Transition Metals & Aqueous Ions in Solution

Question 1

What is the mechanism for heterogenous catalysts?

Answer 1

• Reactants diffuse to the surface and adsorb to the active site
• The reaction occurs at the surface (Rate determining step)
• Products desorb and diffuse away

Question 2

What are catalyst poisons?

Give two examples of catalysts and their poisons

Answer 2

Impurities that strongly adsorb to the active site, blocking it and preventing the reactants from binding

1. Iron is poisoned by sulphur compounds
2. Rhodium in catalytic converters is poisoned by lead oxides

Question 3

What properties allow transition elements to act as catalysts?

Answer 3

• Incomplete d-subshell
  
  • Form weak bonds with the reactants, holding them close together in the correct orientation
• Variable oxidation states

Question 4

How can the efficiency of a heterogenous catalyst be maximised?

Answer 4

Maximise surface area by:

1. Covering an inert support in a thin layer of catalyst
2. Grind the catalyst into a fine powder
3. Use a honeycomb structure

Question 5

Give examples of transition metals which are hetereogenous catalysts and the respective reactions they catalyse

Answer 5

1. Haber process
   
   • Fe: N₂ + 3H₂ ⇌ 2NH₃
2. Contact process
   
   • V₂O₅: 2SO₂ + O₂ ⇌ SO₃
3. Catalytic converters
   
   • Rh: 2NO + 2CO → 2CO₂ + N₂
4. Decomposition of hydrogen peroxide
   
   • MnO₂: 2H₂O₂ → 2H₂O + O₂
5. Manufacturing methanol:
   
   • Cr₂O₃: CO + 2H₂ ⇌ CH₃OH

Question 6

What is the mechanism for homogenous catalysts?

Answer 6

• Catalsyt reacts to form an intermediate
• The intermediate reacts to reform the catalyst

Question 7

Give the mechanism for the contact process when catalysed

Answer 7

1. SO₂ + V₂O₅ → SO₃ + V₂O₄
2. 2V₂O₄ + O₂ → 2V₂O₅

Question 8

Give examples of transition metals which are homogenous catalysts

Answer 8

1. Fe²⁺/Fe³⁺
2. Mn²⁺/Mn³⁺

Question 9

Give the mechanism for the catalysis involving Fe²⁺ ions

Answer 9

Overall: 2I^- + S₂O₈^2- → I₂ + 2SO₄^2-

1. 2Fe²⁺ + S₂O₈^2- → 2Fe³⁺ + 2SO₄^2-
2. 2Fe³⁺ + 2I^- → 2Fe²⁺ + I₂

Question 10

What is autocatalysis?

Answer 10

Where one of the products of a reaction catalyses a reaction

Question 11

Give an example of an autocatalysis reaction

Identify the catalyst

Identify what reactions take place as part of the catalysed reaction

Answer 11

Reaction: 2MnO₄^- + 5C₂O₄^2- + 16H⁺ → 2Mn²⁺ + 8H₂O + 10CO₂

Catalyst: Mn²⁺

Mechanism:

1. Mn²⁺ reacts with MnO₄^-
2. Mn³⁺ reacts with C₂O₄^2-

Question 12

What are the characteristic features of transition elements?

Answer 12

1. Form complex ions
2. Form coloured ions
3. Have variable oxidation states
4. Act as catalysts

Question 13

What shape is exhibited by complex ions with coordination number 2?

Answer 13

Linear

E.g. Tollens’ reagent

Question 14

What shape is exhibited by complex ions with coordination number 4?

Answer 14

Tetrahedral

E,g, [CuCl₄]^2-

Question 15

What shape is exhibited by complex ions with coordination number 6?

Answer 15

Octahedral

E.g. [Fe(H₂O)₆]³⁺

Question 16

Give examples of unidentate ligands

Answer 16

1. H₂O
2. NH₃
3. Cl^-
4. CN^-
5. OH^-

Question 17

Give examples of multidentate ligands

Answer 17

1. C₂O₄^2- (bi)
2. H₂NCH₂CH₂NH₂ (bi)
3. Haemoglobin (quadra)
4. EDTA^4- (hexa)

Question 18

Why are complexes coloured?

Answer 18

• Ligands split the d-orbitals
• Light is absorbed by the electrons which excites them (moves them to a higher energy level)
• This light provides the energy and is from the visible spectrum
  
  • The energy needed is given by E=hv
• Certain frequencies are absorbed and others reflected
  
  • The reflected frequencies determine the colour of the complex

Question 19

What is the colour of [CuCl4]2-?

Answer 19

Yellow/green solution

Question 20

What is the colour of [CoCl₄]^2-?

Answer 20

Blue solution

Question 21

How can the color of a complex be changed and why does this occur?

Answer 21

Changes in:

• Ligand
• Coordination number
• Oxidation state

will change the colour because they change the value of E and so change which colours are reflected

Question 22

What can be used to reduce transition elements?

Answer 22

Zn/H⁺

Question 23

What can be used to oxidise transition elements?

Answer 23

H₂O₂/OH^-

Question 24

What are the different oxidation states of chromium and what are their colours?

Answer 24

(II) Cr²⁺ - blue solution (only forms if air is excluded)

(III) Cr³⁺ - green solution

(VI) CrO₄^2- - yellow solution

(VI) Cr₂O₇^2- - orange solution

Question 25

Why does chromium have two different compounds in the +6 oxidation state?

Answer 25

Chromate (VI) ions and dichromate (VI) ions exist in equilibrium:

2CrO₄^2- + 2H⁺ ⇌ Cr₂O₇^2- + H₂O

Question 26

What are the different oxidation states of vanadium and what are their colours?

Answer 26

(II) V²⁺ - violet (only forms if air is excluded)

(III) V³⁺ - green

(IV) VO²⁺ - blue

(V) VO₂⁺ - yellow

Question 27

What is an example of a complex ion with coordination number 4 which is not the typical shape? What shape does it exhibit?

Answer 27

Cisplatin

Square planar

Question 28

Why are [M(H₂O)₆]³⁺ ions more acidic in solution than [M(H₂O)₆]²⁺ ions?

Answer 28

• The M³⁺ ion has a higher charge density (bigger charge, smaller size)
• This means it is more polarising (withdraws the electrons from the oxygen atom more)
• This weakens the O-H bond more and so more H⁺ ions are released into solution

Question 29

What colour is Fe²⁺ solution? What observations are seen with:

1. Small amount of OH^-/NH₃
2. Excess OH^-
3. Excess NH₃
4. CO₃^2-

Answer 29

Green solution:

1. Green precipitate of Fe(H₂O)₄(OH)₂
2. Green precipitate of Fe(H₂O)₄(OH)₂
3. Green precipitate of Fe(H₂O)₄(OH)₂
4. Green precipitate of FeCO₃

Question 30

What colour is Co²⁺ solution? What observations are seen with:

1. Small amount of OH^-/NH₃
2. Excess OH^-
3. Excess NH₃
4. CO₃^2-

Answer 30

Pink solution:

1. Blue precipitate of Co(H2O)₄(OH)₂
2. Blue precipitate of Co(H2O)₄(OH)₂
3. Straw solution of [Co(NH₃)₆]²⁺
4. Pink precipitate of CoCO₃

Question 31

What colour is Cu²⁺ solution? What observations are seen with:

1. Small amount of OH^-/NH₃
2. Excess OH^-
3. Excess NH₃
4. CO₃^2-

Answer 31

Blue solution

1. Blue precipitate of Cu(H₂O)₄(OH)₂
2. Blue precipitate of Cu(H₂O)₄(OH)₂
3. Deep blue solution of [Cu(H₂O)₂(NH)₄]²⁺
4. Blue/green preciptate of CuCO₃

Question 32

What colour is Fe³⁺ solution? What observations are seen with:

1. Small amount of OH^-/NH₃
2. Excess OH^-
3. Excess NH₃
4. CO₃^2-

Answer 32

Yellow solution:

1. Brown precipitate of Fe(H₂O)₃(OH)₃
2. Brown precipitate of Fe(H₂O)₃(OH)₃
3. Brown precipitate of Fe(H₂O)₃(OH)₃
4. Brown precipitate of Fe(H₂O)₃(OH)₃ and effervescence (CO₂)

Question 33

What colour is Al³⁺ solution? What observations are seen with:

1. Small amount of OH^-/NH₃
2. Excess OH^-
3. Excess NH₃
4. CO₃^2-

Answer 33

Colourless solution:

1. White precipitate of Al(H₂O)₃(OH)₃
2. Colourless solution of [Al(H₂O)₂(OH)₄]^-
3. White precipitate of Al(H₂O)₃(OH)₃
4. White precipitate of Al(H₂O)₃(OH)₃ and effervesence (CO₂)

Question 34

What colour is Cr³⁺ solution? What observations are seen with:

1. Small amount of OH^-/NH₃
2. Excess OH^-
3. Excess NH₃
4. CO₃^2-

Answer 34

Green solution

1. Green precipitate of Cr(H₂O)₃(OH)₃
2. Green solution of [Cr(OH)₆]^3-
3. Purple solution of [Cr(NH₃)₆]³⁺
4. Green precipitate of Cr(H₂O)₃(OH)₃ and effervescence (CO₂)

Question 35

What happens if [Co(NH₃)₆]²⁺ is left standing in air?

Account any observations

Answer 35

The straw coloured solution turns to a dark brown solution

The Co²⁺ is oxidised by the air to Co3+