Transition Metals: Some Cobalt Chemistry

AQA, Edexcel, OCR A level Chemistry (2017)

Principles of transition metal chemistry

Learning Objectives related to cobalt chemistry.

AQA

H2O, NH3 and Cl− can act as monodentate ligands. The ligands NH3 and H2O are similar in size and are uncharged.

Exchange of the ligands NH3 and H2O occurs without change of co-ordination number (e.g. Co2+ and Cu2+).

Substitution may be incomplete (e.g. the formation of [Cu(NH3)4(H2O)2]2+).

The Cl− ligand is larger than the uncharged ligands NH3 and H2O.

Exchange of the ligand H2O by Cl– can involve a change of co-ordination number (e.g. Co2+, Cu2+ and Fe3+).

Edexcel

15/24. To be able to record observations and write suitable equations for the reactions of Cr3+(aq), Fe2+(aq), Fe3+(aq), Co2+(aq) and Cu2+(aq) with aqueous sodium hydroxide and aqueous ammonia, including in excess.

 

15/25. To be able to write ionic equations to show the difference between ligand exchange and amphoteric behaviour for the reactions in (24) above.

15/26. To understand that ligand exchange, and an accompanying colour change, occurs in the formation of:

i [Cu(ΝΗ3)4(Η2Ο)2]2+ from [Cu(Η2Ο)6]2+ via Cu(OH)2(Η2Ο)4 ii [CuCl4]2− from [Cu(Η2Ο)6]2+
iii [CoCl4]2− from [Co(Η2Ο)6]2+

15/27. To understand that the substitution of small, uncharged ligands (such as H2O) by larger, charged ligands (such as Cl−) can lead to a change in coordination number.

15/28. To understand, in terms of the large positive increase in ΔSsystem, that the substitution of a monodentate ligand by a bidentate or multidentate ligand leads to a more stable complex ion.

These learning objectives also cover OCR chemistry references to cobalt.

Some cobalt chemistry

Introduction

Cobalt is a first row d block element because it’s 3d subshell is partially filled.

Cobalt (Co)    1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s²

It is also a transition metal because it has ions with partially filled 3d orbitals.

Cobalt(II) (Co²⁺)    1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷

Cobalt(III) (Co³⁺)    1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶

Reactions of cobalt(II) with sodium hydroxide (NaOH), ammonia solution (NH₃) and concentrated hydrochloric acid (HCl).

Be prepared for some highly unusual and surprising colour changes.

First, with sodium hydroxide solution:

[Co(H₂O)₆]²⁺ (aq) + 2OH^— (aq)  ⟶    Co(H₂O)₄(OH)₂ (s)   +   2H₂O (l) 

The pink aqueous cobalt(II) chloride solution forms on initially blue precipitate of cobalt (II) hydroxide.  The hydrogens of the water molecules in the hexa aqua cobalt(II) ion are acidic and donate protons to the hydroxide ions forming water. 

The cobalt(II)hydroxide precipitate is blue at first but turns pink on standing. 

Cobalt(II)hydroxide is not amphoteric and therefore does not dissolve in an excess of sodium hydroxide solution. 

Second, with ammonia solution:

Initially ammonia acts as a weak base and reacts with the cobalt(II) ions as an alkali precipitating the cobalt(II) hydroxide blue solid. (again it would turn pink on standing if left)

[Co(H₂O)₆]²⁺(aq) + 2OH^—(aq)  ⟶    Co(H₂O)₄(OH)₂(s)   +   2H₂O(l) 

But the presence of excess ammonia molecules in the mix leads to a ligand substitution reaction.  Since both water molecules and ammonia molecules are of a similar size there is no change to the coordination number of the complex ions.

Co(H₂O)₄(OH)₂(s) + 6NH₃(aq) ⟶ [Co(NH₃)₆]²⁺(aq)+2OH^—(aq) + 4H₂O(l)  

Pink                                             straw coloured

The pink precipitate of cobalt(II)hydroxide dissolves in excess ammonia solution to form a straw coloured solution of hexaamminecobalt(II) ions.

In the dissolved oxygen in the solution, the cobalt(II) compound is oxidised up to a solution of the deep red cobalt(III) compound.

These reactions are driven by the positive changes in entropy resulting from the increased complexity of the product molecules. 

Third, with concentrated hydrochloric acid.

Here the chloride ion is much larger than either the water molecule or the ammonia molecule and so not as many ions can fit around the cobalt(II) cation.

As a result the coordination number falls from 6 to 4.

This is a ligand exchange reaction.

  [Co(H₂O)₆]²⁺(aq) + 4Cl^—(aq)  ⟶    [Co(Cl)₄]^2—(aq)   +   6H₂O(l) 

A pink solution of the hexaaquacobalt(II) ion gives way to a deep royal blue solution of the tetrachlorocobaltate(II) ion.

5 particles form 7 particles and this increase in entropy is the drive for this process.