3.2.6 - Reactions of Ions in Aqueous Solution Flashcards
Describe what happens when transition metal compounds dissolve in water
- (generally 6) water molecules form co-ordinate bonds with each metal ion
- Forms metal-aqua complex ion
Explain how water molecules form co-ordinate bonds with transition metal ions
Water molecules donate a non-bonding pair of electrons from their oxygen
Draw [Fe(H2O)6]2+
Draw [Al(H2O)6]3+
Solutions containing metal-aqua ions are ____
Acidic
Explain why a solution containing metal-aqua 2+ ions is acidic
- There’s a hydrolysis or acidity reaction between metal-aqua ion and water
- Metal-aqua 2+ ions release H+ ions = acidic solution formed
- Only slightly dissociate = weakly acidic
Write an equation for when [Cu(H2O)6]2+ reacts with water
[Cu(H2O)6]2+ + H2O ⇌ [Cu(H2O)5(OH)]+ + H3O+
Explain metal-aqua 3+ ions form more acidic solutions than 2+ metal-aqua ions
Dissociate more than 2+ ions
Explain why metal-aqua 3+ ions dissociate more than 2+ metal-aqua ions
- Metal 3+ ions = small but have big charge
- ∴ high charge density (charge/size ratio)
- Makes 3+ ions more polarising
- Means they attract electrons from oxygen atoms of co-ordinated water molecules more strongly = weakens O–H bond
- So it’s more likely hydrogen ion will be released = more acidic solution
Write an equation for when [Fe(H2O)6]3+ reacts with water
[Fe(H2O)6]3+ + H2O ⇌ [Fe(H2O)5(OH)]2+ + H3O+
What happens when you add OH- ions to solutions of metal aqua ions?
Insoluble metal hydroxides are produced
Explain why adding OH- ions to solutions of metal-aqua 3+ ions produces insoluble metal hydroxides
- In water, metal-aqua 3+ ions (e.g. Fe3+ or Al3+) form equilibrium
- If you add OH- ions to equilibrium, H3O+ ions are removed
- Shifts equilibrium to the right
- Another equilibrium is set up in the solution
- & same thing happens when you add OH-
-
Happens one last time
- Left with insoluble, uncharged metal hydroxide
Hydrolysing Metal-Aqua Ions to form Precipitates
1) Write the equation for when metal-aqua 3+ ions form an equilibrium in water
[M(H2O)6]3+ + H2O ⇌ [M(H2O)5(OH)]2+ + H3O+
Hydrolysing Metal-Aqua Ions to form Precipitates
2) Write the equation for the new equilibrium formed when OH- ions are added to metal-aqua 3+ ions in water
Hydrolysing Metal-Aqua Ions to form Precipitates
3) Write the equation for the final equilibrium formed when OH- ions are added to metal-aqua 3+ ions in water
Hydrolysing Metal-Aqua Ions to form Precipitates
1) Write the equation for when metal-aqua 2+ ions form an equilibrium in water
[M(H2O)6]2+ + H2O ⇌ [M(H2O)5(OH)]+ + H3O+
Hydrolysing Metal-Aqua Ions to form Precipitates
2) Write the equation for the new equilibrium formed when OH- ions are added to metal-aqua 2+ ions in water
Why is there only 2 steps when OH- ions are added to metal-aqua 2+ ions?
∵ only 2 of the water ligands need to be deprotonated to make the +2 complex uncharged (and so insoluble)
What do all metal hydroxides precipitates dissolve in?
Acid
Why do all metal hydroxides precipitates dissolve in acid?
- ∵ acts as Brønsted–Lowry bases and accept H+ ions
- Reverses hydrolysis reactions
Some metal hydroxides are _______
amphoteric
(Dissolve in excess base as well as in acids)
Give an example of a metal hydroxide that is amphoteric
aluminium hydroxide
Describe how aluminium hydroxide behaves in the presence of an acid
Acts as Brønsted–Lowry base & accepts H+ ions from H3O+ ions
Describe how aluminium hydroxide behaves in the presence of a base
- Acts as a Brønsted–Lowry acid & donates H+ ions to OH- ions
- Forming soluble compound