3.2.6 Reactions of ions in aqueous solutions

Question 1

Why are metal aqua 3+ ion more acidic than metal aqua 2+ ions ?

Answer 1

• Metal 3+ ions have a much higher charge density, metal 2+ ions have a much smaller charge density
• This makes the 3+ ions more polarising than the 2+ ions
• More polarising power means they attract e- from the O2 atoms of the coordinated water molecules more strongly
• This weakens the O-H bond
• More likely H+ is released so solution is more acidic

Question 2

What would you observe when OH- or NH3 is added to [Cu(H2O)6]2+

Answer 2

Cu(OH)2(H2O)4
Blue precipitate formed

Question 3

What would you observe when OH- or NH3 is added to [Fe(H2O)6]2+

Answer 3

Forms Fe(OH)2(H2O)4
Green precipitate formed (Goes brown on standing in air)

Question 4

What would you observe when OH- or NH3 is added to [Al(H2O)6]3+

Answer 4

Forms Al(OH)3(H2O)3
White precipitate formed

Question 5

What would you observe when OH- or NH3 is added to [Fe(H2O)6]3+

Answer 5

Forms Fe(OH)3(H2O)3
Brown precipitate formed

Question 6

What would you observe when excess OH- is added to [Al(H2O)6]3+

Answer 6

Forms [Al(OH)4]-
Colourless solution

Question 7

What would you observe when excess NH3 is added to [Cu(H2O)6]2+

Answer 7

Forms [Cu(NH3)4(H2O)2]2+
Deep blue solution