Transition metal CATALYSIS Flashcards
Why do transition metals and their complexes make good catalysts?
Because they can change oxidation states by gaining or losing electrons within their d-orbitals
How do transition metals and their complexes act as catalysts?
They can transfer electrons to speed up reactions
What is made in ‘The Contact Process’ ?
Makes sulphuric acid
What catalyses the contact process?
Vanadium (V) oxide
V2O5
How many steps is the contact process made up of?
2
What is the overall equation of the contact process?
SO2 (g) + 1/2O2(g) –(V2O5 catalyst, solid)—> SO3(g)
What is the 1st step of the contact process?
First, vanadium(V) oxide OXIDISES
SO2 –> SO3 and is reduced in itself to vanadium (IV) oxide
What is the equation for the 1st step of the contact process?
V2O5 + SO2 –> V2O4 + SO3
What happens in the 2nd stage of the contact process?
The reduced catalyst is oxidised by oxygen back to its original state
What is the equation for the 2nd stage of the contact process?
V2O4 + 1/2O2 –> V2O5
How does having variable oxidation states allow vanadium (V) oxide to catalyse the contact process?
V2O5 is able to oxidise SO2 –> SO3 because it can be reduced to V2O4. It is then oxidised back to V2O5 by oxygen, ready to start again. If vanadium didn’t have variable oxidation states it wouldn’t be able to catalyse this reaction
What is a heterogeneous catalyst?
A catalyst that is in a different phase from the reactants and the reaction occurs at active sites on the surface
Give two examples of a heterogeneous catalyst and in what process are they used in?
Iron catalyst (s) which is used in the Haber Process
&
Vanadium (V) oxide which is used in the Contact Process
What is the Haber process used to make?
Ammonia
Give the overall equation for the Haber process:
N2 (g) + 3H2(g) – (Fe(s) catalyst) –> 2NH3 (g)
How does a heterogeneous catalyst work during its reaction?
The catalyst is a solid and the reactants are gases, gases are passed over the surface of the solid catalyst.
How can you increase the rate of reaction when a heterogeneous catalyst is used?
Increasing the surface area of the catalyst increases the number of molecules that can react at the same time which increases the rate of reaction.
What is often used in order to increase the surface area of a heterogeneous catalyst?
A SUPPORT MEDIUM is often used to make the surface area of a catalyst as large as possible.
What are two advantages of a support medium which is used in reactions that involve a heterogeneous catalyst?
maximise the surface area of a heterogeneous catalyst and minimise the cost.
What does a catalytic converter do?
Cleans up emissions from car engines
They convert waste gases to less harmful products
Describe the structure of a catalytic converter:
They contain a ceramic lattice coated with a thin layer of rhodium
What acts as a catalyst in a catalytic converter?
Rhodium
Give the equation for the reaction that occurs in a catalytic converter:
2CO(g) + 2NO(g) –(Rh catalyst)–> 2CO2(g) + N2(g)
How does the structure of the catalytic converter make it more effective and what is the benefit of its structure?
The lattice structure maximises the surface area of the catalyst so that it is more effective.
It also minimises the cost of the catalyst as only a small layer of coating is needed.
During a reaction, what happens between the catalyst and the reactants?
The reactants are adsorbed onto the active sites on the surface of the heterogeneous catalyst
What is catalyst poisoning?
It is when the impurities in the reaction mixture may also bind to the catalyst’s surface and block the reactants from being adsorbed.
THE ACTIVE SITES ARE BLOCKED –> REDUCED EFFICIENCY –> COST IMPLICATIONS
Give three disadvantages of catalyst poisoning:
REDUCES SURFACE AREA of catalyst available to the reactants which slows the reaction down
INCREASES COSTS of chemical processes because less product is made in a certain time or with a certain amount of energy
REPLACING CATALYST as it has been contaminated/poisoned this also COSTS MONEY
How can catalyst poisoning be reduced?
By purifying the reactants - this removes many of the impurities which could have poisoned the catalyst
Give one example of catalyst poisoning: pb
LEAD poisons the catalyst in CATALYTIC CONVERTERS
(think! unleaded petrol)
Lead can coat the surface of the catalyst in the converter so vehicles that have them have to have unleaded petrol
Give a second example of catalyst poisoning: s
SULPHUR poisons the iron catalyst in the Haber process
the hydrogen in the process is obtained from methane which is obtained from natural gases which contain impurities - these include sulphur compounds
Any sulphur that isn’t removed is adsorbed onto the iron, forming iron sulphide. This stops the iron from catalysing the reaction efficiently.
What is a homogeneous catalyst?
A catalyst that is in the same phase as the reactants
When reactants and catalysts are in the same phase, what has to happen?
The reaction proceeds through an intermediate species
What state is a homogeneous catalyst usually in?
Aq
How does a homogeneous catalyst work?
It works by forming an intermediate species
The reactants combine with the catalyst to make an intermediate species which then reacts to form the products and reform the catalyst.
What does the enthalpy profile of a reaction catalysed by a homogeneous catalyst look like?
It causes the enthalpy profile to have two humps in it, corresponding to the two reactions.
Describe the difference in activation energy needed for an uncatalysed reaction and a catalysed one (homogeneous)
The activation energy needed to form the intermediates and products is lower than that needed to make products directly from the reactants.
Describe the structure of the enthalpy profile of a homogeneously catalysed and an uncatalysed reaction:
X axis –> progress of reaction
Y axis –> enthalpy
(uncatalysed) Reactants (down) Products
(catalysed) Reactants (UP) Intermediates (down - past reactants) Products –> TWO HUMPS
Give 2 examples of reactions catalysed by homogeneous catalysts:
Fe2+ catalysing the reaction between S2O8(2-) and I-
&
Mn2+ autocatalysing the reaction between MnO4- and C2O4(2-)
What type of reaction is the reaction between iodide ions and peroxodisulphate ions?
A redox reaction
Why does the reaction between iodide ions and peroxodisulphate ions happen slowly?
Becuase both of the ions are negatively charged - they repel each other meaning that it is unlikely that they will collide and react.
What is the overall equation for the reaction between I- and S2O8(2-)?
S2O8(2-) (aq) + 2I- (aq) –> I2 (aq) + 2SO4(2-) (aq)
How does Fe2+ help speed up the reaction between S2O8(2-) and I- ions?
Because each stage of the reaction involves a positive and negative ion - which means there is no repulsion
What happens in the first stage of the reaction between I- and S2O8(2-) using a homogeneous catalyst?
First, Fe2+ is oxidised to Fe3+ by S2O8(2-) ions
Give an equation for the first stage of the reaction between I- and S2O8(2-) using a homogeneous catalyst?
S2O8(2-) (aq) + 2Fe(2+) (aq) –> 2Fe(3+) (aq) + 2SO4(2-) (aq)
What happens in the second stage of the reaction between I- and S2O8(2-) using a homogeneous catalyst?
Fe3+ can now easily oxidise I- to I2 and the catalyst is regenerated
Give an equation for the second stage of the reaction between I- and S2O8(2-) using a homogeneous catalyst?
2Fe3+ (aq) + 2I- (aq) –> I2(aq) + 2Fe2+ (aq)
What is the test for iodine?
Add starch solution
Turns blue-black if iodine present
Why does the reaction between I- and S2O8(2-) have a high activation energy?
Because they both have negative charges
Give an example of an autocatalysis reaction:
Mn2+ autocatalysing the reaction between MnO4- and C2O4(2-)
What is an autocatalysis reaction?
Where the product of the reaction is also the catalyst of the reaction
How does an autocatalysis reaction work?
As the reaction progresses and the amount of product increases, the reaction speeds up
Give the overall equation for the reaction between MnO4- and C2O4(2-):
2MnO4- (aq) + 16H+ (aq) + 5C2O4(2-) —> 2Mn2+ (aq) + 8H2O (l) + 10CO2(g)
Describe the reaction between MnO4- and C2O4(2-) and why it acts like this?
There isn’t any Mn2+ present at the start of the reaction so the rate of reaction is very slow
During this uncatalysed part the activation energy is very high. This is because the reaction proceeds via the collision of negative ions and this requires a lot of energy to achieve.
Give the equation for the reaction between MnO4- and C2O4(2-) once some of the catalyst has been produced: (1st stage of reaction)
4Mn2+ (aq) + MnO4- (aq) + 8H+ (aq) –> 5Mn3+ (aq) + 4H2O(l)
What is the intermediate in the reaction between MnO4- and C2O4(2-) ?
Mn3+
Give the equation for the reaction between the intermediate and one of the reactants in the reaction between MnO4- and C2O4(2-): 2nd stage of reaction
2Mn3+(aq) + C2O4(2-) (aq) –> 2Mn2+ (aq) + 2CO2(g)
Describe what the graph would look like to represent the reaction between MnO4- and C2O4(2-) and explain why?
X-axis –> TIME
Y-axis –> CONCENTRATION
Looks like a backward ‘s’ but the curves aren’t as prominent. Draw and check
Because Mn2+ autocatalyses the reaction, ROR increases with time as more catalyst is made.
What three labels should be on the concentration-time graph for the reaction between MnO4- and C2O4(2-) ?
Uncatalysed part of reaction
Reaction rate increases as catalyst is made
Graph levels off as all of MnO4- is used up
