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TOPIC 6 ELECTROCHEMISTRY

TOPIC 6 ELECTROCHEMISTRY. By: Chemistry Lecturer School of Allied Health Sciences City University College of Science and Technology. Electrolytes and Non-electrolytes. Electrolytes: Chemical compound that can conduct electricity in the molten state or in aqueous solution

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TOPIC 6 ELECTROCHEMISTRY

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  1. TOPIC 6ELECTROCHEMISTRY By: Chemistry Lecturer School of Allied Health Sciences City University College of Science and Technology

  2. Electrolytes and Non-electrolytes • Electrolytes: Chemical compound that can conduct electricity in the molten state or in aqueous solution • Electricity conducted by free moving ions • Non electrolytes: Chemical compound that cannot conduct electricity in any state

  3. ELECTROLYTES NON-ELECTROLYTES

  4. Electrolytic of a molten compound • Electrolysis is a process whereby a compound is decomposed by electric current • Electrolytic cell consists of two electrodes • Anode – Positive terminal • Anions (-ve charged ions) attracted to anode • Cathode – Negative terminal • Cations (+ve charged ions) attracted to cathode

  5. Electrolytic process • Generally a molten compound electrolytes AnBm produces Am+ cations and Bn- anions AnBm Am+ Bn- • Examples: • PbBr2 Pb2+ + 2Br- • NaCl  Na+ + Cl-

  6. LET’S TRY! • Gives the anion and cation for the electrolytes: • PbCl2 - • AgCl - • CuCl2 - • CuBr2 -

  7. ANSWER • Gives the anion and cation for the electrolytes: • PbCl2 - Pb2+ , Cl- • AgCl - Ag+ , Cl- • CuCl2 - Cu2+ , Cl- • CuBr2 - Cu2+ , Br-

  8. Two steps occur during electrolysis • Movement of ions to the electrodes • Cations (+ve) move towards the cathode • Anions (-ve) move towards the anode • Discharge of ions at the electrodes • Cations discharged by receiving electrons (losing positive charge to become neutral) An+ + ne-  A • Anions discharged by releasing electrons (losing negative charge to become neutral) Bn- B + ne-

  9. Writing half equation

  10. Overall equation • Balance the number of electrons from the half equation Example: Half equation At the Anode: 2Cl-  Cl2 + 2e At the cathode: Na+ + e  Na Overall equation 2Cl-  Cl2 + 2e (Na+ + e  Na) x 2 = 2Na+ + 2e  2Na = 2Cl- +2Na+  Cl2 +2Na

  11. Predicting the products • In the electrolysis of a molten compound, • The metal component of the compound is formed at the cathode • The non-metal component is formed at the anode

  12. Example: NaCl molten electrolytes NaCl Na+ + Cl- Anode: Cl- Cathode: Na+ Half equation At the Anode: 2Cl-  Cl2 + 2e At the cathode: Na+ + e  Na Overall equation 2Cl-  Cl2 + 2e (Na+ + e  Na) x 2 = 2Na+ + 2e  2Na 2Cl- +2Na+  Cl2 +2Na Product Anode: Cl2 gas Cathode: Na metal

  13. Example: PbBr2 molten electrolytes Solution Equation PbBr2 Pb2+ + Br- Anode: Br- Cathode: Pb2+ Half equation Anode: 2Br-  Br2 + 2e Cathode: Pb2+ +2e  Pb Overall equation 2Br-  Br2 + 2e Pb2+ +2e  Pb 2Br- +Pb2+ Br2 +Pb Products Anode: Br2 gas Cathode: Pb metal

  14. Electrolysis of aqueos solution • Aqueos solution consists of TWO types of cations and anions; H+ and OH- • Example: Aqueos sodium chloride (NaCl) solution Cations: Na+ and H+ Anions: Cl- and OH- • However only ONE type of cation and anion will be discharged at each electrode

  15. Factors that determine the products formed • Positions of ions in the electrochemical series • The tendency of ions to be selectively discharged depends on their positions in a series known as electrochemical series (ES). • The lower the position of the ion in the ES, the easier the ion will be discharged.

  16. ELECTROCHEMICAL SERIES

  17. Example: Aqueos sodium chloride (NaCl) solution electrolytes Cations: Na+ and H+ Anions: Cl- and OH- • Half equation Anode: 4OH- 2H2O + O2 + 4e Cathode: 2H+ + 2e  H2 • Overall equation 4OH- 2H2O + O2 + 4e (2H+ + 2e  H2 ) x 2 = 4H+ + 4e  2H2 4OH- +4H+  2H2O + O2 + 2H2 • Products Anode: oxygen gas Cathode: hydrogen gas

  18. Example 2: Aqueos sodium sulphate (Na2SO4) solution electrolytes Cations: Na+ and H+ Anions: SO42- and OH- • Half equation Anode: 4OH- 2H2O + O2 + 4e Cathode: 2H+ + 2e  H2 • Overall equation 4OH- 2H2O + O2 + 4e (2H+ + 2e  H2 ) x 2 = 4H+ + 4e  2H2 4OH- +4H+  2H2O + O2 + 2H2 • Products Anode: oxygen gas Cathode: hydrogen gas

  19. Factors that determine the products formed • Effect of ion concentration • When the concentration of a particular type of ion is higher, ion will more likely to be discharged in electrolysis • Usually concentrated halide (Cl- / Br- / I- ions)

  20. Factors that determine the products formed • Effect of types of electrodes used • The types of electrodes used can determined the type of ions discharged in electrolysis • Using metal electrodes at anode, ions are not discharged instead the metal dissolves by releasing electrons to form metal ions. • Example: using copper (Cu) electrodes. Cu  Cu2+ + 2e Hence mass of anode decrease.

  21. Let’s Try!! • Gives the diagram, anion, cation, half equation, overall equation, products and observation using electrolytes • PbBr2 molten • PbCl2 molten • CuSO4 solution

  22. THANK YOU…

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