§41. The concept of galvanic cells
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 principle of operation of a galvanic cell is quite simple. Copper and zinc plates are immersed in an aqueous solution of sulfuric acid, which then play the role of a positive and negative pole.
The principle of operation of a galvanic cell
When the poles are connected with a conductor, the simplest electrical circuit appears. The current flow inside the element will occur from a negative charge to a positive one, that is, from a zinc plate to a copper one. The movement of charged particles along the external circuit will be carried out 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. In this case, hydrogen transfers the charge to the copper plate, and the rest of the acid to the zinc plate. Thus, the 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 polarization electromotive force. To avoid this phenomenon, a substance is introduced into the composition that is capable of absorbing hydrogen atoms and performing the function of depolarization.
Galvanic cells: advantages and disadvantages
For the manufacture of modern galvanic cells, a variety of materials are used. The most common are materials based on carbon-zinc elements used for fingernails.
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 the use of alkaline elements. Here, not coal, but an alkali solution acts as an electrolyte. When discharging, no gas is released, which ensures complete tightness. Alkaline elements have a higher shelf life.
The general principle of operation of a galvanic cell for all their types is exactly the same. For example, elements based on mercury oxide structurally resemble alkaline ones. They are characterized by increased resistance to high temperatures, high mechanical strength and stable voltage value. The disadvantage is the toxicity of mercury, which requires careful handling of spent elements.
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 stresses, 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, the following processes occur:
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. (one)
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 FROM Ox , (3)
where FROM 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.
In addition to electrolysis, another variant of the redox reaction is possible. 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 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 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
Galvanic element copper - zinc - sulfuric acid
He poured dilute sulfuric acid into a glass, lowered a plate of galvanized sheet into it. Hydrogen evolution has begun. I attached a wire to the plate with a "crocodile", connected with another crocodile with a flattened copper tube. I lowered copper into a glass with zinc and acid - hydrogen evolution began from the copper surface.
We got a galvanic cell: zinc dissolves, electrons pass through the wire to copper, hydrogen ions are discharged (reduced) on the copper surface. Ideally, after copper was immersed in acid, the evolution of hydrogen on the surface of zinc should have ceased, but in reality, hydrogen was released on both copper and zinc.
If you remove the zinc plate from the acid, but leave the copper one, the evolution of hydrogen from the copper surface will stop: copper does not displace hydrogen from sulfuric acid.
I connected the tester electrodes between the plates - the voltage turned out to be 0.8-0.9 V. If one of the plates (copper or zinc) is removed from the solution, the voltage drops to zero (there is no electric current in the system). The same thing will happen if copper and zinc in solution come into contact: electrons will pass from zinc to copper directly - bypassing the wire and the tester.
How can we increase the voltage of our galvanic cell? We will get the answer if we consider the equations of ongoing processes:
Zn 0 => Zn 2+ + 2e -
2H + + 2e - = H 2 0
The electromotive force (EMF) of a galvanic cell is equal to the potential difference of the electrodes, in our case - "hydrogen" and zinc:
EMF \u003d E (2H + / H 2) - E (Zn 2+ / Zn)
The greater the potential of the hydrogen electrode and the less of the zinc electrode, the greater the EMF of the galvanic cell. In both cases, the potential of the electrode - hydrogen or zinc - increases with an increase in the concentration of hydrogen or zinc cations in the solution, respectively.
There are two ways out: to lower the concentration of zinc ions or to increase the concentration of hydrogen ions.
At the initial moment, the concentration of zinc cations is practically zero (there is nowhere to reduce it), but you can increase the concentration of hydrogen cations - add more sulfuric acid to the glass. The potential of the hydrogen electrode will increase, as a result, the potential difference will increase.
And immediately a significant clarification: as the galvanic cell works, the concentration of hydrogen ions in the solution will decrease, and zinc ions will increase (zinc goes into solution, and hydrogen ions are reduced to H 2). Conclusion: The EMF of our galvanic cell will fall over time.
Another option is to replace zinc with any metal that is in the electrochemical series of voltages to the left of zinc (i.e., a metal that is more active than zinc). The potential of an electrode with such a metal is more positive (ceteris paribus). For example, instead of zinc, you can take magnesium.
And what will change if instead of copper we take another, less active metal (which is to the right of copper in the voltage series), for example, silver, platinum, etc.? Will the potential of the galvanic cell increase? No, because we are not dealing with a galvanic cell with zinc and copper electrodes (aka Daniel's cell):
And with a galvanic cell with zinc and hydrogen electrodes.
Zn | ZnSO 4 || H2SO4 | H2.
Zn 0 => Zn 2+ + 2e -
2H + + 2e - = H 2 0
It is easy to see that the material of the electrode on which hydrogen is released is not included in the equations, and therefore does not matter.
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The term "hydrogen electrode" is in quotation marks because the plate in a standard hydrogen electrode is not copper, but platinum - this significantly affects its operation.
Strictly speaking, the material of the electrode on which hydrogen is released matters (how it does). - Otherwise, for a standard hydrogen electrode, there would be no need to use platinum. But let's not complicate the presentation.
The emergence of e. d.s. in a galvanic cell. The simplest copper-zinc Volta 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, 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. f. s, 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, 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. With prolonged operation, the amount of electricity that a galvanic cell can give decreases, since the active chemicals present in it are gradually consumed, which ensure 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 due to the fact that chemical reactions in such elements proceed 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.
