Kinetics - Catalyst

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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 gt increase
  the rate II) negative catalyst (inhibitor) gt
  decrease the rate
• How to change the rate of reaction???

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• By providing an alternative pathway (or
  mechanism) with lower/ higher activation energy.

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• For example,Ea for the pathway with catalyst
  ltEa for the pathway without catalyst

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• The reaction can then be speeded up by increasing
  the fraction of molecules that have energies in
  excess of the Ea for a reaction.

Kinetic energy
Ea1
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Exercise

• H2 and O2 mixed at room temperaturegt no
  reactionaddition of small amount of Pt
  powdergt violent reaction
• a) What is the role of Pt?
• Catalyst
• b) What is the effect of Pt on the enthalpy
  change to the reaction between H2 and O2?
• No change

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• What is the effect of Pt on the activation energy
  of the original pathway?
• No change
• 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
  H2O2 with MnO2 as catalyst
• e.g. hydrogenation of ethene (Ni as catalyst)

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• - provides an active reaction surface for
  reactant gt reaction occurs with a lower Ea
• - are usually transition metal such asPt, Pd,
  V2O5 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 S2O32- with Fe3
  ion as catalyst
• 2I- S2O82- gt I2 2SO42-
• -------------------------------------------
• 2I- 2Fe3 gt 2Fe2 I2
• 2Fe2 S2O82- gt 2Fe3 2SO42-

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• 3. Autocatalysis
• - the product in the reaction be the catalyst
  of the reaction
• - this product is called autocatalyst
• - e.g. 2MnO4- 16H 5C2O42- gt 2Mn2
  8H2O 10CO2

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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?

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The Cost is Very High!!
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• Haber Process3H2 N2 gt 2NH3 (Fe)
• Contact Process2SO2 O2 gt 2SO3 (Pt/V2O5)
• Hydrogenation of CC(hardening of oil -
  vegetable oil to margarine)CH2CH2 H2 gt
  CH3CH3 (Ni/Pd/Pt)

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• B) Catalytic Converters in Car Exhaust
  Systems
• Convert CO, NOx hydrocarbons to harmless
  substances
• Catalyst are coated on a honeycomb gt to
  increase the surface are

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• 3 Kinds of Catalysts

Rhodium (Rd)
Platinum (Pt)
Palladum (Pd)
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Air Pollution
- Take care our environment - Relief the problem
of air pollution
Environmental Department
Test for the car exhaust
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Rhodium

• Catalyses the reduction of nitrogen oxides
• and the oxidation of carbon monoxide

N2
NO
Rd
CO2
CO
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Platinum or Palladium
Catalyses the oxidation of CO and hydrocarbons
Pt/Pd
Pt/Pd
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Biological Catalysts - Enzymes
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Further Exercise

• Download some more exercise from the following
  web-site
• http//nitec.dcu.ie/chemlc/CAL2.html