Half-cell

The half-cell ( half cell ), is part of the galvanic element. It is composed of a metal electrode ( metal rod, sheet metal, etc.) that plunges into its corresponding metal salt solution (electrolyte). A zinc half-cell is obtained by reacting, in a zinc salt solution (eg, zinc sulfate solution ) immersed a zinc electrode. The half-cells of a galvanic element are distinguished according to the respective metal ( cf. zinc half-cell, copper half-cell, etc.) and their function in which they exist in the cell reaction ( Donatorhalbzelle, Akzeptorhalbzelle ).

Of the half-cell for electrochemical element

From two half- cells, a galvanic element can be constructed in an appropriate experimental design, by connecting the two half-cells conductive manner. The connection between the two half-cells consists of an electron conductor ( such as a metal wire ), which connects the two metal electrodes electrically conductive, and an ion conductor (for example, an electrolyte bridge ), which closes the circuit through the ion conductive path between the metal salt solutions. The Daniell cell as an example of a galvanic element is composed of a zinc half-cell and a copper half-cell.

Chemical processes in half-cells

Immediately after the immersion of the metal electrodes in the appropriate metal salt solution play on the metal surface from certain operations, which have a negative charge of the metal electrodes. The cause of the onset of processes is the desire of each metal to oxidize in aqueous solution. As a result of this, referred to as solution pressure behavior of each metal to metal atoms released from the metal mesh of the electrode and go as metal ions in solution. Released during this oxidation of the metal atoms remain bound electrons in the metal at the surface thereof. In this way, the metal surface charges negatively. Since the metal ions formed are always positively charged, they are attached as a result of this negative charge on the metal surface. This results in the phase interface between the metal surface and the metal salt solution is a so-called electrical double layer. In it negative (electrons) and positive charges (metal ions) from the same as the number z of electrons formed per metal ion with the charge number of the corresponding metal ions corresponds. Thus, for example, in a copper half-cell per formed doubly positive copper ion and two electrons free, so that there is always compensate for negative and positive charges.

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

One. In the copper half-cell of the Daniell element is an analogous process takes place. Again oxidize due to the solution tension of the copper atoms copper to copper ions, so that after a short time on the metal surface, the equilibrium

Sets. In this way, the metal electrode load in the two half-cells to negative.

Forming an electrical voltage between two half-cells

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

The reason for this is that the solution pressure of a metal corresponding 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., the electron donation, which are thus freely and reduce other particles ( see reduction) can. This reduction capacity of metals is documented in the redox series of the metals.

The metal zinc, for example, is more volatile and therefore less noble. It has a greater tendency to oxidation-reduction reactions to act as a reducing agent (i.e., to oxidize themselves). His solution pressure in aqueous solution is therefore greater than the more precious metals such as tin, copper or silver. Therefore, the equilibrium lies in the zinc half-cell

Further to the right than the balance of the copper semi- cell of the Daniell element:

Considering the equilibria more closely, one sees that form corresponding to the equilibrium position, different amounts of electrons (electron inventories, electron pressures) in the metal electrodes. The two half- cells can therefore be distinguished now as places of a higher and a lower electron pressure. Since the corresponding above-mentioned equilibrium is due to the zinc electrode is due to the higher solution pressure of zinc to the right, the zinc electrode invites more negative than the copper electrode because copper as more stable and more noble metal has a lower solution pressure (see the position of the metals in the the redox metals). Thus, the zinc electrode is the location of the higher electron pressure, the copper electrode of the location of the lower electron pressure.

In this way formed between the two half- cells of the Daniell element an electric voltage. Frequently one also talks of a potential difference. The reason for this is that the equilibrium position of general equilibrium

With the height of the electron pressure ( see above) also determines the electrochemical potential of a metal ( Me). The more this equilibrium is on the right side, the higher is the electron pressure, and the more negative the electrochemical potential of the metal. The size of the electrochemical potentials of metals under standard conditions is quantitatively documented in the electrochemical series of metals. So, the negative is the standard electrode potential of a metal in the electrochemical series of metals, the greater is the electron pressure, the development of this particular metal in a half-cell of a galvanic element and the greater (as seen qualitatively) be reducing power. Therefore, the position of the metals corresponds to the redox of metals also their position in the electrochemical series of metals. Combining in a galvanic element in accordance with the two half cells of metals of different electrochemical potential, there arises a potential difference, which corresponds to the concept of the electric voltage. Thus, this potential difference corresponds to the electron pressure difference between the two half cells described above.

Factors influencing the magnitude of voltage between two half- cells

In the manner described above, each element develops an electrical voltage. The size of the electric power depends on two main factors that result from the cause of the development of tension, the different equilibrium positions:

1 The magnitude of the voltage depends on the material system. This means that the voltage is determined by the half-cell selection. Thus developed the Daniell cell (ie, the galvanic element of a zinc and a copper half-cell ) with U = 1.11 V ( at standard conditions ) a voltage other than the galvanic element 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 half-cell selection, the differences in the solution Tens ions of the metals are different in size.

2 The magnitude of the voltage depends upon the concentration of the metal salt solutions. Thus, one can self develop in a galvanic element comprises two identical half-cell voltage, when the electrolyte solutions having different concentrations. Such arrangements then called concentration cells and concentration cells. The reason for this factor is that set at the electrodes as described equilibria and this according to the principle of LeChâtelier (see chemical equilibrium ) are disturbed by changes in concentration with respect to their equilibrium position.

Current flow between two half cells and cell reaction

As long as the resistance between the two conductive electrodes connected to one another is high, a current flow as a result of the discharge voltage remains off and thus the voltage resulting constant. But it allows current to flow by the resistance between the two conductive interconnected electrodes is lowered ( for example, instead of a voltmeter connecting a small motor ), so it comes to the reduction of tension and thus an exchange of electrons between the two half cells. Due to the tension then acting between the two electrodes, an electromotive force. Drives the electrons from the location of the higher pressure to the location of the electron- electron lower pressure so that the electrons pressure difference, the voltage gradually equalizes. Thus, electrons are released from the half-cell of the higher pressure to the electrons of the half-cell lower electron pressure. Therefore refers to the half- cell of the higher electron pressure, i.e., with the metal having the more negative electrode potential than Donatorhalbzelle, the other half-cell (called " electron- receiving cell "), as Akzeptorhalbzelle.

Due to the loss of balance at the electrodes by the current flowing the cell reaction of the galvanic element occurs. Thus in the Daniell cell flow electrons from the zinc half-cell to the copper half-cell. The consequence of this is that the size of the negative charge back to the zinc electrode, so that the balance between the above described negative (electrons) and the positive charges ( metal ions) falls into imbalance. Since the negative charge back to the zinc electrode, the zinc ions can be released from the electric double layer and diffuse into the solution thus now. According to the principle of LeChâtelier the equilibrium shifts

Corresponding to the right, that is, in the zinc half-cell takes place, the enhanced oxidation. In the copper half-cell, however, the incoming electrons provide for enhanced reduction of copper ions from the copper salt solution. On the copper electrode thereby reducing copper ions to copper is enhanced instead so that the equilibrium

Further shifts to the left. Thus, the reduction is now in the zinc half-cell enhances the oxidation in the copper half-cell reinforced instead. The zinc electrode is referred to so as anode ( electrode on which oxidation takes place), and the copper electrode as the cathode ( electrode on which reduction takes place). The processes taking place can thus be summarized in the cellular response after

Runs. In this cell reaction, the zinc half-cell is the donor, the copper half-cell the Akzeptorhalbzelle.

Half-cell processes during the cell reaction and reaction end

During the reaction cell, the potential difference is not simply due to the reduced flow, but also because of the processes taking place in the half-cells. The Daniell cell, the oxidation takes place during the cellular response in the zinc half-cell ( Donatorhalbzelle ) amplified instead, i.e. it is formed of zinc ion increased. As a result, during the cell reaction, the mass of the zinc electrode decreases and the concentration of zinc ions in the zinc half- cell. This has an effect on the balance

At the zinc electrode. Because due to the increasing during cell reaction zinc ion concentration shifts this balance according to the principle of Le Chatelier increasingly in the direction of reduction, that is, the first weak reduction gains strength and gets the first strong oxidation in the zinc half-cell gradually. In the course of the cell reaction is on the zinc electrode, so a new equilibrium.

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

Result. According to the principle of Le Chatelier, the equilibrium displaces due to the decreasing concentration of copper ions increasingly in the direction of oxidation, that is, the initially weak oxidation brings the first significant reduction in the copper half-cell gradually. Over the cell reaction is thus also in the copper half-cell a new equilibrium.

The cell reaction, i.e. induced by the electron transfer between the two half-cell reactions at the electrodes, and finally comes to a standstill, where there has been set at the two electrodes as described, the new equilibrium, i.e., at both electrodes, the oxidation and the reduction level are equal. Because then there is no voltage between the electrodes, so that occurs no electron transfer and the cell reaction is thus completed as a redox ( electron transfer reaction). Since the cell reaction is reversible, then the total cell reaction is at equilibrium.

Electrolysis
Galvanic cell

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