ELECTROCHEMISTRY

ELECTROCHEMISTRYElectrolysis , Faraday's Laws of Electrolysis , Electrochemical or Galvanic or Voltaic cellsElectrolysisIt is the process of decomposition of an electrolyte by the passage of electricity through its aqueous solution or molten state.Mechanism of electrolysisWhen ever an electrolyte is dissolved in water or taken in the molten state it dissociates into positive and negative ions. The positive ions are known as cations and the negative ions are known as anions.On passing electric current through the electrolyte cations move towards the cathode and anions move towards the anode.On reaching their respective electrodes these ions loose their charge. On loosing their charge they get deposited on the electrode or discharged as a gas.Let us take an example of electrolysis of aqueous copper sulphate solution using inert electrodes such as platinum electrodes.In the aqueous solution copper sulphate dissociates into its respective ions.CuSO4 Cu2+ + SO4 2-On passing electric current the copper ions(cations) move towards the cathode and get deposited as copper. Simultaneously the sulphate ions(anions) move towards the anode.Faraday's Laws of ElectrolysisFaraday's First Law of ElectrolysisThe mass of a substance deposited or liberated at any electrode is directly proportional to quantity of electric current passed.If W grams of a substance is deposited or liberated on passing Q Coulomb of charge then :W Qand W = Z x Q where Z is the proportionality constant and is called the Electrochemical EquivalentFaraday's Second Law of ElectrolysisWhen the same amount of charge is made to pass through any number of electrolytes, the mass of the substance liberated or deposited at the electrodes are directly proportional to their chemical equivalents.Chemical equivalent = atomic mass / valencem1/m2 = E1/E2 where m1 and m2 are the respective masses liberated or deposited on the electrodesand E1 and E2 are the chemical equivalents of the substances liberated or deposited.Electrochemical or Galvanic or Voltaic CellsAn Electrochemical Cell is a device used to convert chemical energy (produced in a redox reaction) into electrical energy.Electrochemical Cells are also known as Galvanic or Voltaic Cells.If we take a zinc rod and place it in a container filled with copper sulphate solution heat will be produced. This happens due a spontaneous redox reaction given below:Zn(Solid) + CuSO4(Aqueous) ZnSO4(Aqueous) + Cu (Solid) depositedAs the reaction would proceed the zinc rod would get eroded ,copper particles would get deposited and solution would becom warm.It would be useful to be able to convert this chemical energy to electrical energy instead of heat energy. This is done by an electrochemical cell.Construction of an Electrochemical CellLet us use the redox reaction given below to explain the construction of an Electrochemical Cell.Zn(Solid) + CuSO4(Aqueous) ZnSO4(Aqueous) + Cu (Solid)The ionic form of the reaction is:Zn + Cu2+ Zn2+ + CuThis reaction can be split into the following two half reactions.1. Oxidation half reactionZn Zn2+ + 2e- 2. Reduction half reactionCu2+ + 2e- CuAn Electrochemical CellThe oxidation reaction in the zinc rod releases two electrons.These two electrons are taken by the Copper ion in the copper sulphate solution.If these two half reactions can be separated then the electrons can be made to move through a wire.In this manner we can produce electical energy from chemical energy.The salt bridge is a concentrated solution of inert electrolytes. It is required for completing the circuit. It allows the movement of ions from one solution to the other.