Writing More Complex Redox Equations

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Writing More Complex Redox Equations

  1. 1. Writing more complex Redox Equations Aim: To be able construct ionic half equations for reactions involving complex ions.
  2. 2. Starter <ul><li>NaBr + H 2 SO 4 </li></ul>HBr + NaHSO 4 Redox? Yes or no? 2HBr + H 2 SO 4 SO 2 + Br 2 + 2H 2 O Half ionic equations? Oxidation number?
  3. 3. Starter: NaI with Sulphuric acid <ul><li>NaI + H 2 SO 4 HI + NaHSO 4 </li></ul><ul><li>8HI + H 2 SO 4 H 2 S + 4I 2 + 4H 2 O </li></ul>Redox? – Strength of reducing agent?
  4. 4. Starter Disproportionation <ul><li>3NaClO 2NaCl + NaClO 3 </li></ul><ul><li>Ionic equation? </li></ul><ul><li>Disproportionation? </li></ul>
  5. 5. Disproportionation 2 <ul><li>2NaOH + Cl 2 NaOCl + NaCl + H 2 O </li></ul><ul><li>Ionic equation? </li></ul><ul><li>Disproportionation? </li></ul>
  6. 6. Disproportionation 3 <ul><li>In hot alkali </li></ul><ul><li>6NaOH + 3Cl 2 NaClO 3 + 5NaCl + 3H 2 O </li></ul><ul><li>Ionic equation? </li></ul><ul><li>Disproportionation? </li></ul>
  7. 7. Redox reactions – Potassium Permanganate and Sodium thiosulphate. <ul><li>KMnO 4 </li></ul><ul><li>Na 2 S 2 O 3 </li></ul><ul><li>Oxidation number of Potassium? </li></ul><ul><li>Charge on the Manganate ion? </li></ul><ul><li>Oxidation number of Mn in MnO 4 - ? </li></ul><ul><li>Oxidation number of Na? </li></ul><ul><li>Charge on the thiosulphate ion? </li></ul><ul><li>Oxidation number of S in the thiosulphate ion – S 2 O 3 -2 ? </li></ul>
  8. 8. Reaction between MnO 4 - and S 2 O 3 - wine into water! <ul><li>MnO 4 - </li></ul><ul><li>Mn +4 </li></ul><ul><li>Mn +2 </li></ul><ul><li>S 2 O 3 - 2 </li></ul><ul><li>S 4 O 6 -2 </li></ul><ul><li>Observation : Purple to colourless. </li></ul><ul><li>Redox? </li></ul>
  9. 9. Reduction of the Manganate ion MnO 4 - <ul><li>MnO 4 - Mn +2 </li></ul><ul><li>Step 1: balance the oxygen by using water molecules. </li></ul><ul><li>MnO 4 - Mn +2 + 4H 2 0 </li></ul><ul><li>Step 2: Balance the hydrogen using H +1 ions. </li></ul><ul><li>MnO 4 - + 8H + Mn +2 + 4H 2 0 </li></ul><ul><li>Balance for Charge using electrons: </li></ul><ul><li>MnO 4 - + 8H + + 5e - Mn +2 + 4H 2 0 </li></ul><ul><li>Reduction or oxidation? </li></ul>
  10. 10. Oxidation of the Thiosulphate ion to Tetrathionate ion. <ul><li>2S 2 O 3 - 2 S 4 O 6 -2 + 2e - </li></ul><ul><li>Sulphur change in oxidation number? </li></ul><ul><li>S +2 S +2.5 </li></ul><ul><li>Reduction or oxidation? Why? </li></ul>
  11. 11. Overall redox equation <ul><li>MnO 4 - + 8H + + 5e - Mn +2 + 4H 2 0 </li></ul><ul><li>2S 2 O 3 - 2 S 4 O 6 -2 + 2e - </li></ul>
  12. 12. Ionic half equation for the reduction of the dichromate ion <ul><li>Cr 2 O 7 2- 2Cr +3 </li></ul><ul><li>Step 1 balance Oxygen using water </li></ul><ul><li>Cr 2 O 7 2- 2Cr +3 + 7H 2 O </li></ul><ul><li>Step 2 Balance Hydrogen using H +1 ions </li></ul><ul><li>Cr 2 O 7 2- + 14H +1 2Cr +3 + 7H 2 O </li></ul><ul><li>Step 3 Balance for Charge using electrons </li></ul><ul><li>Cr 2 O 7 2- + 14H +1 +6e - 2Cr +3 + 7H 2 O </li></ul>
  13. 13. Some examples <ul><li>Write ionic half equations for the following that take place in acidic solution. </li></ul><ul><li>Iodate (v) ions IO 3 -1 to iodine. </li></ul><ul><li>Manganese dioxide MnO 2 to Mn +2. </li></ul><ul><li>VO 2 + ions to VO +2 </li></ul><ul><li>H 2 SO 3 to Sulphuric acid </li></ul>
  14. 14. Answers <ul><li>2IO 3 -1 +12H + +10e - I 2 + 6H 2 O </li></ul><ul><li>MnO 2 + 4H + +2e - Mn +2 + 4H 2 O </li></ul><ul><li>VO 2 + + 2H + + 1e - VO 2+ + H 2 O </li></ul><ul><li>SO 3 -2 + H 2 O SO 4 -2 + 2H + + 2e - </li></ul>
  15. 15. Reaction between Iodine and Sodium Thiosulphate <ul><li>2Na 2 S 2 O 3 + I 2 2NaI + Na 2 S 4 O 6 </li></ul><ul><li>Half ionic equations to identify redox? </li></ul><ul><li>Iodine: </li></ul><ul><li>Thiosulphate: </li></ul><ul><li>How will you know when this reaction has gone to completion? </li></ul><ul><li>How could you prove that the reaction is consistent with the above equation using titration? </li></ul>I 2 + 2e -1 2I -1 2S 2 O 3 -2 S 4 O 6 2- + 2e -
  16. 16. A Redox Titration 0.01M sodium thiosulphate 10ml of 0.01M I 2 <ul><li>Titrate 10ml samples of 0.01M iodine solution with 0.01M Na 2 S 2 O 3 </li></ul><ul><li>The end point for the titration is when the yellow iodine turns colourless. </li></ul><ul><li>The end point can be made more accurate by adding a few drops of 1% starch when the iodine colour becomes very pale. A dark blue colour is produced which suddenly disappears at the end point. </li></ul>
  17. 17. Titration Results. Titre cm 3 Burette reading initial Burette reading final 4 3 2 1 Burette readings
  18. 18. Interpreting the Results <ul><li>Mean Titre Sodium Thiosulphate = </li></ul><ul><li>Calculate the number of moles of sodium thiosulphate that reacted with the iodine. </li></ul><ul><li>Calculate the number of moles of Iodine used in the reaction. </li></ul><ul><li>Prove that this is consistent with the equation for the reaction. </li></ul><ul><li>2Na 2 S 2 O 3 + I 2 2NaI + Na 2 S 4 O 6 </li></ul>
  19. 19. Redox questions <ul><li>Separate the following into their 2 half ionic equations </li></ul><ul><li>2FeCl 2 + Cl 2 2FeCl 3 </li></ul><ul><li>2KBr + Cl 2 2KCl + Br 2 </li></ul><ul><li>MnO 2 + 4HCl MnCl 2 + Cl 2 + 2H 2 O </li></ul>
  20. 20. Further redox questions – separate into half ionic equations <ul><li>MnO 4 - + 5Fe +2 + 8H + Mn +2 +5Fe +3 +4H 2 O </li></ul><ul><li>IO 3 - + 5I -1 + 6H + 3I 2 + 3H 2 O </li></ul>
  21. 21. Home work <ul><li>Questions 1 -12 Question sheet. </li></ul><ul><li>Revise for mock exam on January 3 rd . </li></ul><ul><li>See revision guide. </li></ul>

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