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EMF of a Cell

The electromotive force of a cell or EMF of a cell is the maximum potential difference between
two electrodes of a cell. It can also be defined as the net voltage between the oxidation and
reduction half-reactions. The EMF of a cell is mainly used to determine whether an
electrochemical cell is galvanic or not.

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We will learn more about this topic, including important formulas and how to calculate the EMF
of an electrochemical cell, in this lesson.

What Is an Electrochemical Cell?

An electrochemical cell is a device that generates electricity from a chemical reaction.
chemical reaction.
Essentially, it can be defined as a device that converts chemical energy
energyinto
intoelectrical
electrical energy.
energy. A A
chemical reaction that involves the exchange of electrons is required foran
d for anelectrochemical
electrochemical
cell to operate. Such reactions are called redox reactions.
A cell is characterised by its voltage. A particular kind of cell generates the same voltage
irrespective of its size. The only thing that depends on the cell voltage is the chemical
composition of the cell, given the cell is operated at ideal conditions.

Normally, the cell voltage may be different from this ideal value due to several factors like
temperature difference, change in concentration, etc. The Nernst equation formulated by
Walther Nernst can be used to calculate the EMF value of a given cell, provided the standard
cell potential of the cell.

Types of Electrochemical Cell

Galvanic Cell
The Galvanic Cell is named after Luigi Galvani, an Italian scientist. A galvanic cell is an
important electrochemical cell that forms the base of many other electrochemical cells, like the
Daniell cell. It constitutes of two different metallic conductors called electrodes immersed in
their own ionic solutions. Each of these arrangements is a half-cell. Alone, a half-cell is not able
to generate a potential difference, but combined, they generate a potential difference. A salt
bridge is used to combine two cells chemically. It serves the required amount of electrons to
the electron-deficient half-cell and accepts electrons from the electron-rich half-cell.

Read More: Galvanic Cell

For ease of understanding, let’s look at the theory of the Daniell cell and derive the Nernst
equation for the same.

Daniell Cell
The Daniell cell is an adaptation of the galvanic cell. It is constituted of zinc and copper
electrodes immersed in zinc sulphate and copper sulphate solutions, respectively. Two half-
cells are connected together using a salt bridge, the zinc electrode as an anode, and copper
act as a cathode.

The zinc metal is top of the electrochemical series when compared to the copper metal, owing
to the higher value of the oxidation potential of the metal. Hence, zinc undergoes oxidation;
consequently, two electrons and a zinc ion are generated. This electrode acquires a negative
potential due to the release of electrons when compared to the other electrode. We call it an
anode.

However, copper undergoes reduction owing to its higher reduction potential. The copper ion in
the solution of the copper half-cell accepts two electrons from the electrode and becomes
copper metal, and gets deposited in the electrode. As this electrode uses up electrons, we
consider this electrode as a positive electrode, and we call it a cathode.

The anode reaction is represented as follows:

Zn(s) → Zn2+ (aq) + 2e–

The cathode reaction is represented as follows:

Cu2+ (aq) +2e– → Cu(s)

The combined cell reaction or overall cell reaction is as follows:

Zn(s) + Cu2+(aq) → Zn2+ (aq) + Cu(s)

Electrode Potential
When a metal electrode is immersed in a solution containing its own ions, a potential difference
is set up across the interface. This potential difference is called the electrode potential.

Consider the case of the zinc electrode immersed in a zinc sulphate solution. The zinc metal
gets oxidised by releasing two electrons and is dissipated in the solution. The presence of
electrons in the electrode and ions in the solution creates a potential difference, and in the
same way, copper develops a positive potential. The combination of these two cells is owing to
the cell potential.

In reality, we are not able to determine the potential of a single half-cell alone. To determine the
potential of a single half-cell, we always need a standard half-cell whose potential value is
already known. This standard half-cell is then connected with the unknown half-cell to
determine the overall potential.

This overall 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 inherently set to zero volts. The standard hydrogen electrode is connected with an unknown
half-cell, and the potential difference is measured. As SHE has zero volts, the measured value
will be the potential difference of the unknown half-cell.

The picture below represents the method to find the standard electrode potential of zinc.
Electrochemical Series
In addition, the standard potential values of different metals are calculated and arranged in the
increasing order of the potential, and we obtain the electrochemical series.

The electrochemical series is essential for the determination of cell potential. It also helps in
selecting electrode metals for the construction of a cell.

The electrochemical series table shows the arrangement of a few elements based on the
increasing order of their reduction potential. Lithium usually has the least reduction potential,
and fluorine has the most. Hydrogen has a zero reduction potential. This is because all other
elements are compared against hydrogen to obtain their standard electrode potential.

Representation of an Electrochemical Cell

An electrochemical cell can be represented using special notations. This is useful in
understanding the composition as well as its quantity in the cell.

The above-given Daniell cell can be represented as follows:

Zn | Zn2+ (1M) || Cu2+ (1M) | Cu

Let us break this down and understand its components:

● The left side of the notation represents the anode. At the anode, Zn is converted by releasing a
set of two electrons per zinc atom. As the solution used is of 1M concentration, we include that,
too, in the representation.

Zn | Zn2+ (1M)

● On the right side, we have the cathode. Here, the electrolyte absorbs a pair of electrons from
the electrode and gets converted to Cu metal. Same as before, we are using a 1M copper
sulphate solution.

Cu2+ (1M) | Cu

● These two half-cells are combined using a salt bridge. The salt bridge is represented using
the two vertical bars.
Zn | Zn (1M) || Cu (1M) | Cu