Galvanic cell copper - zinc - sulfuric acid

He poured diluted sulfuric acid into a glass, dipped a plate of galvanized sheet into it. The evolution of hydrogen began. I attached a "crocodile" wire to the plate, connected with another crocodile to a flattened copper tube. I dipped copper into a glass with zinc and acid - hydrogen evolution began from the surface of the copper.

We received a galvanic cell: zinc dissolves, electrons pass through the wire to copper, hydrogen ions are discharged (reduced) on the copper surface. Ideally, after immersion of copper in acid, the evolution of hydrogen on the zinc surface should have stopped, but in reality, hydrogen was released on both copper and zinc.

If you remove the zinc plate from the acid, but leave the copper plate, 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 you remove one of the plates (copper or zinc) from the solution, the voltage drops to zero (there is no electric current in the system). The same will happen if copper and zinc in solution come into contact: electrons will go from zinc to copper directly - bypassing the wire and the tester.

How can we increase the voltage of our galvanic cell? We get the answer if we consider the equations of the ongoing processes:

Zn 0 \u003d\u003e Zn 2+ + 2e -
2H + + 2e - \u003d 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 lower the potential 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 the concentration of hydrogen cations can be increased by adding 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 operates, 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 drop over time.

Another option is to replace zinc with any metal that stands in the electrochemical series of voltages to the left of zinc (i.e., for a metal more active than zinc). The potential of an electrode with such a metal is more positive (all other things being equal). For example, magnesium can be used instead of zinc.

And what will change if instead of copper we take another - less active metal (which in the series of voltages is to the right of copper), for example - silver, platinum, etc.? Will the potential of the electrochemical cell increase? No, since 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 || H 2 SO 4 | H 2.
Zn 0 \u003d\u003e Zn 2+ + 2e -
2H + + 2e - \u003d 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.

__________________________________________________
The term "hydrogen electrode" is put in quotation marks because in a standard hydrogen electrode the plate is not copper, but platinum - this significantly affects its operation.

Strictly speaking, the material of the electrode on which hydrogen is released matters (as it does). - Otherwise, there would be no need to use platinum for a standard hydrogen electrode. But let's not complicate the presentation.

The emergence of e. etc. with. 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 an aqueous solution of sulfuric acid H 2 S0 4. When sulfuric acid dissolves in water, the process of electrolytic dissociation occurs, that is, 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 dissolves in sulfuric acid. When this electrode dissolves, the positive zinc ions Zn + pass into the solution and combine with the negative ions SO 4 - the acid 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. 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, which prevents further transition of positive zinc ions into the electrolyte; the dissolution of the zinc electrode stops. The copper electrode is practically insoluble 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 em. etc. with. E of the considered galvanic cell.

The electromotive force 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. etc. with. the highest, however, in almost all elements used, it does not exceed 1.1 -1.5 V.

When any receiver of electrical energy 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 cell) to the zinc (negative pole). In the electrolyte at this time, the movement of positive ions of zinc Zn + and hydrogen H 2 + 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 enter the electrolyte from the cathode, maintaining a negative charge on this electrode; on the copper electrode, new positive ions will be deposited. Thus, there will always be a potential difference between the anode and cathode, which is necessary for the current to pass through the electrical circuit.

Polarization. The considered Volta galvanic cell cannot work for a long time due to the harmful polarization phenomenon that occurs in it. The essence of this phenomenon is as follows. Positive hydrogen ions H 2 +, heading towards the copper electrode 1, interact with the free electrons available on it and turn into neutral hydrogen atoms. These atoms cover the surface of the copper electrode with a continuous layer 4, which degrades the operation of the galvanic cell for two reasons. First, additional emis- sion arises between the hydrogen layer and the electrolyte. etc. with. (e.d. polarization), directed against the main e. etc. with. element, therefore its resulting e. etc. with. E decreases. Second, 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, special substances are placed around the positive electrode in all galvanic cells - depolarizerswhich readily react with hydrogen. They absorb hydrogen ions approaching the positive electrode, preventing them from being deposited on this electrode.

The industry produces galvanic cells of various types (with different electrodes and electrolytes), which have different designs. The most common are carbon-zinc elements, in which carbon and zinc electrodes are immersed in an aqueous solution of ammonium chloride (ammonia) or sodium chloride, 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 electric flashlights, 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 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 capacity. Capacity is understood as the amount of electricity stored in galvanic cells or batteries. Capacity is measured in ampere-hours. The nominal capacity of a chemical current source is equal to the product of the nominal (calculated) discharge current (in amperes) given by the chemical current source when a load is connected to it, by the time (in hours) until its e. etc. with. will not reach the minimum allowable value. With prolonged operation, the amount of electricity that can be given by the galvanic cell decreases, since the active chemical substances present in it are gradually consumed, which ensure the generation of e. d. with; at the same time, e. etc. with. element and its capacity and its internal resistance increases.

A galvanic cell has a nominal capacity only if a relatively short time has elapsed after its manufacture. The capacity of a galvanic cell gradually decreases, even if it does not release 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 chemical substances stored in them are constantly consumed.

The 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.

Chemical sources of electric current or galvanic cells convert the energy released during the course of redox reactions into electrical energy. Galvanic cells serve as direct current sources. They are classified into chemical and concentration.

The simplest chemical galvanic cell can be made up of two metal electrodes with different electrode potentials and connected in a closed circuit.

On the electrode, which has a lower value of the electrode potential, the oxidation process will take place. Such an electrode is called differently anode.

On the electrode, which has a greater value of the electrode potential, the recovery process will take place. Such an electrode is called differently cathode.

Let us consider in more detail the principle of operation of galvanic cells using the example of a cell composed of zinc and copper electrodes. Such an element is called differently jacobi-Daniel element (fig. 94).

Figure: 94. Diagram of a copper-zinc galvanic cell

Each electrode consists of a metal plate dipped into a salt solution: ZnSO 4 and CuSO 4, respectively.

The salt solutions are separated from each other by a porous partition through which metal ions and SO 4 2- can easily pass. Often, instead of a porous partition, they use " salt bridge »- a curved glass tube filled with a saturated solution of KCl (Fig. 95). In this case, the electrodes are not in contact with each other, each of them is in a separate vessel, which are connected using a salt bridge.

Figure: 95. Diagram of a copper-zinc cell with a salt bridge: 1 - zinc plate; 2 - copper plate; 3 - salt bridge

In this case, the oxidation process takes place on the zinc electrode:

Zn 0 - 2ē \u003d Zn 2+,

as a result of which zinc ions from the plate pass into solution. Excess electrons pass through a metal conductor from a zinc plate to a copper one and reduce the Cu 2+ ions contained in the solution

Cu 2+ + 2ē \u003d Cu 0,

which in the form of neutral atoms are deposited on the plate. The remaining free sulfate ions of the copper electrode and the excess Zn 2+ ions of the zinc electrode move towards each other through the porous partition or salt bridge. Thus, electric charges are transferred in the circuit and an electric current arises.

In this element, electrical energy is obtained as a result of a chemical reaction

Zn + CuSO 4 \u003d Cu + ZnSO 4

The main characteristic of a galvanic cell is electromotive force (emf) , on which the current in the circuit depends. It is equal to the difference of electrode potentials

emf \u003d E 2 - E 1

where E 1 and E 2 - respectively, the potential of the anode and cathode.

For a Jacobi-Daniel cell, the electromotive force is

emf \u003d E Cu - E Zn

The higher the emf value element, the greater the current in its circuit.

According to the Nernst equation, the potential of copper and zinc electrodes is calculated by the formulas:

E Cu \u003d E Cu 0 +

E Zn \u003d E Zn 0 +

Subtracting the second equation from the first, we obtain an expression for calculating the emf. copper-zinc galvanic cell

emf \u003d E Cu 0 - E Zn 0 + \u003d

E Cu 0 - E Zn 0 +

For any other element made up of two metal electrodes, and the operation of which is based on a chemical reaction, the electromotive force can be calculated by the formula:

emf \u003d E 2 0 - E 1 0 +

where E 2 0 and E 1 0 - standard electrode potentials, respectively, of the cathode and anode; n 2 and n 1 - the magnitude of the charges of the ions involved in the half-reactions that occur at the cathode and anode; a 2 and a 1 are the activities of metal ions in solutions at the cathode and anode, respectively).

For a temperature of 298K, when substituting the values \u200b\u200bof the constants R and F and when switching from natural logarithm to decimal, our equation will be written differently:

emf \u003d E 2 0 - E 1 0 + 0.059

Galvanic cells can be designated as a diagram. On the left, an electrode or half-cell with a lower value of the electrode potential (anode) is usually shown, and on the right - with a higher value of the electrode potential (cathode).

When recording electrodes, the solid phase is indicated first (for example, metal in the case of a metal or redox electrode), and then the substances dissolved in the liquid phase. The phases are separated from each other by a single vertical line. If one phase contains several components, then they are written separated by commas.

The interface between the solutions of two electrodes is depicted by a dotted vertical line or two solid lines ½½ (if the solutions are separated from each other by a salt bridge).

In accordance with the above rules, the Jacobi-Daniel element diagram looks like this:

Zn ½ ZnSO 4 ½½ CuSO 4 ½ Cu

A galvanic cell can also be composed of two redox electrodes with different redox potential values. Such cells are also called redox galvanic cells. They also belong to chemical galvanic cells, because their action is based on the course of a chemical reaction.

A galvanic cell, in which the source of energy is not a chemical reaction, but the work of equalizing the concentrations (activities) of ions, is called concentration ... It can consist of two identical metal electrodes immersed in solutions of the same salt, but with different concentration (activity) of metal ions (Fig. 96), for example:

Zn ½ ZnSO 4 ½½ ZnSO 4 ½ Zn or Ag ½ AgNO 3 ½½ AgNO 3 ½ Ag

Figure: 96. Zinc concentration chain: M - salt bridge containing potassium chloride

The electrode, which is in a more dilute solution, dissolves, its ions pass into the solution:

Cu - 2ē ® Cu 2+

Ag - ē ® Ag +

The electrode itself is charged negatively.

On the contrary, metal ions are deposited on an electrode immersed in a more concentrated solution, and it is charged positively. Thus, on both electrodes, processes occur that lead to the equalization of the concentration of metal ions in solutions.

In this case, the potentials of the electrodes are equal:

E 1 \u003d E 0 +; E 2 \u003d E 0 +

Subtracting the first equation from the second, we obtain the formula for calculating the emf. from the concentration element:

emf \u003d E 2 - E 1 \u003d

The concentration element will work until the activities of metal ions in both solutions become equal; at a 1 \u003d a 2 its emf will be equal to 0.

O.S.ZAITSEV

LEARNING BOOK ON CHEMISTRY

FOR SECONDARY SCHOOL TEACHERS,
STUDENTS OF PEDAGOGICAL UNIVERSITIES AND SCHOOL PERSONS OF 9-10 CLASSES,
DECIDING TO DEDICATE CHEMISTRY AND NATURAL SCIENCE

LABORATORY PRACTICE BOOK SCIENTIFIC STORIES FOR READING

Continuation. See # 4-14, 16-28, 30-34, 37-44, 47, 48/2002;
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,
24, 25-26, 27-28, 29, 30, 31, 32, 35, 36, 37, 39, 41, 42, 43, 44, 46, 47/2003;
1, 2, 3, 4, 5, 7, 11, 13, 14, 16, 17, 20, 22, 24, 29, 30, 31, 34/2004

§ 8.2. Metal-solution interface reactions

(continued)

Let's make a circuit of two electrodes, for example, copper and zinc. Let's discuss three options for such a galvanic cell.
Let's say right away that we will not be interested in the first option. Immerse the zinc and copper plates in a glass with a solution of their salts - zinc and copper sulfates (Fig. 8.6). We connect the electrodes with conductors through a voltage measuring device - a voltmeter, which is indicated in the figure with the symbol "B".

Both zinc and copper send their ions into the solution, but the equilibrium of the corresponding reactions is shifted towards metals, since they are not in pure water, but in a solution containing the ions of these metals. Despite this, zinc has a higher ability to send ions into the solution and has a higher negative electrode potential. Therefore, copper ions will rush to the zinc electrode, and copper is formed on the zinc:

Zn + Cu 2+ \u003d Zn 2+ + Cu.

The transition of electrons occurs directly on the zinc surface, the potential difference between the plates does not arise, and the voltmeter will not show voltage.
Let's change the experience. Place a porous ceramic partition in the vessel (Figure 8.7).

The electrons leave the zinc and pass through the conductor through the voltmeter to copper, where they interact with copper ions, as a result of which copper is deposited on the copper electrode. At the same time, zinc ions pass into solution.
The porous baffle serves to prevent the approach of copper ions to zinc and thereby prevent the direct transfer of electrons from zinc to copper ions instead of passing through the conductor. As the reaction progresses, zinc ions move from zinc to copper, the same happens with copper ions.
The porous septum poorly prevents mixing of solutions, and, in addition, the manufacture of vessels with a porous septum is difficult, so you can proceed as follows. Take two glasses, pour solutions into them, which we connect with an electrolytic bridge - a U-shaped glass tube filled with a saturated solution of potassium chloride (Fig. 8.8).
Cotton swabs are inserted into the ends of the tubes so that the liquid from the bridge does not pour out.

So, the porous baffle has been replaced with an electrolytic bridge. In it, chloride ions move to the zinc electrode, and potassium ions move to the copper electrode. The bridge separates the electrode spaces, prevents electrical conductivity due to the movement of zinc and copper ions, and lowers the potential that occurs when two different solutions come into contact. An additional potential also arises when ions move at different speeds, and potassium and chloride ions move at almost the same speeds.
Let's make a chain (see Fig. 8.8) of standard copper and zinc electrodes (concentration of metal ions in solutions of 1 mol / l). Let's determine the direction of the reaction in this galvanic cell and its EMF:

The potential of the zinc electrode has a negative sign, and that of the copper one is positive. Consequently, the zinc electrode has a greater ability to donate electrons, and an opposite reaction will take place on it, and the copper electrode will receive electrons:

Thus, if we immerse a piece of metallic zinc in a solution of copper sulfate, the zinc will pass into the solution in the form of ions and at the same time a layer of copper will be deposited on it.
There is a reaction in the list of electrode potentials:

2H + (10 –7 M, water) + 2 e \u003d H 2 (g), E \u003d -0.41 V.

This is the potential of a hydrogen electrode in water. All metals that are located in the list above and whose electrode potentials have higher negative values \u200b\u200bmust react with water (“dissolve”) to form hydrogen. But you know perfectly well that iron, chromium, zinc, aluminum do not react with water under normal conditions. Magnesium reacts with hot water, while sodium, calcium, potassium, and lithium react with water under normal conditions. This is due to the fact that poorly soluble oxide films are formed on iron, chromium, zinc, aluminum, excluding the access of water to the metal. When the oxide layer is removed, these metals begin to interact with water. Oxides or hydroxides of sodium, calcium, potassium, lithium are soluble in water and do not protect metals from contact with water.
For electrode reactions, potentials and EMF, all those formulas that we previously derived for redox reactions are applicable:

G \u003d–nЕF= H – TS = –RTln K \u003d–nЕ 96 484 = –2,303 8,314 Tlg TO.

When calculating the equilibrium constant, remember that crystalline phases (metals) are not written into the expression for the equilibrium constant, because the concentration of the crystalline substance does not depend on its amount, i.e. constant. For example:

Electrode potentials and EMF of electrochemical reactions are highly dependent on the concentration of ions and the pH of the medium. Therefore, the direction of the process often predicted for standard conditions does not coincide with that which is carried out under the given conditions.

For information on how to determine the direction of a reaction under non-standard conditions, see high school chemistry textbooks.

List of new and forgotten concepts and words

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 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, a DES also appears on a copper plate and equilibrium is established

Cu ⇄ Cu 2+ + 2ē

Therefore, an electrode potential of copper appears, and the electrode circuit will have the form Cu | CuSO 4 or Cu | Cu 2+.

On the Zn electrode (electrochemically more active), the oxidation process takes place: Zn - 2ē → Zn 2+. On the Cu-electrode (electrochemically less active), the reduction process takes place: Cu 2+ + 2ē → Cu.

Figure: 6 Diagram 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 scheme of a chemical galvanic cell is written according to the "right plus" rule, the scheme of the Jacobi – Daniel cell will have the form

The double line in the diagram denotes 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, a 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 down conductor.

Figure: 7 Diagram of dry zinc-manganese cell

1 - anode (zinc cup), 2 - cathode (mixture of manganese dioxide with graphite), 3 - graphite down conductor with a metal cap,

4 - electrolyte

The electrolyte used in zinc-manganese cells, containing ammonium chloride, due to hydrolysis of NH 4 CI has a weakly acidic reaction. In an acidic electrolyte, a current-forming process takes place on the positive electrode:

MnO 2 + 4H + + 2ē → Mn 2+ + 2H 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 an incomplete manganese (III) hydroxide - manganite.

As hydrogen ions are consumed in the current-forming process, the electrolyte becomes neutral or even alkaline from acidic. It is not possible to maintain an acidic reaction in the salt electrolyte during the discharge of the cells. It is impossible to add acid to the salt electrolyte, as this will cause 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 - getaerolite, and the solution is acidified:

2MnOOH + Zn 2+ → ZnO ∙ Mn 2 O 3 + 2Н +

The formation of getaerolyte protects the electrolyte from alkalizing too much when the cell is discharged.