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© www.chemsheets.co.uk A2 044 12-Jul-12. INTRODUCTION Catalyst = substance that speeds up reaction without being used up. They provide an alternative.

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Presentation on theme: "© www.chemsheets.co.uk A2 044 12-Jul-12. INTRODUCTION Catalyst = substance that speeds up reaction without being used up. They provide an alternative."— Presentation transcript:

1 © A Jul-12

2 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. © A Jul-12

3 INTRODUCTION

4 HETEROGENEOUS CATALYSTS Usually a solid catalyst and gaseous reactants. Most industrial catalysts are like this (e.g. Haber, Contact). © A Jul-12

5

6 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). © A Jul-12

7 HETEROGENEOUS CATALYSTS

8 Strength of adsorption: Too strong (e.g. W) Reactants cannot move round surface. Products cannot desorb. Too weak (e.g. Ag) Reactants not adsorbed. Ideal (e.g. Ni, Pt) © A Jul-12

9 Cu catalyst - dehydrogenation CH 3 CH 2 OH → CH 3 CHO + H 2 Spacing of sites suitable to bond O-H Al 2 O 3 catalyst - dehydration CH 3 CH 2 OH → CH 2 =CH 2 + H 2 O Spacing of sites suitable to bond C-O HETEROGENEOUS CATALYSTS Specificity

10 HETEROGENEOUS CATALYSTS Nature of catalyst Large surface area. Spread thinly over ceramic honeycomb.

11 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 © A Jul-12

12 HOMOGENEOUS CATALYSTS Most examples involve reactions in solution with catalyst in solution. Some gas phase examples. © A Jul-12

13 HOMOGENEOUS CATALYSTS e.g. acid catalyst 1) X + H + → HX + 2) HX + + Y → products + H + X + Y  products 1)Catalyst reacts with a reactant to form intermediate. 2)Intermediate reacts to form product faster than original reactant (and regenerates catalyst). © A Jul-12

14 HOMOGENEOUS CATALYSTS e.g. acid catalyst in esterification e.g. CH 3 COOH(l) + CH 3 OH(l)  CH 3 COOCH 3 (l) + H 2 O(l) No H + catalyst SLOW step With H + catalyst FASTER © A Jul-12

15 HOMOGENEOUS CATALYSTS e.g. transition metal catalyst 1) 2 Fe 2+ + S 2 O 8 2- → 2 Fe SO ) 2 Fe I - → 2 Fe 2+ + I 2 works by metal varying oxidation state Fe 2+ or Fe 3+ e.g. 2 I - + S 2 O 8 2- → I SO 4 2- Slow because both –ve ions so repel. © A Jul-12

16 V V S 2 O e -  2 SO V Fe 3+ + e -  Fe 2+ HOMOGENEOUS CATALYSTS I e -  2 I - © A Jul-12

17 HOMOGENEOUS CATALYSTS e.g. transition metal catalyst works by metal varying oxidation state Mn 2+ e.g. 2 MnO H C 2 O 4 2- → 2 Mn H 2 O + 10 CO 2 the reaction is catalysed by one of the products (Mn 2+ ) – this is called autocatalysis © A Jul-12


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