Galvanic cell

A galvanic cell [ gal ː nɪʃə t͡sɛlə ], galvanic cell or galvanic cell is a device for spontaneous conversion of chemical energy into electrical energy. Each combination of two electrodes and an electrolyte is referred to as a galvanic cell, and serve as direct- voltage sources. The characteristic value is the impressed voltage. Under the capacity of a galvanic element is defined as the product of discharge current multiplied by time.

The name goes back to the Italian physician Luigi Galvani. He discovered that a wetted with instruments dissimilar metal frog legs nerve triggers muscle twitching, because as a galvanic element thus formed redox system builds tension so that current flows.

General

Galvanic cells are systematically divided into three groups:

• Primary cells, also colloquially referred to as a battery. It is characteristic that after assembling the cell is charged and can be discharged once. The discharge is irreversible, and the primary cell can not be electrically charged.
• Secondary cells, also colloquially referred to as an accumulator or battery short. After a discharge secondary cells can be recharged by a counter-rotating with respect to the discharge current direction again. The chemical processes in the cell run, limited by the number of cycles, reversible from. The energy density of the secondary cell is smaller as compared with the primary cells with an identical temperature.
• Fuel cells, referred to as tertiary cells. In these galvanic cells, the chemical energy is not stored in the cell, but provided externally continuously available. The external supply enables a continuous and perpetual operation in principle.

The function of the galvanic cells based on a redox reaction. Reduction and oxidation run spatially separated from each in a half-cell ( half-element ). By joining the two half-cells with an electron conductor and an ionic conductor, the circuit is closed. The voltage of the electric current can be calculated by the Nernst equation. It depends on the kind of metal ( electrochemical series ), the concentration in solution of the respective half-cell as well as the temperature. Unlike electrolysis, for example, in electroplating, in the galvanic cell electrical energy can be recovered during the electrolysis requires electrical energy. In most cases the anode is the negative pole and, accordingly, the cathode of the positive pole, however the distinction is easier if one remembers that the reduction takes place at the anode, and the oxidation of the cathode. The galvanic cell provides so long as a voltage to the electrochemical equilibrium is reached.

Examples

Whenever two dissimilar metals are in an electrolyte solution, a voltage is created ( galvanic cell). This is to go to the particular tendency of the metals in solution, thereby forming ions, the so-called solution tension due. Next to the Daniell cell (copper / zinc) can for example also of copper and silver electrodes, a galvanic cell is generated: the copper electrode is immersed in a copper sulfate solution, the silver electrode in the silver nitrate solution, and is connected, this will be a wire ( electron conductor ) with voltmeter and an ion conductor.

However, when both electrodes are connected to one another via an electrical conductor, the solution pressure of the different electrodes ensures that the reaction can continue. Since the redox potential of the copper (reducing agent) is lower than that of silver (oxidizing agent ), going to the copper electrode more ions in solution and on the silver electrode. Therefore, the negative charges in the electrode is higher than the copper in the silver electrode, that is, it produces a voltage at which the electrons to the silver electrode to be " pushed " out. This means that the solution of the silver atoms is stopped, instead, the excess electrons react with the Ag ions of silver nitrate solution and ensure that this set than regular silver atoms on the silver electrode.

Of the silver electrode so the silver ions are reduced to elemental silver:

The silver electrode so that the cathode ( electrode at which the reduction occurs ) and the positive terminal of the voltaic cell ( since a shortage of electrons generated ).

At the copper electrode, however, the following oxidation takes place:

The copper electrode is the anode ( electrode on which oxidation takes place), and the negative terminal of the electrochemical cell ( as there is a surplus of electrons generated ).

In the galvanic cell contains a redox reaction takes place, which reaction components are, however, separated from one another spatially.

Although, the two electrodes so now connected electrically conductive, so there is a tension, but there is still no current flows. The reason for this is that in the copper sulfate solution, an excess of Cu2 ions is formed and the solution is charging strongly positive, which further prevents copper atoms can dissolve.

A similar thing happens with the silver nitrate solution, which negatively charged, since the neutral silver nitrate only the negatively charged nitrate ions remain ( while attach the positive silver ions in the silver electrode by incorporating therein one electron ).

Silver nitrate solution: c [ NO3- ] >> c [Ag ]

Copper sulphate solution: c [ SO42 - ] << c [ Cu2 ]

Therefore, the electrode chambers on an ion bridge ( salt bridge ) are connected together, which is necessary to close the circuit. The ion bridge is often an U- tube is filled with an electrolyte, and whose ends are provided with a membrane or diaphragm. About the salt bridge carried the ion exchange to counteract the charging of individual cells. Another way of separating the electrode spaces from each other, consists in an selektivpermeablen (selected permeable ) membrane which also provides a load balancing. Ions migrate through the bridge so that the salt ions (in this case, nitrate ions ) from the cathode to the anode, that of the silver half cell to the copper half-cell.

There are also galvanic cells with two identical half-cells, which differ in their concentration, this concentration is called member.

The Deflagrator is also a Galvanic cell.

Shortened Form

A galvanic cell is also sometimes described in abbreviated form. The graphic shown above Daniell element would look in this notation as follows:

Both half- cells are in a galvanic cell separated by a diaphragm, which is a thin, semi- permeable membrane ( semipermeable membrane). Through this membrane, the negatively charged anions are almost exclusively pass through. In the case of the element are Daniell SO42 - ions ( sulfate ions) which are present in the salt solutions of both half-cells.

This diaphragm is in condensed form by the double pipe | | shown. Right and left of this double line is that the two half-cells of the galvanic cell and the fact the reactions taking place are shown in abbreviated form. In addition, the concentration of the metal salt solution, that the concentration of dissolved metals is given in the two half cells. The anode half-cell is usually left.