Writing More Complex Redox Equations
- 1. Writing more complex Redox Equations Aim: To be able construct ionic half equations for reactions involving complex ions.
- 2. Starter NaBr + H 2 SO 4 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. Starter: NaI with Sulphuric acid NaI + H 2 SO 4 HI + NaHSO 4 8HI + H 2 SO 4 H 2 S + 4I 2 + 4H 2 O Redox? – Strength of reducing agent?
- 4. Starter Disproportionation 3NaClO 2NaCl + NaClO 3 Ionic equation? Disproportionation?
- 5. Disproportionation 2 2NaOH + Cl 2 NaOCl + NaCl + H 2 O Ionic equation? Disproportionation?
- 6. Disproportionation 3 In hot alkali 6NaOH + 3Cl 2 NaClO 3 + 5NaCl + 3H 2 O Ionic equation? Disproportionation?
- 7. Redox reactions – Potassium Permanganate and Sodium thiosulphate. KMnO 4 Na 2 S 2 O 3 Oxidation number of Potassium? Charge on the Manganate ion? Oxidation number of Mn in MnO 4 - ? Oxidation number of Na? Charge on the thiosulphate ion? Oxidation number of S in the thiosulphate ion – S 2 O 3 -2 ?
- 8. Reaction between MnO 4 - and S 2 O 3 - wine into water! MnO 4 - Mn +4 Mn +2 S 2 O 3 - 2 S 4 O 6 -2 Observation : Purple to colourless. Redox?
- 9. Reduction of the Manganate ion MnO 4 - MnO 4 - Mn +2 Step 1: balance the oxygen by using water molecules. MnO 4 - Mn +2 + 4H 2 0 Step 2: Balance the hydrogen using H +1 ions. MnO 4 - + 8H + Mn +2 + 4H 2 0 Balance for Charge using electrons: MnO 4 - + 8H + + 5e - Mn +2 + 4H 2 0 Reduction or oxidation?
- 10. Oxidation of the Thiosulphate ion to Tetrathionate ion. 2S 2 O 3 - 2 S 4 O 6 -2 + 2e - Sulphur change in oxidation number? S +2 S +2.5 Reduction or oxidation? Why?
- 11. Overall redox equation MnO 4 - + 8H + + 5e - Mn +2 + 4H 2 0 2S 2 O 3 - 2 S 4 O 6 -2 + 2e -
- 12. Ionic half equation for the reduction of the dichromate ion Cr 2 O 7 2- 2Cr +3 Step 1 balance Oxygen using water Cr 2 O 7 2- 2Cr +3 + 7H 2 O Step 2 Balance Hydrogen using H +1 ions Cr 2 O 7 2- + 14H +1 2Cr +3 + 7H 2 O Step 3 Balance for Charge using electrons Cr 2 O 7 2- + 14H +1 +6e - 2Cr +3 + 7H 2 O
- 13. Some examples Write ionic half equations for the following that take place in acidic solution. Iodate (v) ions IO 3 -1 to iodine. Manganese dioxide MnO 2 to Mn +2. VO 2 + ions to VO +2 H 2 SO 3 to Sulphuric acid
- 14. Answers 2IO 3 -1 +12H + +10e - I 2 + 6H 2 O MnO 2 + 4H + +2e - Mn +2 + 4H 2 O VO 2 + + 2H + + 1e - VO 2+ + H 2 O SO 3 -2 + H 2 O SO 4 -2 + 2H + + 2e -
- 15. Reaction between Iodine and Sodium Thiosulphate 2Na 2 S 2 O 3 + I 2 2NaI + Na 2 S 4 O 6 Half ionic equations to identify redox? Iodine: Thiosulphate: How will you know when this reaction has gone to completion? How could you prove that the reaction is consistent with the above equation using titration? I 2 + 2e -1 2I -1 2S 2 O 3 -2 S 4 O 6 2- + 2e -
- 16. A Redox Titration 0.01M sodium thiosulphate 10ml of 0.01M I 2 Titrate 10ml samples of 0.01M iodine solution with 0.01M Na 2 S 2 O 3 The end point for the titration is when the yellow iodine turns colourless. 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.
- 17. Titration Results. Titre cm 3 Burette reading initial Burette reading final 4 3 2 1 Burette readings
- 18. Interpreting the Results Mean Titre Sodium Thiosulphate = Calculate the number of moles of sodium thiosulphate that reacted with the iodine. Calculate the number of moles of Iodine used in the reaction. Prove that this is consistent with the equation for the reaction. 2Na 2 S 2 O 3 + I 2 2NaI + Na 2 S 4 O 6
- 19. Redox questions Separate the following into their 2 half ionic equations 2FeCl 2 + Cl 2 2FeCl 3 2KBr + Cl 2 2KCl + Br 2 MnO 2 + 4HCl MnCl 2 + Cl 2 + 2H 2 O
- 20. Further redox questions – separate into half ionic equations MnO 4 - + 5Fe +2 + 8H + Mn +2 +5Fe +3 +4H 2 O IO 3 - + 5I -1 + 6H + 3I 2 + 3H 2 O
- 21. Home work Questions 1 -12 Question sheet. Revise for mock exam on January 3 rd . See revision guide.