A2- Catalysts
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Why are transition metals able to act as catalysts?
Variable oxidation states so are able to temporarily donate or accept electrons
How do catalysts speed up rate of reaction?
• Provide an alternative reaction pathway with lower Ea • more particles have E>Ea • more collisions lead to a reaction
What is a heterogeneous catalyst?
a catalyst in a different phase from the reactants
What is a homogeneous catalyst?
a catalyst in the same phase as the reactants
Where do reactions with catalysts occur?
at active sites on the surface
Why do active sites exist?
vacant d-orbitals that can form weak, temporary bonds
What makes a catalyst suitable?
forms strong enough bonds to adsorb reactants, but weak enough that products can de-adsorb
What is the contact process used for?
forming sulfuric acid
What catalyst is used in contact process? WHat type of catalyst is this?
V2O5 (s) heterogeneous
Give 2 equations showing the action of V2O5 in the contact process…
V2O5 (s) + SO2 (g) –> V2O4 (s) + SO3 (g) V2O4 (s) + 1/2O2 (g) –> V2O5 (s)
What catalyst is used in the Haber process?
Fe (s)
What is the overall equation for the Haber process?
N2 (g) + 3H2 (g) –> 2NH3 (g)
How does iron catalyse the Haber process (in 4 steps)?
1) reactants adsorb into active sites on Fe 2) bonds in reactants weaken 3) new bonds form in products 4) products de-adsorb from the surface
What is an autocatalytic reaction?
Where one of the products of the reaction catalyses the reaction
Sketch a graph of an autocatalytic reaction. Explain why it has this shape
graph is a sigmoid shape, starting on y-axis and ending on/above x-axis - reaction rate starts slow - catalyst increases the rate of reaction - reaction rate decreases as reactants are used up
Give an example of a homogeneous catalyst…
Fe2+ in reaction of peroxodisulfate ions with iodide ions S2O82- + 2I- –> 2SO42- + I2
Why is a catalyst needed in the reaction between Iodide ions and peroxodisulfate ions?
very high activation energy as both ions are negatively charged and repel in collisions
Give 2 equations showing the action of Fe2+ in the reaction between Iodide ions and peroxodisulfate ions…
S2O82- + 2Fe2+ –> 2SO42- + 2Fe3+ 2Fe3+ + 2I- –> 2Fe2+ + I2
Give an example of an autocatalytic reaction…
manganate (VII) ions with ethanedioate ions
What is the overall equation for the reaction of manganate (VII) ions with ethanedioate ions?
2MnO4- + 5C2O42- + 16H+ –> 2Mn2+ + 10CO2 + 8H2O
What 2 reactions show the action of Mn2+ in the reaction of manganate (VII) ions with ethanedioate ions?
5Mn2+ + MnO4- + 8H+ –> 5Mn3+ + Mn2+ + 4H2O 2Mn3+ + C2O42- –> 2Mn2+ + 2CO2