Introduction

There exists a potential difference between the two cells’ electrodes. The maximum potential difference between the two electrodes is referred to as the electromotive force abbreviated as EMF. In other words, the EMF is the total voltage between the reduction and the oxidation half-reactions. The two electrodes in the discussion form what we call an electrolytic cell which makes use of electrical energy to perform the chemical reaction. Such cells act as transducers that convert the electrical energy to stored chemical energy and vice versa. In this article, we shall define the electrochemical cell and then discuss the two types of electrochemical cells namely galvanic and electrolytic cells. We shall further have a candid discussion on calculating the potential difference of the electrochemical cell and solve an example. Let us begin!

Defining an Electrochemical Cell

This is an electrical device that can generate electric power through a chemical reaction. Essentially it can be classified as a transducer due to its benefit of converting stored chemical energy to electric energy. The chemical reaction in the electrochemical cell involves the exchange or transfer of electrons for the cell to be operational. This type of reaction is referred to as the redox reaction.

Electrochemical Cells Types

Electrochemical cells can be classified into two: Galvanic and Daniell cells.

Galvanic Cell

The cell is made up of two conducting electrodes that are immersed in the ionic solution. Each arrangement of the electrode and the ionic solution forms half a cell. A single half a cell cannot generate a potential difference and therefore, a salt bridge is incorporated to join the given cells chemically. The work of the salt bridge is to deliver electrons to the cell that lacks electrons and receive electrons from the cell with more electrons. See figure 1 below.

Figure 1: Diagram of the Galvanic Cell Courtesy of Simon Mugo

• ·     Anode– This is where the oxidation chemical reaction occurs
• ·      Cathode– This is the electrode where the reduction chemical reaction occurs
• ·      Half-Cells- This is the compartment separation of the oxidation and the reduction reactions.
• ·      Salt Bridge- This is where the electrolyte that is necessary for circuit completion of the galvanic cell
• ·      Load- this is the part that initiates the flow of the electron in the circuit to fulfill its function
• ·      External Circuit- the channel that allows the electrons to flow in the circuit.

 

Daniell Cell

This is an improvement of the galvanic cell. The cell is made up of copper and zinc electrodes that are dipped in copper sulfate and zinc sulfate solutions in that order. It has two half-cells that are in connection using a salt bridge. The electrode of zinc acts as the anode while that of copper acts as the cathode.

Compared to copper, Zinc metal is more reactive and on top when considering the electrochemical series. This is attributed to the fact that zinc metal has a higher oxidation potential value. Therefore, zinc undergoes oxidation generating zinc ions and two electrons. This makes the zinc electrode require a negative potential when compared to the copper electrode hence we call it the anode.

For the copper metal electrode, the process undertaken is the reduction one and this is attributed to the higher reduction potential. The half-cell where the copper electrode is dipped is known as the copper half-cell which is made up of the copper ion solution that receives the two electrons discharged from the zinc electrode to form copper metal that is deposited on the copper electrode. The copper electrode consumes the uses the electrons and hence it is referred to as the cathode.

Figure 2: The Daniell’s Electrolysis Cell Courtesy of Simon Mugo

 

Electrochemical Cell Representation

In the field of power and energy engineering, we can represent an electrochemical cell using provided special notations. The representation is very important and helps the user to know the composition of the cell as well as each substance quantity in the same cell.

The Daniell cell in Figure 2 can be represented as shown below.

Let us have a breakdown of the representation above to gain more understanding.

·      The notations left side are a representation of the anode electrode. At this electrode, Zn releases two electrons for every zinc atom, and because the solution used for the electrolyte has a concentration of 1M then the same is included in the notation.

 

·      The right side of the notation is the cathode. This part accepts two pairs of electrons from the connection electrode and hence converted to the metal of copper.

 

·      Then the two half-cells are combined using a connector known as a salt bridge which is represented by two vertical bars ||.

 

Electrochemical Cell Potential Calculation

To calculate the EMF or the cell potential of this cell, we have to consider the electrode potential of both the half-cells.

Three methods of care are used to determine the cell potential namely

• ·      Considering the anode oxidation potential and the cathode reduction potential
• ·      By considering both cells’ electrodes reduction potential
• ·      By considering both cells’ electrodes’ oxidation potential.

For the galvanic cell, we can find the standard potential of the cell by taking into account the two cells’ standard reduction potential

Therefore, the galvanic cell, cell potential is calculated as the right-side half-cell potential subtracting the left-side half-cell potential

For the Daniell cell, the cell potential is given by,

Let us invoke the Nernst equation which helps to compare the balancing state potential of the half-cell with the half-cell standard electrode potential, coefficient of reaction, temperature, and the reacting species activities.

Let us have a look at an electrode with a reaction

We know the standard values of R, F, and T at room temperature which can be substituted in the equation above to get;

 

Let us assume that we have an arbitrary cell that has electrodes of M and N where M has more reduction potential compared to N which means M is our cathode while N is our anode.

But we know that the galvanic cell potential is given by the right-side half-cell potential subtracting the left-side half-cell potential. That is to say;

 

The above is the galvanic cell standard cell potential at the given STP conditions.

The electrons involved in the Daniell cell process are given by 2 and thus n=2

 

Summary

• ·      Potential difference that exists between two half-cell electrodes gives us enough electromotive force that generate power.
• ·      The EMF is the total voltage that can be measured between the reduction and oxidation half-reactions.
• ·      Electrochemical cell is an electric device that can generate power through a well-thought chemical reaction involving reduction and oxidation.
• ·      There exist two types of electrochemical cells namely the galvanic and the Daniell cells.
• ·      Galvanic cell has two electrodes submerged on the electrolyte where each arrangement of the electrode forms a half-cell.
• ·      The galvanic cell parts include the anode, the cathode, the salt bridge, half-cells, loads, and the external circuit.

·      Daniells cell is similar to the galvanic cell but only that its reaction is accompanied by the circulation of two electrons.

·      The anode chemical reaction of the Daniells cell is given by: