CATALYSIS A guide for A level students

1. CATALYSIS A guide for A level students 2008 SPECIFICATIONS KNOCKHARDY PUBLISHING

2. 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... www.knockhardy.org.uk/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

3. CATALYSIS • CONTENTS • Enthalpy changes • Activation Energy • Heterogeneous catalysis • Specificity • Catalytic converters • Homogeneous catalysis • Autocatalysis • Enzymes

4. 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

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. If the… FINAL ENTHALPY < INITIAL ENTHALPY it is an EXOTHERMIC REACTION andENERGY IS GIVEN OUT ENTHALPY CHANGE DURING AN EXOTHERMIC REACTION

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. If the… FINAL ENTHALPY < INITIAL ENTHALPY it is an EXOTHERMIC REACTION and ENERGY IS GIVEN OUT FINAL ENTHALPY > INITIAL ENTHALPY it is anENDOTHERMIC REACTION andENERGY IS TAKEN IN ENTHALPY CHANGE DURING AN EXOTHERMIC REACTION

8. 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 theACTIVATION ENERGY ACTIVATION ENERGY Ea 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

10. 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

11. MAXWELL-BOLTZMANN DISTRIBUTION DUE TO THE MANY COLLISONS TAKING PLACE IN GASES, THERE IS A SPREAD OF MOLECULAR ENERGY AND VELOCITY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER Ea MOLECULAR ENERGY 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.

12. MAXWELL-BOLTZMANN DISTRIBUTION DUE TO THE MANY COLLISONS TAKING PLACE IN GASES, THERE IS A SPREAD OF MOLECULAR ENERGY AND VELOCITY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY EXTRA NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER Ea MOLECULAR ENERGY 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 curveafterEashowing that more molecules have energies in excess of the Activation Energy

13. 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

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

15. 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

16. 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

17. Heterogeneous Catalysis For an explanation of what happens click on the numbers in turn, starting with 

18. 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

19. 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

20. 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

21. 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

22. Heterogeneous Catalysis ANIMATION

23. 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

24. EXAMPLES OF CATALYSTS Metals Ni, Pt hydrogenation reactions Fe Haber Process Rh, Pd catalytic converters Oxides Al2O3 dehydration reactions V2O5 Contact Process FormatFINELY DIVIDED increases the surface area provides more collision sites IN A SUPPORT MEDIUM maximises surface area and reduces costs

25. Specificity In some cases the choice of catalyst can influence the products Ethanol undergoes different reactions depending on the metal used as the catalyst. Thedistance between active sitesandtheirsimilarity with the length of bonds determines the method of adsorption and affects which bonds are weakened. CLICK HERE FOR ANIMATION

26. Specificity In some cases the choice of catalyst can influence the products Ethanol undergoes different reactions depending on the metal used as the catalyst. Thedistance between active sitesandtheirsimilarity with the length of bonds determines the method of adsorption and affects which bonds are weakened.

27. Specificity In some cases the choice of catalyst can influence the products Ethanol undergoes different reactions depending on the metal used as the catalyst. Thedistance between active sitesandtheirsimilarity with the length of bonds determines the method of adsorption and affects which bonds are weakened. Copper Dehydrogenation (oxidation) Alumina Dehydration C2H5OH ——> CH3CHO + H2 C2H5OH ——> C2H4 + H2O

28. Specificity Ethanol undergoes two different reactions depending on the metal used as the catalyst. COPPERDehydrogenation (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 ALUMINADehydration (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

29. 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 Haber process Lead catalytic converters in cars

30. Catalytic converters PURPOSE removing the pollutant gases formed in internal combustion engines POLLUTANTSCARBON MONOXIDE NITROGEN OXIDES UNBURNT HYDROCARBONS

31. Catalytic converters PURPOSE removing the pollutant gases formed in internal combustion engines POLLUTANTS CARBON MONOXIDE NITROGEN OXIDES UNBURNT HYDROCARBONS CONSTRUCTIONmade 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

32. 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)

33. 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

34. 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 Removal2CO(g) + O2(g) ——> 2CO2(g) 2CO(g) + 2NO(g) ——> N2(g) + 2CO2(g)

35. 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)

36. 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 NO2NO2 ——> NO + O ii) ozone is produced O + O2 ——> O3

37. 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 NO2NO2 ——> NO + O ii) ozone is produced O + O2 ——> O3 Removal2CO(g) + 2NO(g) ——> N2(g) + 2CO2(g)

38. 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)

39. 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

40. 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

41. 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 ——> 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 can destroy thousands of ozone molecules before the termination stage.

42. Transition metal compounds These work because of theirability 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+

43. Transition metal compounds These work because of theirability 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+ ——> 2SO42- + 2Fe3+ step 2 2Fe3+ + 2I¯ ——> 2Fe2+ + I2 overall S2O82- + 2I¯ ——> 2SO42- + I2

44. 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

45. ENZYMES Actionenzymes 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

46. 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

47. 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

48. CATALYSIS The End ©2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING