Transcript Kinetics - Catalyst

Kinetics -
Catalyst
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???

By providing an alternative pathway (or
mechanism) with lower/ higher activation
energy.

For example,
Ea for the pathway with catalyst <
Ea for the pathway without catalyst

The reaction can then be speeded up by
increasing the fraction of molecules that have
energies in excess of the Ea for a reaction.
Ea2
Ea1
Kinetic energy
Exercise



H2 and O2 mixed at room temperature
==> no reaction
addition of small amount of Pt powder
==> 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
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)
- provides an active reaction surface for reactant
==> reaction occurs with a lower Ea
- are usually transition metal such as
Pt, Pd, V2O5 and Ni
2.Homogeneous Catalyst
- catalyst with the same phase as the
reactant
- usually in aqueous state
e.g. Oxidation of I- ion by S2O32with Fe3+ ion as catalyst
2I- + S2O82- ==> I2 + 2SO42------------------------------------------2I- + 2Fe3+ ==> 2Fe2+ + I2
2Fe2+ + S2O82- ==> 2Fe3+ + 2SO42-
3. Autocatalysis
- the product in the reaction be the
catalyst of the reaction
- this product is called autocatalyst
- e.g. 2MnO4- + 16H+ + 5C2O42==> 2Mn2+ + 8H2O + 10CO2
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 pressure
High temperature
High Concentration

Haber Process
3H2 + N2 ==> 2NH3

(Fe)
Contact Process
2SO2 + O2 ==> 2SO3 (Pt/V2O5)

Hydrogenation of C=C
(hardening of oil - vegetable oil to margarine)
CH2CH2 + H2 ==> CH3CH3 (Ni/Pd/Pt)
B)
Catalytic Converters in Car Exhaust
Systems
Convert CO, NOx & hydrocarbons to harmless
substances
Catalyst are coated on a honeycomb
==> to increase the surface are
3 Kinds of Catalysts:
Rhodium
(Rd)
Platinum
(Pt)
Palladum
(Pd)
Air Pollution
- Take care our environment
- Relief the problem of air pollution
Environmental
Department
Test for the car exhaust
Biological Catalysts - Enzymes
- fermenatation
- baking
- washing powder
contain enzymes
Further Exercise
Download some more exercise from the
following web-site
http://nitec.dcu.ie/~chemlc/CAL2.html