CATALYSIS A guide for A level students KNOCKHARDY PUBLISHING Click here to advance

CONTENTS 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 - 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. ENTHALPY CHANGE DURING AN EXOTHERMIC REACTION If the… FINAL ENTHALPY < INITIAL ENTHALPY it is anEXOTHERMIC REACTION and ENERGY IS GIVEN OUT

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 If the… FINAL ENTHALPY < INITIAL ENTHALPY it is anEXOTHERMIC REACTION and ENERGY IS GIVEN OUT FINAL ENTHALPY > INITIAL ENTHALPY it is anENDOTHERMIC REACTION andENERGY IS TAKEN IN

CATALYSTS - background ACTIVATION ENERGY - E a 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 E a 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 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

NUMBER OF MOLECUES WITH A PARTICULAR ENERGY MOLECULAR ENERGY EaEa 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 E a 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 - E a will move to the left.

The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. Lowering the Activation Energy, E a, results in a greater area under the curve after E a showing that more molecules have energies in excess of the Activation Energy EaEa EXTRA NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER MAXWELL-BOLTZMANN DISTRIBUTION NUMBER OF MOLECUES WITH A PARTICULAR ENERGY MOLECULAR ENERGY 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 E a A GREATER PROPORTION OF PARTICLES WILL HAVE ENERGIES IN EXCESS OF THE MINIMUM REQUIRED SO MORE WILL REACT WITHOUT A CATALYSTWITH A CATALYST

PRINCIPLES OF CATALYTIC ACTION The two basic types of catalytic action are … HETEROGENEOUS CATALYSIS and HOMOGENEOUS CATALYSIS

FormatCatalysts are in a different phase to the reactants e.g. a solid catalyst in a gaseous reaction Actiontakes 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...AdsorptionReactionDesorption 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 orbitals too strong W molecules remain on the surface preventing further reaction just 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, Pthydrogenation reactions FeHaber Process Rh, Pdcatalytic converters Oxides Al 2 O 3 dehydration reactions V 2 O 5 Contact Process FormatFINELY DIVIDEDincreases the surface area provides more collision sites IN A SUPPORT MEDIUMmaximizes surface area and reduces costs

Catalytic converters PURPOSE To remove pollutant gases formed in internal combustion engines 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 POLLUTANTS CARBON MONOXIDE, NITROGEN OXIDES, UNBURNT HYDROCARBONS

Pollutant gases Carbon monoxide CO Originincomplete combustion of hydrocarbons in petrol because there wasn’t enough oxygen present to convert all the carbon to carbon dioxide C 8 H 18 (g) + 8½O 2 (g) ——> 8CO(g) + 9H 2 O(l) Effectpoisonous combines with haemoglobin in blood prevents oxygen being carried Oxidation of carbon monoxide Removal 2CO(g) + O 2 (g) ——> 2CO 2 (g) 2CO(g) + 2NO(g) ——> N 2 (g) + 2CO 2 (g)

Pollutant gases Oxides of nitrogen NO x - NO, N 2 O and NO 2 Originnitrogen and oxygen combine under high temperature conditions nitrogen combines with oxygenN 2 (g) + O 2 (g) ——> 2NO(g) nitrogen monoxide is oxidised2NO(g) + O 2 (g) ——> 2NO 2 (g) Effectphotochemical 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 NO 2 NO 2 ——> NO + O ii) ozone is produced O + O 2 ——> O 3 Removal2CO(g) + 2NO(g) ——> N 2 (g) + 2CO 2 (g)

Pollutant gases Unburnt hydrocarbons C x H y Originhydrocarbons that have not undergone combustion due to insufficient oxygen Effecttoxic and carcinogenic (causes cancer) Removal catalyst aids complete combustion C 8 H 18 (g) + 12½O 2 (g) ——> 8CO 2 (g) + 9H 2 O(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 AcidsEsterificaton Conc. sulphuric acid catalyses the reaction between acids and alcohols CH 3 COOH + C 2 H 5 OH CH 3 COOC 2 H 5 + H 2 O 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 GasesAtmospheric OZONE breaks down naturally O 3 ——> O + O 2 - 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 of UV light to form chlorine radicalsCCl 2 F 2 ——> Cl + CClF 2 b. chlorine radicals then react with ozoneO 3 + Cl ——> ClO + O 2 New intermediate - ClO c. chlorine radicals are regeneratedClO + O ——> O 2 + Cl Overall, chlorine radicals are not used up so a small amount of CFC's can destroy 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 2MnO 4 ¯ + 16H + + 5C 2 O 4 2- ——> 2Mn H 2 O + 10CO 2 The titration needs to be carried out at 70°C because the reaction is slow as Mn 2+ is formed the reaction speeds up; the Mn 2+ 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 ENZYMES Action 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.

ENZYMES Action 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 A Only species with the correct shape can enter the active site in the enzyme B B Once in position, the substrate can react with a lower activation energy C C The new products do not have the correct shape to fit so the complex breaks up

ENZYMES ANIMATED ACTION A A Only species with the correct shape can enter the active site in the enzyme B B Once in position, the substrate can react with a lower activation energy C 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

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CATALYSIS The End © 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING