CATALYSIS
A guide for A level students
KNOCKHARDY PUBLISHING
2015
SPECIFICATIONS
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...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
either
or
clicking on the grey arrows at the foot of each page
using the left and right arrow keys on the keyboard
CATALYSIS
CONTENTS
• Enthalpy changes
• Activation Energy
• Heterogeneous catalysis
• Specificity
• Catalytic converters
• Homogeneous catalysis
• Autocatalysis
• Enzymes
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
CATALYSTS - background
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
CATALYSTS - background
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
CATALYSTS - background
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
CATALYSTS - background
ACTIVATION ENERGY - Ea
• 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
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
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
MAXWELL-BOLTZMANN DISTRIBUTION
NUMBER OF MOLECUES WITH
A PARTICULAR ENERGY
DUE TO THE MANY COLLISONS
TAKING PLACE IN GASES, THERE
IS A SPREAD OF MOLECULAR
ENERGY AND VELOCITY
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.
MAXWELL-BOLTZMANN DISTRIBUTION
NUMBER OF MOLECUES WITH
A PARTICULAR ENERGY
DUE TO THE MANY COLLISONS
TAKING PLACE IN GASES, THERE
IS A SPREAD OF MOLECULAR
ENERGY AND VELOCITY
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
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
PRINCIPLES OF CATALYTIC ACTION
The two basic types of catalytic action are …
HETEROGENEOUS CATALYSIS
and
HOMOGENEOUS CATALYSIS
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
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
Heterogeneous Catalysis
For an explanation of what happens click on the numbers in turn, starting with 
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.
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.
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.
Desorption (STEP 4)
There is a re-arrangement of electrons and the products are then released from the active sites
Heterogeneous Catalysis
ANIMATION
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.
Desorption (STEP 4)
There is a re-arrangement of electrons and the products are then released from the active sites
Heterogeneous Catalysis
ANIMATION
STRENGTH OF ADSORPTION
The STRENGTH OF ADSORPTION is critical ...
too weak
too strong
just right
Ag
W
Ni/Pt
little adsorption - few available d orbitals
molecules remain on the surface preventing further reaction
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
EXAMPLES OF CATALYSTS
Format
Metals
Ni, Pt
Fe
Rh, Pd
hydrogenation reactions
Haber Process
catalytic converters
Oxides
Al2O3
V2O5
dehydration reactions
Contact Process
FINELY DIVIDED
increases the surface area
provides more collision sites
IN A SUPPORT MEDIUM
maximises surface area and reduces costs
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.
CLICK HERE FOR
ANIMATION
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.
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)
C2H5OH ——>
CH3CHO + H2
Alumina Dehydration
C2H5OH ——>
C2H4 + H2O
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 ——>
C2H4 + H2O
The active sites are the same distance
apart as the length of a C-O bond
It breaks to release an OH group
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
Lead
Haber process
catalytic converters in cars
Catalytic converters
PURPOSE
removing the pollutant gases formed in
internal combustion engines
POLLUTANTS
CARBON MONOXIDE
NITROGEN OXIDES
UNBURNT HYDROCARBONS
Catalytic converters
PURPOSE
removing the pollutant gases formed
in internal combustion engines
POLLUTANTS
CARBON MONOXIDE
NITROGEN OXIDES
UNBURNT HYDROCARBONS
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
Pollutant gases
Carbon monoxide CO
Origin
incomplete combustion of hydrocarbons in petrol when not enough
oxygen is present to convert all the carbon to carbon dioxide
C8H18(g) + 8½O2(g) ——> 8CO(g) + 9H2O(l)
Pollutant gases
Carbon monoxide CO
Origin
incomplete combustion of hydrocarbons in petrol when not enough
oxygen is present to convert all the carbon to carbon dioxide
C8H18(g) + 8½O2(g) ——> 8CO(g) + 9H2O(l)
Effect
poisonous
combines with haemoglobin in blood
prevents oxygen being carried to cells
Pollutant gases
Carbon monoxide CO
Origin
incomplete combustion of hydrocarbons in petrol when not enough
oxygen is present to convert all the carbon to carbon dioxide
C8H18(g) + 8½O2(g) ——> 8CO(g) + 9H2O(l)
Effect
poisonous
combines with haemoglobin in blood
prevents oxygen being carried to cells
Removal
2CO(g) + O2(g) ——> 2CO2(g)
2CO(g) + 2NO(g) ——> N2(g) + 2CO2(g)
Pollutant gases
Oxides of nitrogen NOx - NO, N2O and NO2
Origin
nitrogen and oxygen combine under high temperature conditions
nitrogen combines with oxygen
nitrogen monoxide is oxidised
N2(g) + O2(g) ——> 2NO(g)
2NO(g) + O2(g) ——> 2NO2(g)
Pollutant gases
Oxides of nitrogen NOx - NO, N2O and NO2
Origin
nitrogen and oxygen combine under high temperature conditions
nitrogen combines with oxygen
nitrogen monoxide is oxidised
Effect
N2(g) + O2(g) ——> 2NO(g)
2NO(g) + O2(g) ——> 2NO2(g)
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
ii) ozone is produced
NO2 ——> NO + O
O + O2 ——> O3
Pollutant gases
Oxides of nitrogen NOx - NO, N2O and NO2
Origin
nitrogen and oxygen combine under high temperature conditions
nitrogen combines with oxygen
nitrogen monoxide is oxidised
Effect
N2(g) + O2(g) ——> 2NO(g)
2NO(g) + O2(g) ——> 2NO2(g)
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
ii) ozone is produced
Removal
NO2 ——> NO + O
O + O2 ——> O3
2CO(g) + 2NO(g) ——> N2(g) + 2CO2(g)
Pollutant gases
Unburnt hydrocarbons CxHy
Origin
insufficient oxygen for complete combustion
Effect
toxic and carcinogenic (causes cancer)
Removal
catalyst aids complete combustion
C8H18(g) + 12½O2(g) ——>
8CO2(g) + 9H2O(l)
Homogeneous Catalysis
Action • catalyst and reactants are in the same phase
• reaction proceeds through an intermediate species of lower energy
• there is usually more than one reaction step
• transition metal ions are often involved - oxidation state changes
Example
Acids
Esterificaton
Conc. H2SO4 catalyses the reaction between acids and alcohols
CH3COOH + C2H5OH
CH3COOC2H5 + H2O
NB Catalysts have NO EFFECT ON THE POSITION OF EQUILIBRIUM
but they do affect the rate at which equilibrium is reached
Homogeneous Catalysis
Action • catalyst and reactants are in the same phase
• reaction proceeds through an intermediate species of lower energy
• there is usually more than one reaction step
• transition metal ions are often involved - oxidation state changes
Homogeneous Catalysis
Action • catalyst and reactants are in the same phase
• reaction proceeds through an intermediate species with of 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
chlorine radicals then react with ozone
O3 + Cl• ——> ClO• + O2
chlorine radicals are regenerated
ClO• + O ——> O2 + Cl•
——> Cl• + • CClF2
Overall, chlorine radicals are not used up so a small amount of CFC's can
destroy thousands of ozone molecules before the termination stage.
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
Cu2+
+
V3+
——>
Cu+
step 2
Fe3+
+
Cu+
——>
Fe2+ + Cu2+
overall
Fe3+
+
V3+
——>
Fe2+
+
+
V4+
V4+
Transition metal compounds
These work because of their ability to change oxidation state
2. Reaction between I¯ and S2O82A slow reaction because REACTANTS ARE NEGATIVE IONS  REPULSION
Addition of iron(II) catalyses the reaction
step 1
S2O82- + 2Fe2+ ——>
step 2
2Fe3+
overall
S2O82- + 2I¯
+ 2I¯
2SO42- + 2Fe3+
——>
2Fe2+ +
——>
2SO42- + I2
I2
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¯ + 16H+ + 5C2O42- ——>
2Mn2+ + 8H2O + 10CO2
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
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
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
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
CATALYSIS
The End
©2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING