GET THIS RIGHT!! Basis of Electrolysis Definition: Breaking down of a compound into its elements using an electric current Positive terminal Negative terminal Electrons are supplied by the battery from negative terminal Electrodes: Electrolyte: Plates/Rods that can conduct Molten or ionic solutions that electricity have ions that are free to Eg. graphite, platinum, copper move electrons Eg. salts, bases, acids ANODE ANODE CATHODE ANION CATION CATHODE electrode connected to the positive terminal electrode connected to the negative terminal attracts anions attracts cations allows oxidation only allows reduction only electrons are lost electrons are gained OXI. RED. LOSE GAIN ions 2 1 Electrolysis OF BINARY COMPOUND Aqueous solutions Electrolysis of Molten Sodium Chloride Electrode: Carbon/Platinum (Because it is unreactive) When a salt (or an acid) is dissolved in water, water molecules dissociate to form hydrogen ions (H+) and hydroxide ions (OH-) Electrolyte: Molten sodium chloride Ions present: Cation: Na+ ANODE H2O (l) Anion: ClCATHODE Sodium ions form sodium molecules by losing electrons. atoms by gaining electrons. 2 à Cl2 + 2e OXIDATION + OH- (aq) In any salt solution, there will be TWO cations and TWO anions in the electrolyte. Chloride ions form chlorine Cl- H+ (aq) Na+ + e à Na REDUCTION Product formed: Product formed: Chlorine gas Sodium metal Observation: Observation: Greenish-yellow bubbles. Silvery-grey coating on the cathode. Example 1: Example 2: Aqueous sodium chloride, NaCl (aq) Hydrochloric acid, HCl (aq) cations anions cations anions Na+ Cl- H+ Cl- H+ OH- H+ OH- ONLY ONE ion each will be selected to be discharged (or produce a product). 3 4 Example 1: FACTORS AFFECTING SELECTIVITY Electrolysis of Aqueous Sodium Chloride a) Factor 1: Position in Electrochemical Series ANION Electrode: Carbon/Platinum CATION Electrolyte: Dilute aqueous sodium chloride Potassium Ions present: Cations: Na+ Sodium Fluoride Ions LOWER on Calcium Sulfate the Magnesium Nitrate electrochemical Zinc Chloride series will be Iron Bromide discharged Lead Iodide Anions: Cl- CATHODE Hydroxide ions chosen to form Hydrogen ions chosen to water and oxygen molecules form hydrogen molecules by by losing electrons. gaining electrons. 2 H+ + 2e à H2 OXIDATION Copper Silver OH- ANODE 4 OH-à 2 H2O + O2 + 4e Hydrogen Hydroxide H+ REDUCTION Product formed: Product formed: IMPORTANT NOTE: IMPORTANT NOTE: Oxygen gas Hydrogen gas Whenever water is present, hydroxide Whenever water is present, hydrogen are always discharged as it is the is always discharged unless copper ions Observation: Observation: lowest already. or silver ions are present Colourless bubbles. Colourless bubbles. 5 6 b) Factor 2: Concentration of halide ions Example 2: Electrolysis of Aqueous Copper (II) Sulfate Example: Electrode: Carbon/Platinum Electrolysis of Concentrated Sodium Chloride Electrolyte: Aqueous copper (II) sulfate Electrode: Carbon/Platinum Ions present: Cations: Cu2+ ANODE H+ Electrolyte: Concentrated aqueous sodium chloride Anions: SO42- OH- Ions present: Cations: Na+ CATHODE ANODE H+ Anions: Cl- OHCATHODE Hydroxide ions chosen to form Copper (II) ions chosen to water and oxygen molecules form copper atoms by Chloride ions chosen to form Hydrogen ions chosen to by losing electrons. gaining electrons. chlorine molecules by losing form hydrogen molecules electrons. by gaining electrons. 4 OH-à 2 H2O + O2 + 4e OXIDATION Cu2+ + 2e à Cu 2 REDUCTION Product formed: Product formed: Oxygen gas Copper metal Observation: Observation: Colourless bubbles. Brown coating on the cathode. 7 Cl- à Cl2 + 2e 2 H+ + 2e à H2 OXIDATION Product formed: REDUCTION Product formed: Chlorine gas Hydrogen gas Observation: Observation: Greenish-yellow bubbles. Colourless bubbles. 8 c) Factor 3: Type of Electrodes ELECTROLYSIS IN INDUSTRIES Electrolysis of Aqueous Copper (II) Sulfate a) Extraction of Aluminium from Bauxite Electrode: Copper plates Ions present: Cation: Electrolyte: Aqueous copper (II) sulfate Ions present: Cations: Cu+ H+ ANODE Anions: SO42- OH- Copper (II) ions chosen to copper (II) ions by losing form copper atoms by electrons. gaining electrons. Cu à Cu2+ + 2e OXIDATION Cu2+ Anion: CATHODE Oxide ions form Aluminium ions form oxygen molecules by aluminium atoms by losing electrons. gaining electrons. 2 O2- Al3+ + 3e à Al à O2 + 4e OXIDATION + 2e à Cu REDUCTION REDUCTION Product formed: Product formed: Oxygen gas Aluminium metal Product formed: Product formed: Special industrial issues and solutions: Copper (II) ions Copper metal • Observation: Observation: Anode becomes thinner. Cathode becomes thicker 9 1 10 Definition A device which converts chemical energy to electrical energy. c) Purification of Metals 2 Process of removing impurities from metals Electrochemical Series A metal higher in electrochemical series will be oxidised by losing electrons. CATHODE Copper metal dissolves Copper (II) ions chosen to into copper (II) ions by form copper atoms by losing electrons. gaining electrons. OXIDATION Carbon anode has to be replaced timely as they reacts with oxygen gas. GALVANIC CELL Process of coating one metal with another, with the help of electricity Cu à Cu2+ + 2e Bauxite’s extremely high melting point is lowered by adding cryolite (also acts as a solvent) • b) Electroplating ANODE O2- ANODE CATHODE Copper metal dissolves into Al3+ Amount of electrical energy produced is based the difference in the electropotential. K, Na, Ca, Mg, Zn, Fe, Sn, Pb, H, Cu, Ag more reactive Cu2+ + 2e à Cu small voltage large voltage no voltage REDUCTION Product formed: Product formed: Copper (II) ions Copper metal Observation: Observation: Anode becomes thinner. Brown coating on cathode 11 12 3 Galvanic Cell vs Electrolytic Cell 4 Simple Cell Galvanic Electrolytic ANODE Electrical à Chemical Anode (Positive Terminal) Cathode (Negative Terminal) Copper (II) ions form zinc ions by losing copper atoms by gaining electrons. electrons. OXIDATION Oxidation at ANODE, Reduction at CATHODE Chemicalà Electrical Zinc metal dissolves into Zn à Zn2+ + 2e Electrons transferred from ANODE to CATHODE Cathode (Positive Terminal) Anode (Negative Terminal) CATHODE Observation: Zinc becomes thinner. Cu2+ + 2e à Cu zinc copper negative terminal positive terminal REDUCTION Observation: Cathode becomes thicker. Important note: The more electropositive metal ALWAYS behave as the negative terminal and anode 13 5 Daniell Cell a) In a simple cell, zinc displaces copper out of solution directly. A layer of copper is formed on zinc, reducing current rapidly. b) Porous pot allows flow of ions but prevent solutions from mixing. c) Salt bridge contains inert ions (ie. sodium chloride) to complete the circuit. aqueous zinc sulfate aqueous copper (II) sulfate Model 1: Porous pot aqueous zinc sulfate aqueous copper (II) sulfate Model 2: Salt bridge 15 14