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Voltaic Cell

The arrangement where a metal rod is immersed in an electrolyte and once equilibrium is reached, a potential difference between the rod and the electrolyte prevents further dissolution of metals getting into solution in ionic forms is better known as voltaic cell. The magnitude of the electrode potential or potential difference completely depends upon the metal and the electrolyte. Therefore two rods of different metal in an electrolyte give rise to potential difference between the rods.

Voltaic cell contains two half cells which are connected to each other electrically and each of these half-cell in the voltaic cell are termed as electrode. A simple half-cell is made up by dipping a metal rod into its own ions $\frac{M}{M^{n+}}$. In a voltaic cell the electrons move from the anode, which is also called as the site of oxidation carried out through external circuit to the cathode, the site of reduction. The charge balance is achieved in each half cell by the ionic migration through the salt bridge.

The negative ions move from half-cell of reduction to the half-cell of oxidation while the positive ions move in the opposite direction.

 

Voltaic Cell Definition

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In a voltaic cell the element at one electrode gets into the solution by losing electrons which is considered as de-electronation or rather oxidation and the electrode for this oxidation process is known as the anode. The other electrode of the voltaic cell where ions gain electrons and get discharged shows the reduction process is termed as reduction electrode or cathode.

The anode will have negative charge in the voltaic cell as electrons flow from it whereas the cathode will show positive charge as electrons are drawn from it which is in contrast to the electrolytic cell where anode shows positive charge and cathode with negative charge.

The two half cells in a voltaic cell are connected to each other through a salt bridge to complete the circuit. This leads to the idea that voltaic cells could be either reversible or irreversible.
  • In a reversible voltaic cell the cathodic and anodic reactions are found to be in equilibrium state. The reversible voltaic cell also clarifies the conditions
  • If the voltaic cell is connected to external emf source equivalent to that of the voltaic cell then the reaction of cell ceases as no current will flow in the voltaic cell
  • In case the emf of external source is found to be more than voltaic cell emf then the direction of current is external to voltaic cell and reaction gets reversed
  • Finally in case the external emf is lesser than voltaic cell emf then current flows from the voltaic cell into the external source

Simple Voltaic Cell

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In a simple voltaic cell two half cells are connected by a salt bridge which allows cations and anions to move in between these half cells. The electrolyte that is chosen for the salt bridge needs to contain ions which will not react with chemicals or reagents in either of the half cells. In a voltaic cell the sign assigned to anode is negative while the cathode is marked positive as the oxidation occurring at anode producing electrons gives it a negative sign.

It is also essential to take care that neither reactants nor the products could be used as electrode material. Graphite is considered as it could electricity but would not oxidize easily. Moreover, it is cheap to acquire while platinum and gold used in research labs are inert chemically but are generally costly to be used under commercial operations.

Voltaic Cell Notation

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The notations in voltaic cell are important as the polarity is different from electrolytic cells.

For a voltaic cell the following electrochemical conventions are followed:

Anode: Polarity ---- negative, function: oxidation

Cathode: Polarity ----- positive, function: reduction

A voltaic cell notation would follow the convention of providing vertical double bar to show the separation of anode on left and cathode on right. A single vertical bar would show the separation of two phases while a comma would show the separation of two ionic species which are present in the aqueous phase.

2H+ (aq) + 2e- $\rightarrow$ H2 (g)

Zn (s) $\rightarrow$ Zn2+ (aq) + 2e-

Oxidation at anode has occurred so the cell notation for the above reaction is as follows:
 
$Zn (s)\mid zn^{2+} \parallel H^{+} (aq) \mid H^{2} (g) \mid Pt $

Voltaic Cell Diagram

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The diagram shows the oxidation process taking place at anode with copper losing electrons while the reduction of silver at cathode gaining electrons.
Voltaic Cell

Voltaic Cell Experiment

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In voltaic cell experiment we will use the above mentioned diagram where the strategy would be to use the conventions, the different parts of voltaic cell and their functions.

Oxidation at anode:
The oxidation half-cell is shown on the left side where copper half-cell of the voltaic cell shows the loss of electrons by copper.

Cu (s) $\rightarrow$ Cu2+ (aq) + 2e-

Reduction at cathode:
The process of reduction is occurring at electrode on the right half-cell and silver half-cell is gaining electrons.

Ag+ (aq) + e- $\rightarrow$ Ag (s)

The convention needs to be followed and the overall chemical reaction requires balancing with no excess electrons and hence the silver half reaction needs to be multiplied by 2 before adding it to copper half reaction in order to cancel the electrons.

The net ionic equation for the chemical reaction in the voltaic reaction is as follows.

Cu (s) + 2 Ag+ (aq) $\rightarrow$ Cu2+ (aq) + 2 Ag (s)

The copper half-cell is the oxidation point or the anode and the polarity is negative.

The net charge in left cell goes up as copper gets oxidized to copper ion. The negative ions move from salt bridge into the left part of the voltaic cell.
Moreover, sodium cations are moving to opposite directions which also supply the positive charge which is lost due to loss of Ag+ ion.

Voltaic Cell Simulation

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  • The simulation of voltaic cells is basically the demonstration of how this voltaic cell works. In all research labs the voltaic cell consist of two half cells. Each of these is an ideal electrolyte and an electrode which are separated by a porous barrier. 
  • The whole idea behind this simulated investigation is to demonstrate and design an operational voltaic cell utilised for all kinds of scientific research.
  • To begin a proper working voltaic cell we need three different metals / ionic electrolyte of metals combinations needed for voltaic cell simulations.
  • The entire simulation is set up for anode and cathode solution of same concentration of three different metals and their respective electrolyte.
  • For the entire set up and corresponding electrolyte a voltmeter is used to identify which of the electrode is considered as positive and negative.
  • Once the voltmeter is connected the various parts of voltaic cell are replaced after each voltage is recorded. During the simulation the caution must be taken to check the positive potentials or else the corresponding electrodes are reversed.
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