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

Una reazione elettrochimica è un processo in cui gli elettroni vengono trasferiti tra due sostanze, con una che perde elettroni e l'altra che li guadagna. Quando una sostanza perde elettroni, diciamo che ha subito un'ossidazione o che è stata ossidata. Quando guadagna elettroni, diciamo che ha subito una riduzione o è stato ridotto. Per aiutarvi a distinguere tra i due, ricordate la frase "piattaforma petrolifera", che sta per ossidazione sta perdendo, riduzione sta guadagnando. Alcune reazioni elettrochimiche avvengono spontaneamente, che possiamo studiare in una cella galvanica.
La cella galvanica è composta da quattro parti principali. Innanzitutto, ci sono due elettrodi metallici, ognuno dei quali è immerso in una diversa soluzione elettrolitica. Le soluzioni sono costituite dagli ioni necessari per le semireazioni, sia la reazione di ossidazione che quella di riduzione, che si verificano in ciascun elettrodo. Le soluzioni sono collegate da un ponte salino, che consente il flusso di ioni e mantiene la neutralità di carica delle soluzioni. Gli elettrodi sono collegati da un circuito esterno, attraverso il quale gli elettroni viaggiano durante la reazione.
L'anodo è l'elettrodo che cede elettroni nella reazione o viene ossidato. Il catodo è l'elettrodo che guadagna elettroni e si riduce. Per aiutarti a distinguere tra queste due etichette, ricorda la frase "gatto rosso", poiché la riduzione avviene al catodo.
Quindi, come facciamo a sapere quale metallo funge da anodo e quale funge da catodo? Possiamo utilizzare il potenziale elettrodico standard del metallo, che è una misura della tendenza del metallo a perdere elettroni. Un potenziale elettrodico più negativo significa che il metallo ha una maggiore tendenza a perdere elettroni. Gli elettroni fluiscono sempre dal metallo con il potenziale standard più basso al metallo con il potenziale standard maggiore.
In una cella galvanica, gli elettroni fluiscono dall'anodo al catodo, quindi l'anodo è il metallo con il potenziale standard più basso e il catodo è il metallo con il potenziale standard più alto. Quando gli elettroni viaggiano tra i due elettrodi, viene misurato un potenziale elettrico. L'entità della tensione prodotta è uguale al potenziale standard del catodo meno il potenziale standard dell'anodo.
Quindi, se nella cella galvanica vengono utilizzati due metalli con potenziali elettrodici standard simili, come ad esempio piombo e stagno, l'entità della tensione prodotta sarà bassa. Se i metalli hanno diversi potenziali standard, come l'argento e il magnesio, la tensione prodotta sarà elevata.
In questo laboratorio, costruirai una cella galvanica utilizzando due elettrodi metallici e misurerai il potenziale prodotto quando avvengono le reazioni di ossidazione e riduzione. Identificherai quindi gli elettrodi metallici sconosciuti utilizzando potenziali standard noti e determinerai l'entità della tensione prodotta.

celle galvaniche

Galvanic Cells

elettrochimica
L'elettrochimica è una branca della chimica che studia la relazione tra l'energia elettrica e un cambiamento chimico. Queste reazioni chimiche coinvolgono il movimento di elettroni da una specie all'altra. Questo movimento genera corrente o è azionato dalla corrente applicata.
La reazione chiave in elettrochimica è la reazione di ossidoriduzione, o redox. La reazione redox è composta da due semireazioni; la reazione di ossidazione, in cui una sostanza perde elettroni, e la reazione di riduzione, in cui una sostanza guadagna elettroni. Questa reazione chimica fa sì che ogni sostanza cambi il suo stato di ossidazione.
L'atomo o la molecola che perde elettroni, o viene ossidata, è l'agente riducente. L'atomo o la molecola che accetta gli elettroni, o viene ridotta, è l'agente ossidante. Un metodo per ricordare queste relazioni è attraverso la frase "petroliera", che significa "oxidation is losing – reduction i s gaining".
<h4>Celle elettrochimiche</h4>
Una cella elettrochimica è un dispositivo che genera una corrente elettrica dall'energia rilasciata da una reazione redox. Una cella elettrochimica è costituita da una o più camere di reazione e da due elettrodi conduttivi: un anodo e un catodo. L'anodo e il catodo sono collegati elettricamente e la camera di reazione è riempita con un elettrolita. Tra le due camere c'è un ponte di sale, che completa il circuito e permette agli ioni di trasferirsi tra gli elettrodi.
La riduzione avviene a livello del catodo, mentre l'ossidazione avviene a livello dell'anodo. Questo può essere facilmente ricordato usando il mnemonico "gatto rosso", il che significa che l'uction rossasi verifica al cathode.
Esistono due tipi di celle elettrochimiche. Una è la cella elettrolitica, che utilizza l'energia elettrica per guidare una reazione non spontanea. In questo tipo di cella, l'energia elettrica viene fornita da una fonte di alimentazione esterna. L'altro tipo di cella è la cella galvanica, che utilizza una reazione spontanea per generare energia elettrica. Le batterie sono un esempio di celle galvaniche.
<h4>Celle galvaniche</h4>
Le celle galvaniche, note anche come celle voltaiche, sono costituite da due semicelle. Ogni semicella contiene un elettrodo metallico immerso in un elettrolita. Un circuito esterno collega i due elettrodi e un ponte salino collega le due soluzioni elettrolitiche. Gli elettroni fluiscono dall'anodo al catodo. La semireazione di ossidazione avviene a livello dell'anodo, mentre la semireazione di riduzione avviene a livello del catodo. Ad esempio, in una cella galvanica tra rame e magnesio, al catodo si verifica la seguente semireazione:
Cu2+ + 2e- → Cu
E la seguente semireazione si verifica all'anodo:
Mg → Mg2+ + 2e-
Quando gli elettroni vengono persi durante l'ossidazione all'anodo, viaggiano attraverso il circuito esterno per ridurre il catodo, generando corrente.
Quando l'anodo viene ossidato, la concentrazione di cationi nell'elettrolita aumenta. Allo stesso modo, quando il catodo si riduce, la concentrazione di anioni aumenta nell'elettrolita. Per mantenere la neutralità elettrica, gli ioni viaggiano attraverso il ponte di sale. Quando i cationi vengono creati sull'anodo, gli anioni viaggiano dalla soluzione sul lato dell'anodo utilizzando il ponte salino. Sul lato del catodo si creano anioni, spingendo i cationi a viaggiare dal ponte salino alla soluzione sul lato del catodo. È importante ricordare che gli elettroni viaggiano attraverso i fili del circuito esterno e gli ioni fluiscono attraverso il ponte salino e le soluzioni.
<h4>Potenziale di riduzione</h4>
Alcuni metalli hanno una tendenza maggiore di altri a perdere elettroni. Pertanto, l'entità della corrente elettrica prodotta da una cella galvanica dipende dai tipi di elettrodi metallici. Il potenziale elettrodico standard (Eo) di una sostanza è la misura della tendenza di una sostanza a perdere elettroni. Se vengono utilizzati due metalli con potenziali elettrodici quasi uguali, l'entità della corrente prodotta sarà piccola. Se vengono utilizzati due metalli con potenziali elettrodici molto diversi, l'entità della corrente sarà grande. Maggiore è il potenziale di riduzione, più è probabile che il metallo venga ridotto e agisca come agente ossidante.
Tornando alla cella galvanica rame-magnesio, il potenziale elettrodico standard del rame è di 0,337 volt, mentre il potenziale elettrodico standard del magnesio è di -2,370 volt. In questo esempio, il rame è il catodo e il magnesio è l'anodo.
Quando si esegue un esperimento con una cella galvanica, la differenza di potenziale tra i due elettrodi viene monitorata utilizzando un multimetro. La tensione misurata è uguale alla differenza di potenziale tra le due semireazioni.
ΔEº= Eº catodo -Eº anodo
Il potenziale dell'elettrodo standard presuppone che entrambe le semicelle si trovino in condizioni standard di 1 M, 1 bar e 298,15 K (25 °C). La tensione dipende dalla concentrazione delle soluzioni elettrolitiche, che può essere determinata utilizzando l'equazione di Nernst.
<img src="/CDNSource/lm/labs/52/52_Concepts_4.jpg">
Qui, E corrisponde alla differenza di potenziale o alla tensione misurata, Eo è il potenziale di riduzione standard, R è la costante universale dei gas (8,314462618 J/mol·K), T è la temperatura misurata in Kelvin, F è la costante di Faraday (96.485,33212 C⋅mol-1), n è il numero di elettroni trasferiti nella reazione, e Qh è il quoziente di reazione. Il quoziente di reazione è l'equivalente elettrochimico della costante di equilibrio.
<h4>referenze</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimica e reattività chimica. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chimica: la natura molecolare della materia e il cambiamento. Boston, MA: McGraw Hill.</li>
	<li style="dimensione del carattere:17px">Harris, D.C. (2015). Analisi chimica quantitativa. Sesta edizione. New York, NY: W.H. Freeman e compagnia.</li>
</ol>

Celle galvaniche

Elettrochimica
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JoVE Lab Manual

Chemistry

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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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concentration dependence

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temperature dependence

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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

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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

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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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Separating Mixtures by Polarity

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Carbonyl Tests for Aldehydes and Ketones

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chemistry

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Battery Power from Redox Reactions

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