In: Chemistry
Write a balanced equation for the oxidation of Co(II) to Co(III) by hydrogen peroxide. Why is this oxidation process assisted by the presence of ammonia?
balanced equation :
2[Co(NH3)6]2+(aq) + H2O2(g/aq) + 2H+(aq) --------------> 2[Co(NH3)6]3+(aq) + 2H2O(l)
Explanation :
When excess ammonia is added to a cobalt(II) salt solution, the hexamine complex is formed but this is unstable in the presence of dissolved oxygen and is oxidised to the cobalt(III) complex. This change in cobalt's oxidation state from +2 to +3 via an oxidising agent is quite common if a complexing agent is present too.
[Co(H2O)6]2+(aq) + 6NH3(aq) ==> [Co(NH3)6]2+(aq) + 6H2O(l)
The uncharged ligand molecules ammonia NH3 and water H2O are similar in size and ligand exchange occurs without change in co–ordination number (stays at 6).
Oxidation then follows from dissolved oxygen, or you can add hydrogen peroxide for a more efficient job.
1) 4[Co(NH3)6]2+(aq) + O2(g/aq) + 4H+(aq) ==> 4[Co(NH3)6]3+(aq) + 2H2O(l)
2) 2[Co(NH3)6]2+(aq) + H2O2(g/aq) + 2H+(aq) ==> 2[Co(NH3)6]3+(aq) + 2H2O(l)