Starter: 1)Definition of catalysts: 2) Difference between homogeneous and heterogeneous catalyst.
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Starter:1)Definition of catalysts:
2) Difference between homogeneous and heterogeneous catalyst.
3) Do catalyst last for ever?
4) Give examples of catalysts.
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Catalysis
L.O.: What heterogeneous and homogeneous catalysts are.How heterogeneous catalyst be made more efficient.How a homogeneous catalyst works.
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INTRODUCTION
• Catalyst = substance that speeds up reaction without being used up.
• They provide an alternative mechanism with lower activation energy.
• For an equilibrium, they speed up both reactions equally and so do not effect equilibrium position.
• Heterogeneous catalyst is in different phase to the reactants.
• Homogeneous catalyst is in the same phase as the reactants.
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INTRODUCTION
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HETEROGENEOUS CATALYSTS
• Usually a solid catalyst and gaseous reactants.
• Most industrial catalysts are like this (e.g. Haber, Contact).
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HETEROGENEOUS CATALYSTS
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HETEROGENEOUS CATALYSTS
1) Reactants adsorbed onto surface (onto active sites).• weakens bonds• brings molecules closer• more favourable orientation
2) Reaction takes place.
3) Products are desorbed (leave the surface).
Animation
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Discuss in groups: how could a heterogeneous catalysts be made more efficient?
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HETEROGENEOUS CATALYSTS
Nature of catalyst
• Large surface area.
• Spread thinly over ceramic honeycomb.
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HETEROGENEOUS CATALYSTS
Poisoning
• Some substances may block active sites (i.e. they adsorb and will not come off). Can ruin catalyst.
e.g. S in Haber process
e.g. Pb in catalytic converters
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HOMOGENEOUS CATALYSTS
e.g. acid catalyst in esterification
e.g. CH3COOH(l) + CH3OH(l) CH3COOCH3(l) + H2O(l)
R C
O
OH
O H
R
No H+ catalystSLOW step
R C
O
OH
H+
R C
OH
OH
O H
R
R C
O
O R-H+
With H+ catalystFASTER
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HOMOGENEOUS CATALYSTS
e.g. transition metal catalyst
1) 2 Fe2+ + S2O82- → 2 Fe3+ + 2 SO4
2-
2) 2 Fe3+ + 2 I- → 2 Fe2+ + I2
works by metal varying oxidation state
Fe2+ or Fe3+
e.g. 2 I- + S2O82- → I2 + 2 SO4
2-
Slow because both –ve ions so repel.
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0
+ 1.51 V
+
+ 0.77 V
S2O82- + 2 e- 2
SO42-
+ 0.54 V
Fe3+ + e- Fe2+
HOMOGENEOUS CATALYSTS
I2 + 2 e- 2 I-
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HOMOGENEOUS CATALYSTS
e.g. transition metal catalyst
works by metal varying oxidation state
Mn2+
e.g. 2 MnO4- + 16 H+ + 5 C2O4
2- → 2 Mn2+ + 8 H2O + 10 CO2
the reaction is catalysed by one of the products (Mn2+) – this is called autocatalysis
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Worksheet 14.4
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Catalysis
L.O.: What heterogeneous and homogeneous catalysts are.How heterogeneous catalyst be made more efficient.How a homogeneous catalyst work.