Electrolysis: Chemistry of Electrolytes PDF

Electrolysis Electrolysis It is the process of decomposition of a chemical compound in aqueous solution or in a molten state, accompanied by a chemical change using direct current. Electrolyte It is an ionic compound which in the fused state or in the aqueous solution allows the passage of an elec...

Electrolysis Electrolysis It is the process of decomposition of a chemical compound in aqueous solution or in a molten state, accompanied by a chemical change using direct current. Electrolyte It is an ionic compound which in the fused state or in the aqueous solution allows the passage of an electric current and is decomposed by it. Strong Electrolytes The compounds which in their aqueous solution or in the fused state are almost completely ionised are called strong electrolytes. Examples: Mineral acids, alkalis and salts Weak Electrolytes The compounds which in their fused state or aqueous solution are feebly ionised and are poor conductors of electricity are called weak electrolytes. Examples: Acetic acid, oxalic acid Non-electrolytes The compounds which neither in solution nor in the molten state allow an electric current to pass through them are called non-electrolytes. Examples: Kerosene, carbon disulphide Electrolytic Cell It is a device used to convert electrical energy into chemical energy. Electrochemical Cell It is a device used to convert chemical energy into electrical energy. Electrodes The graphite, metal plates or gas carbon rods immersed in the electrolyte through which current enters and leaves the electrolytic cell are called electrodes. Cathode The electrode connected to the negative terminal of the battery is called a cathode. Anode The electrode connected to the positive terminal of the battery is called an anode. Ions The atoms or groups of atoms which carry a positive or negative charge are known as ions. Cations Atoms which carry a positive charge are called cations. Anions Atoms which carry a negative charge are called anions. Oxidation It is a chemical process which involves the addition of oxygen or the removal of hydrogen. Oxidising Agents It is a substance which oxidises other substances either by accepting electrons or by providing oxygen or an electronegative ion. Reduction It is a chemical process which involves the removal of oxygen or the addition of hydrogen. Reducing Agent It is a substance which reduces other substances by providing electrons, hydrogen or an electropositive ion. Dissociation The process due to which an ionic compound dissociates into ions in the fused state or in the aqueous solution is called electrolytic dissociation. Example: Electrovalent compound such as NaCl. NaCl → Na+ + Cl– Ionisation The process by which polar covalent compounds are converted into ions in water solution is called ionisation. HCl H+ + Cl– Electrochemical Series It is a series in which metals are arranged based on the ease with which atoms of metals lose electrons to form positively charged ions. Cation Anion K + Discharged Discharged SO42− with most with most Ca 2+ difficulty difficulty NO31− + Na Mg2+ Cl1− Al3+ Zn2+ Br1− Fe2+ Pb2+ I1− H+ Cu2+ Hg2+ Ag+ Discharged Discharged OH1− most easily most easily Selective Discharge of Ions The preferential discharge of ions present in an electrolyte at the respective electrodes is known as selective discharge of ions. It depends on the following factors: i. Relative position of ions in an electrochemical series ii. Concentration of the ions iii. Nature of the electrode Electrolysis of Fused Lead Bromide Electrolyte: Molten lead bromide (PbBr2) Anode: Graphite Cathode: Graphite Overall reaction: PbBr2  Pb2+ + 2Br– Reaction at the cathode: Pb2+ + 2e−  Pb Reaction at the anode: Br– − e  − Br Br + Br  Br2 Electrolysis of Acidified Water Electrolyte: Acidified water  Anode: Platinum Cathode: Platinum Ionisation of acidified water: H2O H+ + OH– H2SO4 2H+ + SO42– Ions present: H+, SO42–, OH– Reaction at the cathode: H+ + e−  H H + H  H2 (Hydrogen molecule) – Reaction at the anode: OH − e−  OH × 4 – OH ion discharge in preference to SO42– 4OH  2H2O + O2 (Oxygen molecule) Electrolysis of Aqueous Copper Sulphate Electrolyte: Aqueous copper sulphate solution Anode: Copper Cathode: Copper Dissociation of aqueous copper sulphate: CuSO4 Cu2+ + SO42– H2O H+ + OH– Reaction at the cathode: Cu2+ + 2e−  Cu Cu being lower in the electrochemical series. Reaction at the anode: Cu − 2e–  Cu 2+ 2– SO4 and OH– are not discharged. Applications of Electrolysis i. Electroplating with metals ii. Electrorefining of metals iii. Extraction of metals Electroplating It is a process in which a thin film of a metal, such as gold, silver or nickel, gets deposited on another metallic article with the help of electricity. Reasons for Electroplating i. Decoration purposes ii. To protect from rusting and corrosion Electroplating with Nickel Electrolyte: Aqueous solution of nickel sulphate Dissociation: NiSO4 Ni2+ + SO42− H2O H+ + OH− Cathode: Article to be electroplated Anode: Block of pure nickel Reaction at cathode: Ni2+ + 2e−→ Ni (deposited) Reaction at anode: Ni − 2e−→ Ni2+ Note: Article to be electroplated is always kept at the cathode. Electrolytic refining of metals It is a process by which metals containing impurities are purified electrolytically to give a pure metal. Electrolytic refining of copper Electrolyte: Copper sulphate solution and dil. sulphuric acid Cathode: Thin strip of pure copper Anode: Impure copper Reaction at cathode: Cu2+ + 2e−→ Cu Reaction at anode: Cu − 2e− → Cu2+ Electrometallurgy It is the process of extraction of metals by electrolysis. Activity Series Acids, Bases and Salts as Electrolytes They can be classified as strong or weak electrolytes depending on the degree of dissociation. Degree of dissociation = Number of molecules dissociated × 100 Total number of molecules