starter: 1)definition of catalysts: 2) difference between homogeneous and heterogeneous catalyst

Starter:1)Definition of catalysts:

2) Difference between homogeneous and heterogeneous catalyst.

3) Do catalyst last for ever?

4) Give examples of catalysts.

Catalysis

L.O.: What heterogeneous and homogeneous catalysts are.How heterogeneous catalyst be made more efficient.How a homogeneous catalyst works.

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.

INTRODUCTION

HETEROGENEOUS CATALYSTS

• Usually a solid catalyst and gaseous reactants.

• Most industrial catalysts are like this (e.g. Haber, Contact).

HETEROGENEOUS CATALYSTS

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

Discuss in groups: how could a heterogeneous catalysts be made more efficient?

HETEROGENEOUS CATALYSTS

Nature of catalyst

• Large surface area.

• Spread thinly over ceramic honeycomb.

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

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

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.

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-

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

Worksheet 14.4

Catalysis

L.O.: What heterogeneous and homogeneous catalysts are.How heterogeneous catalyst be made more efficient.How a homogeneous catalyst work.