Catalysis

CAT AL YSI S A guide for A level students

KNOCKHARDY PUBLISHING

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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CATALYSIS 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

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 N2(g) + O2(g) ——> 2NO(g) nitrogen monoxide is oxidised 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 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 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 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 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+ +

I2

2SO42- + 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

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 Click on the button to return to the menu

WELL DONE! Try some past paper questions

CAT AL YSI S The End

© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING