at the cathode and anode. Depending on the metal used and the selected electrolyte solution, the service life of the element changes, as well as the value of the rated voltage. Currently, lithium and cadmium galvanic cells that have a fairly long service life are in demand.