Catalysts Flashcards
Define a heterogeneous catalyst
The catalyst is in a different phase to the products and reactants
Define a homogeneous catalyst
The catalyst in the same phase as the products and reactants
How do heterogeneous catalysts work?
- Reactants are adsorbed into active site which weakens bonds, brings molecules closer and to a favourable orientation
- Reaction takes place
- Products are desorbed
Examples of heterogeneous catalysts
- Reduction using nickel/hydrogen
- Catalytic converters
- Haber process
Describe the poisoning of catalysts
Some substances may block active sites (strong adsorption and won’t desorb from active site) which poisons catalyst
Reactions in the contact process
2SO₂ + O₂ ⇌ 2SO₃ [V₂O₅, 450°C, 1-2 atm]
SO₂ + V₂O₄ → SO₃ + V₂O₄
2V₂O₄ + O₂ → 2V₂O₅ (reformed)
Why does the depletion of the ozone layer show the involvement of a homogeneous catalyst?
Because Cl• is reformed
Reactions involved in the oxidation of I⁻ by S₂O₈²⁻
Overall: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂
S₂O₈²⁻ + 2Fe²⁺ → 2SO₄² + 2Fe³⁺
2I⁻ + 2Fe³⁺ → I₂ + 2Fe²⁺
What is the catalyst involved in the oxidation of I⁻ by S₂O₈²⁻?
Fe²⁺/Fe³⁺
What is being reduced in the oxidation of I⁻ by S₂O₈²⁻?
Peroxide groups that have an oxidation state of 1
Why does the oxidation of I⁻ by S₂O₈²⁻ have a high Ea?
Both negative ions that repel each other
What is autocatalysis?
When a reaction supplies its own catalyst
What shape is the curve conc/time for the redox titration of MnO₄⁻ and C₂O₄²⁻?
Sigma curve
- Slow at start due to repulsion
- Faster due to autocatalysis
- Slow again due to low conc.
Half equations for redox titration of MnO₄⁻ and C₂O₄²⁻
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
C₂O₄²⁻ → 2CO₂ + 2e⁻
2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻→ 2Mn²⁺ + 8H₂O + 10CO₂
Catalyst equations for redox titration of MnO₄⁻ and C₂O₄²⁻
a) MnO₄⁻ + 8H⁺ + 4Mn²⁺ → 5Mn³⁺ + 4H₂O
h.e.: MnO₄⁻ + 8H⁺ + 4e⁻ → Mn³⁺ + 4H₂O
Mn²⁺ → Mn³⁺ + e⁻
b) C₂O₄²⁻ + 2Mn³⁺ → 2CO₂ + 2Mn²⁺
