Half cell

The half cell ( half element) is part of the galvanic element . It consists of an electrode (e.g. metal rod, sheet metal, graphite) that is immersed in an electrolyte (often the metal salt solution corresponding to a metal electrode). A zinc half-cell is obtained by immersing a zinc electrode in an acid, alkali or salt solution ( e.g. zinc sulfate solution). The half-cells of a galvanic element are differentiated according to the respective metal (cf. zinc half-cell, copper half-cell, etc.) and their function, which they assume in the cell reaction ( donor half-cell , acceptor half-cell ).

From the half-cell to the galvanic element

A galvanic element can be built from two half-cells by connecting the two half-cells to one another via an ion conductor, e.g. B. with an electrolyte bridge or by immersion in a common electrolyte. In order to use the galvanic element, the electrodes of the two half-cells also have to be connected electrically with an electron conductor - for example with metal wire. The circuit outside the cells is closed by the electron conductor and between them by the ion-conducting connection between the metal salt solutions. The Daniell element as an example of a galvanic element is composed of a zinc half-cell and a copper half-cell. B. be connected by an ion-conductive, solution-saturated porous earthenware crucible.

Chemical processes in half cells

Immediately after the metal electrodes are immersed in the corresponding metal salt solution, processes take place on the metal surface that cause the metal electrodes to charge. Base metals have a tendency to oxidize in aqueous solution; in this case, metal atoms detach from the metal lattice of the electrode and go into solution as metal ions. The electrons released during the oxidation of the metal atoms remain bound in the metal on its surface. In this way the metal surface becomes negatively charged. Since the metal ions formed are always positively charged, they are bound to the metal surface as a result of this negative charge. This creates a so-called electrical double layer within the phase interface between the metal surface and the metal salt solution. Negative (electrons) and positive charges (metal ions) are balanced in it, since the number z of electrons formed per metal ion corresponds to the number of charges of the corresponding metal ions. In a zinc half-cell, for example, two electrons are released for each doubly positive zinc ion formed, so that negative and positive charges are always balanced.

Within the electrical double layer, a dynamic equilibrium between the corresponding redox pairs metal elements / metal ions is established after a short time in each half-cell . In the zinc half-cell of the Daniell element, equilibrium is established on the metal surface after a short time

${\displaystyle Zn\;{\overrightarrow {\leftarrow }}\;Zn^{2+}+2e^{-}}$

a. In the copper half-cell of the Daniell element there is a copper sulphate solution, which is usually saturated with a new cell. Here, too, equilibrium is established on the metal surface after a short time

${\displaystyle Cu\;{\overrightarrow {\leftarrow }}\;Cu^{2+}+2e^{-}}$

a.

Creation of an electrical voltage between two half-cells

The decisive factor with regard to the negative charge, however, is that the equilibria mentioned above have a different equilibrium position in the equilibrium state. This equilibrium position depends on the size of the solution tension of the corresponding metal, which is very different in size depending on the metal. This in turn is related to the position of the metals in the redox series of metals .

The reason for this is that the dissolution tension of a metal corresponds to the tendency of the metal atoms to act as a reducing agent. Because the solution tension corresponds to the oxidation of the metal atoms, i.e. H. the release of electrons, which are thus released and can reduce other particles (see reduction ). This reducing power of metals is documented in the redox series of metals .

The metal zinc, for example, is less stable and therefore less noble. It has a greater tendency to act as a reducing agent (i.e., to oxidize itself) in redox reactions. Its dissolution tension in aqueous solution is therefore greater than that of more noble metals such as tin, copper or silver. Therefore the balance lies in the zinc half-cell

${\displaystyle Zn\;{\overrightarrow {\leftarrow }}\;Zn^{2+}+2e^{-}}$

further to the right than the equilibrium in the copper half- cell of the Daniell element :

${\displaystyle Cu\;{\overrightarrow {\leftarrow }}\;Cu^{2+}+2e^{-}}$

If you take a closer look at the equilibria, you can see that, depending on the equilibrium position, different amounts of excess electrons (electron reserves, electron pressures ) are also accumulated in the metal electrodes. The two half-cells now differ as locations of higher and lower electron pressure. Since the above-mentioned equilibrium on the zinc electrode is further to the right due to the higher dissolution tension of the zinc, the zinc electrode is more negatively charged than the copper electrode, because copper, as a more stable and noble metal, has a lower dissolution tension (cf. the position of the metals in Redox series of metals ). Thus the zinc electrode is the location of the higher electron pressure, the copper electrode the location of the lower electron pressure.

In this way, an electrical voltage is created between the two half-cells of the Daniell element. One often speaks of a potential difference . The reason is that the equilibrium position of general equilibrium

${\displaystyle Me\;{\overrightarrow {\leftarrow }}\;Me^{z+}+ze^{-}}$

with the level of the electron pressure (see above) also the electrochemical potential of a metal (Me) is determined. The further this equilibrium is to the right, the higher the electron pressure and the more negative the electrochemical potential of the metal. The magnitude of the electrochemical potentials of metals under standard conditions is documented quantitatively in the voltage series of the metals . The more negative the standard electrode potential of a metal in the voltage series of metals, the greater the electron pressure that this corresponding metal develops in a half-cell of a galvanic element and the greater (qualitatively speaking) its reducing power. Therefore the position of the metals in the redox series of the metals also corresponds to their position in the voltage series of the metals. If you combine two half-cells of metals with different electrochemical potential in a galvanic element, a potential difference arises which corresponds to the concept of electrical voltage. This potential difference thus corresponds to the electron pressure difference described above between the two half-cells.

Factors influencing the voltage level between two half-cells

Each galvanic element develops an electrical voltage in the manner described above. The size of the electrical voltage depends on two essential factors that result from the cause of the generation of the voltage, the different equilibrium positions:

1. The size of the tension depends on the material system. This means that the voltage is determined by the half-cell selection. Thus, the Daniell element (i.e. the galvanic element made of a zinc and a copper half-cell) with U = 1.11 V (under normal conditions) develops a different voltage than the galvanic element made of a magnesium and a silver half-cell with U = 3.06 V (under standard conditions). The reason for this is that, depending on the choice of half-cell, the differences in the solution dimensions of the metals are different.
2. The size of the voltage depends on the concentration of the metal salt solutions. This means that a voltage can be developed even in a galvanic element made up of two identical half cells if the electrolyte solutions have different concentrations. Such arrangements are then called concentration cells or concentration chains. The reason for this factor is that equilibria are established at the electrodes as described and, according to the LeChâtelier principle (see chemical equilibrium ), these are disturbed by changes in concentration with regard to their equilibrium position.

Current flow between two half cells and cell reaction

As long as the resistance between the two conductively connected electrodes is high, there is no current flow as a result of the discharge of the voltage and thus the resulting voltage remains constant. However, if a current flow is enabled by lowering the resistance between the two conductively connected electrodes (e.g. connecting a small motor instead of a voltmeter), the voltage is reduced and thus an exchange of electrons between the two half-cells occurs. As a result of the voltage, an electromotive force then acts between the two electrodes. It drives the electrons from the location of the higher electron pressure to the location of the lower electron pressure, so that the electron pressure difference, the voltage, gradually equalizes. Thus, electrons are released from the half-cell of the higher electron pressure to the half-cell of the lower electron pressure. Hence the half-cell of the higher electron pressure is called. H. with the metal that has the more negative electrode potential as the donor half-cell , the other half-cell (as the "electron-receiving cell") as the acceptor half-cell .

The cell reaction of the galvanic element takes place as a result of the imbalance on the electrodes caused by the current flow. This means that electrons flow from the zinc half-cell to the copper half-cell in the Daniell element . The consequence of this is that the size of the negative charge on the zinc electrode decreases, so that the balance described above between negative (electrons) and positive charges (metal ions) becomes unbalanced. Since the negative charge on the zinc electrode is reduced, zinc ions can now detach from the electrical double layer and diffuse into the solution. According to the LeChâtelier principle, the balance shifts

${\displaystyle Zn\;{\overrightarrow {\leftarrow }}\;Zn^{2+}+2e^{-}}$

accordingly to the right, d. H. The oxidation takes place to a greater extent in the zinc half-cell. In the copper half-cell, on the other hand, the electrons flowing in ensure an increased reduction of copper ions from the copper salt solution. The reduction of copper ions to copper thus takes place at the copper electrode, so that the equilibrium is achieved

${\displaystyle Cu\;{\overrightarrow {\leftarrow }}\;Cu^{2+}+2e^{-}}$

further to the left. Thus, the oxidation takes place more intensely in the zinc half-cell and the reduction takes place more intensely in the copper half-cell. The zinc electrode is called the anode (electrode on which the oxidation takes place) and the copper electrode as the cathode (electrode on which the reduction takes place). The processes taking place can thus be summarized in the cell reaction that follows

${\displaystyle Zn+Cu^{2+}\;{\overrightarrow {\leftarrow }}\;Zn^{2+}+Cu}$

runs. In this cell reaction, the zinc half-cell is the donor and the copper half-cell is the acceptor half-cell .

Half-cell processes during the cell reaction and the end of the reaction

During the cell reaction, the potential difference is not only reduced simply as a result of the current, but above all because of the processes taking place in the half-cells. In the Daniell element, oxidation takes place to a greater extent during the cell reaction in the zinc half-cell (donor half-cell). H. zinc ions are increasingly formed. As a result, the mass of the zinc electrode decreases during the cell reaction and the concentration of zinc ions in the zinc half-cell increases. This has an effect on the balance

${\displaystyle Zn\;{\overrightarrow {\leftarrow }}\;Zn^{2+}+2e^{-}}$

on the zinc electrode. Because of the increasing zinc ion concentration during the cell reaction, this equilibrium shifts increasingly in the direction of reduction, according to the principle of Le Châtelier. H. the initially weak reduction gains strength and gradually overtakes the initially strong oxidation in the zinc half-cell. In the course of the cell reaction, a new equilibrium is established at the zinc electrode.

The reverse process takes place in the copper half-cell. As a result of the strong reduction of copper ions to copper there, the mass of the copper electrode increases during the cell reaction and the concentration of copper ions in the copper salt solution decreases. This also has an effect on the equilibrium in the copper half-cell

${\displaystyle Cu\;{\overrightarrow {\leftarrow }}\;Cu^{2+}+2e^{-}}$

result. According to Le Châtelier's principle, the equilibrium shifts increasingly in the direction of oxidation as a result of the decreasing copper ion concentration, i.e. H. the initially weak oxidation gradually overtakes the initially strong reduction in the copper half-cell. In the course of the cell reaction, a new equilibrium is established in the copper half-cell.

The cell response, i.e. H. The reactions on the electrodes caused by the electron exchange between the two half-cells finally come to a standstill when the new equilibrium has been established on the two electrodes as described, i.e. H. the oxidation and reduction strength are identical at both electrodes. Because then there is no longer any voltage between the electrodes, so that no more electron transfer takes place and the cell reaction as a redox reaction ( electron exchange reaction ) is thus ended. Since the cell reaction can also be reversed, the entire cell reaction is then in a state of equilibrium.