catalysts (transition metal chemistry) Flashcards
why do transition metals act as catalysts (2 marks)
- transition metals are able to form variable oxidation states and temporally donate/accept electrons
- they provide an alternative reaction pathway with a lower activation energy, therefore more particles have energy greater than or equal to activation energy therefore more collisions result in a reaction
define heterogeneous
catalyst is in different phase from reactants
define catalyst
something that speeds up a chemical reaction without being used up
they provide an alternative reaction pathway with a lower activation energy, therefore more particles have energy greater than or equal to activation energy therefore more collisions result in a reaction
where do reactions occur on a catalyst
reactions occur at active sites on the surface
these exist due to vacant d orbitals that can form weak temporary bonds
what makes a catalyst suitable
must form strong enough bonds to adsorb reactants but weak enough that their products can de adsorb
give the equations for the contact process and how vanadium acts as a catalyst
V2O5 (s) + SO2(g) —-> V2O4 (s) + SO3(g)
V2O4 (s) + 1/2 O2 (g) —–> V2O5 (s)
what catalyst is needed for the Haber process
iron
write an equation for the Haber Process in the presence of a iron catalyst
N2 (g) + 3H2 (g) —–> 2NH3 (g)
define homogenous catalyst
catalyst and reactant are in the same phase
(an intermediate species is formed)
write the overall equation for the reaction of iodide ions and peroxodisulfate ions
S2O8 2- + 2I- —–> 2SO4 2- + I2
why does the reaction of iodide ions and peroxodisulfate ions have a high activation energy
reaction has high activation energy because both ions are negatively charged and repel in collisons
write the 2 intermediate equations for the reaction of iodide ions and peroxodisulfate ions in presence of Fe2+
S2O8 2- + 2Fe 2+ —-> 2SO4 2- + 2Fe3+
2Fe 3+ + 2I- —-> 2Fe2+ + I2
what is the catalyst in the reaction of iodide ions and peroxodisulfate ions
Fe2+
how does Fe2+ act as a catalyst in the reaction of iodide ions and peroxodisulfate ions
Fe ions change oxidation state
either reaction can go first so Fe2+ or Fe3+ ions will catalyse the reaction
give 3 factors that affect the colour of a transition metal
- oxidation state
- ligands
- coordination number
Explain how the efficiency and cost effectiveness of V2O5
is increased in the contact process
used as a fine powder sprayed onto a wire mesh (increased
SA and low amount of an expensive material)
Explain with the aid of equations how V2O5 acts as a
catalyst for the contact process
V2O5 + SO2 ⎯→ V2O4 + SO3
V2O4 + ½O2 ⎯→V2O5
overall SO2 + ½O2 ⎯→ SO3
This question is about iron and its ions.
Discuss the role of iron as a heterogeneous catalyst in the Haber process.
3 H2 + N2 ⇌ 2 NH3
Your answer should include:
- the meaning of the term heterogeneous catalyst
- how iron acts as a heterogeneous catalyst
- the factors that affect the efficiency and lifetime of the catalyst.
heterogenous means reactant in different phase from catalyst
catalyst speeds up chemical reaction and is left unchanged or catalyst lowers activation energy
hydrogen and nitrogen (reactants) adsorb onto the surface/active sites of iron
bonds weaken/reaction takes place
products de adsorb from iron
large surface area of iron by using powder or pellets
catalyst poisoned by sulfur/ sulfur binds to catalyst
active sites are blocked
Platinum acts as a heterogeneous catalyst in the reaction between ammonia
and oxygen. It provides an alternative reaction route with a lower activation
energy.
Describe the stages of this alternative route. [3 marks]
- reactants adsorb onto platinum surface/platinum provides active sites
- bonds breaking/ bonds making occurs on the surface
- de adsorption of product
Write an equation for the reaction of peroxodisulfate ions S2O82- with iodide
ions
S2O82- + 2I- ⎯→ 2SO42- + I2
Explain why the activation energy for this reaction is so high (therefore low
rate)
Both ions are negatively charged, therefore repel on collision
What is an autocatalytic reaction?
Where one of the products of the reaction catalyses the reaction
The reaction between C2O42– ions and MnO4– ions is autocatalysed.
write the overall equation for the reaction
Overall reaction
2MnO4- + 5C2O42- +16H+ ⎯→ 2Mn2+ + 10CO2 + 8H2O
The reaction between C2O42– ions and MnO4– ions is autocatalysed.
write the 2 step reaction equation involving catalyst
5Mn2+ + MnO4- + 8H+ ⎯→ 5Mn3+ + Mn2+ + 4H2O
2Mn3+ + C2O42- ⎯→ 2Mn2+ + 2CO2
outline the steps involved in Fe acting as a heterogenous catalyst in the Haber process
1- reactants adsorb to active sites on surface
2. weakens bonds in reactants
3. new bonds form in products
4. products deadsorb from surface
explain how heterogenous catalysts can become poisoned and explain how this reduces effectiveness of catalysts
some impurities stick to enzyme and block active sites
(impurities may permanently adsorb and block active sites
