6. Reactions of ions in aqueous solution
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When does complex formation occur?
When a positively charged transition metal ion attracts a ligand with a lone pair of electrons, which forms a coordinate bond with the central ion
What is a Lewis base?
An electron pair donor
What is a Lewis acid?
An electron pair acceptor
Does a ligand act as a Lewis acid or base?
Lewis base
Does a transition metal ion in a complex act as a Lewis acid or base?
Lewis acid
In a complex, what acts as the Lewis acid and what acts as the Lewis base?
• ligand - Lewis base * transition metal ion - Lewis acid
How do metal ions exist in aqueous solution?
As hexaaquaions
Which metal ions form 2+ hexaaquaions?
Cu2+ and Fe2+
Which metal ions form 3+ hexaaquaions?
Al3+ and Fe3+
Is the formation of hexaaquaions exo or endothermic? Why is this?
Exothermic as bonds are being formed
Is the formation of 2+ or 3+ hexaaquaions more exothermic? Why is this?
Formation of 3+ hexaaquaions because the bonds are stronger
What happens when a solution of hexaaquaions is evaporated?
Water of crystallisation - hexaaquaion remains intact as the coordinate bond between the water molecules and metal ion is so strong that they are not broken when the solution is evaporated
What colour is iron (II) sulphate in solution?
Green
What colour is iron (III) chloride in solution?
Violet
What colour is copper (II) nitrate in solution?
Blue
What colour is aluminium chloride in solution?
Colourless
In a hexaaqua ion, what are the types of bonds that could be broken?
• coordinate * covalent
What happens if the coordinate bond is broken in a hexaaqua ion?
Ligand substitution - water ligand is replaced by another ligand
What is hydrolysis?
When the O-H in the water molecule of a hexaaqua ion is broken
What effect does water have when it is added to a hexaaqua ion?
It strengthens the metal oxygen bond and the O-H bond weakens. Once the OH bond breaks a H⁺ ion is released from a H₂O ligand
What happens when the OH bond breaks in a hexaaqua ion?
A H⁺ ion is released from a H₂O ligand
What effect does the metal ion in a hexaaqua ion have on electron density in the ligands?
The positive charge pulls electron density away from O atom, and O is electronegative therefore pulls electron density away from H atom
What does the degree of acidity of a solution after hydrolysis depend on?
How many H₃O⁺ are formed
Where does the position of equilibrium for an M²⁺ hexaaquaion lie, and what does this mean for the acidity of the resulting solution?
Lies to left, so not many H₃O⁺ formed, so not very acidic