Electrolyze

The electrolysis is a method which makes it possible to carry out chemical reactions thanks to an electric activation. It is the process of conversion of electrical energy into chemical energy. It allows in addition, in the chemical Industrie, the separation of element S or the synthesis of chemical compounds. The first electrolysis (electrolysis of water) was carried out on May 2nd 1800 by two British chemists, (1753-1815) and (1768-1842), a few days after the invention of the first Battery (publication submitted on March 20th, 1800 in a French letter to the president of the Royal Society, Joseph Banks) by Alessandro Volta and thanks to this one.

Principle

The Matter to be broken up or transfer is dissolved in a Solvant suitable, or molten so that its constitutive Ion S is available in the solution.

  • an electric difference of Potentiel is applied between two electrode S immersed in this solution.
  • the Cathode is the seat of a Réduction and, the Anode the seat of a Oxydation. (Potential of the anode being higher - or equal in a pile circuitée- to the potential of cathode one runs can say that the anode is the positive terminal and cathode the negative terminal; attention it is the reverse with the piles!)
  • At the time of the passage of a continuous Electric current, the electrodes attract with them the ions of opposite load. But it is false to say that electrolysis summarizes with the Oxydation Anion S with the Anode and with the Réduction of the Cation S with the Cathode. Indeed, it is also possible to oxidize Cation S with the Anode. For example:
\ mathrm {Fe^ {2+} _ {aq} \ longrightarrow \ Fe^ {3+} _ {aq} + \ e^-} or to reduce Anion S to the Cathode, for example hexacyanoferrate (III) gives hexacyanoferrate (II):
\ mathrm {Fe (CN) _6^ {3} + \ e^- \ longrightarrow \ Fe (CN) _6^ {4}} .

In the same way, of the neutral molecules can react to the electrodes; for example para-quinone is reduced as a para-dihydroquinone:

\ mathrm {+ \ 2e^- + \ 2:00 ^+ \ longrightarrow \}

That is possible because the transport of the species to the electrode can be done by three different mechanisms: by migration which involves the displacement of the chemical species in charge (ions) in the electric field within the cell of electrolysis, by diffusion which relates to all the chemical species for which it exists a gradient of concentration and by natural or forced Convection according to whether the agitation of the solution is due to thermal agitation or a mechanical agitation.

Applications

Production of dihydrogene per electrolysis of water

See also: Electrolysis of water

Electrolysis can be used to break up the Eau (H2O) into Dihydrogène (H2) and into Dioxygène (O2).

Reactions:

  • Oxidation with the Anode (connected to the pole + of the generator):

\ begin {matrix} & \ \ 2 \, \ mbox {H} _2 \ mbox {O (liquid)}& \ overrightarrow {\ qquad} & \ mbox {O} _2 \ mbox {(gas)} + 4 \, \ mbox {H} ^+ \ mbox {(aqueous)} + 4 \, e^- \ \ \ \ end {matrix}

  • Reduction with the Cathode (connected to the pole - generator):

\ begin {matrix} & \ \ 4 \, \ mbox {H} _2 \ mbox {O (liquid)} + 4 \, e^- \ overrightarrow {\ qquad} & 2 \, \ mbox {H} _2 \ mbox {(gas)} + 4 \, \ mbox {OH} ^- \ mbox {(aqueous)} \ \ \ \ end {matrix}

  • All in all, we have:

\ begin {matrix} & \ \ 2 \, \ mbox {H} _2 \ mbox {O (liquid)}\ overrightarrow {\ qquad} & 2 \, \ mbox {H} _2 \ mbox {(gas)} + \ mbox {O} _2 \ mbox {(gas)} \ \\\end{matrix}

Nevertheless, if this method is effective (Rendement Faraday of 90% in alkaline solution), it is not the least expensive to produce dioxygene and dihydrogene industrially. One often prefers to extract dioxygene from the air, and dihydrogene by reforming of the Méthane: CH4. But contrary to electrolysis, this method also produces CO2 (see the article Dihydrogène)

Note: when water is electrolyzed, the heavy Eau (containing the heavy isotope Deutérium in the place of 1H) tends to dissociate at the end of the electrolysis, since the connection C is slightly stronger than the connection 1H-O of light water (water " normale" 1H2O).

Production of Chlorine

The dichlore is a toxic yellowish gas. One currently uses electrolysers which use the electrolysis of an aqueous NaCl solution to produce ions hypochlorites with the use of the water treatment of the swimming pools.

Plating

The Technique of the plating by electrolysis consists in covering with the objects of a thin regular layer of a generally invaluable metal or an alloy: Gilding, silvering, Nickel plating, Chromium plating, Cadmium-coating, coppering…

For example, a rings is plunged in a solution of Cerargyrite (Ag+ + Cl-) connected to a Générateur.

The money ions will settle on the ring according to the reaction of Oxydo-réduction: Ag+ (aq) + e- - > Ag (S) . One obtains a plated ring thus money.

More generally, the chemical derivatives of a given metal can be used at ends of plating with this last. Thus, one can cover a metal object of copper with the copper sulfate bath which one usually uses to treat the vine.

One can test with the following experiment with:

  • a source of D.C. current adjustable, able to deliver from 1 to 10 Amps.
  • a container for the bath (does not have to be metal).

The Cathode (-) is connected at piece-rates to plate, the Anode (+) is connected to a piece of Cuivre which will be used to feed plating. The two electrodes are plunged in the bath, with a few centimetres one of the other and, one runs the current as much of time than necessary.

Factors supporting the phenomenon:

  • Concentration of the bath: The more the bath is concentrated, the more plating is fast.
  • Surface of the part to be plated: the larger surface of the part is, the more the electric intensity must be high.
  • the speed of the deposit: it is proportional to the quantity of current crossing the part to be plated.
  • the Temperature: heat supports the output of the Bain.

the chromium plating of steel part proceeds as follows:

· the metal part constituting CATHODE is plunged with a chromium ANODE in a solution (the ELECTROLYTE) containing chromium ions (Cr3+); · the generator positioned between the anode and cathode delivers a D.C. current; · the metal part gains electrons and attracts the chromium ions which adherent on its surface.

Production of copper and aluminum

See also: electrolytic Reduction of aluminum

The primary Aluminum is obtained by electrolysis of alumina. The process consists in reducing by electrolysis of dissolved alumina in a bath of Cryolithe (double Fluorure of aluminum and Sodium - AlF3, 3NaF) melted to approximately 950 °C, in a tank crossed by an electric current of high intensity. Subjected to the action of the continuous electric current, the Ions are transported to the two electrodes.

The positive electrode attracts the negative ions, oxygen (O--). The anode is placed in top of the crucible where electrolysis is carried out; oxygen can be thus released in the form of gas CO2, after having burned the Carbone which constitutes the anode.

The negative electrode, attracts the positive ions, aluminum (Al+++). Cathode is located at the bottom of the crucible where aluminum heavier than the bath settles and remains in the form of a molten layer of metal at high temperature. This molten metal regularly is aspired and transported in foundry, to be solidified in the form of semi-finished product (ingot, billet, plate of rolling).

The total phenomenon results in the reaction: 2 Al 2O3 + 3 C = 4 Al + 3CO2

The industrial refining of the Cuivre gross is carried out by electrolysis of black copper anodes in a copper sulfate bath. Pure copper (up to 99,95%) is fixed on the Cathode, the impurities remaining in the bath.

Founders

The scientists pionners of electrolysis are:

  • (1753-1815) and (1768-1840) carried out the first electrolyzes on May 2nd 1800.
  • Michael Faraday, scientist British, is one of the creators of the electrochemistry with Humphry Davy.
  • Dr. Charles Eugene Michel, American ophthalmologist, had the idea in 1875 to insert a fine needle charged with galvanic current in the pilous follicules at one of its patient, at ends of depilation.
  • Professor Paul Mr. Kree, technician and inventor, set up in 1916 the type of apparatus at multiple needles.
  • Van Zeynik was in 1899 the first to observe that the organic fabrics can be heated by a high frequency current.

Sources

  • electrochemical Cells: thermodynamic aspects and kinetics , Didier Devilliers and Eric Mahé, chemical topicality - January 2003 - Article on line.

See too

French-speaking forum dealing with the possibilities of increase in the output of the electrolysis of water

Simple: Electrolysis

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