1. Here we’ll work through another example of a type 2 electrolytic cell - Electrolysis of an aqueous solution using unreactive or inert electrodes.

2. We’re given that an aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. Because we have an aqueous solution and inert electrodes, we know this is a Type 2 cell. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. b)What is the product at the cathode? c)What will happen to the pH near the cathode? d)Write the half-reaction taking place at the anode. e)What is the product at the anode? f)Write the overall redox equation

3. We can visualize this solution as mobile Magnesium 2+ ions, iodide ions, and a large number of water molecules, all moving in the solution. Now, we make a diagram based on what we have. Power Supply + – e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– e–e– 1 M MgI 2(aq) – + Anode Cathode I–I– Mg 2 + I–I– H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O I – H H O

4. In this solution, we have Mg 2+ ions An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. Mg 2+ H 2 O I –

5. I minus ions An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. Mg 2+ H 2 O I –

6. and water molecules An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. Mg 2+ H 2 O I –

7. We’ll represent the cathode here with a C minus and (click) the anode here with an A + An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. Mg 2+ H 2 O I – C– A+

8. Question (a) asks us to write the half-reaction taking place at the cathode. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. b)What is the product at the cathode? c)What will happen to the pH near the cathode? d)Write the half-reaction taking place at the anode. e)What is the product at the anode? f)Write the overall redox equation

9. The cathode is where reduction takes place. We could have either the (click) reduction of Mg 2+ ions or (click) the reduction of Neutral water. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. Mg 2+ H 2 O I – C– A+

10. Remember, if we have an aqueous solution, (click) for the cathode, we use the overpotential arrow on the left side of the table For the Cathode In Aqueous Solution

11. Any cations ABOVE this overpotential arrow WILL be reduced from an aqueous solution, even though water is present. Cations ABOVE this arrow WILL be reduced from aqueous solution For the Cathode In Aqueous Solution

12. But any cations BELOW this overpotential arrow WILL be NOT reduced from an aqueous solution. If these are present, Water will be reduced instead. Cations ABOVE this arrow WILL be reduced from aqueous solution Cations BELOW this arrow Will be NOT reduced from aqueous solution For the Cathode In Aqueous Solution

13. Here’s the black arrow on the left we use for the reduction overpotential of neutral water. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode.

14. Notice the reduction half-reaction for magnesium ions is below this black arrow. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode.

15. Therefore, magnesium ions cannot be reduced in aqueous solution. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode.

16. And water will be reduced instead, according to this half-reaction. And we can (click) replace the question mark by the H2O in the diagram with a check to indicate that water is the species reduced at the cathode in this cell. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode.

17. So we’ll make a note up here that the cathode half-reaction is 2H 2 O + 2e –  H 2(g) + 2OH – An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. Cathode: 2H 2 O + 2e –  H 2(g) + 2OH –

18. Question (b) asks what are the products at the cathode. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. b)What are the products at the cathode? c)What will happen to the pH near the cathode? d)Write the half-reaction taking place at the anode. e)What is the product at the anode? f)Write the overall redox equation

19. We’ll write the half-reaction taking place at the cathode in a larger form here. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. b)What are the products at the cathode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH –

20. One of the products is hydrogen gas. Because it is a gas, (click) bubbles will form on the surface of the cathode. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. b)What are the products at the cathode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH –

21. The other product is hydroxide ions An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. b)What are the products at the cathode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH –

22. Question (c) asks us what will happen to the pH near the cathode. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. b)What are the products at the cathode? c)What will happen to the pH near the cathode? d)Write the half-reaction taking place at the anode. e)What is the product at the anode? f)Write the overall redox equation

23. Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Remember, one of the products at the cathode is hydroxide ions. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. c)What will happen to the pH near the cathode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH –

24. Hydroxide ions make a solution basic, An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. c)What will happen to the pH near the cathode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Make the solution Basic

25. Which means the pH in the solution surrounding the cathode will increase. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. c)What will happen to the pH near the cathode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Make the solution Basic pH will INCREASE

26. Question (d) asks us to write the half-reaction taking place at the anode. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. b)What are the products at the cathode? c)What will happen to the pH near the cathode? d)Write the half-reaction taking place at the anode. e)What is the product at the anode? f)Write the overall redox equation

27. We’ll go back to this diagram, An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. d)Write the half-reaction taking place at the anode. Mg 2+ H 2 O I – C– A+

28. And focus on the anode. We see that either (click) water, or (click) the iodide ion is oxidized at the anode. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. d)Write the half-reaction taking place at the anode. Mg 2+ H 2 O I – C– A+

29. When we’re considering the anode, we go to the right side of the reduction table. (click) In aqueous solutions for the anode half-reaction, we use this overpotential arrow on the right. For the Anode In Aqueous Solution

30. Remember, how it works is any anions BELOW this overpotential arrow on the right WILL be oxidized from aqueous solution at the anode of an electrolytic cell. For the Anode In Aqueous Solution Anions BELOW this arrow WILL be oxidized from aqueous solution

31. But any anions ABOVE this overpotential arrow on the right will NOT be oxidized from aqueous solution. If one of these are present, water will be oxidized instead. For the Anode In Aqueous Solution Anions BELOW this arrow WILL be oxidized from aqueous solution Anions ABOVE this arrow Will be NOT oxidized from aqueous solution

32. We see that that iodide ion is well below the overpotential arrow on the right.

33. This means iodide will be oxidized at the anode. So we can (click) replace the question mark in the diagram by a checkmark, to indicate that iodide is the species oxidized at the anode in this cell.

34. Because oxidation occurs at the anode, this half-reaction must occur in reverse, so (click) the half- reaction at the anode is 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Anode: 2I –  I 2(s) + 2e – Oxidation

35. We’ll write this half-reaction a little larger here, An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. d)Write the half-reaction taking place at the anode. Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Anode: 2I –  I 2(s) + 2e –

36. So we’ve now answered question (d). The half-reaction at the anode is 2I –  I 2(s) + 2e – An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. d)Write the half-reaction taking place at the anode. Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Anode: 2I –  I 2(s) + 2e –

37. Question (e) asks what is the product at the anode. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. b)What are the products at the cathode? c)What will happen to the pH near the cathode? d)Write the half-reaction taking place at the anode. e)What is the product at the anode? f)Write the overall redox equation

38. Looking at the half-reaction, (click) we see that the product at the anode is solid iodine. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. e)What is the product at the anode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Anode: 2I –  I 2(s) + 2e –

39. Because the electrode is surrounded by an aqueous solution, the iodine that forms dissolves partially in water to form a reddish brown colour. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. e)What is the product at the anode? Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Anode: 2I –  I 2(s) + 2e –

40. Question (f) asks us to write the overall redox equation An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. a)Write the half-reaction taking place at the cathode. b)What are the products at the cathode? c)What will happen to the pH near the cathode? d)Write the half-reaction taking place at the anode. e)What is the product at the anode? f)Write the overall redox equation

41. To do this, we take the half-reactions at the cathode and the anode, An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH –

42. And add them up An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

43. We see that water gains 2 electrons and (click) Iodide loses 2 electrons An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

44. So electrons can be cancelled. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

45. On the left side, we have 2H 2 O and 2I minus An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

46. And on the right side, we have H2 (gas), 2 OH minus, and I 2 Solid. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

47. So we’ve now completed the overall redox equation. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

48. Checking it for balancing, we have 4 H’s on both sides, An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

49. 2 O’s on both sides, An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

50. 2 I’s on both sides, An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation

51. And a negative 2 charge on both sides, so it is balanced. An aqueous solution of magnesium iodide (MgI 2(aq) ) is electrolyzed using inert platinum electrodes. f)Write the overall redox equation Anode: 2I –  I 2(s) + 2e – Cathode: 2H 2 O + 2e –  H 2(g) + 2OH – :: 2H 2 O + 2I –  H 2(g) + 2OH – + I 2(s) Overall Redox Equation