3. What is a catalyst ?
A substance that increases the speed of a
reaction without being used up itself.
(https://www.youtube.com/watch?v=m_9bpZep1QM) 3
4. What is a catalyst ?
A catalyst opens an alternative pathway for a chemical
reaction – with lower activation energy
4
8. What kind in these industrial applications ?
http://www.ncert.nic.in/NCERTS/l/lech105.pdf
8
9. A catalyst does NOT change the free enthalpy G and therefore
NOT the equilibrium constant K !
Remember: G = - RT ln K
It only lowers the activation energy Ea and therefore the SPEED
G
Transition
state
9
10. Activation Energy and Speed
The relation between activation energy and speed is given by:
http://www.chemguide.co.uk/physical/basicrates/arrhenius.html 10
11. Calculate the speed constant when we can lower the
activation energy to 25 kJ/mol
11
13. How does it work ?
When we want to increase the reaction speed, normally we heat
up the mixture.
In this way we create more molecule collisions.
BUT: in an exothermic reaction, increasing the heat reduces the
yield :
=> Higher speed BUT lower yield ! (Le Chatelier’s Principle)
13
14. How a system a equilibrium reacts
to changes in temperature
(Le Chatelier’s Prinicple)
“A chemical system at equilibrium always responds to
changes in order to maintain that equilibrium”
Examle: an exothermic reaction with H = - 75 kJ/mol
https://www.youtube.com/watch?v=XhQ02egUs5Y
14
15. An INCREASE in temperature would favor the endothermic
back-reaction:
So the equilibrium is shifted to the left side !
15
18. How about heat ?
We can include heat release in an exothermic reaction equation:
A + B <===> C + heat (H < 0)
When we add heat from outside, we shift the equilibrium
left or right ?
How about an endothermic reaction:
A + B + heat <===> C
18
20. Heterogeneous catalysis
Advantages:
• Easy separation between catalyst and product
• Catalyst can be re-used many times
• Higher selectivity towards a product
Disadvantages:
• Lower activity than homogeneous (surface !)
• Less specific for one reaction pathway
20
22. 1. Porous catalytic particles
For example cracking of hydrocarbons
https://www.youtube.com/watch?v=jeUW6h2oVEY 22
23. Typical example of supported catalyst
Most often transition metals, finely distributed on a
support.
“Inert” porous materials which
carry the metal atoms or ions.
Most often used are
• Aluminumoxide
• Titaniumoxide
• Active Charcoal
23
25. 3. Monolithic catalysts
example: Haber Process
N2 + 3 H2 == 2 NH3 H = - 92 kJ/mol
The process needs high pressure
– explain from the entropy !
http://www.chemguide.co.uk/physical/equilibria/haber.html
25
27. Application of Le Chatelier’s Principle
https://www.youtube.com/watch?v=Tt6diRJli6Q
27
28. Steps in catalytic reactions
https://www.youtube.com/watch?v=Gg5drDzrWwo
The steps 3 – 5
define the rate
28
29. (1) Adsorption Isotherms
(1) Langmuir: molecules occupy step by step the
whole surface, but do not sit upon each other
Typical for gas
molecules on a
metal surface
29
30. (2) Freundlich Isotherm:
more common in liquids
How could we design an experiment to find the Freundlich parameters of
the adsorption of a dye on charcoal ?
(max 80 mg dye could be a adsorbed by 1g of charcoal)
30
31. Practical example
How could we design an experiment to find the Freundlich parameters of
the adsorption of a dye on charcoal ?
(max 80 mg dye could be a adsorbed by 1g of charcoal)
We can re-write the relation: to:
So we can measure the solution conc. C at different initial concentrations
after adsorption took place:
Example: we can put 10 mg dye in 1000 ml + 1 g Charcoal
=> 8 mg should be adsorbed x/m = 8 mg/g
=> we measure C = 2 ppm = 2 mg/L
Adsorbed
mg/g
Equilibrium
conc. mg/L
31
32. Example data
Initial C eq C x/m
(1) 2 ppm 1.8 ppm 0.2 mg/g
(2) 6 ppm 5.7 ppm 0.3 mg/g
(3) 12 ppm 11.6 ppm 0.4 mg/g
(4) 20 ppm 19.5 ppm 0.5 mg/g
Calculate log x/m and log C and draw a graph.
What is k and n ?
from
preparation
measure
this
Calculate as
difference
amount
32