EMF of Cell

An Electrochemical cell is used to generate electricity from a chemical reaction. It can be defined as a device that converts chemical energy into electrical energy. A chemical reaction that involves the exchange of electrons is required for an Electrochemical cell to operate. These kinds of reactions are called redox reactions.

A cell is characterized by its voltage. A particular type of cell generates the same voltage irrespective of the size of the cell. The chemical composition of the cell depends on the cell voltage, given the cell is operated at ideal conditions. The cell voltage may differ due to several factors such as temperature difference, change in concentration, etc. 

The Nernst equation by Walther Nernst can be used to calculate the EMF value of a given cell, providing the standard cell potential of the cell.

History 

Michael Faraday proved that chemical reactions at each of two Electrodes–electrolyte interfaces provide the "seat of emf" for the voltaic cell, around 1830. These reactions drive the current in the circuit which is an open case, charge separation continues until the electrical field from the separated charges is sufficient enough to arrest the reactions. 

Electromotive force or EMF of a cell is the maximum Potential difference between two Electrodes of a cell. The electromotive force of a cell can also be defined as the net voltage between the oxidation and reduction half-reactions. The electromotive force of a cell is mainly used to determine whether an Electrochemical cell is Galvanic or not.

Electrochemical Cell

An Electrochemical cell is a device that is capable of generating electrical energy from the chemical reaction taking place in it or using the electrical energy supplied to it to enable a chemical reaction in it. These devices can convert chemical energy to electrical energy and vice versa and are used to power many electrical devices such as TV remote controls and watches. 

Cells that can generate an electric current from the chemical reactions that occur there are called Galvanic or voltaic cells. 

Electrode Potential 

Immersing a metal Electrode in a solution containing its ions creates a Potential difference across the interface which is called the Electrode Potential. 

Consider the case of a zinc Electrode when immersed in a zinc sulfate solution is oxidized by the emission of two electrons and released into solution. The presence of electrons in the Electrode and ions in the solution creates a Potential difference. Similarly, Copper also has a positive Potential. The combination of these two cells by cell Potential. 

To determine the Potential of an individual half cell, you always need a standard half cell whose Potential value is already known. Then connect this standard half cell to an unknown half cell to determine the total Potential. 

This total Potential is the difference between the Potentials of the two half cells. The standard hydrogen electrode (SHE) is an example of such a standard half-cell. The Potential Value of SHE is naturally set to zero volts. Connect a standard hydrogen Electrode to an unknown half-cell and measure the Potential difference. Since SHE is  zero volts, the measured value is an unknown half-cell Potential difference.

EMF of a Cell 

The maximum Potential difference that exists between the two electrodes of a cell is defined as the electromotive force of the cell or the EMF of the cell. This is also known as the net voltage between the oxidation and reduction half-reactions.

Types of Electrochemical Cell

1. Galvanic Cell

Galvanic cell, also known as a voltaic cell, is a device that can generate Electricity by a spontaneous redox reaction. 

Given below is the reaction of zinc metal with aqueous copper sulphate solution is used here. 

\[Zn(s) + Cu^{2+}(aq) \rightarrow  Zn^{2+} (aq) + Cu(s)\]

It consists of a zinc Electrode and a copper Electrode soaked in a zinc sulfate solution and a copper sulfate solution, respectively. The zinc Electrode acts as the anode and the copper acts as the cathode. In the container on the left, a  zinc metal rod (anode) is immersed in a zinc sulfate solution. In the container on the right, the copper rod (cathode) is immersed in a copper sulfate solution. The copper cable connects the zinc Electrode and the copper Electrode. The solution in the anode and cathode compartments is connected to the potassium sulfate solution by a salt bridge.

Oxidation half-reaction takes place in the anode compartment 

\[Zn(s) \rightarrow  Zn^{2+} (aq) + 2e^{-} \]

Cathode reaction occurs as- 

\[Cu^{2+} (aq) + 2e^{-} \rightarrow Cu(s)\] 

The movement of electrons from the zinc Electrode to the copper cathode takes place. Zinc dissolves in the anode solution to form \[Zn^{2+}\] ions. \[Cu^{2+}\] ions in a cathode half cell take in electrons and are converted to Cu atoms. At the same time, \[SO{_{4}}^{2-}\] ions move from the cathode half cell across the salt bridge to the anode half cell. Similarly, \[Zn^{2+}\] ions move from the anode half cell to the cathode half cell. The circuit is completed by the transfer of ions from one half-cell to the other, guaranteeing a constant power supply. The cell will continue to operate until either zinc metal or copper ions are exhausted. 

2. Daniel Cell 

It is not different from Galvanic Cell. It is also the same Copper-Zinc cell as described for Galvanic Cell. The only key difference is that Daniel's cell can only use Zinc and copper as Electrodes whereas for Galvanic cells it is not limited to Zinc and Copper Electrodes only, it can use a variety of metals as Electrodes. 

In Daniel Cell, the electrolytes used are copper(II) sulfate and zinc sulfate whereas for the Galvanic cell the salts of metals of each Electrode are the electrolytes used.