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Kinetics - Catalyst
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Definition of Catalyst + A substance that alters the reaction rate of a particular chemical reaction + chemically unchanged at the end of the reaction + 2 classes :I) positive catalyst ==> increase the rate II) negative catalyst (inhibitor) ==> decrease the rate + How to change the rate of reaction???
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n By providing an alternative pathway (or mechanism) with lower/ higher activation energy.
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n For example, Ea for the pathway with catalyst < Ea for the pathway without catalyst
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n The reaction can then be speeded up by increasing the fraction of molecules that have energies in excess of the Ea for a reaction. Ea1 Ea2 Kinetic energy
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Exercise n H 2 and O 2 mixed at room temperature ==> no reaction addition of small amount of Pt powder ==> violent reaction n a) What is the role of Pt? Catalyst n b)What is the effect of Pt on the enthalpy change to the reaction between H 2 and O 2 ? No change
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n What is the effect of Pt on the activation energy of the original pathway? No change n What is the effect of Pt on the activation energy in the reaction? Give another pathway with lower activation energy
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Types of Catalyst 1.Heterogeneous Catalyst - catalyst with different phase as the reactant -usually solid state e.g. decomposition of H 2 O 2 with MnO 2 as catalyst e.g. hydrogenation of ethene (Ni as catalyst)
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- provides an active reaction surface for reactant ==> reaction occurs with a lower Ea -are usually transition metal such as Pt, Pd, V 2 O 5 and Ni
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2.Homogeneous Catalyst -catalyst with the same phase as the reactant -usually in aqueous state e.g. Oxidation of I - ion by S 2 O 3 2- with Fe 3+ ion as catalyst 2I - + S 2 O 8 2- ==> I 2 + 2SO 4 2- ------------------------------------------- 2I - + 2Fe 3+ ==> 2Fe 2+ + I 2 2Fe 2+ + S 2 O 8 2- ==> 2Fe 3+ + 2SO 4 2-
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3. Autocatalysis -the product in the reaction be the catalyst of the reaction -this product is called autocatalyst -e.g.2MnO 4 - + 16H + + 5C 2 O 4 2- ==> 2Mn 2+ + 8H 2 O + 10CO 2
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Application of Catalysts A)Usage of Catalysts in Chemical Industries Cost is always the greatest concerns of manufacturers + How can we get the highest yield of product? High pressureHigh temperatureHigh Concentration
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n Haber Process 3H 2 + N 2 ==> 2NH 3 (Fe) n Contact Process 2SO 2 + O 2 ==> 2SO 3 (Pt/V 2 O 5 ) n Hydrogenation of C=C (hardening of oil - vegetable oil to margarine) CH 2 CH 2 + H 2 ==> CH 3 CH 3 (Ni/Pd/Pt)
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B) Catalytic Converters in Car Exhaust Systems +Convert CO, NO x & hydrocarbons to harmless substances Catalyst are coated on a honeycomb ==> to increase the surface are
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3 Kinds of Catalysts : Rhodium (Rd) Palladum (Pd) Platinum (Pt)
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Air Pollution - Take care our environment - Relief the problem of air pollution Test for the car exhaust Environmental Department
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Rhodium Catalyses the reduction of nitrogen oxides and the oxidation of carbon monoxide Rd NO CO N2N2 CO 2
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CxHy +O 2 CO 2 +H 2 O CO 2 CO+ O 2 Platinum or Palladium Catalyses the oxidation of CO and hydrocarbons Pt/Pd
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Biological Catalysts - Enzymes - fermenatation - baking - washing powder contain enzymes
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Further Exercise Download some more exercise from the following web-site http://nitec.dcu.ie/~chemlc/CAL2.html
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