Electrochemistry is the branch of physical chemistry which deals with the relationship between electrical energy and chemical changes taking place in redox reactions
Electrolytes and Electrolysis
(1) Definition: “The substances whose aqueous solution undergo decomposition into ions when electric current is passed through them are known as electrolytes and the whole process is known as electrolysis or electrolytic decomposition.”
Solutions of acids, bases, salts in water and fused salts etc. are the examples of electrolytes. Electrolytes may be weak or strong. Solutions of cane sugar, glycerine, alcohol etc., are examples of non-electrolytes.
(2) Electrolytic cell or Voltameter: The device in which the process of electrolysis or electrolytic decomposition is carried out is known as electrolytic cell or voltameter.
(i) Voltameter convert electrical energy into chemical energy.
(ii) The electrode on which oxidation takes place is called anode (or +ve pole) and the electrode on which reduction takes place is called cathode (or –ve pole)
(iii) During electrolysis in voltameter cations are discharged on cathode and anions on anode.
(iv) In voltameter, outside the electrolyte electrons flow from anode to cathode and current flow from cathode to anode.
For voltameter, and
(v) The anions on reaching the anode give up their electrons and converted into the neutral atoms.
At anode : (Oxidation)
(vi) On the other hand cations on reaching the cathode take up electrons supplied by battery and converted to the neutral atoms.
At cathode: (Reduction)
This overall change is known as primary change and products formed is known as primary products.
The primary products may be collected as such or they undergo further change to form molecules or compounds. These are called secondary products and the change is known as secondary change.
(3) Preferential discharge theory: According to this theory “If more than one type of ion is attracted towards a particular electrode, then the ion is discharged one which requires least energy or ions with lower discharge potential or which occur low in the electrochemical series”.
The potential at which the ion is discharged or deposited on the appropriate electrode is termed the discharge or deposition potential, (D.P.). The values of discharge potential are different for different ions.
The decreasing order of discharge potential or the increasing order of deposition of some of the ions is given below,
For cations :
For anions :