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

Eine elektrochemische Reaktion ist ein Prozess, bei dem Elektronen zwischen zwei Substanzen übertragen werden, wobei eine Elektronen verliert und die andere sie gewinnt. Wenn eine Substanz Elektronen verliert, sagen wir, dass sie oxidiert oder oxidiert wurde. Wenn es Elektronen gewinnt, sagen wir, dass es eine Reduktion erfahren hat oder reduziert wurde. Um Ihnen bei der Unterscheidung zwischen den beiden zu helfen, denken Sie an den Ausdruck "Bohrinsel", der für Oxidation bedeutet, zu verlieren, zu reduzieren, zu gewinnen. Einige elektrochemische Reaktionen laufen spontan ab, die wir in einer galvanischen Zelle untersuchen können.
Die galvanische Zelle besteht aus vier Hauptteilen. Zunächst gibt es zwei Metallelektroden, die jeweils in eine andere Elektrolytlösung getaucht sind. Die Lösungen bestehen aus den Ionen, die für die Halbreaktionen, entweder die Oxidations- oder die Reduktionsreaktion, benötigt werden, die an jeder Elektrode ablaufen. Die Lösungen sind durch eine Salzbrücke verbunden, die den Fluss von Ionen ermöglicht und die Ladungsneutralität der Lösungen aufrechterhält. Die Elektroden sind durch einen externen Stromkreis verbunden, den die Elektronen während der Reaktion durchlaufen.
Die Anode ist die Elektrode, die bei der Reaktion Elektronen abgibt oder oxidiert wird. Die Kathode ist die Elektrode, die Elektronen aufnimmt und reduziert wird. Um Ihnen bei der Unterscheidung zwischen diesen beiden Bezeichnungen zu helfen, denken Sie an den Ausdruck "rote Katze", da die Reduktion an der Kathode erfolgt.
Woher wissen wir also, welches Metall als Anode und welches als Kathode fungiert? Wir können das Standard-Elektrodenpotential des Metalls verwenden, das ein Maß für die Tendenz des Metalls ist, Elektronen zu verlieren. Ein negativeres Elektrodenpotential bedeutet, dass das Metall eine höhere Tendenz hat, Elektronen zu verlieren. Elektronen fließen immer von dem Metall mit dem niedrigeren Standardpotential zum Metall mit dem größeren Standardpotential.
In einer galvanischen Zelle fließen Elektronen von der Anode zur Kathode, so dass die Anode das Metall mit dem niedrigeren Standardpotential und die Kathode das Metall mit dem höheren Standardpotential ist. Während sich die Elektronen zwischen den beiden Elektroden bewegen, wird ein elektrisches Potential gemessen. Die Größe der erzeugten Spannung ist gleich dem Standardpotential der Kathode abzüglich des Standardpotentials der Anode.
Wenn also zwei Metalle mit ähnlichen Standardelektrodenpotentialen in der galvanischen Zelle verwendet werden, wie z. B. Blei und Zinn, ist die Größe der erzeugten Spannung gering. Wenn die Metalle unterschiedliche Standardpotentiale haben, wie Silber und Magnesium, ist die erzeugte Spannung hoch.
In diesem Labor konstruieren Sie eine galvanische Zelle mit zwei Metallelektroden und messen das Potential, das bei den Oxidations- und Reduktionsreaktionen entsteht. Anschließend identifizieren Sie unbekannte Metallelektroden anhand bekannter Standardpotentiale und bestimmen die Größe der erzeugten Spannung.

galvanische zellen

Galvanic Cells

Elektrochemie
Die Elektrochemie ist ein Teilgebiet der Chemie, das die Beziehung zwischen elektrischer Energie und einer chemischen Veränderung untersucht. Diese chemischen Reaktionen beinhalten die Bewegung von Elektronen von einer Spezies zur anderen. Diese Bewegung erzeugt entweder Strom oder wird durch angelegten Strom angetrieben.
Die Schlüsselreaktion in der Elektrochemie ist die Oxidations-Reduktions- oder Redoxreaktion. Die Redoxreaktion setzt sich aus zwei Halbreaktionen zusammen; die Oxidationsreaktion, bei der eine Substanz Elektronen verliert, und die Reduktionsreaktion, bei der eine Substanz Elektronen gewinnt. Diese chemische Reaktion führt dazu, dass jede Substanz ihren Oxidationszustand ändert.
Das Atom oder Molekül, das Elektronen verliert oder oxidiert wird, ist das Reduktionsmittel. Das Atom oder Molekül, das die Elektronen aufnimmt oder reduziert wird, ist das Oxidationsmittel. Eine Methode, sich an diese Beziehungen zu erinnern, ist die Formulierung "Bohrinsel", die so viel bedeutet wie "Oxidation is losing – reduction is gaining".
<h4>Elektrochemische Zellen</h4>
Eine elektrochemische Zelle ist ein Gerät, das aus der Energie, die durch eine Redoxreaktion freigesetzt wird, einen elektrischen Strom erzeugt. Eine elektrochemische Zelle besteht aus einer oder mehreren Reaktionskammern und zwei leitfähigen Elektroden: einer Anode und einer Kathode. Anode und Kathode sind elektrisch miteinander verbunden, die Reaktionskammer ist mit einem Elektrolyten gefüllt. Zwischen den beiden Kammern befindet sich eine Salzbrücke, die den Kreislauf schließt und den Transfer der Ionen zwischen den Elektroden ermöglicht.
Die Reduktion erfolgt an der Kathode, während die Oxidation an der Anode erfolgt. Dies kann man sich leicht mit der Eselsbrücke "rote Katze" merken, was bedeutet, dass die roteSaugung an der Katzenladestattfindet.
Es gibt zwei Arten von elektrochemischen Zellen. Eine davon ist die Elektrolysezelle, die elektrische Energie nutzt, um eine nichtspontane Reaktion auszulösen. Bei diesem Zelltyp wird elektrische Energie von einer externen Stromquelle zugeführt. Der andere Zelltyp ist die galvanische Zelle, die durch eine spontane Reaktion elektrische Energie erzeugt. Batterien sind ein Beispiel für galvanische Zellen.
<h4>Galvanische Zellen</h4>
Galvanische Zellen, auch Voltaikzellen genannt, bestehen aus zwei Halbzellen. Jede Halbzelle enthält eine Metallelektrode, die in einen Elektrolyten eingetaucht ist. Ein externer Stromkreis verbindet die beiden Elektroden und eine Salzbrücke verbindet die beiden Elektrolytlösungen. Die Elektronen fließen von der Anode zur Kathode. Die Oxidationshalbreaktion findet an der Anode statt, während die Reduktionshalbreaktion an der Kathode stattfindet. In einer galvanischen Zelle zwischen Kupfer und Magnesium findet beispielsweise an der Kathode folgende Halbreaktion statt:
Cu2+ + 2e- → Cu
Und an der Anode findet folgende Halbreaktion statt:
Mg → Mg2+ + 2e-
Da Elektronen bei der Oxidation an der Anode verloren gehen, wandern sie durch den externen Schaltkreis, um die Kathode zu reduzieren und Strom zu erzeugen.
Wenn die Anode oxidiert wird, steigt die Konzentration der Kationen im Elektrolyten an. In ähnlicher Weise nimmt die Konzentration der Anionen im Elektrolyten zu, wenn die Kathode reduziert wird. Um die elektrische Neutralität zu wahren, wandern die Ionen über die Salzbrücke. Wenn Kationen an der Anode erzeugt werden, wandern Anionen von der Lösung auf der Anodenseite über die Salzbrücke. Auf der Kathodenseite werden Anionen erzeugt, die dazu führen, dass Kationen von der Salzbrücke zur Lösung auf der Kathodenseite wandern. Es ist wichtig, sich daran zu erinnern, dass die Elektronen durch die externen Schaltungsdrähte wandern und die Ionen durch die Salzbrücke und die Lösungen fließen.
<h4>Reduktionspotenzial</h4>
Einige Metalle neigen stärker als andere, Elektronen zu verlieren. Daher hängt die Größe des von einer galvanischen Zelle erzeugten elektrischen Stroms von der Art der Metallelektroden ab. Das Standardelektrodenpotential (Eo) einer Substanz ist das Maß für die Tendenz einer Substanz, Elektronen zu verlieren. Wenn zwei Metalle mit nahezu gleichen Elektrodenpotentialen verwendet werden, ist die Größe des erzeugten Stroms gering. Wenn zwei Metalle mit sehr unterschiedlichen Elektrodenpotentialen verwendet werden, dann ist die Größe des Stroms groß. Je größer das Reduktionspotenzial ist, desto wahrscheinlicher ist es, dass das Metall reduziert wird und als Oxidationsmittel wirkt.
Zurück zur galvanischen Kupfer-Magnesium-Zelle: Das Standardelektrodenpotential von Kupfer beträgt 0,337 Volt, während das Standardelektrodenpotential von Magnesium -2,370 Volt beträgt. In diesem Beispiel ist Kupfer die Kathode und Magnesium die Anode.
Bei einem galvanischen Zellexperiment wird die Potentialdifferenz zwischen den beiden Elektroden mit einem Multimeter überwacht. Die gemessene Spannung ist gleich der Potentialdifferenz zwischen den beiden Halbreaktionen.
ΔEº =Eº Kathode - EºAnode
Das Standardelektrodenpotential geht davon aus, dass sich beide Halbzellen unter Standardbedingungen von 1 M, 1 bar und 298,15 K (25 °C) befinden. Die Spannung ist abhängig von der Konzentration der Elektrolytlösungen, die mit Hilfe der Nernst-Gleichung bestimmt werden kann.
<img src="/CDNSource/lm/labs/52/52_Concepts_4.jpg">
Dabei entspricht E der Potentialdifferenz bzw. der gemessenen Spannung, Eo ist das Standardreduktionspotential, R ist die universelle Gaskonstante (8,314462618 J/mol·K), T ist die gemessene Temperatur in Kelvin, F ist die Faradaysche Konstante (96.485,33212 C⋅mol-1), n ist die Anzahl der in der Reaktion übertragenen Elektronen, und Qh ist der Reaktionsquotient. Der Reaktionsquotient ist das elektrochemische Äquivalent der Gleichgewichtskonstante.
<h4>Referenzen</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr., P.M., Townsend, J.R. (2012). Chemie und chemische Reaktivität. Belmont, Kalifornien: Brooks/Cole, Cengage Lernen.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chemie: Die molekulare Natur von Materie und Veränderung. Boston, MA: McGraw Hill.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Quantitative chemische Analyse. Sechste Auflage. New York, NY: W.H. Freeman und Company.</li>
</ol>

Galvanische Zellen

Elektrochemie
Die Elektrochemie ist ein Teilgebiet der Chemie, das die Beziehung zwischen elektrischer Energie und einer chemischen Veränderung untersucht. Diese chemischen ...

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JoVE Lab Manual

Chemistry

Chemie

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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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

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column chromatography

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identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

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chemistry

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Learn how galvanic cells use redox reactions, salt bridges, and electrode potentials to generate current and measure voltage in batteries.

Battery Power from Redox Reactions | JoVE K12

Bereiten Sie den Unterricht zu Galvanische Zellen mit diesem 6-minütigen Leitfaden für Lehrkräfte vor. Behandelt die wichtigsten Vorbereitungsschritte für das Chemistry-Labor auf JoVE Lab Manual.

Galvanische Zellen Lehrervorbereitung in Chemistry - 3.9K+ Aufrufe | JoVE Lab Manual

Folgen Sie den Schritt-für-Schritt-Verfahren von Galvanische Zellen in diesem 14-minütigen Studentenprotokoll-Video. Teil der Chemistry-Sammlung auf JoVE Lab Manual.

Galvanische Zellen Studentenprotokoll in Chemistry - 26K+ Aufrufe | JoVE Lab Manual

Battery Power from Redox Reactions

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