Week 7.2   electrolytic cell
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Week 7.2 electrolytic cell

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Week 7.2 electrolytic cell Presentation Transcript

  • 1. Chapter 3 Oxidation and Reduction Oxidation-Reduction Concepts Electrochemical Cells Voltaic Cell Electrolytic Cell Prepared by: Mrs Faraziehan Senusi PA-A11-7C Corrosion & Prevention Reference: Chemistry: the Molecular Nature of Matter and Change, 6th ed, 2011, Martin S. Silberberg, McGraw-Hill
  • 2. Electrolytic Cell • The principle of an electrolytic cell is: electrical energy from an external source drives a nonspontaneous reaction. • This process is called electrolysis.
  • 3. Electrolytic Cell • In voltaic cell, Sn anode will gradually become oxidized to Sn2+ ions, and the Cu2+ ions will gradually be reduced and plate out on the Cu cathode because the cell reaction is spontaneous in that direction:
  • 4. Electrolytic Cell • Therefore, the reverse cell reaction is nonspontaneous and never happens, as the negative E°cell and positive ΔG° indicate: • However, we can make this process happen by supplying from an external source an electric potential greater than E°cell • In effect, we have converted the voltaic cell into an electrolytic cell and changed the nature of the electrodesanode is now cathode, and cathode is now anode
  • 5. Electrolytic Cell • Note that in an electrolytic cell, oxidation takes place at the anode and reduction takes place at the cathode, but the direction of electron flow and the signs of the electrodes are reversed.
  • 6. Electrolytic Cell • In a voltaic cell, electrons are generated at the anode, so it is negative, and electrons are consumed at the cathode, so it is positive. • In an electrolytic cell, the electrons come from the external power source, which supplies them to the cathode, so it is negative, and removes them from the anode, so it is positive.
  • 7. Electrolytic Cell • Lets consider: THE ELECTROLYSIS OF MOLTEN SODIUM CHLORIDE. • Solid sodium chloride does not conduct electricity. However, molten (melted) NaCl is an excellent conductor because its ions are freely mobile. The melting point is 801°C.
  • 8. Electrolytic Cell • Consider a cell in which a source of direct current is connected by wires to two inert graphite electrodes. They are immersed in a container of molten (melted) sodium chloride. When the current flows, we observe the following: 1. 2. A pale green gas, which is chlorine, Cl2, is liberated at one electrode. Molten, silvery white metallic sodium, Na, forms at the other electrode and floats on top of the molten sodium chloride.
  • 9. Electrolytic Cell • Chlorine must be produced by oxidation of Cl ions, and the electrode at which this happens must be the anode. • Metallic sodium is produced by reduction of Na ions at the cathode, where electrons are being forced into the cell. • The formation of metallic Na and gaseous Cl2 from NaCl is nonspontaneous except at temperatures very much higher than 801°C. • The direct current (dc) source must supply electrical energy to force this reaction to occur.
  • 10. The Downs Cell …    Industrial method electrolytic cell of molten NaCl electrolysis The iron screen prevent Na to spontaneously react with Cl2 Most practical, highly effective method of obtaining metallic Na, but operating cost is very high (construction cost, electricity cost, heating cost)
  • 11. FARADAY’S LAW OF ELECTROLYSIS • The amount of substance that undergoes oxidation or reduction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the cell. Faraday’s Law of Electrolysis • A quantitative unit of electricity is called the faraday. • A smaller electrical unit commonly used is the coulomb (C). • One coulomb is defined as the amount of charge that passes a given point when 1 ampere (A) of electric current flows for 1 second.
  • 12. FARADAY’S LAW OF ELECTROLYSIS • Problems based on Faraday's law often ask you to calculate current, mass of material, or time.
  • 13. FARADAY’S LAW OF ELECTROLYSIS Example: How long does it take to produce 3.0g of Cl2 (g) by electrolysis of aqueous NaCl using a power supply with a current of 12A? Solution: From half-reaction, we loss of 2 mol of electrons produces 1 mol of chlorine gas: So, we find the total charge: Then, use relationship between charge and current to find time needed: