TM: Catalysis
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What is a catalysis
Increases the rate of a chemical reaction (but is not used up) by providing an alternative pathway
What is a heterogenous catalyst?
A catalyst in a different phase to the reactants. Usually solid and on the surface.
How does a heterogenous catalyst work?
One of the reactants is adsorbed onto a surface’s active sites. Can move around the surface and bond to different sites.
Advantages of a heterogenous catalyst
Increases concentration of reactants so more liklehood of collisions. Weaken bonds to make reaction easier. Position the molecule in a favourable orientation.
What would happen if the adsorption of a heterogenous catalyst is too weak
Not many molecules adsorbed
What would happen if the adsorption of a heterogenous catalyst is too strong
Molecules could not move around the active sites so less likely to collide with another reactant and react
Catalyst in the Contact process? Is it heterogenous?
V2O5 Yes
Catalyst in the Haber process? Is it heterogenous?
Fe Yes
How is a support medium used to maximise a catalysis?
Increase the surface area whilst using the thinnest layer possible. Cost - saving.
What is catalyst poisoning
Active sites blocked due to impure molecules
Effect of catalyst poisoning
Lowers the effciency of the catalyst Difficult to remove so expensive to replace
What is the contact process?
Vandium oxide (V2O5) catalysing the conversion of SO2 to SO3 for the production of H2SO4
Contact process reactions
V2O5 + SO2 → V2O4 + SO3 V2O4 + 1/2O2 → V2O5 Overall: SO2 + 1/2O2 → SO3
What is a homogenous catalyst?
A catalyst in the same phase as the reactants within solution. Involve an intermediate species
Why is reacting I- and S2O82- slow? Catalyst used?
Two negative ions repel each other Fe2+ / Fe3+
Reaction when reacting I- and S2O82- using a Fe catalyst
2Fe2+ + S2O82- → 2Fe3+ + 2SO42- 2Fe3+ + 2I- → 2Fe2+ + I2
Explain why an aq solution containing [Fe(H2O)6]3+ ions have a lower pH than an aq solution containing [Fe(H2O)6]2+ ions. [3]
[1] Fe3+ is smaller (than Fe2+) OR Fe3+ has a greater charge [2] Fe3+ ions are more polarising [3] So more O-H bonds (in the water ligands) break OR more H+ ions released
The reaction between C2O42- ions and MnO4- ions is very slow at first. Explain why the reaction is initially slow. [3]
[1] There is no/very little catalyst at the start OR the reaction only speeds up when the catalyst is produced [2] Two negative ions (MnO4- and C2O42-) repel [3] The activation energy for the reaction is high
A solution of KMnO4 has an uknown concentration. Describe briefly how colorimetry can be used to determine the concentration of this solution. [3]
[1] Make some known concentrations [2] Plot a graph of absorbance vs concentration [3] Read/compare unknown concentration from calibration curve/graph
The redox reaction, in aq solution, between acidified potassium manganate(VII) and sodium ethanedioate is autocatalysed. Write an equation for the redox reaction, identify the catalyst species and explain it’s properties which enable this. [6]
[1] 2MnO4− + 16H+ + 5C2O42− → 2Mn2+ + 8H2O + 10CO [2] Mn2+ OR Mn3+ [3] Mn can exist in variable oxidation states [4] Ea lowered because oppositely charged ions attract [5] Mn3+ (reduced) to Mn2+ by C2O42− [6] Mn2+ (oxidised (back)) to Mn3+ by MnO4−