Electrolysis or Electrolysis process that separates the elements of a compound by means of electricity. Certain substances ( acids , hydroxides, salts and some dissolved or molten metal oxides ) are conductors of electricity at the same time that they decompose by the passage of electric current, these substances are called electrolytes. Such a phenomenon is called electrolysis and it fundamentally constitutes an oxidation-reduction process that develops “not spontaneously”, that is, a set of transformations that imply an increase in the free energy of the system, and therefore, requires for its realization the concurrence of an external force of energy.
As in electrochemical cells , an electrolysis reaction can be considered as the set of two half-reactions, an anodic oxidation and a cathodic reduction. When we connect the electrodes with a power source (direct current generator), the electrode that is attached to the positive terminal of the generator is the anode of electrolysis, and the electrode that is attached to the negative terminal of the generator is the cathode.
Summary
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- 1 Electrolysis.
- 2 Electrolysis of aqueous solutions
- 3 Electrolysis production
- 4 The electrolysis scheme is as follows:
- 5 Aqueous solutions
- 6 Electrolysis and other oxidation-reduction processes
- 7 Decomposition Potential:
- 8 The decomposition potential is calculated as:
- 9 Diffusion polarization
- 10 Change of transformation conditions
- 11 Overvoltage depends on different magnitudes
- 12 Rules for Choosing Anodic and Cathodic Reactions
- 13Fuentes
Electrolysis.
The reactions that take place in the electrodes of electrolysis are generally determined by energy laws, as well as in the battery , the reaction in each electrode is the one that corresponds to a reaction that produces the maximum decrease in free energy, in electrolysis it is will produce the reactions that correspond to an overall reaction that produces the least increase in free energy. In the case of electrolysis of molten salts such as sodium chloride (NaCl), only one reaction can be verified at the cathode, which is the reduction of sodium ions (Na), since the chlorine ion (Cl) can only be oxidized, and at the At the anode, the oxidation of Chlorine ions (Cl) occurs , since Sodium (Na) can only be reduced.
Electrolysis of aqueous solutions
When it comes to electrolysis of aqueous solutions of various electrolytes, the reactions that take place at the anode must be chosen according to the energetic principles that we have referred to before, since there is more than one possible oxidation at the anode, more than one possible reduction. in the cathode, because in addition to the ionic species produced by the electrolytes, water molecules are present and can be oxidized and reduced in a similar way to salts.
For example, during the electrolysis of the copper sulfide (CuSO 4 ) solution (with smooth platinum electrodes) on the cathode, the separation of metallic copper is observed. On the other hand, the water molecules leave their charge on the anode and not the sulfur ions (SO 4 ).
electrolysis production
Electrolysis occurs in a device called an electric cell, which is made up of a tank containing the electrolyte and in which the electrodes, generally metallic, are immersed and are connected to an electric generator. The electrode connected to the positive pole is the anode and the one connected to the negative pole is the cathode. This cell is a closed electrical circuit, in which there are, on the one hand, the metallic conductors and, on the other hand, the conductors formed by the molten salts or by the electrolytic solutions. The current consists of a flow of electrons in the metallic part of the circuit and a flow of ions in the liquid part.
The electrolysis scheme is as follows:
- On the cathode(-): 2Cu + 4e 2Cu
- Sobre el ánodo(+): 3H 2O 2H 3 O + 1/2O2 + 2e
On the anode in this case the oxygen is separated, which is eliminated as a gas, and in the solution, in the vicinity of the anode, the hydrogen ions (H) accumulate, which may be present only with an equivalent amount of some anions. such anions are sulfides (SO 4 ), which move during electrolysis towards the anode and accumulate in the vicinity of it together with hydrogen ions (H). Therefore, at the anode, in addition to oxygen, sulfuric acid (as corresponding ions) is also formed, ie the solution becomes acidic.
aqueous solutions
In general, in the case of aqueous solutions, it should be kept in mind that water molecules can be oxidized or reduced according to the following equations:
Oxidation : 3 H 2 O 2H 3 O + 1/2 O2 + 2e E0 = 1.23V
Reduction : H 2 O + 1e 1/2 H 2 + OH E = – 0.82 V (at pH = 7)
Note that in those cases where the solutions have a pH different from zero (like this second one) the potentials are no longer called E0 but they have varied according to the values of (H 3 O) and (OH) and must be adjusted with the well-known Nernst equation.
Electrolysis and other oxidation-reduction processes
In electrolyses , other oxidation-reduction processes can also take place without the solid phase being deposited on the electrode; thus, Iron (Fe) and iodine (I) ions are oxidized to Iron (Fe) and iodine (I 2 ), while Iron (Fe) and iodine (I 2 ) are reduced on the cathode to Iron (Fe) and iodine (I), etc.
If the anode is not made of platinum, but of any other metal, it can also participate in oxidation-reduction processes, which take place during electrolysis. Thus, it had been seen that during the electrolysis of the Copper Sulfide solution (CuSO, 4), using a platinum anode, the water molecules are oxidized on it to Oxygen (O 2 ). If the platinum anode is replaced by a copper one, on it, during the electrolysis, the water molecules will no longer oxidize, but the material of the electrode itself, that is, metallic copper, which loses electrons even more easily than the molecules. of water. Consequently, the anode will dissolve with the formation of the ions Copper (Cu):
Cu – 2e Cu
Simultaneously, an equivalent amount of copper will be deposited on the cathode. In other words, a kind of passage of copper from the anode to the cathode will take place.
Decomposition Potential:
Consider as an example the electrolysis of copper sulfide (CuSO 4 ) solution with platinum electrodes. When the electric current passes through the solution, electrolysis products are released at the electrodes, which, being present simultaneously with the ions that gave rise to them, form oxidation-reduction pairs. In the example, the Cu /Cu pair is formed at the cathode and O 2 + H / H 2 O at the anode. As soon as the current begins to flow, the release of O 2 at the anode and the deposition of Cu at the cathode convert the device into a galvanic cell: Pt / Cu / Cu2+, H+ / O 2/ Pt which has its own electromotive force (EMF). The direction of this EMF is opposite to that of the external EMF, which is applied in electrolysis. The operation of the cell tries to make the current flow in the opposite direction to the current with which the electrolysis of the solution is attempted. In order to counteract this “opposing emf”, the applied emf must be greater than that of the cell whose reaction is opposite to the desired electrolysis reaction.
The minimum voltage that is necessary to apply to the electrodes to cause continuous electrolysis of the given electrolyte is called Decomposition Potential (Ed).
The breakdown potential is calculated as:
Ed = Ec – Ea (1)
where:
Ea = potential of the pair that occurs at the anode.
Ec = potential of the pair that occurs at the cathode.
However, it is often necessary to increase the emf to a point considerably above this value in order to drive the reaction at any appreciable rate. This excess potential is called overvoltage and depends on the nature of the electrodes, the current per unit area, and the composition of the solution.
Polarization by concentration and overvoltage or overvoltage.
In an electrode that is in a state of equilibrium, the discharge of ions and their formation take place at the same rate at the same time that the net current flowing is zero. However, if as a result of the application of an external EMF there is a real flow of current, the electrode will be disturbed from its equilibrium condition. This disturbance of equilibrium associated with current flow is called electrolytic polarization and results from the slowness of some of the processes that take place at the electrodes during the discharge or formation of an ion. Polarization is classified depending on the step that controls the kinetics of the electrochemical process, this can be classified as polarization by diffusion, polarization by transfer, by chemical reaction, among others.
A simple type of polarization that is essentially due to the slow diffusion of ions in solution resulting from concentration variations that occur in the vicinity of an electrode during electrolysis; this is known as diffusion polarization.
Diffusion polarization
Diffusion polarization in the vicinity of the anode where the ion concentration is increased by oxidation of the anode, the practical potential must be greater than the theoretical potential. On the other hand, in the vicinity of the cathode where the reduction occurs, there is a lower concentration of ions and therefore the practical potential in this electrode is lower than its theoretical potential. It is denoted by the letter P.
In the electrolysis studied, the polarization value for the anode is designated as Pa-Cu, and for the cathode we denote it as Pc-Cu (These are determined experimentally because they depend on the density of the current, the nature of the metal used as electrode, the state of the electrode surface, the temperature and the composition of the medium), then the calculation of the real potentials for each electrode would be as follows:
Ea = E0 + Pa-Cu
Ec = E0 – Pc-Cu
Sometimes the magnitude of the decomposition potential Ed found by formula (1) is lower than that found experimentally for reasons other than diffusion polarization phenomena. The reason for this is that another type of polarization known as activation polarization has not been taken into account when calculating Ed.
If during electrolysis a gaseous evolution occurs, as is the case of oxidation, and/or reduction of water: 2H /H 2 , O 2 + 4H / 2H 2 O, or any other gas such as Cl 2 /2Cl, etc., different conditions are produced than those created by determining their normal potentials. Indeed, when determining the normal potentials, a platinum platinum plate (ie coated with a layer of platinum black) is always used as the electrode. During electrolysis, gas evolution occurs on the surface of a smooth (shiny) platinum plate (or wire).
Change of transformation conditions
Experimentally it has been shown that this change of transformation conditions of H into elemental hydrogen or water into elemental oxygen and H , leads to the variation of the potentials of the corresponding pairs. For example, while the normal potential of the 2H /H 2 pair in platinum platinum is equal (by hydrogen scale) to zero, at the same concentration of H ions and pressure of hydrogen gas on the smooth platinum electrode, it is equal to -0.07 V. In the same way, the potential of this pair also changes when using electrodes of other metals, for example, copper , lead, mercury, etc.
Such a change in potential of the given pair, when substituting the platinum electrode for some other electrode, is called over-voltage of the corresponding element (hydrogen, oxygen, chlorine, etc.) in the given electrode.
The overvoltage depends on different magnitudes
The overvoltage depends on different magnitudes: on the density of the current, on the nature of the metal used as electrode, on the state of the electrode surface and on the temperature. For example, the overvoltage of Hydrogen in copper constitutes -0.85 V at a current density of 0.1 A/cm, while at 0.01A/cm it is equal to -0.58 V.
If the existence of overvoltage is taken into account, tension, calculating the decomposition potential, one must take into account not only the magnitudes of the oxidation-reduction potentials of the pairs that form at the anode (Ea) and at the cathode (Ec), but also the overvoltages corresponding to the indicated electrodes (a and c). The formula for calculating the decomposition potential looks like this:
Ed = (Ec – c) – (Ea + a) (2)
Rules for choosing anodic and cathodic reactions
The rules that apply to choose the anodic and cathodic reactions after having taken into account the effects of electrochemical polarization and overvoltage in the potential settings will be:
- At the anode, the most probable oxidation is chosen, that is, the one with the highest potential.
- At the cathode, the most probable reduction is chosen, that is, the one with the lowest oxidation potential.
