Consider a Jacobi-Daniel galvanic cell (the circuit is shown in Fig. 2). It consists of a zinc plate immersed in a zinc sulfate solution and a copper plate immersed in a copper sulfate solution. To prevent direct interaction between the oxidizing agent and the reducing agent, the electrodes are separated from each other by a porous partition.
In a galvanic cell, an electrode made of a more active metal, i.e. metal, located to the left in a series of voltages, is called anode, and an electrode made of a less active metal - cathode.
A double electric layer appears on the surface of the zinc electrode (anode) and equilibrium is established:
Zn 0 – 2 ē ←→ Zn2+.
As a result of this process, the electrode potential of zinc arises.
A double electric layer also appears on the surface of the copper electrode (cathode) and equilibrium is established:
Cu 2+ + 2 ē ←→ Cu 0 .
As a result, the electrode potential of copper arises.
Since the potential of the zinc electrode has a more negative value than the potential of the copper electrode, when the external circuit is closed, i.e. when connecting zinc to copper with a metal conductor, electrons will move from zinc to copper. As a result of this process, the equilibrium on the zinc electrode shifts to the right, so an additional amount of zinc ions will pass into the solution. At the same time, the equilibrium on the copper electrode will shift to the left and the copper ions will be discharged.
Thus, when the external circuit is closed, spontaneous processes of zinc dissolution on the zinc electrode and copper precipitation on the copper electrode occur. These processes will continue until the potentials are equalized or all the zinc dissolves or all the copper precipitates on the copper electrode.
So, during the operation of the Jacobi-Daniel galvanic cell, following processes:
1. Anode process, oxidation process:
Zn 0 – 2 ē → Zn2+ .
2. Cathodic process, recovery process:
Cu 2+ + 2 ē → Cu 0 .
3. Movement of electrons in an external circuit.
4. Movement of ions in solution: SO 4 2– anions to the anode, Cu 2+ cations to the cathode. The movement of ions in solution closes the electrical circuit of the galvanic cell.
Summing up the electrode reactions, we get:
Zn + Cu 2+ = Zn 2+ + Cu.
As a result of this reaction in a galvanic cell, the movement of electrons in the external circuit and ions inside the cell occurs, i.e. electricity. Therefore, the total chemical reaction occurring in a galvanic cell is called current-forming reaction.
The electric current in the galvanic cell arises due to the redox reaction, which proceeds in such a way that the oxidation and reduction processes are spatially separated: the oxidation process occurs on the negative electrode (anode), and the reduction process occurs on the positive electrode (cathode).
A necessary condition for the operation of a galvanic cell is the potential difference of the electrodes. The maximum potential difference of the electrodes that can be obtained during the operation of a galvanic cell is called the electromotive force (EMF) of the cell. It is equal to the difference between the potential of the cathode and the potential of the anode of the element:
EMF = E To - E a. (1)
The EMF of the element is considered positive if the current-generating reaction in this direction proceeds spontaneously. Positive EMF also corresponds to a certain order in the record of the element circuit: the electrode written on the left must be negative. For example, the Jacobi-Daniel element scheme is written as:
Zn │ ZnSO 4 ║ CuSO 4 │ Cu.
1.4. Electrode potential equation (Nernst equation)
As a result of studying the potentials of various electrode processes, it was found that their values depend on the following factors:
1) on the nature of substances - participants in the electrode process;
2) on the ratio between the concentrations (activities) of these substances;
3) on the temperature of the system.
Under standard conditions (temperature 298 K or 25 °C, pressure 101.3 kPa or 1 atm, molar concentration of the electrolyte solution 1 mol/l), the electrode potentials have certain standard values. If the electrolyte concentration or temperature is different from the standard, the electrode potentials can be calculated from the standard potentials using the Nernst equation:
E Ox/Red= E 0 Ox/Red + ln , (2)
Where T - absolute temperature (273 + t), TO; F- Faraday number (96485 C/mol); n- the number of electrons involved in the oxidation-reduction reaction; [Ox] is the concentration of the oxidized form (for a metal electrode, this is the concentration of metal ions in solution), mol/l; - concentration of the restored form; R- universal gas constant (8.314 J/mol deg).
At a temperature of 25 °C and provided that the reduced form represents the metal in the elemental state, the following equation can be used
E Ox/Red= E 0 Ox/Red + lg WITH Ox , (3)
Where WITH Ox - concentration of metal ions in solution, mol/l.
Example. Calculate the EMF of a galvanic cell formed by a zinc electrode immersed in a 0.01M solution of zinc nitrate Zn(NO 3) 2 and a silver electrode immersed in a 0.001M solution of silver nitrate AgNO 3 . Temperature 25 °C. Give a schematic representation of the element and write down the electrode processes occurring at the cathode and anode.
Solution. Comparing the standard reduction potentials of zinc and silver, we find that the silver electrode will act as the cathode in the indicated galvanic cell, and the zinc electrode will act as the anode.
Schematic representation of this galvanic cell:
Zn │ Zn(NO 3) 2 ║ AgNO 3 │ Ag.
Anode process: Zn 0 – 2 ē → Zn2+ .
Cathodic process: Ag++ ē → Ag0 .
The EMF of the galvanic cell is calculated by the formula (1), and the cathode and anode potentials are calculated by the Nernst equation in a simplified form (3):
E Zn 2 + / Zn 0 \u003d - 0.762 + lg0.01 \u003d - 0.82 B
E Ag + / Ag 0 \u003d - 0.90 + log0.001 \u003d + 0.62 B
EMF \u003d 0.62 - (-0.82) \u003d 1.44 V.
The emergence of e. d.s. in a galvanic cell. The simplest copper-zinc galvanic cell Volta (Fig. 156) consists of two plates (electrodes): zinc 2 (cathode) and copper 1 (anode), lowered into electrolyte 3, which is water solution sulfuric acid H 2 S0 4. When sulfuric acid is dissolved in water, the process of electrolytic dissociation occurs, i.e., part of the acid molecules decomposes 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, the positive zinc ions Zn+ go into solution and combine with the negative ions SO 4 - the acid residue, forming neutral molecules of zinc sulfate ZnSO4. In this case, the remaining free electrons will accumulate on the zinc electrode, as a result of which this electrode acquires a negative charge. In the electrolyte, a positive charge is formed 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 that prevents further transition of positive zinc ions into the electrolyte; at the same time, 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 electrodes with an open external circuit is e. d.s. E of the considered galvanic cell.
E. d. s created by a galvanic cell depends on chemical properties electrolyte and metals from which the electrodes are made. Usually, such combinations of metals and electrolyte are selected, in which e. d.s. the largest, however, in almost all the elements used, it does not exceed 1.1 -1.5 V.
When connected to the electrodes of a galvanic cell of any receiver of electrical energy (see Fig. 156), current I will begin to flow through the external circuit from the copper electrode (the positive pole of the element) to the zinc electrode (negative pole). In the electrolyte at this time, positive zinc ions Zn + and hydrogen H 2 + will begin to move from the zinc plate to the copper and negative ions of the acid residue S0 4 - from the copper plate to the zinc. As a result, the balance of electric charges between the electrodes and the electrolyte will be disturbed, 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 the cathode, there will always be a potential difference necessary for the passage of current through the electrical circuit.
Polarization. The considered galvanic cell of Volta cannot work 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 +, heading to the 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 worsens the operation of the galvanic cell for two reasons. First, an additional e.m. appears between the hydrogen layer and the electrolyte. d.s. (emf of polarization), directed against the main e. d.s. element, so its resulting e. d.s. E decreases. Secondly, the hydrogen layer separates the copper electrode from the electrolyte and prevents new positive ions from approaching it. This sharply increases the internal resistance of the galvanic cell.
To combat polarization in all galvanic cells, special substances are placed around the positive electrode - depolarizers which readily 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 various electrodes and electrolytes) with 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 common salt, and manganese peroxide is used as a depolarizer.
dry items. A type of galvanic cell is a dry cell (Fig. 157), used in batteries of pocket electric torches, radio receivers, etc. In this cell, the liquid electrolyte is replaced by a pasty 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 the operation of the element, a gas outlet tube is provided in it.
Capacity. The ability of chemical current sources to give off electrical energy is characterized by their capacitance. Capacity refers to the amount of electricity stored in galvanic cells or batteries. Capacitance is measured in amp-hours. The nominal capacitance of a chemical current source is equal to the product of the nominal (calculated) discharge current (in amperes) given off by the chemical current source when a load is connected to it, by the time (in hours) until its e. d.s. will not reach the minimum allowable value. During prolonged operation, the amount of electricity that a galvanic cell can give decreases, as the active elements present in it are gradually consumed. chemical substances, providing the occurrence of e. d.s; while decreasing e. d.s. element and its capacitance 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 give off electrical energy (after 10-12 months of storage, the capacity of dry cells decreases by 20-30%). This is explained by chemical reactions in such cells flow continuously and the active chemicals stored in them are constantly consumed.
The decrease in the capacitance of chemical current sources over time is called self-discharge. The capacity of a galvanic cell also decreases when it is discharged with a large current.
In order to draw up a diagram of a galvanic cell, it is necessary to understand the principle of its action, structural features.
Consumers rarely pay attention to accumulators and batteries, while these current sources are the most in demand.
Chemical current sources
What is a galvanic cell? Its circuit is based on an electrolyte. The device includes a small container where the electrolyte is located, adsorbed by the separator material. In addition, the scheme of two galvanic cells assumes the presence. What is the name of such a galvanic cell? The scheme linking two metals together suggests the presence of a redox reaction.
The simplest galvanic cell
It implies the presence of two plates or rods made of different metals, which are immersed in a strong electrolyte solution. During the operation of this galvanic cell, an oxidation process is carried out on the anode, associated with the return of electrons.
At the cathode - reduction, accompanied by the acceptance of negative particles. There is a transfer of electrons along 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 operating on the basis of the energy released during the interaction of copper sulfate with zinc.
This galvanic cell, the scheme of which will be given below, is called the Jacobi-Daniel cell. It includes which is immersed in a solution of copper sulfate (copper electrode), and it also consists of a zinc plate in a solution of its sulfate (zinc electrode). The solutions are in contact with each other, but in order to prevent their mixing, a partition made of a porous material is used in the element.
Operating principle
How does a galvanic cell function, 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 contact surface with a 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 taking place on 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, depositing them in the form of a metal. This process looks like this:
If we add the two reactions discussed above, we get a summary equation that describes the operation of a zinc-copper galvanic cell.
The anode is a zinc electrode, the cathode is copper. Modern galvanic cells and batteries require the use of a single electrolyte solution, which expands the scope of their application, makes their operation more comfortable and convenient.
Varieties 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, so 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 a low cost, but low power. When the temperature drops, they reduce their efficiency, and when it rises, the electrolyte gradually dries out.
Alkaline elements involve the use of an alkali solution, so they have quite a few applications.
In lithium cells, an active metal acts as an anode, which has a positive effect on the service life. Lithium has a negative therefore, with small dimensions, such elements have a maximum rated voltage. Among the disadvantages of such systems is the high price. Opening lithium current sources is explosive.
Conclusion
The principle of operation of any galvanic cell is based on redox processes occurring at the cathode and anode. Depending on the metal used, the selected electrolyte solution, the service life of the element changes, as well as the value of the rated voltage. Currently, lithium, cadmium galvanic cells with a sufficiently long service life are in demand.
An example of a chemical galvanic cell is the Jacobi-Daniel cell (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). A DES appears on the surface of a zinc plate and an equilibrium is established
Zn ⇄ Zn 2+ + 2ē
In this case, the electrode potential of zinc arises, and the electrode circuit will look like Zn|ZnSO 4 or Zn|Zn 2+ .
Similarly, a DES also appears on a copper plate and an equilibrium is established
Cu ⇄ Cu 2+ + 2ē
Therefore, an electrode potential of copper arises, and the electrode circuit will look like Cu|CuSO 4 or Cu|Cu 2+ .
On the Zn electrode (electrochemically more active), the oxidation process proceeds: Zn - 2ē → Zn 2+. On the Cu-electrode (electrochemically less active) the reduction process takes place: Cu 2+ + 2ē → Cu.
Rice. 6 Scheme of a copper-zinc galvanic cell
The overall equation of the electrochemical reaction:
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, then the Jacobi–Daniel cell circuit will look like
The double bar in the diagram indicates the electrolytic contact between the electrodes, usually carried out by means of a salt bridge.
In a manganese-zinc galvanic cell (Fig. 7), as in a copper-zinc one, the zinc electrode serves as an anode. The positive electrode is pressed from a mixture of manganese dioxide with graphite and acetylene soot in the form of an “agglomerate” column, in the middle of which a carbon rod is placed - a current collector.
Rice. 7 Scheme of a dry manganese-zinc cell
1 - anode (zinc cup), 2 - cathode (a mixture of manganese dioxide with graphite), 3 - graphite current collector with a metal cap,
4 - electrolyte
The electrolyte used in manganese-zinc cells containing ammonium chloride, due to the hydrolysis of NH 4 CI, has a slightly acidic reaction. In an acidic electrolyte, a current-generating process takes place 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 proceed with the participation of water:
MnO 2 + H 2 O + ē → MnOOH + OH -
MnOOH is incomplete manganese (III) hydroxide - manganite.
As hydrogen ions are consumed in the current-forming process, the electrolyte becomes acidic, neutral or even alkaline. It is not possible to keep the acid reaction in the saline electrolyte during the discharge of the elements. It is impossible to add acid to the salt electrolyte, as this will cause a strong 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 - hetaaerolite, and the solution is acidified:
2MnOOH + Zn 2+ → ZnO∙Mn 2 O 3 + 2H +
The formation of hetaaerolite prevents the electrolyte from becoming too alkaline when the cell is discharged.
In addition to electrolysis, another variant of the flow is possible redox reactions. In this case, the electrons from the reducing agent to the oxidizing agent pass through the metal conductor through an external electrical circuit. As a result, an electric current appears 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 application. practical use as chemical current sources.
In a galvanic cell, chemical energy is converted into electrical energy. The simplest galvanic cell consists of two vessels with CuSO 4 and ZnSO 4 solutions, in which copper and zinc plates are immersed, respectively. The vessels are interconnected by a tube called a salt bridge filled with an electrolyte solution (for example, KCl). Such a system is called copper-zinc galvanic element.
Schematically, the processes occurring in a copper-zinc galvanic cell, or, in other words, a circuit of a galvanic cell, are shown in the figure below.
Diagram of a galvanic cell
Zinc oxidation occurs at the anode:
Zn - 2e - \u003d Zn 2+.
As a result, zinc atoms turn 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 received from the zinc atoms) unlike in the electrolysis process where it is connected to the positive pole of an external battery.
Electrons from zinc atoms move along an external electrical circuit (metal conductor) to the cathode, where the process of reducing copper ions from a solution of its salt takes place:
Cu 2+ + 2e - \u003d Cu.
As a result, copper atoms are formed, which are deposited on the cathode surface, and its mass increases. The cathode in a galvanic cell is a positively charged electrode.
The overall equation of 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 takes place. The same reaction can be carried out in another way - by immersing 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 recoil and attachment of electrons are spatially separated. The processes of recoil (oxidation) and attachment (reduction) of electrons do not occur during direct contact of the Zn atom with the Cu 2+ ion, but in different places of the system - respectively on the anode and on the cathode, which are connected by a metal conductor. With this method of carrying out this reaction, the 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 occurs in the external circuit of the galvanic cell. You need JavaScript enabled to vote
