Electrochemistry is the study of electricity production from energy released during spontaneous chemical reactions and how the electrical energy usage brings about non-spontaneous chemical transformations. A large number of metals such as sodium and magnesium, compounds like sodium oxide, and gases like chlorine and fluorine are produced by electrochemical processes. Batteries and fuel cells convert chemical energy into electrical energy and are used in large scale and devices. Let’s get introduced to some important terms related to an electrochemical cell, chapter 20 electrochemistry for Science subject of class 12.
During oxidation, a loss of electrons takes place whereas during reduction, a gain of electrons is happening. Both oxidation and reduction reactions take place simultaneously in a redox reaction. Direct redox reactions involve a redox reaction in the same vessel and chemical energy gets converted to heat energy. During indirect redox reactions, oxidation and reduction takes place in different vessels and chemical energy is converted into electrical energy and the device where this takes place is called an electrochemical cell.
An electrochemical cell is a device which is capable of either producing an electric current due to chemical action or of producing chemical action due to the passage of electricity.
There are two types of electrochemical cells, named as follows:
Voltaic Cells: In voltaic cells, the chemical energy of a spontaneous redox reaction is converted into electrical energy. These are also called galvanic cells. The electrical energy produced by such batteries can be used for powering cell phones, radios, and other devices.
Electrolytic Cells: In electrolytic cells, electrical energy is needed to carry out a non-spontaneous chemical reaction. When we charge a cell phone battery, it’s like running an electrolytic cell.
Voltaic Cell: It consists of a half cell where oxidation takes place and it is known as oxidation half-cell; the other half-cell where reduction takes place is called reduction half-cell. Two half cells must be connected to build a voltaic cell. Oxidation takes place at negatively charged anode and reduction takes place at positively charged cathode. There will be a transfer of electrons taking place from anode to cathode when electric current flows in the opposite direction. An electrochemical cell also consists of an electrode which is prepared by dipping the metal plate into the electrolytic solution of its soluble salt. A salt bridge is a U shaped tube consisting of an inert electrolyte in agar-agar and gelatine. It maintains electrical neutrality and also allows the flow of electric current by completing the electric circuit.
Anode is on the left side while the cathode is written on the right sife.
Anode represents the oxidation half-cell and can be written as metal/metal ion (concentration).
Cathode represents the reduction half-cell and is written as metal ion (concentration)/metal
There’s a salt bridge which is indicated by placing double vertical lines between the anode and the cathode.
Electrode potential is defined as the potential difference developed between the electrode and its electrolyte. It is represented as P.D. and P.D. between the metal and the solution of its ions is the result of the separation of charges at the equilibrium state and it is the measure of tendency of an electrode in the half cell in losing or gaining electrons.
Standard Electric or Electrode Potential: When the concentration of all the species involved in a half cell is unity, it is called standard electrode potential.
Anode is a negative electrode in the voltaic cell and cathode is a positive electrode in a voltaic cell.
The half reaction for the Daniel cell are written as follows:
Left electrode : Zn(s) → Zn₂+ (aq, 1 M) + 2 e–
Right electrode: Cu₂ + (aq, 1 M) + 2 e– → Cu(s)
The net ionic equation for the daniel cell is as follows:
Zn(s) + Cu₂ + (aq) → Zn₂ + (aq) + Cu(s)
Zinc metal reacts with Copper ions to form Zinc ions and copper metal.
Electrical potential multiplied by total charge passed is equal to the electrical work done in one second. If we need to obtain maximum work from a galvanic cell then charge has to be passed reversibly. The reversible work done by a galvanic cell is equal to the amount decreased in its Gibbs energy and therefore, if the EMF of the cell is E and amount of charge passed is nF, delta G is the Gibbs energy of the reaction. Then,
ΔrG = – nFE(cell)
1. What is Electronic or Metallic Conductance?
Electrical conductance through metals is called electronic or metallic conductance and it is due to the movement of electrons. It depends on three factors, namely, the nature and structure of the metal, the number of valence electrons per atom and temperature. It decreases with increase in temperature. As the electrons enter at one end and go out through the other end, the compositions of the metallic conductor will not change. The mechanism of conductance through semiconductors is more complex.
2. What are the Factors on Which the Conductivity of Electrolytic or Ionic Solutions Depend?
The conductivity of electrolytic (ionic) solution depends on the following factors:
The nature of the electrolyte added
The solvent’s nature and its viscosity
Ions’ size and their solvation
Concentration of the electrolyte
Temperature (Conductivity increases with the increase in temperature).
3. What are the Characteristics of Secondary Batteries?
A secondary battery after use can be recharged by passing current through it in the opposite direction and it can be used again. A large number of discharging and charging cycles will undergo in a good secondary cell or battery. The most important secondary cell known is the lead storage battery which is used in inverters and automobiles. It consists of a lead anode and also a grid of lead packed with lead dioxide as cathode. A 38 percent solution of sulphuric acid is used as an electrolyte.