Reactions of Inorganic Compounds in Aqueous Solutions

Question 1

Define ‘Lewis acid’

Answer 1

• A Lewis acid is an electron pair acceptor.
• Metal ions acts as Lewis acids in aqueous solution as they accept electron pairs from water molecules around them.
• The water molecules, like any ligands, are electron pair donors so they must be Lewis bases.

Question 2

Define ‘Lewis base’

Answer 2

• A Lewis base is an electron pair donor.
• Donates a pair of electrons to form a coordinate bond; always involves one substance donating an electron pair to another.

Question 3

What occur when transition metal compounds mix with water?

Answer 3

Metal-aqua ions are formed in aqueous solution, the transition metal compounds dissolve in water, with the water molecules forming coordinate bonds with the metal ion.

Six water molecules form cooridnate bonds with each ion, by donating a non-bonding pair of electrons from their oxygen.

• [M(H2O)6]2+ formed by Fe, Co and Cu.
  (2+ is charge of the metal ion)
• [M(H2O)6]3+ formed by Al, Cr and Fe.
  (3+ is charge of the metal ion)

Question 4

What is the equation of the hydrolysis/acidity reaction between the metal-aqua ion Fe(H₂O)₆²⁺ and water?

Answer 4

[M(H₂O)₆]²⁺+ + H₂O ⇄ [M(H₂O)₅(OH)]⁺ + H₃O⁺

• The metal-aqua 2+ ions release H+ ions, thus a weakly acidic solution is formed.
  (only slight dissociation)
• Product has positive charge each, water molecule expelled, OH from dissociation.

Question 5

What is the equation of the hydrolysis/acidity reaction between the metal-aqua ion Al(H₂O)₆3+ and water?

Answer 5

[M(H₂O)₆]³⁺+ + H₂O ⇄ [M(H₂O)₅(OH)]²⁺ + H₃O⁺

• The metal-aqua 3+ ions release H+ ions, thus an acidic solution is formed.
  (more dissociation)
• Product has positive charge each 2+, +, water molecule expelled, OH from dissociation.

Question 6

Why is the acidity of [M(H2O)6]³⁺ greater than that of [M(H2O)6]²⁺?

Answer 6

Charge/size ratio of metal ion

• Metal 3+ ions are pretty small but have a big charge; they have a high charge density (charge/size ratio).
• Metal 2+ ions have a much lower charge density.
• This 3+ ions are much more polarising than 2+ ions; more polarising power means that they attract electrons from the oxygen atoms of the coordinated water molecules more strongly, weakening the O-H bond.
• More likely that a hydrogen ion is released.

Question 7

What occurs when metal 2+ ions react with sodium carbonate?

Answer 7

• Metal 2+ ions react with sodium carbonate to form insoluble metal carbonates:

M(H₂O)₆²⁺ + CO₃^2- ⇄ MCO₃ + 6H₂O

• Metal 3+ ions are stronger acids so they always form hydroxide precipitates when you add sodium carbonate. The carbonate ions react with the H3O+ ions removing them from solution like OH- ions do.

Question 8

Describe the amphoteric character of metal hydroxides

Answer 8

• Amphoteric; can act as both acids and bases, dissolving in both.
• Alumnium hydroxide and chromium(III) hydroxide are amphoteric.
• Act as Bronsted-Lowry acids and donate H+ ions to the OH- ions, forming soluble compounds.

Question 9

Know this equilibrium reaction:
2CrO42- + 2H+ ⇄ Cr2O72- + H2O

Answer 9

Do you know it? Know it.
2CrO42- + 2H+ ⇄ Cr2O72- + H2O

Question 10

What occurs during the ligand exchange of H2O and NH3?

Answer 10

Ligands are similar in size and are thus unchanged; ligand exchange occurs without the change of the:

• coordination number of the complex ion
• shape

Question 11

What occurs between the ligand exchange of H2O and Cl-?

Answer 11

Ligands are different sizes; there’s a change in coordination number and a change in shape.

The Cl- ligand is larger than the uncharged ligands, thus coordination number changes.

Question 12

What does the substituion of a unidentate ligand with a bidentate/multidentate yield?

Answer 12

A more stable complex; ligand exchange reactions can be easily reversed unless the new complex ion is much more stable than the old one, in this case.

Question 13

Why is the substiution of a multidentate for a unidentate ligand harder to reverse?

Answer 13

Due to a positive entropy change:

• when unidentate ligands are substituted with multidentate ligands, the number of particles increases - the more particles, the greater the entropy. Reations that result in an increase in entropy are more likely to occur. This is known as the chelate effect.
• Going from multi to uni would have to be a decrease in entropy which is difficult to achieve.