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CATALYSIS BY- SANCHIT
DHANKHAR
CONTENTS
 Introduction
 Basis
 Importance
 Classification
 Homogeneous catalysis
Mechanism
Example
 Heterogeneous catalysis
Mechanism
Examples
2
 Promoters
 Catalytic Poisoning
 Autocatalysis
 Enzyme catalysis
 Enzymes
 References
3
Catalysis
Catalyst: -
The substances that alter the rate of a
reaction but itself remains chemically
unchanged at the end of the reaction is
called a Catalyst.
The process is called Catalysis.
4
Basis of Catalysis
WITHOUT A CATALYST WITH A CATALYST
A catalyst lower the activation barrier for a transformation, by
introducing a new reaction pathway
5
6
Examples of Catalysts
Metals Ni, Pt hydrogenation reactions
Fe Haber Process
Rh, Pdcatalytic 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
7
Properties
 A small quantity of the catalyst is enough to
catalyze a large quantity of reactants into products.
 Most catalysts are highly selective.
 Though reactants are same, the type of catalyst used
may decide the products formed. 8
Eg.
C2H2 + 2H2 C2H6
C2H2 + H2 C2H4
Pt
Lindlar’s
Catalyst 8
 A catalyst cannot start the reaction by itself.
 Catalytic activity increases as surface area of
catalyst increases.
 Catalysts are thermolabile, this effect is very well
pronounced in enzymes.
 Catalytic activity is maximum at a catalyst’s
optimum temperature.
 A catalyst does not alter the position of the
equilibrium, instead it helps in achieving the
equilibrium faster.
9
 If two catalysts can catalyze the same reaction, the
better catalyst is the one which produces a greater
reduction in activation energy.
e.g. Activation energy for the decomposition of H2O2 for
different catalysts.
Catalyst EA, kJ/mol
None 75
MnO2 58
I- 56
Colloidal Pt 49
10
 The reaction occurs on the surface of the catalyst, at
special sites called active sites. Increasing surface
area, increases the number of active sites available for
catalysis.
 In homogenous catalysis, esp. in liquids, the rate of
the reaction is proportional to the concentration of the
catalyst.
11
So how do catalysts work..?
 All molecules posses energy, and during reaction they
undergo collision.
 Only those collisions result in chemical reaction, in
which the colliding molecules are associated with a
certain minimum amount of energy, called the
Threshold Energy.
 The excess of energy that molecules have to gain to
reach the threshold energy is called Activation
energy.
 Ea = Et - Ei
12
 Catalysts provide an alternate pathway for reaction.
 This alternate pathway requires lower activation
energy and is easier to achieve, and a larger number of
molecules can react and yield products.
 All these factors aid in increasing the rate of the
reaction, and that is what catalysis is all about.
13
2 14
Classification
There are two types of catalytic processes: -
1. Homogeneous catalysis
2. Heterogeneous catalysis
These two processes have industrial importance.
There is another mechanism involving catalysis i.e.
enzyme catalysis which possess biological
importance. 15
HomogeneousCatalysis
Action • catalyst and reactants are in the same phase
• the catalyst is evenly distributed throughout.
• 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
16
Transition metal compounds participate in homogeneous
catalysis as they have the ability to change their oxidation
states.
1. Reaction between iron(III) and vanadium(III)
The reaction is catalysed by Cu2+
step 1 Cu2+ + V3+ ——> Cu+ + V4+
step 2 Fe3+ + Cu+ ——> Fe2+ + Cu2+
overall Fe3+ + V3+ ——> Fe2+ + V4+
17
HeterogeneousCatalysis
 Catalyst is in different physical phase from the
reactants.
 It is also called Contact catalysis.
 It possesses great industrial importance.
It works in three steps: -
- Adsorption
- Reaction
- Desorption 18
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.
19
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.
20
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.
21
Examples of Heterogeneous Catalysis: -
1. Gas Phase
2SO2 + O2 + [Pt] 2SO3 + [Pt]
2. Liquid Phase
H2O2 + [Pt] 2 H2O + O2 + [Pt]
3. Solid Phase
2KClO3 + [MnO2] 2KCl + 3O2 + [MnO2]
22
Promoters
A substance which, though itself not a
catalyst, promotes the activity of a
catalyst is called a Promoter.
Example: -
N2 + 3H2 2NH3
23
Explanation of Promotion Action
1. Change of Lattice Space: The lattice spacing of the
catalyst is changed thus enhancing the spacing
between the catalyst particles. The adsorbed
molecules of the reactant are further weakened and
cleaved. This makes the reaction go faster.
2. Increase in peaks and cracks: Promoters increase the
peaks and cracks on the surface of the catalyst
thereby increasing the concentration of reactant
molecules and hence the rate of reaction.
24
Catalytic Poisoning
A substance which destroys the activity of the catalyst
to accelerate a reaction, is called a poison and the
process is called Catalytic Poisoning.
Example: -
2SO2 + O2 + [Pt] 2SO3
This is poisoned by As2O3
25
Explanation of Catalytic Poisoning
1. The poison is adsorbed on the catalyst surface in
preference to the reactants.
2. The catalyst may combine chemically with the
impurity.
Fe + H2S FeS + H2
26
AutoCatalysis
When one of the products of a reaction itself acts as a
catalyst for that reaction the phenomenon is called
autocatalysis.
Examples of autocatalysis: -
(a) Hydrolysis of an ester
CH3COC2H5 + H2O CH3COOH + C2H5OH
Here CH3COOH is acting as a catalyst.
27
Negative Catalysis
 When a catalyst reduces the rate of reaction, it is
called a Negative catalyst or Inhibitor.
 A negative catalyst is used to slow down or stop
altogether an unwanted reaction.
Example: -
4CHCl3 + 3O2 4COCl2 + 2H2O + 2Cl2
Chloroform (anaesthetic) on oxidation by air forms
carbonyl chloride (poisonous).
Ethanol when added to chloroform acts as a negative
catalyst.
28
Explanation of Negative Catalysis: -
1. By poisoning a catalyst.
2. By breaking a chain reaction.
Cl2 Cl. + Cl.
H2 + Cl. HCl + H.
H. + Cl2 HCl + Cl.
NCl3 breaks the chain of reactions by absorbing Cl. and the reaction stops.
NCl3 + Cl. ½ N2 + 2Cl2
29
Importance of catalysis
Many major industrial chemicals are prepared with
the aid of catalysts. Many fine chemicals are also
made with the aid of catalysts.
– Reduce cost of production
– Lead to better selectivity and less
waste
30
31
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” mech.
makes enzymes very specific as to what they catalyse.
32
MECHANISM OF ENZYME ACTION
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
33
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
34
ENZYME CATALYSIS
The catalysis brought about by enzymes is known as Enzyme
catalysis.
Examples-
1. Inversion of cane sugar
C12H22O11 +H2o
invertase
C6H12O6 + C6H12O6
GLUCOSE FRUCTOSE
C12H22O11
ZYMASE
+
C2H5OH CO2
ETHANOL
2. Conversion of glucose to ethanol
3. Hydrolysis of urea
NH2
C
O
N
H2 + H2
urease
NH3 + co2
35
36

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Catalysis- Physical Chemistry

  • 2. CONTENTS  Introduction  Basis  Importance  Classification  Homogeneous catalysis Mechanism Example  Heterogeneous catalysis Mechanism Examples 2
  • 3.  Promoters  Catalytic Poisoning  Autocatalysis  Enzyme catalysis  Enzymes  References 3
  • 4. Catalysis Catalyst: - The substances that alter the rate of a reaction but itself remains chemically unchanged at the end of the reaction is called a Catalyst. The process is called Catalysis. 4
  • 5. Basis of Catalysis WITHOUT A CATALYST WITH A CATALYST A catalyst lower the activation barrier for a transformation, by introducing a new reaction pathway 5
  • 6. 6
  • 7. Examples of Catalysts Metals Ni, Pt hydrogenation reactions Fe Haber Process Rh, Pdcatalytic 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 7
  • 8. Properties  A small quantity of the catalyst is enough to catalyze a large quantity of reactants into products.  Most catalysts are highly selective.  Though reactants are same, the type of catalyst used may decide the products formed. 8 Eg. C2H2 + 2H2 C2H6 C2H2 + H2 C2H4 Pt Lindlar’s Catalyst 8
  • 9.  A catalyst cannot start the reaction by itself.  Catalytic activity increases as surface area of catalyst increases.  Catalysts are thermolabile, this effect is very well pronounced in enzymes.  Catalytic activity is maximum at a catalyst’s optimum temperature.  A catalyst does not alter the position of the equilibrium, instead it helps in achieving the equilibrium faster. 9
  • 10.  If two catalysts can catalyze the same reaction, the better catalyst is the one which produces a greater reduction in activation energy. e.g. Activation energy for the decomposition of H2O2 for different catalysts. Catalyst EA, kJ/mol None 75 MnO2 58 I- 56 Colloidal Pt 49 10
  • 11.  The reaction occurs on the surface of the catalyst, at special sites called active sites. Increasing surface area, increases the number of active sites available for catalysis.  In homogenous catalysis, esp. in liquids, the rate of the reaction is proportional to the concentration of the catalyst. 11
  • 12. So how do catalysts work..?  All molecules posses energy, and during reaction they undergo collision.  Only those collisions result in chemical reaction, in which the colliding molecules are associated with a certain minimum amount of energy, called the Threshold Energy.  The excess of energy that molecules have to gain to reach the threshold energy is called Activation energy.  Ea = Et - Ei 12
  • 13.  Catalysts provide an alternate pathway for reaction.  This alternate pathway requires lower activation energy and is easier to achieve, and a larger number of molecules can react and yield products.  All these factors aid in increasing the rate of the reaction, and that is what catalysis is all about. 13
  • 14. 2 14
  • 15. Classification There are two types of catalytic processes: - 1. Homogeneous catalysis 2. Heterogeneous catalysis These two processes have industrial importance. There is another mechanism involving catalysis i.e. enzyme catalysis which possess biological importance. 15
  • 16. HomogeneousCatalysis Action • catalyst and reactants are in the same phase • the catalyst is evenly distributed throughout. • 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 16
  • 17. Transition metal compounds participate in homogeneous catalysis as they have the ability to change their oxidation states. 1. Reaction between iron(III) and vanadium(III) The reaction is catalysed by Cu2+ step 1 Cu2+ + V3+ ——> Cu+ + V4+ step 2 Fe3+ + Cu+ ——> Fe2+ + Cu2+ overall Fe3+ + V3+ ——> Fe2+ + V4+ 17
  • 18. HeterogeneousCatalysis  Catalyst is in different physical phase from the reactants.  It is also called Contact catalysis.  It possesses great industrial importance. It works in three steps: - - Adsorption - Reaction - Desorption 18
  • 19. 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. 19
  • 20. 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. 20
  • 21. 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. 21
  • 22. Examples of Heterogeneous Catalysis: - 1. Gas Phase 2SO2 + O2 + [Pt] 2SO3 + [Pt] 2. Liquid Phase H2O2 + [Pt] 2 H2O + O2 + [Pt] 3. Solid Phase 2KClO3 + [MnO2] 2KCl + 3O2 + [MnO2] 22
  • 23. Promoters A substance which, though itself not a catalyst, promotes the activity of a catalyst is called a Promoter. Example: - N2 + 3H2 2NH3 23
  • 24. Explanation of Promotion Action 1. Change of Lattice Space: The lattice spacing of the catalyst is changed thus enhancing the spacing between the catalyst particles. The adsorbed molecules of the reactant are further weakened and cleaved. This makes the reaction go faster. 2. Increase in peaks and cracks: Promoters increase the peaks and cracks on the surface of the catalyst thereby increasing the concentration of reactant molecules and hence the rate of reaction. 24
  • 25. Catalytic Poisoning A substance which destroys the activity of the catalyst to accelerate a reaction, is called a poison and the process is called Catalytic Poisoning. Example: - 2SO2 + O2 + [Pt] 2SO3 This is poisoned by As2O3 25
  • 26. Explanation of Catalytic Poisoning 1. The poison is adsorbed on the catalyst surface in preference to the reactants. 2. The catalyst may combine chemically with the impurity. Fe + H2S FeS + H2 26
  • 27. AutoCatalysis When one of the products of a reaction itself acts as a catalyst for that reaction the phenomenon is called autocatalysis. Examples of autocatalysis: - (a) Hydrolysis of an ester CH3COC2H5 + H2O CH3COOH + C2H5OH Here CH3COOH is acting as a catalyst. 27
  • 28. Negative Catalysis  When a catalyst reduces the rate of reaction, it is called a Negative catalyst or Inhibitor.  A negative catalyst is used to slow down or stop altogether an unwanted reaction. Example: - 4CHCl3 + 3O2 4COCl2 + 2H2O + 2Cl2 Chloroform (anaesthetic) on oxidation by air forms carbonyl chloride (poisonous). Ethanol when added to chloroform acts as a negative catalyst. 28
  • 29. Explanation of Negative Catalysis: - 1. By poisoning a catalyst. 2. By breaking a chain reaction. Cl2 Cl. + Cl. H2 + Cl. HCl + H. H. + Cl2 HCl + Cl. NCl3 breaks the chain of reactions by absorbing Cl. and the reaction stops. NCl3 + Cl. ½ N2 + 2Cl2 29
  • 30. Importance of catalysis Many major industrial chemicals are prepared with the aid of catalysts. Many fine chemicals are also made with the aid of catalysts. – Reduce cost of production – Lead to better selectivity and less waste 30
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  • 32. 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” mech. makes enzymes very specific as to what they catalyse. 32
  • 33. MECHANISM OF ENZYME ACTION 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 33
  • 34. 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 34
  • 35. ENZYME CATALYSIS The catalysis brought about by enzymes is known as Enzyme catalysis. Examples- 1. Inversion of cane sugar C12H22O11 +H2o invertase C6H12O6 + C6H12O6 GLUCOSE FRUCTOSE C12H22O11 ZYMASE + C2H5OH CO2 ETHANOL 2. Conversion of glucose to ethanol 3. Hydrolysis of urea NH2 C O N H2 + H2 urease NH3 + co2 35
  • 36. 36