OCR Advanced GCE in Chemistry/Chemistry of Transition metals

Vanadium

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

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⁺

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⁺

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:

e.g.

[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.