Name: battery power from redox reactions
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 3 min 31 s
Channel: JoVE

Une réaction électrochimique est un processus dans lequel des électrons sont transférés entre deux substances, l’une perdant des électrons et l’autre en gagnant. Lorsqu’une substance perd des électrons, on dit qu’elle a subi une oxydation ou qu’elle a été oxydée. Lorsqu’il gagne des électrons, on dit qu’il a subi une réduction ou qu’il a été réduit. Pour vous aider à faire la distinction entre les deux, rappelez-vous l’expression « plate-forme pétrolière », qui signifie que l’oxydation est perdante, la réduction est gagnante. Certaines réactions électrochimiques se produisent spontanément, que nous pouvons étudier dans une pile galvanique.
La cellule galvanique est composée de quatre parties principales. Tout d’abord, il y a deux électrodes métalliques, chacune étant immergée dans une solution électrolytique différente. Les solutions sont constituées des ions nécessaires aux demi-réactions, soit la réaction d’oxydation, soit la réaction de réduction, qui se produisent à chaque électrode. Les solutions sont reliées par un pont salin, qui permet la circulation des ions et maintient la neutralité de charge des solutions. Les électrodes sont reliées par un circuit externe, à travers lequel les électrons voyagent au cours de la réaction.
L’anode est l’électrode qui cède des électrons dans la réaction ou qui est oxydée. La cathode est l’électrode qui gagne des électrons et qui est réduite. Pour vous aider à distinguer ces deux étiquettes, rappelez-vous l’expression « chat rouge », car la réduction se produit au niveau de la cathode.
Alors, comment savoir quel métal fait office d’anode et lequel fait office de cathode ? Nous pouvons utiliser le potentiel d’électrode standard du métal, qui est une mesure de la tendance du métal à perdre des électrons. Un potentiel d’électrode plus négatif signifie que le métal a une plus grande tendance à perdre des électrons. Les électrons circulent toujours du métal ayant le potentiel standard le plus faible vers le métal ayant le potentiel standard le plus élevé.
Dans une pile galvanique, les électrons circulent de l’anode à la cathode, donc l’anode est le métal avec le potentiel standard le plus faible, et la cathode est le métal avec le potentiel standard le plus élevé. Lorsque les électrons se déplacent entre les deux électrodes, un potentiel électrique est mesuré. L’amplitude de la tension produite est égale au potentiel standard de la cathode moins le potentiel standard de l’anode.
Ainsi, si deux métaux avec des potentiels d’électrode standard similaires sont utilisés dans la pile galvanique, comme le plomb et l’étain, par exemple, l’amplitude de la tension produite sera faible. Si les métaux ont des potentiels standard différents, comme l’argent et le magnésium, la tension produite sera élevée.
Dans ce laboratoire, vous construirez une cellule galvanique à l’aide de deux électrodes métalliques et mesurerez le potentiel produit lors des réactions d’oxydation et de réduction. Vous identifierez ensuite les électrodes métalliques inconnues à l'aide de potentiels standard connus et déterminerez l'amplitude de la tension produite.

cellules galvaniques

Galvanic Cells

électrochimie
L’électrochimie est une branche de la chimie qui étudie la relation entre l’énergie électrique et un changement chimique. Ces réactions chimiques impliquent le mouvement d’électrons d’une espèce à une autre. Ce mouvement génère du courant ou est entraîné par le courant appliqué.
La réaction clé en électrochimie est la réaction d’oxydoréduction, ou réaction redox. La réaction d’oxydoréduction est composée de deux demi-réactions ; la réaction d’oxydation, où une substance perd des électrons, et la réaction de réduction, où une substance gagne des électrons. Cette réaction chimique entraîne le changement d’état d’oxydation de chaque substance.
L’atome ou la molécule qui perd des électrons, ou qui est oxydé, est l’agent réducteur. L’atome ou la molécule qui accepte les électrons, ou qui est réduit, est l’agent oxydant. L’une des méthodes pour se souvenir de ces relations consiste à utiliser l’expression « plate-forme pétrolière », qui signifie « oxidation is losing – reduction i s gaining ».
<h4>Cellules électrochimiques</h4>
Une pile électrochimique est un dispositif qui génère un courant électrique à partir de l’énergie libérée par une réaction d’oxydoréduction. Une cellule électrochimique se compose d’une ou plusieurs chambres de réaction et de deux électrodes conductrices : une anode et une cathode. L’anode et la cathode sont connectées électriquement, et la chambre de réaction est remplie d’un électrolyte. Entre les deux chambres se trouve un pont salin, qui complète le circuit et permet aux ions de passer entre les électrodes.
La réduction se produit au niveau de la cathode, tandis que l’oxydation se produit au niveau de l’anode. Cela peut être facilement mémorisé à l’aide du mnémonique « chat rouge », ce qui signifie que l’induction rougese produit au niveau de la cage du chat.
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 spontanée pour générer de l’énergie électrique. Les batteries sont un exemple de cellules galvaniques.
<h4>Cellules galvaniques</h4>
Les cellules galvaniques, également appelées cellules voltaïques, sont constituées de deux demi-cellules. Chaque demi-cellule contient une électrode métallique immergée dans un électrolyte. Un circuit externe relie les deux électrodes, et un pont salin relie les deux solutions d’électrolytes. Les électrons circulent de l’anode à la cathode. La demi-réaction d’oxydation a lieu à l’anode, tandis que la demi-réaction de réduction se produit au niveau de la cathode. Par exemple, dans une pile galvanique entre le cuivre et le magnésium, la demi-réaction suivante se produit au niveau de la cathode :
Cu2+ + 2e- → Cu
Et la demi-réaction suivante se produit à l’anode :
Mg → Mg2+ + 2e-
Comme les électrons sont perdus lors de l’oxydation à l’anode, ils voyagent dans le circuit externe pour réduire la cathode, générant du courant.
Au fur et à mesure que l’anode est oxydée, la concentration de cations augmente dans l’électrolyte. De même, à mesure que la cathode est réduite, la concentration d’anions augmente dans l’électrolyte. Pour maintenir la neutralité électrique, les ions traversent le pont salin. Lorsque des cations sont créés à l’anode, les anions se déplacent de la solution du côté de l’anode en utilisant le pont salin. Du côté de la cathode, des anions sont créés, incitant les cations à se déplacer du pont salin à la solution du côté de la cathode. Il est important de se rappeler que les électrons voyagent à travers les fils du circuit externe et que les ions circulent à travers le pont salin et les solutions.
<h4>Potentiel de réduction</h4>
Certains métaux ont plus tendance que d’autres à perdre des électrons. Par conséquent, l’amplitude du courant électrique produit par une pile galvanique dépend des types d’électrodes métalliques. Le potentiel d’électrode standard (Eo) d’une substance est la mesure de la tendance d’une substance à perdre des électrons. Si deux métaux avec des potentiels d’électrode presque égaux sont utilisés, l’amplitude du courant produit sera faible. Si deux métaux avec des potentiels d’électrode très différents sont utilisés, l’amplitude du courant sera grande. Plus le potentiel de réduction est grand, plus le métal est susceptible d’être réduit et d’agir comme agent oxydant.
Pour en revenir à la pile galvanique cuivre-magnésium, le potentiel d’électrode standard du cuivre est de 0,337 volts, tandis que le potentiel d’électrode standard du magnésium est de -2,370 volts. Dans cet exemple, le cuivre est la cathode et le magnésium est l’anode.
Lors de la réalisation d’une expérience sur une pile galvanique, la différence de potentiel entre les deux électrodes est surveillée à l’aide d’un multimètre. La tension mesurée est égale à la différence de potentiel entre les deux demi-réactions.
ΔEº =Eº cathode -Eº anode
Le potentiel d’électrode standard suppose que les deux demi-cellules sont dans des conditions standard de 1 M, 1 bar et 298,15 K (25 °C). La tension dépend de la concentration des solutions d’électrolyte, qui peut être déterminée à l’aide de l’équation de Nernst.
<img src="/CDNSource/lm/labs/52/52_Concepts_4.jpg" font-size>Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.
	<li style="font-size :17px">Silderberg, M.S. (2009). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>
	<li style="font-size :17px">Harris, D.C. (2015). Analyse chimique quantitative. Sixième édition. New York, NY : W.H. Freeman et compagnie.</li>

Cellules galvaniques

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L’électrochimie est une branche de la chimie qui étudie la relation entre l’énergie électrique et un changement chimique. Ces produits chimiques ...

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battery power from redox reactions

Electrochemistry studies how chemical change and electrical energy are linked. In this branch of chemistry, electrons move from one species to another. That movement can either produce current or be driven by an applied current. The main reaction in electrochemistry is the oxidation-reduction, or redox, reaction. A redox reaction has two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. These changes also shift the oxidation state of each substance. The species that loses electrons is the reducing agent, and the species that gains electrons is the oxidizing agent. A common memory aid is “oil-rig,” which means oxidation is losing and reduction is gaining.

An electrochemical cell is a device that turns the energy from a redox reaction into electric current. It includes reaction chamber or chambers and two conductive electrodes, an anode and a cathode. The electrodes are connected electrically, and the chamber contains an electrolyte. A salt bridge sits between the two chambers. It completes the circuit and lets ions move between the electrodes. Reduction occurs at the cathode, and oxidation occurs at the anode. Students often remember this with “red cat,” which means reduction occurs at the cathode. The two main kinds of electrochemical cells are electrolytic cells and galvanic cells. An electrolytic cell uses external electrical energy to drive a nonspontaneous reaction. A galvanic cell uses a spontaneous reaction to generate electrical energy. Batteries are examples of galvanic cells.

Galvanic cells, also called voltaic cells, have two half-cells. Each half-cell contains a metal electrode immersed in an electrolyte. An external circuit connects the electrodes, and a salt bridge connects the two electrolyte solutions. Electrons flow from the anode to the cathode. Oxidation happens at the anode, and reduction happens at the cathode. In a copper-magnesium galvanic cell, copper is reduced at the cathode through the half-reaction Cu2+ + 2e- → Cu. Magnesium is oxidized at the anode through the half-reaction Mg → Mg2+ + 2e-. As electrons move through the external circuit, they reduce the cathode and generate current. As the anode is oxidized, the concentration of cations increases in that electrolyte. As the cathode is reduced, the concentration of anions increases in that electrolyte.

The salt bridge keeps the cell electrically neutral. When cations are created at the anode, anions move from the anode side through the salt bridge. On the cathode side, anions are created, so cations move from the salt bridge into that solution. It is important to separate electron flow from ion flow. Electrons travel through the external wires. Ions move through the salt bridge and the solutions.

Some metals lose electrons more easily than others. For that reason, the current produced by a galvanic cell depends on the metals used for the electrodes. The standard electrode potential, E°, measures this tendency. If two metals have nearly equal electrode potentials, the current is small. If their electrode potentials are very different, the current is larger. A larger reduction potential means a metal is more likely to be reduced and act as the oxidizing agent. In the copper-magnesium cell, copper has a standard electrode potential of 0.337 volts, and magnesium has a standard electrode potential of -2.370 volts. That is why copper is the cathode and magnesium is the anode.

In a galvanic cell experiment, a multimeter is used to measure the potential difference between the two electrodes. The measured voltage equals the difference between the two half-reactions, written as ΔE° = E° cathode - E° anode. Standard electrode potential assumes standard conditions of 1 M, 1 bar, and 298.15 K, or 25 °C. The voltage also depends on electrolyte concentration. That dependence can be found with the Nernst equation. In the equation, E is the measured potential difference, E° is the standard reduction potential, R is the universal gas constant, T is temperature in Kelvin, F is Faraday’s constant, n is the number of electrons transferred, and Qh is the reaction quotient. The reaction quotient is the electrochemical equivalent of the equilibrium constant.

Battery Power from Redox Reactions

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Balancing Equations and Reactant Limits

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Oxidation States in Redox Reactions

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concentration dependence

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temperature dependence

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galvanic cells

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electrolytic cells

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proper lab notebook keeping

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boiling points

Boiling Point and Vapor Pressure

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recrystallization

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extraction

Separating Mixtures with Solvent Extraction

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simple distillation

Separating Liquids by Boiling Point

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steam distillation

Steam Distillation and Low-Boiling Separation

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thin-layer chromatography

TLC Separation by Polarity

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column chromatography

Separating Mixtures by Polarity

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identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

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Battery Power from Redox Reactions

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