Name: electroplating with electrolytic cells
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 2 min 52 s
Channel: JoVE

Les réactions électrochimiques sont essentielles à de nombreux processus de la technologie et du corps humain et impliquent le transfert d’électrons d’une espèce chimique à une autre. Ces réactions sont appelées réactions de réduction-oxydation ou réactions d’oxydoréduction.
Lorsqu’une espèce perd des électrons, elle subit une oxydation, et on dit qu’elle est oxydée. Lorsqu’une espèce gagne des électrons, elle a subi une réduction, et on dit qu’elle est réduite. Dans une pile galvanique électrochimique, des réactions d’oxydoréduction se produisent spontanément, qui génèrent de l’énergie électrique. Cependant, dans une cellule électrolytique, de l’énergie électrique est appliquée, ce qui provoque les réactions d’oxydoréduction.
Une cellule électrolytique est constituée d’une solution réactionnelle appelée électrolyte, qui contient toutes les espèces chimiques oxydées ou réduites ainsi que les ions nécessaires pour permettre la circulation des électrons. La pile électrolytique comporte deux électrodes métalliques qui sont immergées dans la solution électrolytique. Un circuit externe relie les deux électrodes, ce qui complète le circuit et nous permet d’appliquer une tension ou un courant.
Le courant ou la tension appliquée est ce qui provoque la réaction électrochimique. La demi-réaction d’oxydation se produit au niveau de l’anode et la demi-réaction de réduction se produit au niveau de la cathode. Pour vous aider à faire la différence entre les deux, rappelez-vous l'expression « chat rouge », qui signifie « réduction à la cathode ».
Mais comment déterminer quelle électrode est laquelle ? Nous pouvons utiliser le potentiel de réduction standard des électrodes métalliques, qui est une mesure de la tendance du métal à perdre des électrons. Plus le potentiel de réduction est élevé, plus il est probable que le métal soit réduit.
Disons que nous plaçons une électrode d'argent et une électrode de cuivre dans une cellule d'électrolyse et que nous appliquons du courant. L’électrode d’argent a un potentiel de réduction plus élevé, elle est donc réduite et agit comme la cathode. L’électrode de cuivre a un potentiel de réduction plus faible, elle est donc oxydée et agit comme l’anode.
L’une des utilisations des cellules électrolytiques est d’effectuer la galvanoplastie, qui est une réaction où un métal est oxydé puis réduit à la surface d’un autre métal. Étant donné que l’anode est le métal qui effectue le placage, dans notre exemple d’argent et de cuivre, l’électrode d’argent est plaquée d’une fine couche de cuivre.
Dans cet atelier, vous allez assembler une cellule électrolytique et effectuer le processus de galvanoplastie en plaquant une clé en laiton avec du cuivre.

cellules électrolytiques

électrochimie
L’électrochimie est une branche de la chimie qui décrit et mesure la relation entre l’énergie électrique et un changement chimique. Les réactions électrochimiques impliquent le déplacement d’électrons d’une espèce à une autre. Si la réaction est spontanée, elle peut entraîner un courant généré. Si la réaction n’est pas spontanée, elle peut être provoquée par l’application d’un courant.
En électrochimie, la réaction clé est la réaction d’oxydoréduction, appelée réaction d’oxydoréduction. La réaction d’oxydoréduction est composée de deux demi-réactions ; l’oxydation, où une substance connue sous le nom d’agent réducteur perd des électrons, et la réduction, où une substance connue sous le nom d’agent oxydant gagne des électrons. Les réactions d’oxydoréduction se produisent toujours par paires et modifient les états d’oxydation des atomes dans les molécules impliquées. Un moyen facile de se souvenir de cette relation est d’utiliser le pneumatique OIL-RIG : Oxidation Is Losing – Reduction Is Gaining.
<h4>Cellules électrochimiques</h4>
Une pile électrochimique est utilisée pour mesurer ou induire des réactions électrochimiques. Il se compose de plusieurs composants : une chambre contenant la solution réactionnelle, deux électrodes conductrices, une solution d’électrolyte conducteur et un circuit externe. Il existe deux types de cellules électrochimiques. L’une est la cellule électrolytique, qui utilise l’énergie électrique pour provoquer une réaction non spontanée. Dans ce type de cellule, l’énergie électrique est fournie par une source d’alimentation externe.
L’autre type de cellule est la pile galvanique, qui utilise une réaction électrochimique spontanée pour générer de l’énergie électrique. Les deux électrodes sont appelées anode et cathode, et elles sont reliées par un circuit externe. La chambre de réaction est remplie d’un électrolyte, qui favorise le passage des ions entre les électrodes. Dans une pile galvanique, un pont salin complète le circuit en permettant le passage des ions entre les solutions électrolytiques contenant les électrodes. Dans une pile électrolytique, il n’y a pas de pont salin car les électrodes sont généralement dans la même solution électrolytique.
La réaction de réduction se produit au niveau de la cathode, tandis que la réaction d’oxydation se produit au niveau de l’anode. On s’en souvient à l’aide de l’outil mnémotechnique « Red Cat », ce qui signifie que l’induction rougese produit au niveau de la cage du chat.
<h4>Cellules électrolytiques</h4>
Les cellules électrolytiques ont généralement une chambre de réaction, qui contient la solution d’électrolyte. L’électrolyte est généralement une solution aqueuse contenant des ions ou des sels dissous. Les ions dans l’électrolyte favorisent le mouvement des ions et des électrons à travers la solution.
Lorsqu’une tension externe est appliquée, les ions de l’électrolyte sont attirés vers l’électrode avec la charge opposée. C’est là que se produisent les deux demi-réactions. L’anode perd des électrons lors de l’oxydation, tandis que la cathode gagne des électrons lors de la réduction.
La galvanoplastie est un processus qui utilise une cellule électrolytique. La galvanoplastie est un processus dans lequel un métal est déposé à la surface d’une électrode, généralement un autre métal. Le scientifique britannique Michael Faraday, qui a donné son nom à la constante de Faraday, a démontré la relation molaire entre l’ion chargé par galvanoplastie et le courant électrique. En regardant la demi-réaction spécifique pour l’argent :
Ag+ + 1e- → Ag
Il faut une mole d’électrons, fournie par un courant externe, pour réduire une mole de cations d’argent en argent massif. Par conséquent, la stœchiométrie à demi-réaction peut déterminer la quantité de matériau de galvanoplastie utilisée pour une quantité connue d’électrons. Rappelons que l’électricité n’est pas mesurée en fonction des moles d’électrons, mais plutôt en coulombs, du nom de l’ingénieur français Charles-Augustin de Coulomb. Un coulomb est la quantité d’électricité passant dans un circuit électrique avec 1 ampère de courant sur un intervalle de 1 seconde, qui est basée sur le principe de la charge élémentaire, ou la charge qu’un seul électron porte. La découverte de Faraday peut donc relier le courant électrique au nombre de moles que le cation de galvanoplastie plaque :
F = eNA
F est la constante de Faraday, e est la charge élémentaire exprimée en coulombs, et NA est le nombre d’Avogadro. Par conséquent, la constante de Faraday est exprimée comme le nombre de coulombs par mole et a une valeur de 96485 coulombs par mole. Comment déterminer le nombre de moles d’électrons transférées à l’anode en fonction du courant ? En utilisant la charge électrique Q et la définition d’un coulomb :
<img src="/CDNSource/lm/labs/53/53_Concepts_3.jpg" text-align center><img src="/CDNSource/lm/labs/53/53_Concepts_4.jpg" text-align center>Cu2+ + 2e- → Cu(s)
Les électrons pour la réaction sont reçus de la borne négative de la batterie. Ainsi, la réaction de réduction se produit au niveau de la clé en laiton, tandis que la réaction d’oxydation se produit au niveau de l’électrode en cuivre. La solution concentrée et acidifiée de sulfate de cuivre augmente la solubilité ; Par conséquent, plus la concentration de la solution est élevée, plus la résistance est faible et plus le courant est élevé. À son tour, un courant plus élevé permet un placage plus important des ions de cuivre sur la clé en laiton.
Certains métaux ont plus tendance que d’autres à perdre des électrons. Le potentiel d’électrode standard (E°) d’une substance est la mesure de la tendance d’une substance à perdre des électrons. Le métal ayant le potentiel de réduction le plus élevé a la plus forte tendance à perdre des électrons ; Ainsi, il est d’abord galvanisé.
<h4>références</h4>
<ol>
	<li style="font-size :17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.</li>
	<li style="font-size :17px">Silberberg, M.S. (2009). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>
</ol>

Cellules électrolytiques

Électrochimie
L’électrochimie est une branche de la chimie qui décrit et mesure la relation entre l’énergie électrique et un changement chimique. ...

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Electrolytic Cells

electroplating with electrolytic cells

Electrochemistry links electrical energy with chemical change, and electrolytic cells show how electricity can drive a nonspontaneous reaction. In these reactions, electrons move from one species to another. The key chemistry is a redox reaction, or oxidation-reduction reaction, which happens in two half-reactions.

Oxidation is the loss of electrons, and reduction is the gain of electrons. The substance that loses electrons is the reducing agent, and the substance that gains electrons is the oxidizing agent. Redox reactions always occur in pairs and change the oxidation state of the atoms involved. A simple memory aid is OIL-RIG: oxidation is losing, reduction is gaining.

An electrochemical cell is used to measure or cause electrochemical reactions. It includes a reaction chamber, two conductive electrodes, a conductive electrolyte, and an external circuit. The two main types are electrolytic cells and galvanic cells. An electrolytic cell uses outside electrical energy to drive a nonspontaneous reaction, while a galvanic cell uses a spontaneous reaction to produce electrical energy. The electrodes are the anode and cathode, and the reaction at the cathode is reduction while the reaction at the anode is oxidation. This is often remembered with “Red Cat,” which means reduction occurs at the cathode.

Electrolytic cells usually have one reaction chamber filled with an electrolyte solution. The electrolyte is often an aqueous solution with ions or dissolved salts. These ions help charge move through the solution. When voltage is applied, ions move toward the electrode with the opposite charge, and the two half-reactions occur there. The anode loses electrons during oxidation, and the cathode gains electrons during reduction.

A major use of an electrolytic cell is electroplating. In electroplating, one metal is deposited as a thin layer on the surface of another electrode. Michael Faraday showed the link between the charge of the ions being plated and the electrical current. For silver, the half-reaction is Ag+ + 1e- -&gt; Ag, so one mole of electrons reduces one mole of silver cations to solid silver. This means half-reaction stoichiometry can be used to calculate how much plating occurs for a known amount of charge.

Electricity is measured in coulombs, not in moles of electrons. One coulomb is the amount of electricity that passes through a circuit with 1 ampere of current in 1 second. Faraday’s constant connects these ideas. It is equal to the elementary charge times Avogadro’s number, and it has a value of 96485 coulombs per mole. The electrical charge is found by multiplying current, I, by time in seconds, and dividing Q by Faraday’s constant gives the moles of electrons transferred.

This relationship shows how current controls electroplating. The amount of metal plated depends on the current applied and on how much electroplating cation is available. An external power source, such as a battery, pushes electrons from the anode to the cathode and from the positive terminal to the negative terminal on the battery. In a copper plating example, a copper electrode, a brass key, and aqueous copper sulfate are used. Copper from the electrode and from the solution is deposited onto the brass key.

For copper to coat the key, the solid copper electrode must first be oxidized to copper ions. Then copper cations in the solution are reduced to solid copper on the brass key, following Cu2+ + 2e- -&gt; Cu(s). The electrons come from the negative terminal of the battery. So the brass key is the site of reduction, and the copper electrode is the site of oxidation. A concentrated and acidified copper sulfate solution increases solubility, lowers resistance, and raises the current, which allows more copper to plate onto the key.

Some metals lose electrons more easily than others. The standard electrode potential, E°, measures a substance’s tendency to lose electrons. In electroplating, the metal with the highest reduction potential has the highest tendency to lose electrons, so it is plated first.

Electroplating with Electrolytic Cells

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Separating Mixtures with Solvent Extraction

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Electroplating with Electrolytic Cells

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