Electro Chemistry

Conductors pass electricity (metals and ionic compounds (melted or in solution)) Insulators do not pass electricity (Plastics, wood, non-metal)

Types of conductors If electric current is passed due to flow of electrons, it is electronic conduction (metals and graphite) If ions are responsible for the electric flow, it is ionic conduction (Molten ionic compounds and ionic solutions)

Electrode used to send current in and take current out of electrolyte Anode is the positive electrode Cathode is the negative electrode Electrolysis is the process of chemical change when electricity is passed through substances Electrolyte is the substance which undergoes chemical change during electrolysis

Electrolysis of molten ionic compounds Lead bromide (PbBr 2 ): Molten lead(ll) bromide contains lead ions Pb 2+ and bromide ions Br - At cathode: Lead ion receives 2 electrons (reduction) to become Lead atom Pb e -  Pb reduction At anode: Bromide ions loose an electron each to form bromine atoms (oxidation). Bromine atoms join to form Bromine molecule 2Br -  Br 2 + 2e- oxidation

Sodium chloride: molten sodium chloride contains sodium ions (Na + ) and chloride ions Cl - At cathode: Sodium ion receives an electron (reduction) to be Sodium atoms Na + + e -  Na reduction At anode: chloride ions loose an electron each to form chlorine atoms (oxidation). Chlorine atoms join to form chlorine molecule 2Cl -  Cl 2 + 2e- oxidation

Electrolysis of concentrated solutions Concentrated solutions contain +ive ions and – ive ions from the substance and hydrogen ions (H+) and hydroxide (OH-) ions from water. At cathode positive ions compete and metal or hydrogen will be collected At anode oxygen or a halogen will be collected

Activity series Potassium Sodium Calcium Magnesium Aluminium Zinc Iron Lead Hydrogen Copper Silver Gold If Hydrogen ions compete with Copper, Silver or Gold ions, metal ions win. If hydrogen ions compete with any other metal ions, Hydrogen wins At the anode hydroxide ions compete with chloride, bromide or iodide ions (halide), halide wins. If hydroxide ions compete with any other ions, hydroxide ions win and oxygen gas will be produced

Concentrated sodium chloride solution At cathode: Sodium ions(Na + ) and hydrogen ions (H + ). Hydrogen ions win. 2H + + 2e -  H 2 reduction At anode: Chloride ions and hydroxide ions. Chlorine collected. 2Cl - - 2e-  Cl 2 (green yellow gas) oxidation Chlorine bleaches the colour

Concentrated hydrochloric acid solution At cathode: Only hydrogen ions (H + ). No competition 2H + + 2e -  H 2 reduction At anode: chloride ions and hydroxide ions. Chlorine collected. 2Cl - - 2e-  Cl 2 oxidation

Electrolysis of dilute solutions At cathode, the rule is same as that of concentrated solutions. At anode, the anions from the ionic substance are not many in a dilute solution. So Oxygen gas from hydroxide ions will be collected always.

Copper (II)sulphate solution: At cathode: copper ions (Cu 2+ ) and Hydrogen ions (H + ). Copper is lower than hydrogen and so copper is formed at cathode Cu e -  Cu reduction At anode: Sulphate ions (SO 4 2+ ) and hydroxide ions. Oxygen from hydroxide ions will be collected. 4OH - - 4e-  H 2 O + O 2 oxidation The solution remaining will be sulphuric acid

Acidified water : Pure water is not a conductor. Water contains a few drops of acid is an electrolyte. At cathode: Only hydrogen ions 2H + + 2e -  H 2 reduction At anode: anions from acid and hydroxide ions. Few drops of acid contains very small number of ions from acid. Oxygen from hydroxide ions will be collected. 4OH - -4e-  H 2 O + O 2 oxidation OR 2O 2-  O 2 + 4e-

Electrolysis of Brine Brine a concentrated solution of sodium chloride It is made by dissolving salt in salt mines by water pumped in. The products are Hydrogen gas(cathode), Chlorine gas(anode) and sodium hydroxide solution is left. All the above products are useful

Electrolysis of brine

Electrolysis of brine: Uses of products Hydrogen gas -making hydrogen peroxide (used in hair dye) -manufacture of margarine from oil -making nylon -used as rocket fuel

Chlorine gas -Making different types of plastics (PVC) -making medical drugs -to make pesticides and weed killers -making bleaching powder and bleaches -To kill germs in water(swimming pools)

Sodium hydroxide - Making soaps -Making detergents -different types of textile fibres (Rayon) -manufacture of medical drugs -making ceramic products (tiles) -to make household bleaches

Electrolysis to Purify metals Metals like copper is purified by electrolysis Impure copper is used as anode, a thin sheet of pure copper is used as cathode and a solution of copper(II) sulphate is used the electrolyte. At anode: Copper atoms change into copper ions and get into the electrolyte Cu  Cu e - oxidation At cathode: Copper ions accept electrons to form copper atoms Cu e -  Cu reduction

Electrolysis of copper(ll) sulphte solution using copper electrodes

Electroplating A metal is coated over another metal. Electroplating is used to make something look better or to prevent corrosion. Examples: -Gold and silver plated things are attractive. -Chromium plating prevents iron from rusting and also it looks nice.

Electroplating Which metal to electroplate (eg silver, chromium, gold etc) is used as the anode The article on which we electroplate is the cathode A salt solution of the metals is the electrolyte To Electroplate silver on a iron spoon, a piece of silver is the anode, iron spoon is the cathode and a solution of silver nitrate is the electrolyte

Anode: Ag  Ag + + e - oxidation Cathode: Ag + + e -  Ag reduction

Uses of electrolysis To make highly reactive metals like sodium, potassium calcium, magnesium and aluminium To make commercially important substances like hydrogen gas, chlorine gas and sodium hydroxide Electroplating Purification of metals (copper)