Metal Aqua Ions (3.2.6)

Question 1

What are metal aqua ions?

Answer 1

• A central metal ion with six water ligands

- [M(H2O)6]2+ or 3+

Question 2

What reactions can metal ions undergo?

Answer 2

1) Hydrolysis - Loss of H+ from H2O ligands
2) Substitution - Replacement of H2O by other ligands
3) Redox - Metal changes oxidation state

Question 3

What are neutral complex ions?

Answer 3

• The neutral complexes formed by hydrolysis of a metal aqua ion –> precipitates (insoluble)
• Known as metal (II)/metal (III) hydroxides
• Sometimes written as M(H2O)4(OH)2 or M(OH)2

Question 4

What is the pH of a metal aqua ion solution?

Why?

Answer 4

• 2+ = less acidic solutions (pH-6)
• 3+ = more acidic solutions (pH-3)
• This is because 3+ ions have a high charge density, so are more polarising, meaning they attract electrons from the oxygen of water molecules more strongly
• This weakens the O-H bond and it is more likely a Hydrogen ion will be released –> more acidic

Question 5

Reaction of metal 2+ aqua ions with small vol. NaOH (aq)

Answer 5

-Hexaaqua iron (II): Green sol –> Green ppt
-Hexaaqua copper: Blue sol –> Blue ppt
[M(H2O)6]2+ + 2OH- –> [M(H2O)4(OH)2] + 2H2O

Question 6

Reaction of metal 2+ aqua ions with Xs NaOH (aq)

Answer 6

-No reaction

Question 7

Reaction of metal 3+ aqua ions with small vol. NaOH (aq)

Answer 7

-Iron (III): Yellow sol –> Brown ppt
-Aluminium: Colourless sol –> white ppt
[M(H2O)6]3+ + 3OH- –> [M(H2O)3(OH)3] + 3H2O

Question 8

Reaction of metal 3+ aqua ions in Xs NaOH (aq)

Answer 8

-Iron (III): No reaction
-Al: White ppt redissolves into colourless sol
[Al(H2O)3(OH)3] + OH-
–> [Al(H2O)2(OH)4]- + H2O

Question 9

Reaction of metal (II) aqua ions with small amount of NH3 (aq)

Answer 9

-Iron (II): Green sol –> Green ppt
-Copper: Blue sol –> Blue ppt
[M(H2O)6]2+ + 2NH3
–> [M(H2O)4(OH)2] + 2NH4+

Question 10

Reaction of metal (II) aqua ions in Xs NH3 (aq)

Answer 10

-Iron (II): no reaction
-Copper: Blue ppt –> Deep blue sol
(Ligand substitution)
[Cu(H2O)4(OH)2] + 4NH3 –> [Cu(NH3)4(H2O)2] + 2H2O + 2OH-

Question 11

Reaction of metal (II) aqua ions with small vol NH3 (aq)

Answer 11

-Iron (II): Yellow sol –> Brown ppt
-Al: Colourless sol –> White ppt
[M(H2O)6]3+ + 3NH3 –>
[M(H2O)3(OH)3 + 3NH4+

Question 12

Reaction of metal (III) aqua ions with Xs NH3 (aq)

Answer 12

-No reaction

Question 13

Reaction of metal (II) aqua ions with any vol Na2CO3 (aq)

Answer 13

-Iron (II):
Green sol –> Green ppt (of metal carbonate)
-Copper:
Blue sol –> Blue ppt (of metal carbonate)
[M(H2O)6]2+ + CO3 2- –> MCO3 + 6H20

Question 14

Reaction of metal (III) aqua ions with any vol Na2CO3 (aq)

Answer 14

-Iron (II): Fizzing & Yellow sol –> Brown ppt (metal hydroxide)
-Al: Fizzing & Colourless sol –> White ppt (metal hydroxide)
2[M(H2O)6]3+ +3CO3 2- –>
2[M(H2O)3(OH)3] + 3H2O + 3CO2

Question 15

• Metal (II) aqua ions react with Na2CO3 (aq) to form…

- By what reaction?

Answer 15

• Precipitates of metal (II) carbonates

- By precipitation

Question 16

• Metal (III) aqua ions react with Na2CO3 (aq) to form…

- By what reaction?

Answer 16

• Bubbles of CO2 gas & precipitates of metal (III) hydroxides
• By hydrolysis

Question 17

Why do metal (II) aqua ions and metal (III) aqua ions react differently with Na2CO3 (aq)?

Answer 17

• 3+ aqua ions are acidic enough to react in acid-base reactions (with CO3 2-) by hydrolysis
• 2+ aqua ions are not acidic enough

Question 18

• What is an amphoteric hydroxide?

- Example

Answer 18

• A hydroxide which reacts with both aciuds and bases

- [Al(H2O)3(OH)3]

Question 19

• What are acidic and basic hydroxides?

- Examples

Answer 19

-Acidic hydroxide: one which only reacts with bases
NO EXAMPLES
-Basic hydroxide: one which only reacts wth acids
[Cu(H2O)4(OH)2]
[Fe(H2O)4(OH)2]
[Fe(H2O)3(OH)3]