Electrochemical cells are devices in which chemical energy is converted into electrical energy or electrical energy is used to bring about chemical changes. These cells operate through redox reactions involving the transfer of electrons between chemical species. An electrochemical cell consists of two electrodes immersed in an electrolyte and connected by an external circuit, allowing the flow of electrons and ions.
Components of an Electrochemical Cell
An electrochemical cell consists of various components that work together to facilitate a redox reaction and the flow of electric current. The main components of an electrochemical cell are electrodes, electrolyte, salt bridge, and an external circuit.
1. Anode: The anode is the electrode at which oxidation occurs. During oxidation, a substance loses electrons and releases them into the external circuit. In a galvanic cell, the anode is the negative electrode because it supplies electrons.
Example: Zn(s) → Zn 2+ (aq) + 2e−
2. Cathode: The cathode is the electrode at which reduction occurs. During reduction, a substance gains electrons from the external circuit. In a galvanic cell, the cathode is the positive electrode.
Example: Cu 2+ (aq) + 2e− → Cu(s)
3. Electrolyte: An electrolyte is a substance that conducts electricity through the movement of ions. The electrolyte provides ions that participate in the electrochemical reactions and helps maintain charge balance within the cell.
Examples: ZnSO4 solution , CuSO4 solution , HCl solution
4. Salt Bridge: A salt bridge is a U-shaped tube containing an inert electrolyte such as KCl, KNO₃, or NH₄NO₃ in a gel medium. It connects the two half-cells and completes the electrical circuit by allowing the migration of ions.
5. External Circuit: The external circuit consists of a conducting wire that connects the two electrodes. Electrons released at the anode travel through the external circuit to the cathode, producing an electric current.
Cell Potential (EMF)
The cell potential, also known as the electromotive force (EMF) of a cell, is the potential difference between the two electrodes of an electrochemical cell. It is a measure of the driving force responsible for the flow of electrons from the anode to the cathode through the external circuit.
- The cell potential depends on the nature of the electrodes, the concentration of ions in the electrolyte, and the temperature of the system.
- A greater difference in electrode potentials results in a higher cell potential.
Under standard conditions (ion concentration = 1 M, gas pressure = 1 bar, and temperature = 298 K), the standard cell potential is represented as:
E^\circ_{cell}=E^\circ_{cathode}-E^\circ_{anode}
Types of Electrochemical Cells
Electrochemical cells are classified into two main types based on the manner in which chemical and electrical energy are interconverted. These are galvanic (voltaic) cells and electrolytic cells.
1. Galvanic (Voltaic) Cell
A galvanic cell is an electrochemical cell in which electrical energy is produced from a spontaneous redox reaction.
- In such cells, oxidation and reduction occur in separate half-cells, and the electrons released during oxidation flow through an external circuit to produce electric current.
- Chemical energy is converted into electrical energy.
- The redox reaction is spontaneous.
Example: Daniell cell.
2. Electrolytic Cell
An electrolytic cell is an electrochemical cell in which electrical energy is used to drive a non-spontaneous chemical reaction. The reaction does not occur on its own and requires an external source of electricity.
- Electrical energy is converted into chemical energy.
- The redox reaction is non-spontaneous. .
- An external power source is required to carry out the reaction.
Examples: Electrolysis of molten sodium chloride, electrolysis of water, and electroplating.
Representation of Electrochemical Cells
An electrochemical cell can be represented in a simplified symbolic form known as cell notation.
- In cell notation, the anode is written on the left-hand side and the cathode is written on the right-hand side.
- The two half-cells are separated by a double vertical line representing the salt bridge.
General Representation:
Anode ∣ Anode Electrolyte ∣∣ Cathode Electrolyte ∣ Cathode
Example: Daniell Cell
Zn(s) ∣ Zn2+(aq) ∣∣ Cu2+(aq) ∣ Cu(s)
In this cell:
- Zinc acts as the anode, where oxidation occurs.
- Copper acts as the cathode, where reduction occurs.
- The salt bridge connects the two half-cells.
