Reactions of cobalt and iron complexes

Question 1

the equation, colour change and type of reaction for the reaction between the hexaaquacobalt(II) ion and aqueous sodium hydroxide solution until no further change is seen is:

Answer 1

[Co(H2O)6]2+ + 2OH- → [Co(H2O)4(OH)2] + 2H2O

pink solution → blue precipitate
when the precipitate is left to stand, it gradually turns pink

acid-base reaction: the two hydroxide ion have removed H+ ions from the H2O ligands

Question 2

the equation, colour change and type of reaction for the reaction between the hexaaquacobalt(II) ion and aqueous ammonia solution is:

Answer 2

[Co(H2O)6]2+ + 2NH3- → [Co(H2O)4(OH)2] + 2NH4+
pink solution → blue precipitate
acid-base

when NH3 is added to excess:
[Co(H2O)4(OH)2] + 6NH3 → [Co(NH3)6]2+ + 4H2O + 2OH-
blue precipitate dissolves → brown solution
ligand exchange
upon standing, the brown solution reacts with O2 in air and the oxidation number of cobalt increases form 2+ to 3+, becoming yellow: [Co(NH3)6]3+

Question 3

the equation, colour change and type of reaction for the reaction between the hexaaquacobalt(II) ion and concentrated hydrochloric acid is:

Answer 3

[Co(H2O)6]2+ + 4Cl- → [CoCl4]2- + 6H2O
pink → blue
ligand exchange and a change in coordination number. note that although the overall charge on the complex ion changes, the Co2+ ‘s oxidation number remains the same

Question 4

the equation, colour change and type of reaction for the reaction between the hexaaquairon(II) ion and aqueous sodium hydroxide is:

Answer 4

[Fe(H2O)6]2+ + 2OH- → [Fe(H2O)4(OH)2] + 2H2O
pale green solution → green precipitate
acid-base reaction

Question 5

the equation, colour change and type of reaction for the reaction between the hexaaquairon(II) ion and aqueous ammonia is:

Answer 5

[Fe(H2O)]2+ 2NH3 → [Fe(H2O)4(OH)2] + 2NH4+
pale green solution → green precipitate
acid-base reaction

upon standing, the colour of the precipitate changes from green to brown as oxygen from the atmosphere causes oxidation, forming the [Fe(H2O)3(OH)3] , the triaaquatrihydroxoiron(III) complex

Question 6

the equation, colour change and type of reaction for the reaction between the hexaaquairon(III) ion and aqueous sodium hydroxide solution until no further change is seen is:

Answer 6

[Fe(H2O)6]3+ + 3OH- → [Fe(H2O3)(OH)] + 3H2O
yellow-brown solution → brown precipitate
acid-base reaction

no further reaction when excess added or when left to stand

Question 7

the equation, colour change and type of reaction for the reaction between the hexaaquairon(III) ion and aqueous ammonia is:

Answer 7

[Fe(H2O)6]3+ + 3NH3 → [Fe(H2O)3(OH)3] + 3NH4+
yelllow-brown solution → brown precipitate

no further reaction when excess added or when left to stand