OCR Advanced GCE in Chemistry/Chemistry of Transition metals
Vanadium is a silver-grey metal which is very resistant to corrosion.
It can form compounds in the +5,+4,+3 and +2 oxidation states
Cobalt (Atomic no 27 relative mass no 59) is an unreactive white metal with a slight blue appearance and forms compounds with +2 and +3 oxidation states. The +2 state is usually the most stable.
Co2+ forms the complex molecule [Co(H2O)6]2+ which is a pink octahedral complex ion.
Co2+ also forms the tetrahedral complex ion [CoCl4]2- which is blue.
Cobalt chloride paper can therefore be used to test for water because the complex [Co(H2O)6]2+ is formed. This causes the blue cobalt chloride paper to turn pink.
This reaction is an equilibrium reaction:
[CoCl4]2-(aq) + 6H2O(l) <---> [Co(H2O)6]2+(aq) + 4Cl-(aq)
The reaction is exothermic in the forward reaction. By adding either heat or a high concentration of Cl- the reverse ligand substitution reaction will occur.
Co3+ forms the aqueous complex [Co(H2O)6]3+ which is blue but is so easily reduced to [Co(H2O)6]2+ that we do not really look at it. Other simple compound of Co3+ become hydrated and are reduced in the same way.
Co3+ does however make other complex ions with different ligands and they prove to be very stable indeed:
[Co(H2O)6]3+ + e- <---> [Co(H2O)6]2+; E= +1.81V
[Co(NH3)6]3+ + e- <---> [Co(NH3)6]2+; E= +0.11V
Here the Electrode potential values show that the Co3+ complex without H2O ligands (Hexaamminecobalt ll) is the more stable. This is because ammonia has only one lone pair of electrons resulting in stronger dative bonds.