Fig. 21.13
Fe(s) Fe2+(aq) + 2 e- [anodic region; oxidation]
O2 (g) + 4 H+(aq) + 4 e- 2 H2O(l) [cathodic region; reduction]
2 Fe(s) + O2 (g) + 4 H+(aq) 2 Fe2+(aq) + 2 H2O(l)