The galvanic cell or voltaic cell is an electrochemical cell that obtains electrical energy from spontaneous redox reactions that take place within it. It acquires its name in honor of Luigi Galvani and Alessandro Volta respectively. They occur through a device in which the transfer of electrons (from the oxidation half-reaction to the reduction half-reaction) occurs through an external circuit instead of occurring directly between the reactants; in this way the flow of electrons ( electric current ) can be used.
Summary
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- 1 History
- 2 Characteristics and uses
- 3 Operation of a galvanic cell
- 1 Components
- 4 Electromotive force of a cell
- 1 Function of the salt bridge in the galvanic cell
- 5 Galvanic in aesthetics
- 6 Fountains
History
In the 17th century , Otto Von Guericke invented the first machine to produce electricity. In the second half of the eighteenth century , Luigi Galvani Aloisio began research on the therapeutic application of electricity. After ten years of published research on the forces of electricity on muscle movements, he concluded that muscles store electricity (in the same way as a Leiden bottle) and nerves are responsible for carrying out electricity. In the 18th century, Alessandro Volta, putting Luigi Galvani’s experience into practice, discovered something curious. He found that if two metalsdifferent metals come into contact with each other, one of the metals being slightly negative and the other slightly positive. A potential difference, a voltage , is established between them . Using this experience as a basis , he conceived of a cell , which he called a voltaic pile .
Characteristics and uses
If there is only one system comprised of the same redox couple (the oxidized species and the same species but reduced) then the potential will be the same at all points and no current will be produced. This implies that each species that can be oxidized or reduced has an oxidized species that will act as an oxidant as long as its redox potential is lower. Note that two oxidants cannot react and neither can two reductants, there must be one oxidizing agent and one reducing agent. The oxidizing agent must have a higher potential than that of the reducing agent pair. When the above condition is met, a difference in electrical potential (electrochemical in this case) is naturally generated, which can be measured in Volts ( Volts ).), that is, the movement of electrons with a potential difference generates electrical energy.
Galvanic cells are normally used as a source of electrical energy. By their very nature they produce current. For example, a lead- acid battery contains a number of galvanic cells. The two electrodes are effectively lead and lead oxide .
In a galvanic or voltaic (spontaneous) cell, from the partial reactions at the electrodes, a potential difference is generated and an electric current is obtained. It typically consists of two dissimilar metals connected by a salt bridge , or individual half-cells separated by a porous membrane. Volta was the inventor of the voltaic battery, the first electric battery. In common usage, the word battery is a single galvanic cell and a battery proper which consists of several cells, connected in series or parallel.
Although the terms galvanic and voltaic are generally used interchangeably to designate spontaneous electrochemical cells, a distinction can be made between them. A galvanic cell refers to the Galvani experiment, since in it, two half cells are separated from each other by a liquid junction (porous wall, salt bridge, etc.), for example the Daniell(a) cell. The voltaic cell, on the other hand, is a spontaneous cell made up of a single liquid (b). Image Daniel
Functioning of a galvanic cell
The operation of the cell is based on the principle that the oxidation of Zn to Zn2+ and the reduction of Cu2+ to Cu can be carried out simultaneously, but in vessels separated by a salt bridge, with the transfer of electrons, e-, through an external metallic conductor wire. The zinc and copper sheets are electrodes.
Oxidations occur in the anodic half-cell, while reductions occur in the cathodic half-cell. The anodic electrode conducts the electrons that are released in the oxidation reaction towards the metallic conductors. These electrical conductors conduct the electrons and take them to the cathode electrode; the electrons thus enter the cathodic half-cell, producing the reduction in it.
Functioning of a galvanic cell
Components
The galvanic cell consists of a sheet of metallic zinc, Zn (anodic electrode), immersed in a solution of zinc sulfate , ZnSO 4 , 1 M (anodic solution) and a sheet of metallic copper, Cu (cathodic electrode), immersed in a solution of copper sulfate, CuSO 4 , 1 M (cathodic solution).
The electrodes are the contact surface between the metallic conductor and the half-cell solution (anodic or cathodic). If the electrode does not participate in the redox reaction (it is neither oxidized nor reduced), it is called an inert or passive electrode. When it participates in the redox reaction, as in this case, it is called an active electrode. Oxidations occur in the anodic half-cell, while reductions occur in the cathodic half-cell. The anodic electrode conducts the electrons that are released.
In the oxidation reaction, towards metallic conductors. These electrical conductors conduct the electrons and take them to the cathode electrode; the electrons thus enter the cathodic half-cell, producing the reduction in it. The electrode at which oxidation occurs is the anode and at which reduction takes place is the cathode. The electrons are freed as the metallic zinc oxidizes at the anode; they flow through the external circuit to the cathode, where they are consumed as Cu2+(aq) is reduced.
As the Zn(s) is oxidized in the cell, the zinc electrode loses mass and the concentration of Zn2+(aq) in the solution increases with cell operation. Similarly, the copper electrode gains mass and the Cu2+(aq) solution becomes less concentrated as it is reduced to Cu(s). Anode (oxidation) Zn(s) → Zn 2+ (aq) Cathode (reduction) Cu 2+ (aq) + 2e- → Cu(s)
The signs that we assign to the electrodes of a voltaic cell. We have seen that electrons are released at the anode as the zinc oxidizes and flow into the external circuit. Since electrons have a negative charge, we attach a negative sign to the anode. Rather, the electrons flow to the cathode, where they are consumed in the reduction of copper. Consequently, a positive sign is conferred on the cathode because it appears to attract negative electrons.
With cell operation, oxidation of Zn introduces additional Zn2+ ions into the anode compartment. Unless a means is provided to neutralize this positive charge, no further oxidation can take place. Similarly, the reduction of Cu2+ at the cathode leaves an excess of negative charge in solution in that compartment. Electrical neutrality is preserved by ion migration through a salt bridge or, as in this case, through a porous barrier that separates the two compartments. Electrons flow from the anode of a voltaic cell to the cathode because of a difference in potential energy .
The potential energy of the electrons is higher at the anode than at the cathode, for this reason, the electrons spontaneously flow from the first to the second through an external circuit. The potential energy difference per electrical charge (the potential difference) between two electrodes is measured in volts. One volt (V) is the potential difference required to impart 1 Joule (J) of energy to a charge of 1 coulomb (C): The potential difference between two electrodes in a voltaic cell provides the driving force that pushes the electrons through the external circuit. Therefore, this difference in potential is called electromotive force (which causes movement of electrons), or EMF.
electromotive force of a cell
The emf of a particular voltaic cell depends on the specific reactions taking place at the cathode and anode, the concentration of the reactants and products, and the temperature. At 25 ºC under standard conditions: 1 M concentration of reactants and products in solution and 1 atm of pressure for gases. Under standard conditions the emf is called the standard emf or standard potential of the cell. For example, for the Zn/Cu voltaic cell, the standard cell potential at 25 ºC is 1.10 V:
Zn(s) + Cu2+(ac, 1 M) → Zn2+(ac, 1 M) + Cu(s) Eocelda = 1,10 V
The EMF of a cell, denoted Ecell, is known as the cell potential. Ecell is measured in volts, we usually refer to it as the cell voltage. When redox reactions are spontaneous, they release energy that can be used to do electrical work. For any cell reaction that takes place spontaneously, such as in a voltaic cell, the cell potential is positive.
Galvanic cells store chemical energy. In these, the reactions at the electrodes occur spontaneously and produce a flow of electrons from the cathode to the anode (through an external conductive circuit). This flow of electrons generates an electrical potential that can be measured experimentally.
Function of the salt bridge in the galvanic cell
A salt bridge is made up of a “U” shaped tube containing a very concentrated solution of an electrolyte , (for example: NaNO 3 (aq), NH 4 NO 3 (aq), NaCl(aq), KNO 3 ( ac), among others) whose ions do not react with the other ions in the cell or with the material of the electrodes.
Flow in the salt bridge.png
If a salt bridge were not used, this potential difference would prevent the flow of more electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the two solutions separate. As the oxidation and reduction of the electrodes occurs, the salt bridge ions migrate to neutralize the charge in the cell compartments. The anions migrate to the anode and the cations to the cathode.
In fact, no measurable flow of electrons through the external circuit will occur unless a means is provided for the ions to migrate through solution from one compartment to the other, thus completing the circuit.
The electrolyte is usually incorporated into a gel so that the electrolyte solution does not run off when the U-tube is inverted.
As the oxidation and reduction of the electrodes occurs, the salt bridge ions migrate to neutralize the charge in the cell compartments. The anions migrate to the anode and the cations to the cathode. In fact, no measurable flow of electrons through the external circuit will occur unless a means is provided for the ions to migrate through solution from one compartment to the other, thus completing the circuit.
Galvanic in aesthetics
They are treatments that consist of applying galvanic current to the skin of the face to achieve great results in facial cleansing. Galvanic current equipment uses two roller or ball-shaped diodes that generate a sustained flow of electrons from one positive pole to the other negative.