Catalysis

Question 1

WHat is a hetrogranous catalyst?

Answer 1

• a catalyst that is present in a reaction in a different phase to the reaction
  
  • usually are solid, whilst reactants are gases or liquids
  • catalytic action occurs on solid surface
  • catalyst is not lost and does not need to be separated

Question 2

How is the efficiency of catalysts increased?

Answer 2

• increase SA
• spread onto inert support medium, or impregnate it into one
• increase surface to mass ratio
  
  • little goes a long way
  • more expensive catalysts used in this way
    
    • e.g. rhodium and platinum on a ceramic metal

Question 3

How do catalysts degrade over time?

Answer 3

• posioning
  
  • surface become covered with unwanted purities
  • e.g. fuel additives in catalytic converters
• finely divided catalyst may gradually be lost from the support medium

Question 4

WHat is the contact process?

Answer 4

• produces sulfuric acid
  
  • vital industrial chemical
• made from sulfur, oxygen and water, the key step being
  
  • 2SO₂ + O₂ ⇌ 2SO₃

Question 5

What is the catalyst for the contact process and what are the steps involved in the catalysis?

Answer 5

• catalyst by vandium(V) oxide, V₂O₅ in 2 steps
• vanadium(V) oxide oxideses sulfur dioxide to sulfur trioxide, itself is reduced to vanadium(IV) oxide
  
  • SO₂ + V₂O₅ → SO₃ + V₂O₄
• Vanadium (IV) oxide oxidised back to vandium(V) oxide by oxgen
  
  • 2V₂O₄ + O₂ → 2V₂O₅
• vandium(V) oxide is regenerated and unchanged
• each step has a lower E_a than the uncatalysed step, therefore the reaction goes faster.

Question 6

What is the Haber process and what is the catalyst?

Answer 6

• ammonia made by the reaction of nitrogen with hydrogen
• the catalyst is iron
  
  • present in pea sized clumps to increase the SA
• the iron catalyst lasts about five years before it becomes poisoned by impurities in the gas stream, such as sulfur compounds, before it has to be replaced.

Question 7

WHat is a homogenous catalyst?

Answer 7

same phase as reactant

Question 8

How does homogenous catalysis occur?

Answer 8

• catalysts proceeds through the reaction as an intermediate

Question 9

In the reaction of peroxodisulfate ions, S₂O₈^2- and 2I^-, Fe(II or III) catalyses the reaction in 2 steps

• what are the 2 steps?

suggest why the rate of reaction increase?

Answer 9

• uncatalysed reaction takes place between two ions of the same negative charge, which repel, giving a high activation energy
• both sides of the catlaysed reaction involve reaction between pairs of oppositely charged ions#
• this is why rate has inceasesd
• S₂O₈^2- (aq) + 2Fe²⁺(aq) → 2SO₄^2- (aq) + 2Fe³⁺
• 2Fe³⁺ (aq) + 2I^- (aq) → 2Fe²⁺ (aq) + 2I₂(aq)

Question 10

What is autocatalysis? describe the change in rate

Answer 10

• where one of the products of the reaction is a catalyst for the reaction
• reaction starts slowly at the uncatalysed rate
• as conc of product that is also the catalyst builds up, the reaction speeds up to the catalysed rate
• the reaction then behaves like a normal reaction, gradually slowing down as the reactants are used up

Question 11

what type of reaction is the reaction between ethanedioic acid by manganate(VII ions)? WHy is it useful?

Answer 11

• autocatalysed
• soln of ethandioic acid and acidified soln of potassium manganate(VII)
• used to find conc of potassium manganate in soln

Question 12

What is the equation for the oxidation of ethanedioic acid by manganate(VII) ions? What is the catalyst and what are the steps of the catalysis?

Answer 12

• 2MnO₄^- (aq) + 16H⁺ (aq) + 5C₂O₄^2- (aq) →2Mn²⁺ (aq) + 8H₂O (l) + 10CO₂ (g)
• Mn²⁺
• reacts with MnO₄^- to produce Mn³⁺ as an intermediate species
  
  • 4Mn²⁺ (aq) + MnO₄^- (aq) + 8H⁺ (aq) → 5Mn³⁺ (aq) + 4H₂O (l)
• Mn³⁺ then reacts w/ C₂O₄^2- ions to reform Mn²⁺
  
  • 2Mn³⁺(aq) + C₂O₄^2- (aq) → 2CO₂ (g) + 2Mn²⁺ (aq)