3.2.5 Catalysis - Transition metals Flashcards
Define a catalyst
A susbtance that increases the rate of a chemical reaction but is not used up in the reaction.
How do catalysts usually work?
By providing an alternative route with a lower activation energy (catalysts do not alter ΔG ΔH ΔS for a reaction, only activation energy.)
Do catalysts alter the position of equilibrium?
No, they speed up the rate of reaction at which an equilibrium is reached by speeding up the forward and backward reaction equally
Why are transition metals good catalysts?
They can exist in variable oxidation states, so can easily provide alternate reaction pathways.
Why are group 1 2 and 3 not as good catalysts?
They only exist as one oxidation state
What are the advantages of using a catalyst for a reaction?
allow reactions to happen at lower temperatures and pressures which saves energy and valuable resources
What metals are used in a catalytic converter and what reactions do they catalyse?
Pt, Rd, Pd
Catalyse 2CO+2NO–>N2+2CO2
and CxH2x+2–> H2O + CO2
Define a heterogenous catalyst
A catalyst that is in a different phase to the reactants (usually a solid with gas/liquid reactants)
How do heterogenous catalysts work?
At least one of the reactants is adsorbed onto the surface (i.e forms bonds to the atoms in the solid surface)
The places on the surface where molecules are adsorbed are called active sites.
What makes an effective catalyst?
When molecules can move about the surface bonding to different active sites
(So adsorbtion must be at the right strength)
In what ways does the adsorbtion of reactants onto the surface result in increased reaction?
- adsorbtion onto the surface effectively concentrates the reactants i.e brings them closer together than in the gas phase so increasing the likelihood of collision
- it may weaken some bonds in the molecu;e making the reaction easier
- it may position the molecule in a favourable orientation for reaction
catalysts
What happens when adsorbtion is too weak?
not many molecules will be adsorbed so the catalyst will have very little effect
catalysts
What happens when adsorbtion is too strong?
molecules will not be able to move around the active sites and so be less likely to meet another reactant and so be less likely to react (also any product will tend to remain adsorbed on the surface)
Describe the trend in adsorbtion strength across the 1st 2nd (3rd) transition series in relation to relative rate
Catalysts are expensive, how can use them efficiently?
Increasing surface area means we can use less quantity of the catalyst to produce the same amount of product
Surface area is maximised by using a very thin coating of the caralyst on some type of support medium ( a support medium is needed as the layer is too thin to support itself) odten a cermaic “honeycomb” structure is used as a support-
NB the catalyst must be able to bind to the support.
What does it mean when a catalyst is poisoned?
where other substances adsorb strongly to the surface blocking the active sites.
this lowers the efficiency of the catalyst or makes it totally ineffective depending on the extent of the poisoning
the poisons are xtremely difficult to remove and the catalyst is ruined, can be very costly particularly with pricey catalysts
Brifly desribe the Contact process
what does it produce
what reacts
what kind of reaction is it
what is the catalyst
what are the equns
- produces concentrated H2SO4
- conversion of SO2 to SO3
- which is a slow reversible reaction
- it is catalysed by vanadium oxide V2O5
- V2O5 + SO2 –> V2O4 + SO3
+V2O4 + 1/2 O2 –> V2O5
= SO2 + 1/2O2–> SO3
What is a homogenous catalyst?
A catalyst that is in the same phase as the reactants (most reactions involving one take place in solution (where all species are either liquids or are dissolved so they are all in the same phase)
How do homogenous catalysts work?
producced via an intermediate species formed from a reactant and the catalyst which then reacts (faster) and regenerates the catalyst
How does a transition metal homogenous catalyst work?
+ example for reaction X + Y –> products (cat H+)
Reactions involving homogeneous catalysis prodceed via an intermediate species formed from a reactants and the catalyst which then further reacts and regenerates the catalyst.
X + Y –> products (cat H+)
X + H+ —> HX+
HX+ + Y —> products + H+
How does a transition metal homogeneous catalyst work? Specifically iron
Transition metal compounds have the ability to vary their oxidation states which allows them to act as catalysts.
Fe2+ (aq) or Fe 3+ (aq) can easily act as a catalyst because it can easily change oxidation states between 2+ and 3+
Why is this reaction slow?
2I- + S2O8 —> I2 + 2SO42-
very slow in the absence of a catalyst because it involves the reaction of two negative ions (they repel each other so collision is difficult)
Explain how Fe2+/Fe3+ catalyses the reaction
2I- + S2O8 —> I2 + 2SO42-
and how come both ions are catalysts
- Fe2+ reduces S2O8 to SO4- (Fe2+ is oxidised to Fe3+)
2Fe2+ + S2O82- –> 2Fe3+ + 2SO42- - Fe3+ then oxidises the I- to I2 reforming Fe2+
2Fe3+ + 2I- –> 2Fe2+ + I2
The reactions can occur either way around hence both Fe2+ and Fe3+ are catalysts
What is autocatalysis?
Reactions which produce their own catalyst are known as autocatalysis reactions
Give an example of an autocatalysis reaction
reaction between manganate (VII) ions and ethanedioate ions
2MnO4 - + 16H + + 5C2O4 2- —> 2Mn 2+ + 8H2O + 10 CO2
The catalyst is produced as a product - this reaction is slow until some Mn2+ is formed but then iy speeds up as catalyst is formed.
