Catalysts Flashcards
What is the most important feature in the mechanism of homogenous catalysts?
Change of oxidation state
Reduction of S2O82- by I-
S2O82- + 2I- -> 2SO + I2
Heterogenous catalysts
Present in a reaction in a different phase than the reactants. Usually present as solids, while the reactants may be gasses or liquids. Catalytic action occurs on the solid surface.
How do we make catalysts more efficient?
- Increase surface area
2. Spread the catalyst onto an inert support medium -> Increases surface-to-mass ratio
Why don’t catalysts last forever?
- Surfaces may become covered with impurities
2. Finely divided catalyst may gradually be lost from support medium
What catalyst is used in the Haber process?
Iron
2 steps of the Contact process
- Vanadium (v) oxide oxidises sulfur dioxide to sulfur trioxide and is itself reduced to vanadium (iv) oxide
SO2 + V2O5 -> SO3 + V2O4 - The vanadium (iv) oxide is then oxidised back to vanadium (v) oxide by oxygen
2V2O4 + O2 -> 2V2O5
What catalyst is used for production of methanol
Cr2O3
Synthesis gas
Made from methane, present in natural gas and steam. It’s a mixture of CO and H2 and is used to make methanol.
Homogenous catalysts
When the catalyst is in the same phase as the reactant, an intermediate species is formed.
Autocatalysis
One of the products of the reaction is a catalyst for the reaction
How does Mn2+ act as a catalyst?
Reacts with MnO4- ions to form Mn3+ as an intermediate species which then reacts with C2O42- ions to reform Mn2+
Why does tungsten have low catalytic efficiency?
Absorption too strong
Why does silver have low catalytic efficiency?
Absorption too weak
What catalysts are used in a catalytic convertor?
Rhodium
Catalytic convertor equation
2CO + 2NO -> 2CO2 + N2
Haber process equation
N2 + 3H2 -> 2NH3
Iron and HCl
Fe + 2H+ -> Fe2+ + H2
Reagent for conversion of [Cr(OH)6]3- into CrO42-
H2O2
3 essential steps in the mechanism of heterogeneous catalysis
- Surface absorption
- Reaction on surface
- Products released from surface
Identify a substance which acts as a poison for a catalyst and explain how poisoning occurs
Sulfur, Blocks active sites
2CrO42-
Yellow
CrO72-
Orange
What are the reagents, conditions and equation for the conversion of potassium dichromate(VI) to Cr3+
H2O2 + dilute H2SO4
Cr2O7 + 14H+ + 6Fe2+ -> Cr3+ +7H2O +6Fe3+
What are the reagents, conditions and equation for the conversion of potassium dichromate(VI) to Cr2+
HCl + absence of air
Cr2O7 +14H+ +4Zn -> Cr2+ + 7H2O + 4Zn2+
Why is the strength of absorption of reactants and products onto the surface of a catalyst important factors in determining catalytic activity?
Too strong -> can’t be desorbed from surface
Too weak - reactants not brought together
Other than strength of absorption, why would the absorption onto a catalyst surface not result in a reaction?
Reactants not correctly orientated
3 characteristic features of a catalyst
- Speeds rate
- Unchanged at the end
- New route with lower Ea
What 2 measures can be taken to maximise catalytic efficiency?
- Increase S.A.
2. Remove catalyst poisons from reactants
Why does iron have 2 common oxidation states in aq chemistry?
Lose 2 4s electrons so it can lose another to give half full shell
