Precipitation Reactions With Hydroxide Flashcards
Colour of copper 2 ion solution in water[Cu(H₂O)₆]²⁺
Blue
Colour of iron 2 solution in water [Fe(H₂O)₆]²⁺
Green
Colour of iron 3 solution in water [Fe(H₂O)₆]³⁺
Brown/yellow
Colour of cobalt 2 solution in water [Co(H₂O)₆]²⁺
Red/pink
Chromate 6+ ion colour CrO4 ²⁻
Yellow
Dichromate (VI) ion colour Cr₂O₇²⁻
Orange
Colour of manganate ions MnO4-
Purple
Metal 2+ ion solution + 2OH- forms?
[M(H₂O)4(OH)2] + 2H2O
Metal 2+ and 3+ ion solution + OH- reaction type
Acid-base reaction
Metal 2+ ion solution + 2OH- explanation of what occurs
2 of the water ligands donate a H+ ion each to the 2OH- to form 2H2O (water ligands act as acid)
So the 2 water ligands become 2OH- ligands and the other 4 water ligands remain
Why does [M(H₂O)4(OH)2] (formed from metal 2+ solution +2OH-) precipitate out of solution?
Because it is neutral because 2 OH- ions cancel out +2 charge so no longer forms ion dipole attractions with water molecules thus can not dissolve in water
Addition of excess sodium hydroxide to [M(H₂O)4(OH)2]
[M(H₂O)4(OH)2] is still insoluble because it is neutral so cannot form ion dipole attractions to any polar solvent
Colour change for [Cu(H₂O)₆]²⁺ + 2OH-
Blue solution forms pale blue precipitate of [Cu(H₂O)4(OH)2]
Colour change for [Fe(H₂O)₆]²⁺ +2OH-
Pale green solution to green precipitate of [Fe(H₂O)4(OH)2]
What colour does [Fe(H₂O)4(OH)2] go on standing?
Brown
What colour does [Cu(H₂O)4(OH)2] go on standing?
Pink
How many OH- does [Fe(H₂O)₆]³⁺ require in the precipitation reaction?
3 because 3 OH- is required to balance the 3+ charge of Fe3+
What happens when [Fe(H₂O)₆]³⁺ +3OH- occurs?
3 water ligands act as acid and donate H+ ion to 3OH- (acts as a base) to form 3H2O
Leaving the Fe3+ complex with 3OH- ligands and 3H2O ligands = [Fe(H₂O)3(OH)3]
Colour change for [Fe(H₂O)₆]3+ + 3OH-
Yellow brown solution to a brown precipitate of [Fe(H₂O)3(OH3)]
Observation of adding aqueous NaOH to [Cu(H2O)4(OH)2]
Stays a blue precipitate that is insoluble
Observation of adding concentrated NaOH to [Cu(H2O)4(OH)2], [Fe(H2O)4(OH)2], [Co(H2O)4(OH)2], [Fe(H2O)3(OH)3]
Stays an insoluble precipitate and will not dissolve
Group 2 cations + aqueous NaOH
Forms a white precipitate
That is more soluble as you go down the group
Group 1 metal ions + aqueous NaOH
Forms no precipitate
