Electrochemical Cells

Definition and Basics:

Electrochemical cells consist of two half-cells. Each half-cell is composed of an electrode (a metal or a graphitic rod) and an electrolyte.
Half-cells are connected together via a salt bridge or a porous disk which allows ions to move between half-cells, maintaining electrical neutrality.
The entirety of a redox reaction in an electrochemical cell is a combination of two half reactions—one oxidation reaction and one reduction reaction.

Types of Electrochemical Cells:

There are two types of electrochemical cells: galvanic (voltaic) cells and electrolytic cells.
A voltaic cell generates electrical energy from the spontaneous reactions occurring within it. The anode (negative electrode) is the site of oxidation, while reduction happens at the cathode (positive electrode).
An electrolytic cell uses electrical energy to facilitate non-spontaneous reactions. Here, the anode (positive electrode) is where oxidation occurs, and reduction happens at the cathode (negative electrode).

Voltaic Cells and Redox Reactions:

The oxidation state of the material in the anode increases, while the oxidation state of the material in the cathode decreases.
Electrons move from the anode to the cathode in the external circuit, creating an electric current.
Ions move through the salt bridge or porous disk to maintain electrical neutrality in the half-cells.

Electrolytic Cells and Electrolysis:

Electrolysis refers to the process of driving a chemical reaction using an electric current.
Electrolytic cells are used for processes like electroplating and the electrolysis of solutions or melts.
In electrolysis, current is applied to the anode, causing a non-spontaneous redox reaction.

The Role of Salt Bridges and Porous Disks:

The salt bridge or porous disk allows the flow of ions from one half-cell to the other to maintain electrical neutrality.
Without this ion transport, the reaction would stop due to the buildup of charge in each half-cell.

Working of Devices Using Electrochemical Cells:

Devices like batteries and fuel cells work based on the principles of electrochemical cells.
Voltaic cells are often used in devices that require spontaneous reactions like a car battery, while electrolytic cells find uses in processes that involve non-spontaneous reactions, such as extraction of reactive metals like sodium and aluminium.

Calculating Cell Potential or Electrode Potential:

Cell potential, or emf (electromotive force), of an electrochemical cell can be calculated by the difference in electrode potentials between the cathode and the anode.
As per the Nernst equation, this cell potential can be altered by factors like temperature and concentration of reactants.
A positive cell potential means the reaction can occur spontaneously, while a negative cell potential means it is non-spontaneous.