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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 rateII) negative catalyst (inhibitor)
==> 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 <Ea 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.
Ea1Ea2 Kinetic energy
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Exercise H2 and O2 mixed at room temperature
==> no reactionaddition of small amount of Pt powder==> violent reaction
a) What is the role of Pt?Catalyst
b) What is the effect of Pt on theenthalpy 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 Catalyst1.Heterogeneous Catalyst
- catalyst with different phase as the reactant
- usually solid statee.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 ==> 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- ==> I2 + 2SO4
2-
-------------------------------------------
2I- + 2Fe3+ ==> 2Fe2+ + I2
2Fe2+ + S2O82- ==> 2Fe3+ + 2SO4
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. 2MnO4- + 16H+ + 5C2O4
2-
==> 2Mn2+ + 8H2O + 10CO2
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Application of CatalystsA) 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 temperature
High Concentration
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Haber Process3H2 + N2 ==> 2NH3 (Fe)
Contact Process2SO2 + O2 ==> 2SO3 (Pt/V2O5)
Hydrogenation of C=C(hardening of oil - vegetable oil to margarine)CH2CH2 + H2 ==> CH3CH3 (Ni/Pd/Pt)
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B) Catalytic Converters in Car Exhaust Systems
Convert CO, NOx & hydrocarbons to harmless subs
tancesCatalyst 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
EnvironmentalDepartment
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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