CATALYSISA guide for A level students

KNOCKHARDY PUBLISHING

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CONTENTS

• Enthalpy changes

• Activation Energy

• Heterogeneous catalysis

• Specificity

• Catalytic converters

• Homogeneous catalysis

• Autocatalysis

• Enzymes

• Check list

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

CATALYSIS

CATALYSTS - backgroundAll 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 DURINGAN EXOTHERMIC REACTION

CATALYSTS - backgroundAll 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 DURINGAN EXOTHERMIC REACTION

If the…

FINAL ENTHALPY < INITIAL ENTHALPY

it is an EXOTHERMIC REACTION

and ENERGY IS GIVEN OUT

CATALYSTS - backgroundAll 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 DURINGAN EXOTHERMIC REACTION

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

CATALYSTS - backgroundACTIVATION 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 FORAN 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

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

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MOLECULAR ENERGY Ea

DUE TO THE MANY COLLISONS TAKING PLACE IN GASES, THERE

IS A SPREAD OF MOLECULAR ENERGY AND VELOCITY

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

MAXWELL-BOLTZMANN DISTRIBUTION

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.

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

Ea

EXTRA NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER

MAXWELL-BOLTZMANN DISTRIBUTION

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MOLECULAR ENERGY

DUE TO THE MANY COLLISONS TAKING PLACE IN GASES, THERE

IS A SPREAD OF MOLECULAR ENERGY AND VELOCITY

DUE TO THE MANY COLLISONS TAKING PLACE IN GASES, THERE

IS A SPREAD OF MOLECULAR ENERGY AND VELOCITY

Catalysts work by providing…

“AN ALTERNATIVE REACTION PATHWAY WHICH HAS A LOWER ACTIVATION ENERGY”

CATALYSTS - lower Ea

A GREATER PROPORTION OF PARTICLES WILL HAVE ENERGIESIN EXCESS OF THE MINIMUM REQUIRED SO MORE WILL REACT

WITHOUT A CATALYST WITH A CATALYST

PRINCIPLES OF CATALYTIC ACTION

The two basic types of catalytic action are …

HETEROGENEOUS CATALYSIS

and

HOMOGENEOUS CATALYSIS

Format Catalysts are in a different phase to the reactantse.g. a solid catalyst in a gaseous reaction

Action takes place at active sites on the surface of a solidgases are adsorbed onto the surfacethey form weak bonds with metal atoms

Catalysis is thought to work in three stages...

Adsorption

Reaction

Desorption

Heterogeneous Catalysis

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

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.

ANIMATION

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.

Heterogeneous Catalysis

ANIMATION

STRENGTH OF ADSORPTION

The STRENGTH OF ADSORPTION is critical ...

too weak Ag little adsorption - few available d orbitalstoo strong W molecules remain on the surface preventing further reactionjust 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

• two reactants are adsorbed alongside each other give a greater concentration

(Molecules will collide more often)

• one species is held in a favourable position for reaction to occur

(Molecules are lined up better)

• adsorption onto the surface allows bonds to break and fragments react quicker

(Catalyst breaks bonds, less energy needed in a collision)

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 areaprovides more collision sites

IN A SUPPORT MEDIUM maximizes surface area and reduces costs

Catalytic convertersPURPOSE To remove pollutant gases formed in internal combustion engines

CONSTRUCTION made from alloys of platinum, rhodium and palladiumcatalyst 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 collisionsinvolves HETEROGENEOUS CATALYSIS

POLLUTANTS CARBON MONOXIDE, NITROGEN OXIDES, UNBURNT HYDROCARBONS

Pollutant gases

Carbon monoxide CO

Origin incomplete combustion of hydrocarbons in petrol because there wasn’tenough oxygen present to convert all the carbon to carbon dioxide

C8H18(g) + 8½O2(g) ——> 8CO(g) + 9H2O(l)

Effect poisonouscombines with haemoglobin in bloodprevents oxygen being carried Oxidation of carbon monoxide

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 N2(g) + O2(g) ——> 2NO(g)

nitrogen monoxide is oxidised 2NO(g) + O2(g) ——> 2NO2(g)

Effect photochemical smog - irritating to eyes, nose and throatproduces low level ozone - affects plant growth

- is irritating to eyes, nose and throat

i) sunlight breaks down NO2NO2 ——> NO + O

ii) ozone is produced O + O2 ——> O3

Removal 2CO(g) + 2NO(g) ——> N2(g) + 2CO2(g)

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) + 12½O2(g) ——> 8CO2(g) + 9H2O(l)

Homogeneous Catalysis

Action • catalyst and reactants are in the same phase

• reaction proceeds through an intermediate species with lower energy

(Different intermediates are created!)

• 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 CH3COOC2H5 + H2O

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 with lower energy

(Different intermediates are created!)

• 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 lower energy

(Different intermediates are created!)

• 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 :-

a. CFC's break down in the presence ofUV light to form chlorine radicals CCl2F2 ——> Cl• + • CClF2

b. chlorine radicals then react with ozone O3 + Cl• ——> ClO• + O2

New intermediate - ClO• c. chlorine radicals are regenerated ClO• + O ——> O2 + Cl•

Overall, chlorine radicals are not used up so a small amount of CFC's candestroy thousands of ozone molecules before the termination stage.

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

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

ENZYMESAction enzymes are extremely effective biologically active catalysts

they are homogeneous catalysts, reacting in solution with body fluidsonly one type of molecule will fit the active site “lock and key” mechanismmakes enzymes very specific as to what they catalyse.

ENZYMESAction enzymes are extremely effective biologically active catalysts

they are homogeneous catalysts, reacting in solution with body fluidsonly one type of molecule will fit the active site “lock and key” mechanismmakes 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

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

You need to go over the relevant topic(s) again

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WELL DONE!

CATALYSIS

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© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING