catalysis a guide for a level students knockhardy publishing

CATALYSISCATALYSISA guide for A level studentsA guide for A level students

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

CATALYSISCATALYSIS

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.argonet.co.uk/users/hoptonj/sci.htm

Navigation is achieved by...

either clicking on the grey arrows at the foot of each page

or using the left and right arrow keys on the keyboard

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

CONTENTSCONTENTS

• Enthalpy changes

• Activation Energy

• Heterogeneous catalysis

• Specificity

• Catalytic converters

• Homogeneous catalysis

• Autocatalysis

• Enzymes

• Check list

CATALYSISCATALYSIS

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

CATALYSISCATALYSIS

CATALYSTS - backgroundCATALYSTS - 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





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 - backgroundCATALYSTS - 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 - backgroundCATALYSTS - background

COLLISION THEORYCOLLISION 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

NU

MB

ER

OF

MO

LE

CU

ES

WIT

H A

PA

RT

ICU

LA

R E

NE

RG

Y

MOLECULAR ENERGY Ea

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 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 MAXWELL-BOLTZMANN DISTRIBUTION

NU

MB

ER

OF

MO

LE

CU

ES

WIT

H A

PA

RT

ICU

LA

R E

NE

RG

Y

MOLECULAR ENERGY





Catalysts work by providing…

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

CATALYSTS - lower ECATALYSTS - lower Eaa

A GREATER PROPORTION OF PARTICLES WILL HAVE ENERGIES

IN EXCESS OF THE MINIMUM REQUIRED SO MORE WILL REACT

WITHOUT A CATALYST WITH A CATALYST

PRINCIPLES OF CATALYTIC ACTIONPRINCIPLES 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...

AdsorptionAdsorption

ReactionReaction

DesorptionDesorption

Heterogeneous CatalysisHeterogeneous Catalysis


For an explanation of what happens click on the numbers in turn, starting with


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



ANIMATIONANIMATION


Desorption (STEP 4)There is a re-arrangement of electrons and the products are then released from the active sites




ANIMATIONANIMATION

STRENGTH OF ADSORPTIONSTRENGTH 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

• 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 CATALYSTSEXAMPLES 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 maximises surface area and reduces costs

SpecificitySpecificityIn some cases the choice of catalyst can influence the products

C2H5OH ——> CH3CHO + H2 C2H5OH ——> C2H4 + H2O

Ethanol undergoes different reactions depending on the metal used as the catalyst.The distance between active sites and their similarity with the length of bondsdetermines the method of adsorption and affects which bonds are weakened.

Alumina DehydrationCopper Dehydrogenation (oxidation)

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 distanceapart 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 distanceapart as the length of a C-O bond

It breaks to release an OH group

SpecificitySpecificity

PoisoningPoisoning

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 processLead catalytic converters in cars

Catalytic convertersCatalytic 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 gasesPollutant 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)


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)


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 CatalysisHomogeneous 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 CH3COOC2H5 + H2O

NB Catalysts have NO EFFECT ON THE POSITION OF EQUILIBRIUM

but they do affect the rate at which equilibrium is reached






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 ofUV light to form chlorine radicals CCl2F2 ——> Cl• + • CClF2

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

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.

Transition metal compoundsTransition metal compoundsThese 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+ + V4+

step 2 Fe3+ + Cu+ ——> Fe2+ + Cu2+

overall Fe3+ + V3+ ——> Fe2+ + V4+

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+

Transition metal compoundsTransition metal compoundsThese 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 S2O82- + 2Fe2+ ——> 2SO4

2- + 2Fe3+

step 2 2Fe3+ + 2I¯ ——> 2Fe2+ + I2

overall S2O82- + 2I¯ ——> 2SO4

2- + I2

2. Reaction between I¯ and S2O82-

A slow reaction because REACTANTS ARE NEGATIVE IONS REPULSION

Addition of iron(II) catalyses the reaction

step 1 S2O82- + 2Fe2+ ——> 2SO4

2- + 2Fe3+

step 2 2Fe3+ + 2I¯ ——> 2Fe2+ + I2

overall S2O82- + 2I¯ ——> 2SO4

2- + I2

Auto-catalysisAuto-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

ENZYMESENZYMESActionAction 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.

ENZYMESENZYMESActionAction 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.

AA B B C C

AA Only species with the correct shape can enter the active site in the enzyme

BB Once in position, the substrate can react with a lower activation energy

CC The new products do not have the correct shape to fit so the complex breaks up

ENZYMESENZYMES

ANIMATED ACTIONANIMATED ACTION

AA Only species with the correct shape can enter the active site in the enzyme

BB Once in position, the substrate can react with a lower activation energy

CC The new products do not have the correct shape to fit so the complex breaks up

REVISION CHECKREVISION 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? CAN YOU DO ALL OF THESE? YES YES NONO

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

Click on the button toClick on the button toreturn to the menureturn to the menu

WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions

catalysis a guide for a level students knockhardy publishing

Documents

energy particles

sufficient energy

minimum energy required

spread of molecular

exothermic reaction

homogeneous catalysis

catalyst slide

velocity slide