15B: Reactions of transition metal elements

Question 1

State the colours of the oxidation states of vanadium (+5, +4, +3 and +2) in its compounds.

Answer 1

+5: Yellow (VO₂⁺)
+4: Blue (VO²⁺
+3: Green (V³⁺)
+2: Violet (V²⁺)

Question 2

Describe the reduction of the dichromate(VI) ion, Cr₂O₇²⁻.

Answer 2

Reduced to Cr³⁺ and Cr²⁺ ions,
Using zinc in acidic conditions.

Question 3

Describe a method to produce the dichromate(VI) ion, Cr₂O₇²⁻.

Answer 3

Oxidation of Cr³⁺ ions,
Using hydrogen peroxide in alkaline conditions (followed by acidification).

Question 4

Give the equilibrium equation that results in the conversion of the dichromate(VI) ion, Cr₂O₇²⁻, into chromate(VI) ions.

Answer 4

2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O

Question 5

Write an ionic equation for the reaction of the complex ion, [M(H₂O)_x]^y+, with aqueous sodium hydroxide, and state the type of reaction.

Answer 5

[M(H₂O)_x]^y+ + yOH^- → [M(H₂O)_x-y(OH)_y] + yH₂O

Proton transfer

Question 6

Write an ionic equation for the reaction of the complex ion, [M(H₂O)_x]^y+, with aqueous ammonia, and state the type of reaction.

Answer 6

[M(H₂O)_x]^y+ + yNH₃ → [M(H₂O)_x-y(OH)_y] + yNH₄⁺

Proton transfer

Question 7

Define the term amphoteric

Answer 7

Can act as an acid or a base

Question 8

Describe the difference between ligand exchange and amphoteric behaviour in terms of the reactions of aqueous complex ions with NaOH or NH₃.

Answer 8

Amphoteric behaviour: H₂O ligands are deprotonated by the base, so the complex ions acts as an acid.

Ligands exchange: NH₃ ligands exchange with H₂O ligands.

Question 9

Describe a reaction that can lead to a change in coordination number.

Answer 9

The substitution of small, uncharged ligands (such as H₂O) by
larger, charged ligands (such as Cl⁻).

Question 10

Explain why the substitution of a monodentate ligand by a bidentate or multidentate ligand leads to a more stable complex ion.

Answer 10

The small ligands are released and have greater freedom of movement,
so there is a large positive increase in ΔS_system.

Question 11

Describe a heterogeneous catalyst

Answer 11

The catalyst is in a different phase from the reactants,
The reaction occurs at the surface of the catalyst.

Question 12

Describe the contact process.

Answer 12

Converts oxygen and sulfur dioxide to sulfur trioxide.

Catalyst: V₂O₅

Question 13

State the role of a catalytic converter

Answer 13

Decreases carbon monoxide and nitrogen monoxide emissions from internal combustion engines

Question 14

Describe how a catalytic converter works

Answer 14

• Adsorption of CO and NO molecules onto the surface of the catalyst,
• Bonds are weakened so a chemical reaction can occur,
• Desorption of CO₂ and N₂ product molecules from the surface of the catalyst.

Question 15

Describe a homogeneous catalyst

Answer 15

The catalyst is in the same phase as the reactants,
The catalysed reaction will proceed via an intermediate species.

Question 16

State the catalyst for the reaction between I⁻ and S₂O₈²⁻ ions

Answer 16

Fe²⁺ ions

Question 17

State the catalyst and the type of catalysis for the reaction between MnO₄⁻ and C₂O₄²⁻ ions

Answer 17

Mn²⁺ ions,
Autocatalysis