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CATALYSIS A guide for A level students KNOCKHARDY PUBLISHING.

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1 CATALYSIS A guide for A level students KNOCKHARDY PUBLISHING

2 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... Navigation is achieved by... either clicking on the grey arrows at the foot of each page orusing the left and right arrow keys on the keyboard KNOCKHARDY PUBLISHING

3 CONTENTS CONTENTS Enthalpy changes Activation Energy Heterogeneous catalysis Specificity Catalytic converters Homogeneous catalysis Autocatalysis Enzymes Check list CATALYSIS

4 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

5 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

6 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

7 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

8 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

9 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

10 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.

11 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

12 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

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

14 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

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

16 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.

17 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.

18 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.

19 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.

20 Heterogeneous Catalysis ANIMATION

21 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 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

22 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 MEDIUMmaximises surface area and reduces costs

23 Specificity In some cases the choice of catalyst can influence the products C 2 H 5 OH > CH 3 CHO + H 2 C 2 H 5 OH > C 2 H 4 + H 2 O 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. AluminaDehydrationCopperDehydrogenation (oxidation)

24 Ethanol undergoes two different reactions depending on the metal used as the catalyst. COPPER Dehydrogenation (oxidation) C 2 H 5 OH > CH 3 CHO + H 2 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) C 2 H 5 OH > C 2 H 4 + H 2 O The active sites are the same distance apart as the length of a C-O bond It breaks to release an OH group Specificity

25 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 examplesSulphurHaber process Leadcatalytic converters in cars

26 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

27 Pollutant gases Carbon monoxide CO Originincomplete combustion of hydrocarbons in petrol because there wasnt 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)

28 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)

29 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)

30 Homogeneous 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 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 NB Catalysts have NO EFFECT ON THE POSITION OF EQUILIBRIUM but they do affect the rate at which equilibrium is reached

31 Homogeneous 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

32 Homogeneous 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 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 :- CFC's break down in the presence of UV light to form chlorine radicalsCCl 2 F 2 > Cl + CClF 2 chlorine radicals then react with ozoneO 3 + Cl > ClO + O 2 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.

33 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 Cu 2+ step 1Cu 2+ + V 3+ > Cu + + V 4+ step 2Fe 3+ + Cu + > Fe 2+ + Cu 2+ overallFe 3+ + V 3+ > Fe 2+ + V Reaction between iron(III) and vanadium(III) The reaction is catalysed by Cu 2+ step 1Cu 2+ + V 3+ > Cu + + V 4+ step 2Fe 3+ + Cu + > Fe 2+ + Cu 2+ overallFe 3+ + V 3+ > Fe 2+ + V 4+

34 Transition metal compounds These work because of their ability to change oxidation state 2. Reaction between I¯ and S 2 O 8 2- A slow reaction because REACTANTS ARE NEGATIVE IONS REPULSION Addition of iron(II) catalyses the reaction step 1S 2 O Fe 2+ > 2SO Fe 3+ step 22Fe I¯ > 2Fe 2+ + I 2 overallS 2 O I¯ > 2SO I 2 2. Reaction between I¯ and S 2 O 8 2- A slow reaction because REACTANTS ARE NEGATIVE IONS REPULSION Addition of iron(II) catalyses the reaction step 1S 2 O Fe 2+ > 2SO Fe 3+ step 22Fe I¯ > 2Fe 2+ + I 2 overallS 2 O I¯ > 2SO I 2

35 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

36 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.

37 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

38 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

39 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

40 You need to go over the relevant topic(s) again Click on the button to return to the menu

41 WELL DONE! Try some past paper questions

42 CATALYSIS The End © 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING


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