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7.5 The Electrolytic Cell • The Structure and Function of the Electrolytic Cell • Comparing Electrochemical and Electrolytic Cells • Electrolytic Cell Types • Electrolytic Applications 7.5 The Electrolytic Cell The Structure and Function of the Electrolytic Cell A chemical is an electrolyte if its aqueous solution conducts electricity. Whenever electricity is conducted through a molten electrolyte or an electrolyte solution, the process is called electrolysis and the apparatus is called an electrolytic cell. Electrolytic cells transform electrical energy into chemical energy. 7.5 The Electrolytic Cell The Structure and Function of the Electrolytic Cell (Graphite) Ni2+ + 2e → Ni (Graphite) 2I ͞ → I2 + 2e 7.5 The Electrolytic Cell The Structure and Function of the Electrolytic Cell The circuit is completed as cations in the molten electrolyte or electrolyte solution pick up the DC source’s electrons while anions or the anode itself give up electrons that return to the DC source. The reactions of electrolytic cells are non-spontaneous (negative Eo). The strongest available reducing agent passes an electron(s) down to the strongest available oxidizing agent in the SRP table. 7.5 The Electrolytic Cell The Structure and Function of the Electrolytic Cell The net Eo provides an estimation of how much voltage will be required to drive the electrolytic cell but this value will normally be less than the voltage actually required. The difference between the voltages the SRP Table predicts will be necessary to drive electrolytic cells and the voltages actually required to drive those cells is referred to as overpotential. 7.5 The Electrolytic Cell Comparing Electrochemical and Electrolytic Cells Electrochemical Cell Electrolytic Cell exothermic ‘makes’ electricity transforms chemical energy into electrical energy endothermic ‘takes’ electricity transforms electrical energy into chemical energy is a DC voltage source requires a DC voltage source 2 half-cells 1 cell spontaneous redox reaction E° is positive non-spontaneous redox reaction E° is negative salt bridge or equivalent no salt bridge 7.5 The Electrolytic Cell V Comparing Electrochemical and Electrolytic Cells Electrochemical Cell Electrolytic Cell V DC The oxidation half-reaction is below the reduction half-reaction in the SRP table. Think of the electrons floating upward. The oxidation half-reaction is above the reduction half-reaction in the SRP table. Think of the electrons being pushed downward. The half-reactions which are the furthest apart in the SRP table will occur, generating the greatest possible voltage. The half-reactions which are the closest together in the SRP table will occur, requiring the least possible voltage to drive the cell. text pages 466-467 6.5 The Electrolytic Cell Electrolytic Cell Types Type 1 cells - inert electrodes immersed in a molten ionic compound. Only the molten ions (1 type of thing) can be oxidized and reduced. Type 2 cells - inert electrodes immersed in an aqueous ionic solution. Only the ions or H2O (2 types of things) can be oxidized and reduced. Be mindful of the overpotential effect. Type 3 cells - non-inert electrodes immersed in an aqueous ionic solution. The ions, H2O, or the electrodes themselves (3 types of things) can be oxidized and the ions or H2O can be reduced. Be mindful of the overpotential effect. 7.5 The Electrolytic Cell Electrolytic Applications Electrowinning is a metallurgical term for the electrolytic recovery of a metal from a solution containing its ions. Electroplating is a form of electrowinning in which a conductive material, usually a metal, is coated with a thin layer of a different metal. Electrorefining is the electrolytic purification of a metal. 7.5 The Electrolytic Cell Electrolytic Applications The Héroult-Hall Process produces aluminum by electrolyzing a molten mixture of aluminum oxide and cryolite which melts at a much lower temperature than the aluminum oxide alone. The chloralkali industry refers to the industrial electrolysis of aqueous sodium chloride to produce sodium hydroxide and chlorine. In impressed current cathodic protection the metal being protected is the cathode of an electrolytic cell, i.e. its hooked up to the negative electrode of a DC source.