Electrochemistry Notes PDF

4. ELECTROCHEMISTRY 4.1 Electrolysis Electrolysis: The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. Electrolyte: Molten or aqueous substance that undergoes electrolysis. Eg: molten salts, Solution of salt in water...

4. ELECTROCHEMISTRY 4.1 Electrolysis Electrolysis: The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. Electrolyte: Molten or aqueous substance that undergoes electrolysis. Eg: molten salts, Solution of salt in water Solutions of acids Solutions of alkalis Electrodes: Metallic or graphite rods which carry the current in to and out of the solution are called electrodes. Positive electrode is called anode and negative electrode is called cathode ANODE – positive electrode CATHODE – negative electrode  Electrons flow from the anode to the battery.  Electrons flow from the battery to the cathode  Anions, negative ions (non-metals except  Cations(usually metal and hydrogen ions) in hydrogen) are attracted to the anode (positive the electrolyte are attracted to the cathode electrode). (negative electrode)  Oxidation occurs at anode  Reduction occurs at cathode Types of electrodes: Electrolysis Using inert electrodes (graphite or platinum ) : Note: Advantages of using inert electrodes like graphite and platinum Platinum is an inert metal electrode and graphite is an inert nonmetal electrode  They are very good conductors of electricity. They do not undergo any chemical reaction. Active electrode: takes part in the reaction. Eg: copper, zinc etc. 1 Difference between metallic conduction and electrolytic conduction: Metallic conduction Electrolytic conduction 1. Due to flow of electrons Due to movement of ions 2. Takes place in solids and liquids In molten or aqueous solution only 3. No chemical change Undergo chemical change Flow chart for the electrolysis of molten and aqueous electrolyte 2 Electrolysis of molten compounds using inert electrodes  A molten substance means that the substance has been melted down. When a molten ionic compound is electrolysed,   Metal is always formed at cathode and the nonmetal is always formed at anode.  The electrons always move in the external circuit. ( from anode to cathode)  The loss or gain of electrons always occurs at the electrodes.  Movement of ions always in the electrolyte. Eg 1. Electrolysis of molten sodium chloride NaCl contains Na+ and Cl- ions. At cathode (-) Na+ + e- Na At anode (+) 2Cl- Cl2+2e- OR 2Cl- - 2e- Cl2 3 2. Electrolysis of molten Calcium chloride CaCl2 contains Ca2+ and Cl- ions. At cathode (-) Ca2+ + 2e- Ca At anode (+) 2Cl- Cl2 + 2e- OR 2Cl- - 2e- Cl2 3.Electrolysis of molten Lead(II) bromide Molten lead bromide contains two ions Pb2+ and Br- At cathode (Negative electrode)  Pb2+ ions are attracted towards the cathode  They will accept two electrons Pb2+ + 2e- Pb At anode (Positive electrode)  Bromide ions are attracted towards the positive electrode  Br- ions give away extra electron to anode and form bromine atoms  Bromine atoms joins up in pairs and form diatomic bromine molecules. 2Br- Br2 + 2e- OR 2Br- - 2e- Br2 4. Electrolysis of molten alumina (Al2O3) – Extraction of Aluminium 4 Aluminium is manufactured by the electrolysis of molten alumina (Al2O3).The ore Bauxite is purified by treating with sodium hydroxide which removes impurities like iron oxide and sand. Purified bauxite is called alumina. Alumina is dissolved in molten cryolite (Na3AlF6). Advantages of using cryolite in Aluminium extraction a) to reduce the melting point of aluminium oxide, b) to increase its conductivity and c) As a solvent for the ore alumina The mixture is electrolysed in Hall Heroult cell using inert electrodes (graphite) Al2O3 contains Al3+ and O2- ions. Reaction at cathode(-) Al3+ + 3e- Al (reduction) Reaction at anode (+) 2 O2- O2+ 4e- (oxidation) The oxygen released at anode react with carbon (graphite) to form CO2.Therefore anode rods have to be replaced at regular intervals. C + O2 CO2 Use of aluminium 1.Manufacture of aircraft (Al has low density) 2.Food containers or cooking foil(Al is resistant to corrosion) 5  ELECTROLYTE Cathode Observation Anode Observation Changes to the (Molten salts) electrolyte Sodium chloride Sodium metal Silvery metal Chlorine Green Used up (NaCl) bubbles Potassium iodide Potassium metal Silvery metal Iodine Purple Used up (KI) bubbles Silvery metal Magnesium Magnesium Chlorine Green Used up chloride(MgCl2) metal bubbles Lead(II) bromide Lead metal Bromine Reddish Silvery metal brown Used up bubbles Copper(II) bromide Copper Bromine Pink solid Reddish Used up brown bubbles ELECTROLYSIS OF AQUEOUS SOLUTIONS In solutions H+ and OH- ions from water compete with the ions from acid or salt to discharge at the electrodes. AT CATHODE  If the metal is more reactive than hydrogen, metal exists as ions and H+ ions will accept electrons to become hydrogen. Hydrogen is the product at cathode.  If the metal is less reactive than hydrogen, Metal ions gain electrons and get reduced to metal. H+ ions will stay in the solution. Respective metal is the product 6 AT ANODE  In concentrated aqueous solution of halides (Cl-,Br-,I-), the halide ion give electrons and get oxidized to Cl2, Br2, I2.  If no halide is present or if halide solution is dilute, OH- ions will give up electrons and oxygen gas will be formed. 4OH- →2 H2O + O2 + 4e- Electrolysis of concentrated sodium chloride solution (brine) The ions present in the electrolyte are H+ and OH- from water and Na+ and Cl- from sodium chloride. At cathode (-) 2H+ + 2e- → H2 Sodium is more reactive than hydrogen, the H+ ions will be discharged at the cathode and hydrogen gas will evolve. Observation: Bubbles of colourless gas. 7 Test for the gas: bringing a lighted splint, if positive it will burn with a pop sound. At the anode (+) 2Cl-→Cl2+2e- Chlorine gas evolves. Observation is bubbles of pale green gas. Test :Chlorine gas bleaches damp litmus paper This leaves the ions Na+ and OH- in the electrolyte forming sodium hydroxide forms in the cathodic compartment. Na+ + OH- → NaOH (Thymolphthalein turns blue) PRODUCTS USES Chlorine gas  Used as a disinfectanct  Used for the synthesis of HCl  Used for the manufacture of plastic Hydrogen gas  Used for the synthesis of HCl  Used as a pollution free fuel Sodium hydroxide  Used for the manufacture of paper Electrolysis of dilute sodium chloride solution The ions present in the electrolyte are H+ and OH- from water and Na+ and Cl- from sodium chloride. At cathode (-) 2H+ + 2e- → H2 At the anode (+) 4OH- → 2H2O + O2 + 4e Other than the formation of gases – another observation is that the bulb glows. 8 Electrolysis of concentrated hydrochloric acid (HCl) At cathode (-) H+ ions are attracted to cathode and reduced to hydrogen gas which is colourless and burns with pop sound with a lighted splint. 2H+ + 2e- → H2 Note : All acid give hydrogen at the negative electrode (cathode) At the anode (+) Chloride ions are discharged at the anode which is pale green gas that bleaches damp litmus paper. 2Cl- → Cl2 + 2e- Electrolysis of dilute sulfuric acid (H2SO4 ) At cathode (-) 2H+ + 2e- → H2 At the anode (+) 4OH- →2 H2O + O2 + 4e- During the electrolysis the concentration of the sulfuric acid increases ( or the pH of solution decreases) as the water is used up. 9  ELECTROLYTE CATHODE OBSERVATION ANODE OBSERVATION CHANGES TO ELECTROLYTE Concentrated Hydrogen Colourless bubbles Chlorine gas Green bubbles Sodium hydroxide NaCl gas Dilute NaCl Hydrogen Colourless bubbles Oxygen gas Colourless Sodium chloride gas bubbles Concentrated Hydrogen Colourless bubbles Chlorine gas Green bubbles Water HCl gas Dilute H2SO4 Hydrogen Colourless bubbles Oxygen gas Colourless Sulfuric acid gas bubbles Con. KBr Hydrogen Colourless bubbles Bromine gas Reddish brown Potassium gas bubbles hydroxide Con.CuCl2 Copper metal Pink/ brown metal Chlorine gas Green bubbles Water deposited General observation in a circuit 1. Bubbles of gases formed (Mention the nature or colourof gas if any) 2. Deposition of metal ( mention the colour) 3.Bulb glows 4.Change in the electrolyte 5. Comparison of concentration of electrolytes-  If the electrolyte is more concentrated the bulb glows brighter and and formation of bubbles or effervescence will be vigorous. 10 Electrolysis of Copper(II) Sulfate Solution: Using inert electrode Ions present in the electrolyte are H+ and OH- from water and Cu+2 and SO4- from copper(II) sulfate. At the Cathode: Cu+2 + 2e- → Cu Cu+2 are discharged because copper is less reactive than hydrogen. A red brown metal is formed. At the anode: 4OH- → 2H2O + O2 + 4e- Oxygen from OH- ions is formed. Bubbles of colourless gas are formed. To test for oxygen, bring a glowing splint, if positive it relights. This leaves us with H+ and SO42- ions which bond together forming sulfuric acid. The blue copper sulfate changes to colourless sulfuric acid. 2. Using active electrodes ( copper ) Electrolysis can be used to refine metals. For example if we have a sample of impure copper that we want to refine, we set up a unique electrolysis cell to do that.  The cathode will be made of pure copper.  The anode will be made of the impure copper sample.  The electrolyte will be a solution of a copper salt (copper sulfate/nitrate) 11 At the anode: Copper anode itself loses electrons to give Cu2+ ions in the solution. Cu → Cu+2 + 2e- At the Cathode: Copper ions are discharged at the cathode as neutral copper atom. Cu+2 + 2e- → Cu The copper anode dissolves and decreases the mass. The concentration of Cu2+ ion remain unchanged. This method can be used for refining or purifying the metal. The impurities of the anode will settle at the bottom of the cell.  Observation at anode : decrease in size as copper goes as positive ions.  Observation at cathode: increase in size as copper metal forms.  Observation in electrolyte: The blue colour of the solution remains the same because the concentration of the electrolyte or Cu 2+ remains unchanged 12 Electroplating It is a process in which one metal is coated with another to improve their appearance and resistance to corrosion. For electroplating  The object which we want to plate is taken as cathode  The plating metal is taken as anode  The electrolyte is a solution containing ions of the plating metal.  Eg: Coating silver on a steel spoon Observation in electrolyte: no change Points to remember to carry out electroplating( PAPER 6) 1. The object must be very clean (rub with a sand paper) and free of grease to make sure that the metal layer sticks firmly. 2. The object must be rotated or agitated so that the layer forms evenly on all sides. 13