3.2.6 Reactions of ions in aqueous solution
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What metal-aqua ions are formed by Fe, Cu and Al in aqueous solution?
[Fe(H2O)6]2+
[Cu(H2O)6]2+
[Al(H2O)6]3+ (colourless)
[Fe(H2O)6]3+
Why is the acidity of [M(H2O)6]3+ greater than that of [M(H2O)6]2+?
M3+ has a higher charge to size ratio than M2+ which increases the polarisation of co-ordinately bonded water molecules. The O-H bond is weakened so the M3+ complex is more able to release H+ ions, hence greater acidity.
Equilibrium [M(H2O)6]3+ ⇌ [M(H2O)5(OH)]2+ + H+ lies further to the RHS compared to that of [M(H2O)6]2+. Higher Ka so more acidic- pH is 3 compared to 6.
Describe and explain the reaction of [Fe(H2O)6]2+ with NaOH
[Fe(H2O)6]2+ + 2OH- → Fe(OH)2(H2O)4 + 2H2O
green solution to green precipitate
Acid-base reaction
Describe and explain the reaction [Fe(H2O)6]2+ with NH3
[Fe(H2O)6]2+ + 2NH3 → Fe(OH)2(H2O)4] + 2NH4+
green solution to green precipitate
Acid-base reaction
Describe and explain the reaction of [Fe(H2O)6]2+ with Na2CO3
[Fe(H2O)6]2+ + CO32- → FeCO3 + 6H2O
green solution to green precipitate
Precipitation reaction
Describe and explain the reaction of [Cu(H2O)6]2+ with NaOH
[Cu(H2O)6]2+ + 2OH- → Cu(OH)2(H2O)4 + H2O
blue solution to blue ppt
Acid-base reaction
Describe and explain the reaction of [Cu(H2O)6]2+ with NH3
[Cu(H2O)6]2+ + 2NH3 → Cu(OH)2(H2O)4 + 2NH4+
blue solution to blue ppt
Acid-base reaction
Describe and explain the reaction of [Cu(H2O)6]2+ with Na2CO3
[Cu(H2O)6]2+ + CO32- → CuCO3 + 6H2O
blue solution to blue-green ppt
Precipitation reaction
Describe and explain the reaction of [Al(H2O)6]3+ with NaOH
[Al(H2O)6]3+ + 3OH- → Al(OH)3(H2O)3 + 3H2O
colourless solution to white ppt
Acid-base reaction
Describe and explain the reaction of [Al(H2O)6]3+ with NH3
[Al(H2O)6]3+ + 3NH3 → Al(OH)3(H2O)3 + 3NH4+
colourless solution to white ppt
Acid-base reaction
Describe and explain the reaction of [Al(H2O)6]3+ with Na2CO3
2[Al(H2O)6]3+ + 3CO32- → 2Al(OH)3(H2O)3 + 3CO2 + 3H2O
colourless solution to white ppt
Acid-base reaction
How does Al(OH)3(H2O)3 show amphoteric character?
Dissolves in both acids and bases
Al(OH)3(H2O)3 + 3H+ → [Al(H2O)6]3+
white ppt to colourless sol.
Al(OH)3(H2O)3 + OH- → [Al(H2O)2(OH)4]- + H2O
white ppt to colourless sol.
Describe and explain the reaction of [Fe(H2O)6]3+ with NaOH
[Fe(H2O)6]3+ + 3OH- → Fe(OH)3(H2O)3 + 3H2O
purple solution to brown ppt
Acid-base reaction
Describe and explain the reaction of [Fe(H2O)6]3+ with NH3
[Fe(H2O)6]3+ + 3NH3 → Fe(OH)3(H2O)3 + 3NH4+
purple solution to brown ppt
Acid-base reaction
Describe and explain the reaction of [Fe(H2O)6]3+ with Na2CO3
2[Fe(H2O)6]3+ + 3CO32- → 2Fe(OH)3(H2O)3 + 3CO2 + 3H2O
purple solution to brown ppt
Acid-base reaction
How and why do [Fe(H2O)6]2+ and [Fe(H2O)6]3+ react differently with Na2CO3?
[Fe(H2O)6]2+ undergoes a precipitation reaction while [Fe(H2O)6]3+ undergoes an acid-base reaction because it is a stronger acid so can release enough H+ ions to react with CO32-
How can [Fe(H2O)6]2+ be identified?
Green ppt that goes brown on standing in air with NaOH, NH3 and Na2CO3
How can [Cu(H2O)6]2+ be identified?
Blue ppt with NaOH and NH3 that dissolves in excess NH3 to form a deep blue solution. Blue-green ppt with Na2CO3
How can [Al(H2O)6]2+ be identified?
White ppt with NaOH, NH3 and Na2CO3 (+ effervescence) that dissolves in excess NaOH
How can [Fe(H2O)6]3+ be identified?
Brown ppt with NaOH, NH3 and Na2CO3 (+effervescence)
Note 
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