transition metal catalysis

why are transition metals good catalysts?

• have variable oxidation states

• so are able to form a range of compounds by gaining/losing e^- w/in their d orbitals

in which process does vanadium act as a catalyst? give the eqns for this process:

Contact Process - industrial process for making H₂SO_{4 :}

• S + O₂ → SO₂

• SO₂ + ½ O₂ → (V₂O₅ catalyst) SO₃

• SO₃ + H₂O → H₂SO₄

give and explain pair of eqns for the use of V₂O₅ as a catalyst in the Contact Process:

V₂O₅ oxidises SO₂ to SO₃ and is itself oxidised to V₂O₄:

• V₂O₅ + SO₂ → V₂O₄ + SO₃

the reduced catalyst is then oxidised back to its original state:

• V₂O₄ + ½ O₂ → V₂O₅

what is a heterogeneous catalyst? how do reactions occur?

catalyst which exists in a different phase from the reactants

explain how a heterogenous catalyst works:

• reactants adsorb onto active sites on the surface of the heterogenous catalyst

• bonds weaken/reaction takes place

• products desorb from surface

give 2 egs of heterogenous catalysts and their eqns:

Fe in the Haber process:

• N_{2 (g)} + 3H_{2 (g)} → (Fe _(s) catalyst) 2NH_{3 (g)}

V₂O₅ in the Contact process:

• SO_{2 (g)} + ½ O_{2 (g)} → (V₂O_{5 (s)} catalyst) SO_{3 (g)}

how can we minimise the cost of a reaction using a heterogenous catalyst?

• maximise SA so increase no. of molecules that can react at the same time

• this can be achieved by using powder/small pellets/support medium

give and explain an example of a support medium and provide the eqn:

• catalytic converters contain a ceramic lattice coating in a thin layer of Rh/Pt/Pd

• 2CO _(g) + 2NO _(g) → (Rh _(s) catalyst) 2CO_{2 (g)} + N_{2 (g)}

what is catalyst poisoning? how does this occur?

• catalysts can be poisoned when impurities adsorb onto the active sites of the catalyst, blocking them

• this decreases the efficiency of catalysis, increasing costs

give 2 examples of catalyst poisoning:

• lead can coat the inner surface of a catalytic converter

• sulfur can poison the active sites on the iron catalyst in the Haber process, forming iron sulfide (sulfur found in fossil fuels used to produce H₂)

what is a homogeneous catalyst?

catalyst which exists in the same phase as the reactants (typically liquids/solutions)

how do homogeneous catalysts work?

• work by forming an intermediate species which then reacts to form the products

• the E_A needed to form the intermediate species is lower than that needed to make the products directly

give an example of a homogeneous catalyst and give the eqn for the reaction it catalyses:

Fe²⁺ ions catalyse the reaction between peronodisulfate ions and iodide ions:

• S₂O₈^2- _(aq) + 2I^-_(aq) → (Fe²⁺ _(aq) catalyst) I_{2 (aq)} + 2SO₄^2- _(aq)

why is the reaction between peronodisulfate ions and iodide ions slow without a catalyst?

-ve charges repel so high E_a

give a pair of eqns to show how Fe²⁺ acts as a catalyst in the reaction between peronodisulfate ions and iodide ions:

S₂O₈^2- _(aq)  + 2Fe²⁺ _(aq) → 2Fe³⁺ _(aq) + 2SO₄^2- _(aq)

2Fe³⁺ _(aq) + 2I^- _(aq) → I_{2 (aq)} + 2Fe²⁺ _(aq)

other than having variable oxidation states, explain why Fe²⁺ ions are good catalysts for the reaction between peronodisulfate ions and iodide ions:

+ve ions attract -ve ions in catalysed process

give an example of a homogeneous catalyst which undergoes autocatalysis and give the overall eqn for the reaction it catalyses:

Mn²⁺ ions autocatalyse the reaction between C₂O₄^2- and MnO₄^- :

• Mn²⁺ is a product of the reaction and acts as a catalyst

• ∴ as the reaction progresses, amount of product increases

• 2MnO₄^- _(aq) + 16H⁺ _(aq) + 5C₂O₄^2- _(aq) → 2Mn²⁺ _(aq) + 8H₂O _(l) + 10CO_{2 (g)}

give a pair of ionic eqns to show how Mn²⁺ ions act as a catalyst in the reaction between C₂O₄^2- and MnO₄^- :

• 4Mn²⁺ _(aq) + MnO₄^- _(aq) + 8H⁺ _(aq) → 5Mn³⁺ _(aq) + 4H₂O _(l) (Mn²⁺ oxidised back to Mn³⁺ by MnO₄^-)

• 2Mn³⁺ _(aq) + C₂O₄^2- _(aq) → 2Mn²⁺ _(aq) + 2CO₂ (g) (Mn³⁺ reduced to Mn²⁺ by C₂O₄^2-)

how does Mn²⁺ lower the E_a of the reaction between C₂O₄^2- and MnO₄^- ?

E_a lowered because oppositely charged ions attract

give and explain the shape of a conc-time graph of MnO₄^-:

• initially gradient shallow as RoR low as reaction is uncatalysed as too few Mn²⁺ ions made ∴ high E_a

• RoR increases w/ time as more of autocatalyst is made

• graph begins to level off as MnO₄^- used up