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A-level Chemistry: 3.2.6 Reactions of Ions in Aqueous Solution

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These flashcards explain how transition metal compounds dissolve in water to form metal-aqua complex ions, how water molecules coordinate with metal ions, and why such solutions are acidic due to hydrolysis releasing H⁺ ions.

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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

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Key Terms

Term
Definition

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 soluti...

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A-level Chemistry: 3.2.6 Reactions of Ions in Aqueous Solution
TermDefinition

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

  1. Metal 3+ ions = small but have big charge

    • ∴ high charge density (charge/size ratio)

  2. Makes 3+ ions more polarising

    • Means they attract electrons from oxygen atoms of co-ordinated water molecules more strongly = weakens O–H bond

  3. 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

  1. 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

  2. Another equilibrium is set up in the solution

    • & same thing happens when you add OH-

  3. ​​​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

[M(H₂O)₆]3+(aq)+3OH−(aq)⇌M(OH)3​(s)+6H2​O(l)

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

[M(H₂O)₆]3+(aq)+4OH−(aq)⇌[M(OH)4​]−(aq)+6H2​O(l)

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

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

[M(H₂O)₆]2+(aq)+2OH−(aq)⇌M(OH)2​(s)+6H2​O(l)

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

Write an equation showing how aluminium hydroxide reacts with base (OH-)

Al(OH)₃(s)+OH−(aq)⇌[Al(OH)4​]−(aq)

Write an equation showing how aluminium hydroxide reacts with acid (H3O+)

Al(OH)₃(s)+3H3​O+(aq)→Al3+(aq)+6H2​O(l)

Name 2 substances you can add to metal-aqua ions to form precipitates besides a strong alkali

  • Ammonia solution

  • Sodium carbonate

Explain how you can use (a small amount of) ammonia solution to add hydroxide ions

When ammonia dissolves in water = equilibrium:

Explain what happens when you add excess ammonia solution to Cu(OH)2(H2O)4

  • Further reaction happens

    • H2O and OH- ligands are displaced by NH3 ligands

    • Forms charged complex which is soluble in water

    • So precipitates dissolves

Write an equation showing how Cu(OH)2(H2O)4 reacts with excess ammonia

What happens when metal 2+ ions react with sodium carbonate

They form insoluble metal carbonates

Write an equation showing what happens when metal 2+ ions react with sodium carbonate


Describe and explain what happens when metal 3+ ions react with sodium carbonate

  • Metal 3+ ions are stronger acids = higher concentration of H3O+ ions in solution

  • Rather than displacing water from metal ions, carbonate ions react with H3O+, removing them from solution & shifting equilibria of reactions to right

  • Precipitate that forms is M(OH)3(H2O)3 rather than M2(CO3)3

Write an equation showing how carbonate ions react with H3O+

Describe how you can use test tube reactions to identify unknown metal ions in solution

  1. Measure out samples of unknown metal ion solution into 3 separate test tubes

  2. To 1st test tube, add NaOH solution dropwise, using a dropping pipette, and record any changes you see

    1. Add more NaOH dropwise so it's in excess

    2. Record any changes

  3. To 2nd test tube, add ammonia solution dropwise, using a dropping pipette, and record any changes you see

    1. Keep adding ammonia so it's in excess

    2. Record any changes

  4. To 3rd test tube, add sodium carbonate solution dropwise

    1. Record your observations

Describe what happens when copper(II), iron(II), iron(III) and aluminium(III) aqua ions are reacted with sodium hydroxide

  • All four metal aqua ions will form precipitates with NaOH

  • But only aluminium hydroxide precipitate dissolves in excess of NaOH

    • ∵ it's amphoteric

Describe what happens when copper(II), iron(II), iron(III) and aluminium(III) aqua ions are reacted with ammonia

  • All 4 metal aqua ions will form precipitates with ammonia

  • But only copper hydroxide precipitate will dissolve in excess ammonia

  • ∵ it undergoes ligand exchange reaction with excess ammonia

Describe what happens when copper(II), iron(II), iron(III) and aluminium(III) aqua ions are reacted with sodium carbonate

  • All 4 metal aqua ions will form precipitates with sodium carbonate

  • Solutions containing Al3+ or Fe3+ will form bubbles as CO2 is formed

    • Differentiating between Fe2+ and Fe3+

    • Fe3+ = gas VS Fe2+ ≠ gas


Why should the metal-aqua ion solutions be freshly made?

∵ if Fe2+ ions are left too long in contact with air they will oxidise to Fe3+ ions

When working with ammonia, carry out the experiment in a ___________

fume cupboard/hood

State the formula and colour of the metal-aqua ion: copper(II)

State the formula and colour of the metal-aqua ion: copper(II) with OH- or NH3

State the effect when excess OH- is added to the metal-aqua ion: copper(II)

State the effect when excess NH3 is added to the metal-aqua ion: copper(II)

State the formula and colour of the metal-aqua ion: copper(II) with Na2CO3

State the formula and colour of the metal-aqua ion: iron(II)

State the formula and colour of the metal-aqua ion: iron(II) with OH- or NH3

State the effect when excess OH- is added to the metal-aqua ion: iron(II)

State the effect when excess NH3 is added to the metal-aqua ion: iron(II)

State the formula and colour of the metal-aqua ion: iron(II) with Na2CO3


State the formula and colour of the metal-aqua ion: aluminium(III)

State the formula and colour of the metal-aqua ion: aluminium(III) with OH- or NH3

State the effect when excess OH- is added to the metal-aqua ion: aluminium(III)

State the effect when excess NH3 is added to the metal-aqua ion: aluminium(III)

State the formula and colour of the metal-aqua ion: aluminium(III) with Na2CO3

State the formula and colour of the metal-aqua ion: iron(III)

State the formula and colour of the metal-aqua ion: iron(III) with OH- or NH3

State the effect when excess OH- is added to the metal-aqua ion: iron(III)

State the effect when excess NH3 is added to the metal-aqua ion: iron(III)

State the formula and colour of the metal-aqua ion: iron(III) with Na2CO3

Write an equation for when [Cu(H2O)6]2+ reacts with (a few drops of) dilute NH3

[Cu(H2O)6]2+ + 2NH3 → Cu(H2O)4(OH)2 + 2NH4+

OR

NH3 + H2O → NH4+ + OH−

[Cu(H2O)6]2+ + 2OH− → Cu(H2O)4(OH)2 + 2H2O

Write an equation for when [Fe(H2O)6]3+ reacts with sodium carbonate

2[Fe(H2O)6]3+(aq) + 3CO32-(aq) → 2Fe(OH)3(H2O)3(s) +3CO2(g) + 3H2O(l)