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

Una reacción electroquímica es un proceso en el que los electrones se transfieren entre dos sustancias, una pierde electrones y la otra los gana. Cuando una sustancia pierde electrones, decimos que ha sufrido oxidación o que se ha oxidado. Cuando gana electrones, decimos que ha sufrido una reducción o se ha reducido. Para ayudarlo a distinguir entre los dos, recuerde la frase 'plataforma petrolera', que significa que la oxidación es perder, la reducción es ganar. Algunas reacciones electroquímicas ocurren espontáneamente, que podemos estudiar en una celda galvánica.
La célula galvánica tiene cuatro partes principales. En primer lugar, hay dos electrodos metálicos, cada uno de los cuales se sumerge en una solución de electrolito diferente. Las soluciones consisten en los iones necesarios para las medias reacciones, ya sea la reacción de oxidación o de reducción, que ocurren en cada electrodo. Las soluciones están conectadas por un puente salino, que permite el flujo de iones y mantiene la neutralidad de carga de las soluciones. Los electrodos están conectados por un circuito externo, a través del cual viajan los electrones durante la reacción.
El ánodo es el electrodo que cede electrones en la reacción o se oxida. El cátodo es el electrodo que gana electrones y se reduce. Para ayudarlo a distinguir entre estas dos etiquetas, recuerde la frase 'gato rojo', ya que la reducción se produce en el cátodo.
Entonces, ¿cómo sabemos qué metal actúa como ánodo y cuál como cátodo? Podemos usar el potencial de electrodo estándar del metal, que es una medida de la tendencia del metal a perder electrones. Un potencial de electrodo más negativo significa que el metal tiene una mayor tendencia a perder electrones. Los electrones siempre fluyen desde el metal con el potencial estándar más bajo hasta el metal con el potencial estándar más grande.
En una celda galvánica, los electrones fluyen del ánodo al cátodo, por lo tanto, el ánodo es el metal con el potencial estándar más bajo y el cátodo es el metal con el potencial estándar más alto. A medida que los electrones viajan entre los dos electrodos, se mide un potencial eléctrico. La magnitud del voltaje producido es igual al potencial estándar del cátodo menos el potencial estándar del ánodo.
Por lo tanto, si se utilizan dos metales con potenciales de electrodo estándar similares en la celda galvánica, como el plomo y el estaño, por ejemplo, la magnitud del voltaje producido será baja. Si los metales tienen diferentes potenciales estándar, como la plata y el magnesio, el voltaje producido será alto.
En este laboratorio, construirá una celda galvánica utilizando dos electrodos metálicos y medirá el potencial producido a medida que ocurren las reacciones de oxidación y reducción. A continuación, identificará electrodos metálicos desconocidos utilizando potenciales estándar conocidos y determinará la magnitud del voltaje producido.

células galvánicas

Galvanic Cells

electroquímica
La electroquímica es una rama de la química que estudia la relación entre la energía eléctrica y un cambio químico. Estas reacciones químicas implican el movimiento de electrones de una especie a otra. Este movimiento genera corriente o es impulsado por la corriente aplicada.
La reacción clave en electroquímica es la reacción de oxidación-reducción, o redox. La reacción redox se compone de dos medias reacciones; la reacción de oxidación, en la que una sustancia pierde electrones, y la reacción de reducción, en la que una sustancia gana electrones. Esta reacción química hace que cada sustancia cambie su estado de oxidación.
El átomo o molécula que pierde electrones, o se oxida, es el agente reductor. El átomo o molécula que acepta los electrones, o se reduce, es el agente oxidante. Un método para recordar estas relaciones es a través de la frase "plataforma petrolífera", que significa "oxidación is losing – reducacion is gaining".
<h4>Celdas electroquímicas</h4>
Una celda electroquímica es un dispositivo que genera una corriente eléctrica a partir de la energía liberada por una reacción redox. Una celda electroquímica consta de una cámara o cámaras de reacción y dos electrodos conductores: un ánodo y un cátodo. El ánodo y el cátodo están conectados eléctricamente y la cámara de reacción está llena de un electrolito. Entre las dos cámaras hay un puente salino, que completa el circuito y permite que los iones se transfieran entre electrodos.
La reducción se produce en el cátodo, mientras que la oxidación se produce en el ánodo. Esto se puede recordar fácilmente usando el mnemónico "gato rojo", lo que significa que la ucción rojaocurre en la manguera del gato.
Hay dos tipos de celdas electroquímicas. Una es la celda electrolítica, que utiliza energía eléctrica para impulsar una reacción no espontánea. En este tipo de celda, la energía eléctrica se suministra desde una fuente de alimentación externa. El otro tipo de célula es la célula galvánica, que utiliza una reacción espontánea para generar energía eléctrica. Las baterías son un ejemplo de celdas galvánicas.
<h4>Células galvánicas</h4>
Las celdas galvánicas, también conocidas como celdas voltaicas, constan de dos medias celdas. Cada media celda contiene un electrodo metálico sumergido en un electrolito. Un circuito externo conecta los dos electrodos y un puente salino conecta las dos soluciones de electrolitos. Los electrones fluyen del ánodo al cátodo. La semirreacción de oxidación tiene lugar en el ánodo, mientras que la semirreacción de reducción se produce en el cátodo. Por ejemplo, en una célula galvánica entre cobre y magnesio, se produce la siguiente semirreacción en el cátodo:
Cu2+ + 2e- → Cu
Y se produce la siguiente semirreacción en el ánodo:
Mg → Mg2+ + 2e-
A medida que los electrones se pierden durante la oxidación en el ánodo, viajan a través del circuito externo para reducir el cátodo, generando corriente.
A medida que el ánodo se oxida, la concentración de cationes aumenta en el electrolito. Del mismo modo, a medida que se reduce el cátodo, aumenta la concentración de aniones en el electrolito. Para mantener la neutralidad eléctrica, los iones viajan a través del puente de sal. Cuando se crean cationes en el ánodo, los aniones viajan desde la solución en el lado del ánodo utilizando el puente salino. En el lado del cátodo, se crean aniones, lo que hace que los cationes viajen desde el puente de sal hasta la solución en el lado del cátodo. Es importante recordar que los electrones viajan a través de los cables del circuito externo y los iones fluyen a través del puente salino y las soluciones.
<h4>Potencial de reducción</h4>
Algunos metales tienen una mayor tendencia que otros a perder electrones. Por lo tanto, la magnitud de la corriente eléctrica producida por una celda galvánica depende de los tipos de electrodos metálicos. El potencial de electrodo estándar (Eo) de una sustancia es la medida de la tendencia de una sustancia a perder electrones. Si se utilizan dos metales con potenciales de electrodo casi iguales, entonces la magnitud de la corriente producida será pequeña. Si se utilizan dos metales con potenciales de electrodo muy diferentes, entonces la magnitud de la corriente será grande. Cuanto mayor sea el potencial de reducción, más probable es que el metal se reduzca y actúe como agente oxidante.
Volviendo a la celda galvánica de cobre-magnesio, el potencial de electrodo estándar del cobre es de 0,337 voltios, mientras que el potencial de electrodo estándar del magnesio es de -2,370 voltios. En este ejemplo, el cobre es el cátodo y el magnesio es el ánodo.
Al realizar un experimento de celda galvánica, la diferencia de potencial entre los dos electrodos se monitorea mediante un multímetro. El voltaje medido es igual a la diferencia de potencial entre las dos semirreacciones.
ΔEº =Eº cátodo - Eºánodo
El potencial de electrodo estándar supone que ambas medias celdas están en condiciones estándar de 1 M, 1 bar y 298,15 K (25 °C). El voltaje depende de la concentración de las soluciones electrolíticas, que se puede determinar mediante la ecuación de Nernst.
<img src="/CDNSource/lm/labs/52/52_Concepts_4.jpg">
Aquí, E corresponde a la diferencia de potencial o al voltaje medido, Eo es el potencial de reducción estándar, R es la constante universal de gas (8.314462618 J/mol·K), T es la temperatura medida en Kelvin, F es la constante de Faraday (96,485.33212 C⋅mol-1), n es el número de electrones transferidos en la reacción, y Qh es el cociente de reacción. El cociente de reacción es el equivalente electroquímico de la constante de equilibrio.
<h4>Referencias</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química y reactividad química. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Química: La naturaleza molecular de la materia y el cambio. Boston, MA: McGraw Hill.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análisis químico cuantitativo. Sexta Edición. Nueva York, NY: W.H. Freeman and Company.</li>
</ol>

Células galvánicas

Electroquímica
La electroquímica es una rama de la química que estudia la relación entre la energía eléctrica y un cambio químico. Estos productos químicos ...

Education

Educación

JoVE Lab Manual

Chemistry

Química

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

lab techniques

Measuring with Accuracy in the Lab

Measuring with accuracy in the lab starts with careful records, the right tools, and correct reading of data. Scientists use lab notebooks to record how an experiment was done. These notes include the amounts of materials used and each step of the procedure. Without that record, an experiment cannot be repeated in the same way. Balances and glassware are used to measure solids and liquids for experiments. The precision of a measurement depends on the equipment used. Digital balances show the...

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stoichiometry-product-yield-and-limiting-reactants

Balancing Equations and Reactant Limits

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redox reactions

Oxidation States in Redox Reactions

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Gas Laws Behind the Ideal Gas Equation

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acid and base concentrations

Acids, Bases, and pH in Titration

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Buffer pH and the Common Ion Effect

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metal flame emission

Flame Test Metal Color Clues

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balmer series

Hydrogen Emission Lines

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beer's law

Absorbance Spectra and Beer-Lambert Analysis

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concentration dependence

Reaction Order and Rate Constants

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temperature dependence

Reaction Rate, Arrhenius, and Catalysts

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galvanic cells

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...

electrolytic cells

Electroplating with Electrolytic Cells

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proper lab notebook keeping

Recording a Reproducible Lab Experiment

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Organic Lab Glassware and Safe Handling

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melting points

How Impurities Change Melting Points

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boiling points

Boiling Point and Vapor Pressure

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recrystallization

Purifying Crystals by Solubility

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extraction

Separating Mixtures with Solvent Extraction

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simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

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column chromatography

Separating Mixtures by Polarity

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hydrolysis of an ester

Soap Formation from Triglycerides

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synthesis of luminol

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esterification

Ester Formation and Everyday Fragrances

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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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uv-vis spectroscopy of dyes

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chemistry

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Battery Power from Redox Reactions

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