1. A guide for A level students KNOCKHARDY PUBLISHING
CATALYSIS
A guide for A level students
KNOCKHARDY PUBLISHING

2. KNOCKHARDY PUBLISHING
CATALYSIS
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
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3. CATALYSIS CONTENTS Check list Enthalpy changes Activation Energy
Heterogeneous catalysis
Specificity
Catalytic converters
Homogeneous catalysis
Autocatalysis
Enzymes
Check list

4. Before you start it would be helpful to…
CATALYSIS
Before you start it would be helpful to…
know how the basics of collision theory
understand the importance of activation energy
understand the importance of increasing the rate of reaction

5. CATALYSTS - background ENTHALPY CHANGE DURING AN EXOTHERMIC REACTION
All reactions are accompanied by changes in enthalpy.
The enthalpy rises as the reaction starts because energy is being put in to break bonds.
It reaches a maximum then starts to fall as bonds are formed and energy is released.
ENTHALPY CHANGE DURING
AN EXOTHERMIC REACTION

6. CATALYSTS - background ENTHALPY CHANGE DURING AN EXOTHERMIC REACTION
All reactions are accompanied by changes in enthalpy.
The enthalpy rises as the reaction starts because energy is being put in to break bonds.
It reaches a maximum then starts to fall as bonds are formed and energy is released.
If the…
FINAL ENTHALPY < INITIAL ENTHALPY
it is an EXOTHERMIC REACTION
and ENERGY IS GIVEN OUT
ENTHALPY CHANGE DURING
AN EXOTHERMIC REACTION

7. CATALYSTS - background ENTHALPY CHANGE DURING AN EXOTHERMIC REACTION
All reactions are accompanied by changes in enthalpy.
The enthalpy rises as the reaction starts because energy is being put in to break bonds.
It reaches a maximum then starts to fall as bonds are formed and energy is released.
If the…
FINAL ENTHALPY < INITIAL ENTHALPY
it is an EXOTHERMIC REACTION
and ENERGY IS GIVEN OUT
FINAL ENTHALPY > INITIAL ENTHALPY
it is an ENDOTHERMIC REACTION
and ENERGY IS TAKEN IN
ENTHALPY CHANGE DURING
AN EXOTHERMIC REACTION

8. CATALYSTS - background ACTIVATION ENERGY Ea FOR AN EXOTHERMIC REACTION
Reactants will only be able to proceed to products if they have enough energy
The energy is required to overcome an energy barrier
Only those reactants with enough energy will get over
The minimum energy required is known as the ACTIVATION ENERGY
ACTIVATION ENERGY Ea FOR
AN EXOTHERMIC REACTION

9. CATALYSTS - background
COLLISION THEORY
According to COLLISON THEORY a reaction will only take place if…
PARTICLES COLLIDE
PARTICLES HAVE AT LEAST A MINIMUM AMOUNT OF ENERGY
PARTICLES ARE LINED UP CORRECTLY
To increase the chances of a successful reaction you need to...
HAVE MORE FREQUENT COLLISONS
GIVE PARTICLES MORE ENERGY or
DECREASE THE MINIMUM ENERGY REQUIRED

10. MAXWELL-BOLTZMANN DISTRIBUTION NUMBER OF MOLECUES WITH
DUE TO THE MANY COLLISONS TAKING PLACE IN GASES, THERE IS A SPREAD OF MOLECULAR ENERGY AND VELOCITY
NUMBER OF MOLECUES WITH
A PARTICULAR ENERGY
NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER
MOLECULAR ENERGY Ea
The area under the curve beyond Ea corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react.
If a catalyst is added, the Activation Energy is lowered - Ea will move to the left.

11. MAXWELL-BOLTZMANN DISTRIBUTION NUMBER OF MOLECUES WITH
DUE TO THE MANY COLLISONS TAKING PLACE IN GASES, THERE IS A SPREAD OF MOLECULAR ENERGY AND VELOCITY
NUMBER OF MOLECUES WITH
A PARTICULAR ENERGY
EXTRA NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER
MOLECULAR ENERGY Ea
The area under the curve beyond Ea corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react.
Lowering the Activation Energy, Ea, results in a greater area under the curve after Ea showing that more molecules have energies in excess of the Activation Energy

12. CATALYSTS - lower Ea Catalysts work by providing…
“AN ALTERNATIVE REACTION PATHWAY WHICH HAS A LOWER ACTIVATION ENERGY”
WITHOUT A CATALYST
WITH A CATALYST
A GREATER PROPORTION OF PARTICLES WILL HAVE ENERGIES
IN EXCESS OF THE MINIMUM REQUIRED SO MORE WILL REACT

13. PRINCIPLES OF CATALYTIC ACTION HETEROGENEOUS CATALYSIS
The two basic types of catalytic action are …
HETEROGENEOUS CATALYSIS
and
HOMOGENEOUS CATALYSIS

14. Heterogeneous Catalysis
Format Catalysts are in a different phase to the reactants
e.g. a solid catalyst in a gaseous reaction
Action takes place at active sites on the surface of a solid
gases are adsorbed onto the surface
they form weak bonds with metal atoms
Catalysis is thought to work in three stages...
Adsorption
Reaction
Desorption

15. Heterogeneous Catalysis
For an explanation of what happens click on the numbers in turn, starting with 

16. Heterogeneous Catalysis
Adsorption (STEP 1)
Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.

17. Heterogeneous Catalysis
Adsorption (STEP 1)
Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.
Reaction (STEPS 2 and 3)
Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.
This increases the chances of favourable collisions taking place.

18. Heterogeneous Catalysis
Desorption (STEP 4)
There is a re-arrangement of electrons and the products are then released from the active sites
Adsorption (STEP 1)
Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.
Reaction (STEPS 2 and 3)
Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.
This increases the chances of favourable collisions taking place.

19. Heterogeneous Catalysis
ANIMATION
Desorption (STEP 4)
There is a re-arrangement of electrons and the products are then released from the active sites
Adsorption (STEP 1)
Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.
Reaction (STEPS 2 and 3)
Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.
This increases the chances of favourable collisions taking place.

20. Heterogeneous Catalysis
ANIMATION

21. STRENGTH OF ADSORPTION
The STRENGTH OF ADSORPTION is critical ...
too weak Ag little adsorption - few available d orbitals
too strong W molecules remain on the surface preventing further reaction
just right Ni/Pt molecules are held but not too strongly so they can get away
Catalysis of gaseous reactions can lead to an increase in rate in several ways
one species is adsorbed onto the surface and is more likely to undergo a collision
one species is held in a favourable position for reaction to occur
adsorption onto the surface allows bonds to break and fragments react quicker
two reactants are adsorbed alongside each other give a greater concentration

22. Metals Ni, Pt hydrogenation reactions
EXAMPLES OF CATALYSTS
Metals Ni, Pt hydrogenation reactions
Fe Haber Process
Rh, Pd catalytic converters
Oxides Al2O3 dehydration reactions
V2O5 Contact Process
Format FINELY DIVIDED increases the surface area
provides more collision sites
IN A SUPPORT MEDIUM maximises surface area and reduces costs

23. In some cases the choice of catalyst can influence the products
Specificity
In some cases the choice of catalyst can influence the products
Ethanol undergoes different reactions depending on the metal used as the catalyst.
The distance between active sites and their similarity with the length of bonds
determines the method of adsorption and affects which bonds are weakened.
Copper Dehydrogenation (oxidation)
Alumina Dehydration
C2H5OH ——> CH3CHO + H2 C2H5OH ——> C2H H2O

24. Specificity
Ethanol undergoes two different reactions depending on the metal used as the catalyst.
COPPER Dehydrogenation (oxidation)
C2H5OH ——> CH3CHO + H2
The active sites are the same distance
apart as the length of an O-H bond
It breaks to release hydrogen
ALUMINA Dehydration (removal of water)
C2H5OH ——> C2H H2O
apart as the length of a C-O bond
It breaks to release an OH group

25. Poisoning
Impurities in a reaction mixture can also adsorb onto the surface of a catalyst thus removing potential sites for gas molecules and decreasing efficiency.
expensive because... the catalyst has to replaced
the process has to be shut down
examples Sulphur Haber process
Lead catalytic converters in cars

26. Catalytic converters
PURPOSE To remove pollutant gases formed in internal combustion engines
CONSTRUCTION made from alloys of platinum, rhodium and palladium
catalyst is mounted in a support medium to spread it out
honeycomb construction to ensure maximum gas contact
finely divided to increase surface area / get more collisions
involves HETEROGENEOUS CATALYSIS
POLLUTANTS CARBON MONOXIDE, NITROGEN OXIDES, UNBURNT HYDROCARBONS

27. Pollutant gases Carbon monoxide CO
Origin incomplete combustion of hydrocarbons in petrol because there wasn’t
enough oxygen present to convert all the carbon to carbon dioxide
C8H18(g) ½O2(g) ——> 8CO(g) H2O(l)
Effect poisonous
combines with haemoglobin in blood
prevents oxygen being carried Oxidation of carbon monoxide
Removal 2CO(g) O2(g) ——> 2CO2(g)
2CO(g) NO(g) ——> N2(g) CO2(g)

28. Oxides of nitrogen NOx - NO, N2O and NO2
Pollutant gases
Oxides of nitrogen NOx - NO, N2O and NO2
Origin nitrogen and oxygen combine under high temperature conditions
nitrogen combines with oxygen N2(g) O2(g) ——> 2NO(g)
nitrogen monoxide is oxidised 2NO(g) + O2(g) ——> 2NO2(g)
Effect photochemical smog - irritating to eyes, nose and throat
produces low level ozone - affects plant growth
- is irritating to eyes, nose and throat
i) sunlight breaks down NO2 NO2 ——> NO + O
ii) ozone is produced O + O2 ——> O3
Removal 2CO(g) NO(g) ——> N2(g) CO2(g)

29. Unburnt hydrocarbons CxHy
Pollutant gases
Unburnt hydrocarbons CxHy
Origin hydrocarbons that have not undergone combustion due to insufficient oxygen
Effect toxic and carcinogenic (causes cancer)
Removal catalyst aids complete combustion
C8H18(g) ½O2(g) ——> 8CO2(g) H2O(l)

30. Homogeneous Catalysis
Action • catalyst and reactants are in the same phase
• reaction proceeds through an intermediate species with lower energy
• there is usually more than one reaction step
• transition metal ions are often involved - oxidation state changes
Example
Acids Esterificaton
Conc. sulphuric acid catalyses the reaction between acids and alcohols
CH3COOH C2H5OH CH3COOC2H H2O
NB Catalysts have NO EFFECT ON THE POSITION OF EQUILIBRIUM
but they do affect the rate at which equilibrium is reached

31. Homogeneous Catalysis
Action • catalyst and reactants are in the same phase
• reaction proceeds through an intermediate species with lower energy
• there is usually more than one reaction step
• transition metal ions are often involved - oxidation state changes

32. Homogeneous Catalysis
Action • catalyst and reactants are in the same phase
• reaction proceeds through an intermediate species with lower energy
• there is usually more than one reaction step
• transition metal ions are often involved - oxidation state changes
Examples
Gases Atmospheric OZONE breaks down naturally O3 ——> O• O2
- it breaks down more easily in the presence of chlorofluorocarbons (CFC's).
There is a series of complex reactions but the basic process is :-
CFC's break down in the presence of
UV light to form chlorine radicals CCl2F2 ——> Cl• + • CClF2
chlorine radicals then react with ozone O3 + Cl• ——> ClO• + O2
chlorine radicals are regenerated ClO• + O ——> O Cl•
Overall, chlorine radicals are not used up so a small amount of CFC's can
destroy thousands of ozone molecules before the termination stage.

33. Transition metal compounds
These work because of their ability to change oxidation state
1. Reaction between iron(III) and vanadium(III)
The reaction is catalysed by Cu2+
step 1 Cu V3+ ——> Cu V4+
step 2 Fe Cu+ ——> Fe Cu2+
overall Fe V3+ ——> Fe V4+

34. Transition metal compounds
These work because of their ability to change oxidation state
2. Reaction between I¯ and S2O82-
A slow reaction because REACTANTS ARE NEGATIVE IONS  REPULSION
Addition of iron(II) catalyses the reaction
step 1 S2O Fe2+ ——> 2SO Fe3+
step 2 2Fe I¯ ——> 2Fe I2
overall S2O I¯ ——> 2SO I2

35. Auto-catalysis
Occurs when a product of the reaction catalyses the reaction itself
It is found in the reactions of manganate(VII) with ethandioate
2MnO4¯ H C2O42- ——> 2Mn H2O CO2
The titration needs to be carried out at 70°C because the reaction is slow as Mn2+ is formed the reaction speeds up; the Mn2+ formed acts as the catalyst

36. ENZYMES
Action enzymes are extremely effective biologically active catalysts
they are homogeneous catalysts, reacting in solution with body fluids
only one type of molecule will fit the active site “lock and key” mechanism
makes enzymes very specific as to what they catalyse.
Activity is affected by ...
temperature - it increases until the protein is denatured
substrate concentration - reaches a maximum when all sites are blocked
pH - many catalysts are amino acids which can be protonated
being poisoned - when the active sites become “clogged” with unwanted

37. ENZYMES
Action enzymes are extremely effective biologically active catalysts
they are homogeneous catalysts, reacting in solution with body fluids
only one type of molecule will fit the active site “lock and key” mechanism
makes enzymes very specific as to what they catalyse.
A B C
A Only species with the correct shape can enter the active site in the enzyme
B Once in position, the substrate can react with a lower activation energy
C The new products do not have the correct shape to fit so the complex breaks up

38. ENZYMES ANIMATED ACTION
A Only species with the correct shape can enter the active site in the enzyme
B Once in position, the substrate can react with a lower activation energy
C The new products do not have the correct shape to fit so the complex breaks up

39. What should you be able to do?
REVISION CHECK
What should you be able to do?
Recall the definition of a catalyst
Explain qualitatively how a catalyst works
Understand the difference between homogeneous and heterogeneous catalysis
Explain how heterogeneous catalysts work
Understand the importance of active sites, poisoning and specificity
Recall and understand the importance of catalytic converters
Explain how homogeneous catalysts work
Work out possible steps in simple reactions involving homogeneous catalysis
Recall and understand how enzymes work
CAN YOU DO ALL OF THESE? YES NO

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42. © 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
CATALYSIS
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