23.5 Transition metal catalysts Flashcards
What is a catalyst
A catalyst is something that speeds up a chemical reaction by providing an alternative reaction pathway with a lower activation energy, and it doesn’t get used up
Why do transition metals make good catalysts
They can change oxidation states easily by gaining or losing electrons in their d orbitals, so they can transfer electrons to speed up reactions
What is a heterogeneous catalyst
Give two examples of reactions using these
It is a catalyst that is in a different phase to the reactants, eg is in a different physical state
The contact process with V2O5
The Haber process with Fe
How can you make a heterogenous catalyst more efficient
. Increase their surface area: The larger the surface area, the better the efficiency
. Spread catalyst onto support medium so increase surface to mass ratio so a little goes a long way.
Do that with expensive catalyst
What is the contact process used for
what is the catalyst for this
It is used industrially to make sulfuric acid
Vanadium (V) oxide V2O5 catalyses this reaction
What product are the reactants in the contact process forming.
How is this turned into sulfuric acid
SO2 is made into SO3
You can react SO3 with water (H2O) to make H2SO4
How does the contact process occur
. Vanadium (v) oxide is the catalyst used, and it occurs in two steps:
- First the V2O5 oxidises SO2 to SO3 in the reaction, and itself is reduced :
V2O5 + SO2 —> SO3 + V2O4
This reduced catalyst is then oxidised back to V2O5 by oxygen to start the reaction again
V2O4 + 1/2 O2 —> V2O5
What type of catalyst is vanadium (V) oxide in this reaction
Why is vanadium (V) oxide the catalyst used in this reaction
It is a heterogeneous catalyst because it is in a different phase to the reactants eg a different physical state
Because it has the right variable oxidation state needed
Why can’t elements in other blocks of the periodic table, eg S block be catalysts in redox reactions
. They generally aren’t used to catalyse redox reactions because they don’t have an incomplete 3d sub shell, and don’t have variable oxidation states
What is the overall reaction for the contact process
SO2(g) + 1/2O2(g) ———-> SO3(g)
With V2O5 catalyst used, over the arrow
What is the Haber process making
Ammonia is made from reacting nitrogen and hydrogen
Describe the Haber process
. Nitrogen and Hydrogen are reacted together
. The iron catalyst Fe(s) is used in the shape as lots of pea sized lumps to. increase surface area
Reaction:
N2(g) + 3H2(g) —> 2NH3(g)
With iron over the arrow
How are the contact process and the Haber process using heterogeneous catalysts
How is the rate of reaction increased
The catalyst in both is a solid whilst the reactants are gases
So the reaction occurs on the surface of the catalyst
So increasing surface area of catalyst increases the number of molecules that can react at the same time, increasing the rate of the reaction
What are catalytic converters and what transition metal is the catalyst
Rhodium is the catalyst in them
They clean up emissions from car engines, by converting nasty waste gases into less harmful products .
They are shaped as a ceramic lattice coated with a thin layer of rhodium on top to increase surface area of catalyst to minimise cost of it because only a small coat is needed
What is catalyst poisoning
During a reaction, reactants are adsorbed onto the surface of heterogeneous catalysts
Impurities in the mixture may also bind to the catalyst surface, blocking the reactants from being absorbed
So it reduces the surface area available for the reactants so slows down the reaction
It can increase cost as less product is made with the certain amount of energy
Why does the iron catalyst in the haber process have to be replaced every 5 years
There are sulfur impurities because the hydrogen used is from methane which is obtained from natural gas
So that sulfur can be adsorbed into the iron forming iron sulfide
What is a homogeneous catalyst
. These are catalysts in the same physical state as the reactants.
Eg an aqueous catalyst for a reaction between two aqueous solutions
How does a homogeneous catalyst work
It forms an intermediate species by the reactants combining with the catalyst
This intermediate species then reacts to form the products and reform the catalyst
So these two reactions make the enthalpy profile for a homogeneously catalysed reaction have two humps in it
The energy needed to form the intermediates (and to form the products from the intermediates) is less than the energy needed to create the products directly from the reactants
What two reactions catalysed by homogeneous catalysts do we need to know
. Fe2+ catalysing the reaction between S2O8 2- ions and I- ions
. Mn2+ autocatalysing the reaction between MnO4- and C2O4 2-
Describe the reaction between S2O82- (peroxodisulfate) and I- ions
The redox reaction is catalysed by iron (Fe2+)
The overall reaction is shown
S2O8 2- + 2I- —> I2 + 2SO42-
why would the reaction between peroxodisulfate ions and iodide ions be really slow without the iron catalyst
Because the reactants are both negatively charged so will repel each other so it is unlikely they will collide and react
How does adding the Fe2+ ion help speed up the reaction between the S2O82- and 2I-
Give the reactions
Because it is positively charged so the reaction will now involve a positive and negative ion
First the Fe2+ ions are oxidised to Fe3+ ions by the S2O8 2- ions:
S2O8 2-(aq) + 2Fe2+ —>
2Fe3+ + 2SO4 2-
The newly formed intermediate Fe3+ ions can now easily oxidise the I- ions to iodine, and the catalyst is regenerated
2Fe3+ + 2I- —->I2 + 2Fe2+
What is the catalyst in the reaction between MnO4- and C2O4 2-
Mn2+ is the catalyst in this reaction, and it is an autocatalysis reaction because Mn2+ is a product and also acts as a catalyst
This means that as the reaction progresses, more product is formed so it speeds up over time
What is the redox equation for the reaction between MnO4- and C2O4 2-
2MnO4- + 5C2O42- + 16H+ —–>
2Mn2+ + 8H2O + 10CO2
The activation energy at the beginning is very high as there is no catalyst
There are two negative ions as reactants which repel eachother
How does the Mn2+ catalyst work in the reaction between MnO4- and C2O4 2-
When some of the Mn2+ is formed, it reacts with the MnO4- ions to make Mn3+ ions and water
4Mn2+ + MnO4- + 8H+ —> 5Mn3+ + 4H2O
These Mn3+ ions are intermediate, so then react with C2O42- ions to make CO2 and reform the Mn2+ catalyst
2Mn3+ + C2O42- –> 2Mn2+ + 2CO2
So rate of reaction increases as time increases in the reaction
What happens if you add ammonia solution to an aqueous solution of cobalt(ii) ions
. First a precipitate is formed by reaction with OH- ions from the ammonia which is alkaline
[Co(H2O)6]2+ + 2OH- –> Co(H2O)4(OH)2(s) + 2H2O(l)
Then the precipitate dissolves in excess ammonia
Co(H2O)4(OH)2(s) + 6NH3 –> [Co(NH3)6]2+ + 2OH- + 4H2O
The resulting complex is oxidised by oxygen in the air to the yellow solution it forms [Co(NH3)6]3+
To get that overall reaction, do two half equations:
[Co(NH3)6]2+ —> [Co(NH3)6]3+ +e-
and
O2 + 2H2O +4e- —> 4OH-
To get
4[Co(NH3)6]2+ + O2 + 2H2O –>
4[Co(NH3)6]3+ + 4OH-
