Electrolysis part 3 aqueous solution

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Electrolysis part 3 aqueous solution

  1. 1. 4S4 6th & 8th July 20 Electrolysis of an aqueous solution
  2. 2. Learning outcomes At the end of the lesson, you should able to: • identify cations and anions in an aqueous solution, • describe the electrolysis of an aqueous solution, • write half equations for the discharge of ions at the anode and the cathode, • predict the products of electrolysis of aqueous solution. • explain using examples factors affecting electrolysis of an aqueous solution
  3. 3. Dilute solution Electrolysis of an aqueous solution
  4. 4. Electrolyte: Molten NaCl Electrolyte: NaCl solution Carbon electrodes NaCl solution Cations Anions Cations Anions Na+ Cl- Na+ Cl- H+ OH-
  5. 5. Sodium chloride solution Cations Anions + Cl- Na + OH - H
  6. 6. Exercise: Solution Cations Anions Dilute sulphuric acid H+ SO42- , OH- H2SO4 Silver bromide solution Ag+, H+ Br- , OH- AgBr Sodium nitrate solution Na+, H+ NO3-, OH- NaNO3
  7. 7. Electrochemical series a series to show the tendency of the ions to donate/ receive electrons.
  8. 8. Cations Anions K+ Na+ F- Ca2+ 2- Electrochemical SO4 series Easier to discharge Mg2+ Al3+ NO3- Zn2+ Cl- Fe 2+ Sn2+ Br- Pb2+ I- H+ Cu2+ OH- Ag+ Au+
  9. 9. Homework for this week Copy & Memorise the ELECTROCHEMICAL SERIES Quiz on next week
  10. 10. Your Electrochemical series own K+ Potassium Pork Potential Pretty Pop Na+ Sodium Sometimes Scientists Sally Stars Ca2+ Calcium Can Can Could Can Mg2+ Magnesium Make Make Marry Make Al3+ Aluminium A A A Any Zn2+ Zinc Zulu Zoo Zulu Zombie Fe 2+ Iron ill ? in in Sn2+ Tin Therefore Tiny Lovely Town Pb2+ Lead Let London Peace Lose H+ Hydrogen Highly ? Honolulu His Cu2+ Copper Clever City Causing Control Ag+ Silver Slay Strange ? Au+ Gold Good Pigs Gazes ?
  11. 11. Anions Your own F- Fat Flower SO42- ‘So So NO3- Nui’ Nice Cl- Clean Can Br- Brother’s Buy I- I-phone’s In OH- ‘O’ ready Orchard
  12. 12. Copper(II) sulphate, CuSO4 solution consists of Ions that are present Cu2+ ions, H+ ions, SO42- ions , OH- ions that move freely.
  13. 13. CATHODE ANODE electrodes Carbon Competing ions Cu2+ ions, H+ ions Cations selectively Cu2+ ions discharged Cu2+ ion is lower than H+ ions Why? in the electrochemistry series. Half equation Cu2+ + 2 e- → Cu Observation A brown solid is deposited at cathode. Products Copper, Cu metal is formed at the cathode.
  14. 14. CATHODE ANODE Carbon electrodes Competing SO42- ions , OH- ions ions Anions OH- ions selectively discharged OH-ion is lower than SO42- ions Why? in the electrochemistry series. Half 4OH- → O2 + 2H2O + 4 e- equation Colourless gas bubbles Observation are released at anode oxygen gas, O2 and water, H2O Products are produced at the anode.
  15. 15. Copper(II) sulphate, CuSO4 solution consists of Ions that are present Cu2+ ions, H+ ions, SO42- ions , OH- ions that move freely. CATHODE ANODE electrodes Carbon Carbon electrodes Competing Competing ions Cu2+ ions, H+ ions SO42- ions , OH- ions ions Cations Anions selectively Cu2+ ions OH- ions selectively discharged discharged Cu2+ ion is lower than H+ ions OH-ion is lower than SO42- ions Why? Why? in the electrochemistry series. in the electrochemistry series. Half Half equation Cu2+ + 2 e- → Cu 4OH- → O2 + 2 H2O + 4 e- equation Observation A brown solid is deposited at Colourless gas bubbles are Observation cathode. released at anode Products Copper, Cu metal is formed at oxygen gas, O2 and water, H2O Products the cathode. are produced at the anode.
  16. 16. Describe Electrolysis of an aqueous solution Example: Copper(II) sulphate, CuSO4 solution 0.1 mol dm-3 Carbon electrodes
  17. 17. • Copper(II) sulphate, CuSO4 solution consists of Cu2+ ions, H+ ions, Cl2- ions and OH- ions that move freely. • During electrolysis, the cations Cu2+ ions and H+ ions move to the cathode; the anions SO42- ions and OH- ions move to the anode.
  18. 18. At CATHODE, • Copper (II) ions , Cu2+ ions are selectively discharged by receiving electrons to form Cu atom. This is because Cu2+ ions has a lower position than H+ ions in the electrochemical series. Cu2+ (aq) + 2 e- → Cu (s) • A brown solid is deposited at cathode. • Copper, Cu metal is formed at the cathode.
  19. 19. At ANODE, • Hydroxide ions , OH- are selectively discharged by donating electrons to form O2 and H2O molecules. This is because OH- ions has a lower position than SO42- ions in the electrochemical series. 4OH- (aq) → O2 (g) + 2H2O(l) + 4 e- • Colourless gas bubbles are released at anode. (The gas relights a glowing wooden splinter.) • Oxygen gas, O2 and water, H2O are produced at the anode.
  20. 20. Overall equation: 2Cu2+ (aq) + 4 e- → 2Cu (s) 4OH- (aq) → O2 (g) + 2H2O(l) + 4 e- 2Cu2+ (aq) + 4OH- (aq) → 2Cu (s)+ O2 (g) + 2H2O(l)
  21. 21. Describe Electrolysis of an aqueous solution Copper(II) chloride, CuCl2 solution 0.1 mol dm-3 Carbon electrodes
  22. 22. Sodium chloride solution Cations Anions + Cl- Na + OH - H
  23. 23. Sodium chloride Copper (II) sulphate solution solution Cations Anions Cations Anions Na+ Cl- Cu2+ SO42- H+ OH- H+ OH- The selection of ions to be discharged is based on: 3 Factors : • Position of ions in the electrochemical series • Concentration of ions in the electrolytes • Types of electrodes used in the electrolysis
  24. 24. Factors that affect the electrolysis of an aqueous solution Factors Ions will be discharged Position of ions The ions that are LOWER in the ELECTROCHEMICAL SERIES in the electrochemical will be selectively discharged. series Concentration of ions The particular ions with HIGHER CONCENTRATION in the electrolytes will be selectively discharged Types of electrodes used ACTIVE ELECTRODES ionises (form ions) in the electrolysis
  25. 25. Position of ions in the electrochemical series Cations Anions K+ F- Na+ Ca2+ SO42- Easier to discharge Mg2+ Al3+ NO3- which ions will be Zn2+ Cl- SELELCTED to be Fe 2+ discharged ? Sn2+ Br- Pb2+ I- H+ Cu2+ OH- Ag+
  26. 26. Observation & Product/s CATHODE ANODE Sodium Cations: Carbon 0.1 Colourless gas Colourless gas sulphate, Na+ mol bubbles are bubbles are Na2SO4 H+ dm-3 released. released. solution, Anions: SO42- OH- H2 gas O2 gas, water Silver Cations: Carbon 0.1 Silvery grey Colourless gas nitrate, Ag+ mol solid is bubbles are AgNO3 H+ dm-3 deposited. released. solution Anions: NO3- Silver, Ag O2 gas, water OH- metal
  27. 27. Concentration of ions in the electrolytes A A 0.001 moldm-3 1.0 moldm-3
  28. 28. Products CATHODE ANODE Dilute HCl Cations: 0.001 Carbon Colourless gas Colourless gas Na+ mol bubbles are bubbles are H+ dm-3 released. released. Anions: Cl- OH- H2 gas O2 gas, water
  29. 29. Products CATHODE ANODE Concentrated Cations: 1.0 Carbon HCl mol Colourless gas Greenish- H+ bubbles are yellow vapour dm-3 released. is released. Anions: Cl- OH- H2 gas Cl2 gas Because: Cl- ions have higher concentration in the electrolyte. (even though Cl- ion has higher position than OH- ion in the electrochemical series.)
  30. 30. Types of electrodes A A Inert electrode Active electrode
  31. 31. Products Change CATHODE ANODE in solution CuSO4 Cations: A brown Carbon 0.1 mol dm-3 solution, Colourless The intensity Cu2+ solid is gas bubbles H+ deposited. are released. of the blue colour of the Anions: electrolyte SO42- decreases. Copper, O2 gas, OH- Cu metal water INERT electrode Because: concentration of the blue Cu2+ ions decreases as more copper is deposited at the cathode
  32. 32. Observation Change CATHODE ANODE in solution CuSO4 Cations: solution, 0.1 mol dm-3 Copper Cu2+ H+ Anions: SO42- OH- ACTIVE electrode  take part in chemical reaction during electrolysis At anode, ions present in the electrolyte will not be discharged.
  33. 33. Observation Change CATHODE ANODE in solution The copper The copper The intensity of electrode electrode the blue colour of becomes dissolves into the the electrolyte THICKER. solution. The anode becomes remain unchanged. THINNER. Cu2+ + 2e- → Cu Cu → Cu2+ + 2e- Discharge ionise Because: concentration of the blue Cu2+ ions REMAIN UNCHANGED. Rate of formation of Cu2+ ions at anode = Rate of discharged of Cu2+ ions at cathode.
  34. 34. Glossary: A solution produced when a compound is dissolved in WATER. Receive/ donate electrons to form atoms and molecules Form ions
  35. 35. K+ Half equation Na+ Ca2+ Mg2+ X n+ + n e- → X Al3+ Zn2+ Eg.: Cu2+ + 2e- → Cu Fe 2+ Sn2+ Pb2+ H+ Cu2+ Ag+ Au+
  36. 36. Observations K Na Ca Mg Al Silvery grey solid is deposited at cathode. Zn Fe Sn Pb Cu Brown solid is deposited at cathode.
  37. 37. Half equations F- 2 Y- → Y2 + 2 e- Cl- Br- Eg.: 2 Br- → Br2 + 2 e- I- OH- 4OH- → O2 + H2O+ 4 e-
  38. 38. Observations A greenish yellow vapour is released at anode. Cl2 The gas turns the blue litmus paper to red and then white. Br2 A brown vapour is released at anode. The gas turns the blue litmus paper to red and then white. I2 A brown liquid is produced at anode. Colourless gas bubbles are released at anode. O2 The gas relights a glowing wooden splinter. Colourless gas bubbles are released at cathode. H2 The gas gives a ‘pop’ sound when tested with a lighted wooden splinter when bring near to the mouth of the test tube.

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