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All about catalysts
Catalysts are substances that increase the rate of a
chemical reaction by reducing the activation energy,
but which is left unchanged by the reaction.
(MODIFIED from URL)
This definition allows for the possibility that small
amounts of the catalyst are lost in the reaction or
that the catalytic activity is slowly lost. However, the
catalyst affects only the rate of the reaction, it
changes neither the thermodynamics of the reaction
nor the equilibrium composition
http://www.lboro.ac.uk/departments/cg/Projects/2002/dhonsi/Theory.ht
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Why are catalysts important?
The principal purpose of catalysis is to increase the rate of the
chemical reactions. It is important to understand the mechanism
by which the reaction takes place.
Catalysis is of crucial importance for the chemical industry: the
number of catalysts applied in industry is very large and
catalysts come in many different forms, from heterogeneous
catalysts in the form of porous solids and homogeneous
catalysts dissolved in the liquid reaction mixture to biological
catalysts in the form of enzymes. A reaction catalysed by a
heterogeneous catalyst can be represented by a flow chart
Making and breaking bonds
absorb:
adsorb?
Reactant molecules are adsorbed at active sites onto the surface of the
catalyst. This involves the formation of weak bonds between reactant
molecules and the catalyst which causes other bonds in the reactant
molecule to be stretched and weakened.
The weakened structure is converted to another complex that is
essentially the product attached to the catalyst. Finally, this complex
breaks down to release the product molecule which moves away to
leave the catalyst surface ready to interact with another reactant
molecule
Mechanisms:
Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism [2]
<reference.html>:
(A) Two molecules adsorb onto the surface.
(B) They diffuse across the surface and interact when they
are close.
(C) A molecule is formed which desorbs.
Eley-Rideal (E-R)
mechanism
A) Molecules adsorbs onto the surface.
(B) Another atom passes by which interacts with the one on the
surface.
(C) A molecule is formed which desorbs.
http://www.docbrown.info/page03/ASA2rates.
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http://www.saskschools.ca/curr_content/chem30_05/graphics/2_graphics/cat
alyst1.gif
http://www.agen.ufl.edu/~chyn/age2062/lect/lect_03/3_4
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Intermediate Compound theory
Catalyst = C
A + B -------------> D slow reaction
C
A + B -------------> D ?
A + C -------------> [AC] intermediate
[AC] + B-------------> D + C
produced quickly
produced quickly
The characteristic double humped profile for an
intermediate compound theory. Notice again the
lower EA
A+B
[AC]
D
modified image!
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ile.gif
http://waynesword.palomar.edu/images/enzyme
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The active site on the enzyme attaches to a substrate
molecule (such as a disaccharide) forming an enzymesubstrate complex. While attached to the substrate, the
enzyme causes a weakening of certain chemical bonds in the
substrate molecule, resulting in a breakdown (hydrolysis) of
the substrate into two smaller product molecules (such as two
monosaccharides). The enzyme is unaltered during the
reaction and is free to catalyze the breakdown of another
substrate molecule. If the active site on the enzyme is blocked
by a poison molecule, this vital hydrolysis reaction cannot
occur. See the next section
Active Site Of Enzyme Blocked By Poison Molecule
In this illustration a poison molecule has bonded to the active site of an enzyme. Now the enzyme is unable to
recognize the substrate molecule that it normally conjugates with because its active site has been blocked. In
locoweed poisoning of livestock, the alkaloid swainsonine blocks a key enzyme called mannosidase. When the
active site of mannosidase is blocked, it cannot catalyze the breakdown of the sugar mannose, resulting in a
lethal accumulation of mannose in vacuoles of cells of the central nervous system (brain and spinal cord). The
actual vacuoles are swollen organelles called lysosomes where the enzymatic breakdown process normally
occurs. The afflicted animal becomes paralyzed and eventually dies. A similar scenario occurs in infants suffering
from the storage disease mannosidosis. In this disease, the vital enzyme mannosidase is lacking due to a mutant
recessive gene. At the present time there is no cure for this disease. It is passed on via heterozygous carriers,
and shows up with a 25 percent probability when two heterozygous individuals have a child. One of the better
known storage diseases is Tay Sachs Disease, in which nerve cells fill up with a lipid called ganglioside or GM2
because they lack the vital enzyme HEX A needed to break down GM2
http://fajerpc.magnet.fsu.edu/Education/2010/Lectures/17_Enzymes_files/imag
e012.jpg