The branch of science which deals with the production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring about non- spontaneous chemical transformations is called electrochemistry.
Oxidation is a process which involves loss of electrons and reduction is a process which involves gain of electrons.
The reactions which involve both oxidation and reduction are called redox reactions. In these reactions, electrons are transferred from one reactant to another.
The substance which can lose one or more electrons (i.e., get oxidised) is called reducing agent or reductant while the substance which can gain one or more electrons (i.e, get reduced) is called oxidising agent or oxidant.
Thus, in a redox reaction, one substance acts as a reducing agent and itself gets oxidised while another substance acts as an oxidising agent and itself gets reduced.
For example: A redox reaction is the reaction between zinc and copper (II) salt occurring in a battery. In this reaction, zinc loses electrons and gets oxidised whereas Cu2+ ions gain electrons and get reduced.
Zn(s) + Cu2+ ———–> Zn2+ (aq) + Cu (s)
Zinc acts as a reducing agent or reductant while Cu2+ ions act as oxidising agent or oxidant.
Some other examples of redox reactions are
Zn + 2HCl ——–> ZnCl2 + H2
5Fe2+ +8 H+ MnO4‾ ———> 5 Fe3+ + Mn2+ + 4 H2O
Metallic and Electrolytic conductance
The substances which allow the passage of electric current are called conductors. The best conductors are metals such as copper, silver, tin, etc.
The substances which do not allow the passage of electric current through them are called non-conductors or insulators.
Some common examples of insulators are rubber, glass, ceramics, wood, wax, etc.
Types of Conductors
The conductors are broadly classified into two types:
(1) Metallic conductors or Electronic conductors
These are metallic substances which allow the electricity to pass through them without undergoing any chemical change. Metals and their alloys have very large conductivity and are called conductors.
For example: copper, silver etc.
The flow of electric current through metallic conductors is due to the flow of electrons in the metal atoms. Electrical conductance through metals is called metallic conductance or electronic conductance.
The electronic conductance depends on
(1) the nature and structure of the metal
(2) the number of valence electrons per atom
(3) the density of metal and
(4) temperature (it decreases with increase of temperature)
(2) Electrolytes or Electrolytic conductors
These are substances which allow the electricity to pass through them in their molten states or in the form of their aqueous solutions and undergo chemical decomposition.
For example: acids, bases and salts are electrolytes.
The flow of electric current through an electrolytic solution is called electrolytic conduction.
In this type of conduction, charge is carried by ions. Therefore, it is also called ionic conductance. The conduction will not occur unless the ions of the electrolyte are free to move.
Therefore, these substances do not conduct electricity in the solid state but conduct electricity in the molten state or in their aqueous solutions due to the movement of ions.
Non-electrolytes: The substances, which do not conduct electricity either in their molten state or through their aqueous solutions are called non-electrolytes.
For example: sugar, glucose, ethyl alcohol, urea, etc.
|It is carried by the movement of electrons
|It is carried by the movement of ions.
|It does not involve the transfer of any matter.
|It involves the transfer of matter as ions.
|It involves no change in the chemical properties of the conductor.
|It involves the decomposition of the electrolyte as a result of the chemical reaction.
|It decreases with increase in temperature.
|It increases with increase in temperature.
Classification of Electrolytes
On this basis , electrolytes are broadly divided into two types : strong electrolytes and weak electrolytes.
(1) Strong electrolytes: The electrolytes which are almost completely dissociated into ions in solution are called strong electrolytes.
For example: NaCl, KCl, HCl, NaOH, NH4NO3 etc.
(2) Weak electrolytes: The electrolytes which do not ionise completely in solution are called weak electrolytes.
For example: CH3COOH, H2CO3, H3BO3, HCN, HgCl2, ZnCl2, NH4OH, etc.
In weak electrolytes, an equilibrium is established between the unionised electrolyte and the ions formed in solution.
The extent of ionisation of a weak electrolyte is expressed in terms of degree of ionisation or degree of dissociation. It is defined as the fraction of total number of molecules of the electrolyte which ionise in the solution. It is generally denoted by alpha (α), for strong electrolytes, α is almost equal to 1 and for weak electrolytes, it is always less than 1.
Factors Affecting Electrical Conductivity
The electrical conductivity of the solutions of electrolytes depends upon the following factors:
(i) Interionic attractions: These depend upon the interactions between the ions of the solute molecules, i.e. solute-solute interactions. If the solute-solute interactions are large, the extent of dissociation will be less. These interactions are also responsible for the classification of electrolytes as strong electrolytes and weak electrolytes.
(ii) Solvation of ions: These depend upon the interactions between the ions of the solute and the molecules of the solvent and are called solute-solvent interactions. If the solute-solvent interactions are strong, the ions of the solute will be highly solvated and their electrical conductivity will be low.
(ii) Viscosity of the solvent: The viscosity of the solvent depends upon the solvent-solvent interactions. Larger the solvent-solvent interactions, large will be the viscosity of the solvent.
The average kinetic energy of the ions of the electrolyte increases with increase in temperature. Consequently, the conductance of electrolytic solutions increases with rise in temperature. The conductance of electronic conductors decreases with increase in temperature.
Conductivity of electrolytic (or ionic) solution depends upon the following factors:
(i) Nature of electrolyte: The conductance of an electrolyte depends upon the number of ions present in the solution. Therefore, the greater the number of ions in the solution, the greater is the conductance. The number of ions produced by an electrolyte depends upon its nature.
The strong electrolytes dissociate almost completely into ions in solutions and therefore, their solutions have high conductance.
Weak electrolytes, dissociate to only small extents and give lesser number of ions. Therefore, the solutions of weak electrolytes have low conductance.
(ii) Nature of the solvent: Electrolytes ionize more in polar solvents. Therefore greater the polarity of the solvent, larger is the ionization and hence greater is the conductance.
(iii) Size of the ions produced and their solvation: If the ions are strongly solvated, their effective size will increase and hence their conductance will decrease.
(iv) Concentration of the electrolytic solution: The molar conductance of electrolytic solution varies with the concentration of the electrolyte. The molar conductance of an electrolyte increases with decrease in concentration or increase in dilution.
(v) Temperature: The conductivity of an electrolyte depends upon the temperature. With increase in temperature, the conductivity of an electrolyte increases.